Grid

• 1: Getting started with Selenium Grid
• 2: When to Use Grid
• 3: Selenium Grid Components
• 4: Configuration of Components
• 4.1: Configuration help
• 4.2: CLI options in the Selenium Grid
• 4.3: TOML configuration options
• 5: Grid architecture
• 6: Advanced features of Selenium
• 6.1: Observability in Selenium Grid
• 6.2: GraphQL query support
• 6.3: Grid endpoints
• 6.4: Customizing a Node
• 6.5: External datastore

1 - Getting started with Selenium Grid

Quick start

1. Prerequisites
   • Java 11 or higher installed
   • Browser(s) installed
   • Browser driver(s)
     • If using Selenium 4.6, Selenium Manager will configure the drivers for Chrome, Firefox, and Edge if they are not found on the PATH.
     • Installed and on the PATH
   • Download the Selenium Server jar file from the latest release
2. Start the Grid
   • java -jar selenium-server-<version>.jar standalone
3. Point* your WebDriver tests to http://localhost:4444
4. (Optional) Check running tests and available capabilities by opening your browser at http://localhost:4444

*Wondering how to point your tests to http://localhost:4444? Check the RemoteWebDriver section.

To learn more about the different configuration options, go through the sections below.

Grid roles

Grid is composed by six different components, which gives you the option to deploy it in different ways.

Depending on your needs, you can start each one of them on its own (Distributed), group them in Hub & Node, or all in one on a single machine (Standalone).

Standalone

Standalone combines all Grid components seamlessly into one. Running a Grid in Standalone mode gives you a fully functional Grid with a single command, within a single process. Standalone can only run on a single machine.

Standalone is also the easiest mode to spin up a Selenium Grid. By default, the server will listen for RemoteWebDriver requests on http://localhost:4444. By default, the server will detect the available drivers that it can use from the System PATH.

java -jar selenium-server-<version>.jar standalone

After starting successfully the Grid in Standalone mode, point your WebDriver tests to http://localhost:4444.

Common use cases for Standalone are:

• Develop or debug tests using RemoteWebDriver locally
• Running quick test suites before pushing code
• Have a easy to setup Grid in a CI/CD tool (GitHub Actions, Jenkins, etc…)

Hub and Node

Hub and Node is the most used role because it allows to:

• Combine different machines in a single Grid
  • Machines with different operating systems and/or browser versions, for example
• Have a single entry point to run WebDriver tests in different environments
• Scaling capacity up or down without tearing down the Grid

Hub

A Hub is composed by the following components: Router, Distributor, Session Map, New Session Queue, and Event Bus.

java -jar selenium-server-<version>.jar hub

By default, the server will listen for RemoteWebDriver requests on http://localhost:4444.

Node

During startup time, the Node will detect the available drivers that it can use from the System PATH.

The command below assumes the Node is running on the same machine where the Hub is running.

java -jar selenium-server-<version>.jar node

More than one Node on the same machine

Node 1

java -jar selenium-server-<version>.jar node --port 5555

Node 2

java -jar selenium-server-<version>.jar node --port 6666

Node and Hub on different machines

Hub and Nodes talk to each other via HTTP and the Event Bus (the Event Bus lives inside the Hub). A Node sends a message to the Hub via the Event Bus to start the registration process. When the Hub receives the message, reaches out to the Node via HTTP to confirm its existence.

To successfully register a Node to a Hub, it is important to expose the Event Bus ports (4442 and 4443 by default) on the Hub machine. This also applies for the Node port. With that, both Hub and Node will be able to communicate.

If the Hub is using the default ports, the --hub flag can be used to register the Node

java -jar selenium-server-<version>.jar node --hub http://<hub-ip>:4444

When the Hub is not using the default ports, the --publish-events and --subscribe-events flags are needed.

For example, if the Hub uses ports 8886, 8887, and 8888

java -jar selenium-server-<version>.jar hub --publish-events tcp://<hub-ip>:8886 --subscribe-events tcp://<hub-ip>:8887 --port 8888

The Node needs to use those ports to register successfully

java -jar selenium-server-<version>.jar node --publish-events tcp://<hub-ip>:8886 --subscribe-events tcp://<hub-ip>:8887

Distributed

When using a Distributed Grid, each component is started separately, and ideally on different machines.

1. Event Bus: enables internal communication between different Grid components.

Default ports are: 4442, 4443, and 5557.

java -jar selenium-server-<version>.jar event-bus --publish-events tcp://<event-bus-ip>:4442 --subscribe-events tcp://<event-bus-ip>:4443 --port 5557

2. New Session Queue: adds new session requests to a queue, which will be queried by the Distributor

Default port is 5559.

java -jar selenium-server-<version>.jar sessionqueue --port 5559

3. Session Map: maps session IDs to the Node where the session is running

Default Session Map port is 5556. Session Map interacts with the Event Bus.

java -jar selenium-server-<version>.jar sessions --publish-events tcp://<event-bus-ip>:4442 --subscribe-events tcp://<event-bus-ip>:4443 --port 5556

4. Distributor: queries the New Session Queue for new session requests, and assigns them to a Node when the capabilities match. Nodes register to the Distributor the way they register to the Hub in a Hub/Node Grid.

Default Distributor port is 5553. Distributor interacts with New Session Queue, Session Map, Event Bus, and the Node(s).

java -jar selenium-server-<version>.jar distributor --publish-events tcp://<event-bus-ip>:4442 --subscribe-events tcp://<event-bus-ip>:4443 --sessions http://<sessions-ip>:5556 --sessionqueue http://<new-session-queue-ip>:5559 --port 5553 --bind-bus false

5. Router: redirects new session requests to the queue, and redirects running sessions requests to the Node running that session.

Default Router port is 4444. Router interacts with New Session Queue, Session Map, and Distributor.

java -jar selenium-server-<version>.jar router --sessions http://<sessions-ip>:5556 --distributor http://<distributor-ip>:5553 --sessionqueue http://<new-session-queue-ip>:5559 --port 4444

6. Node(s)

Default Node port is 5555.

java -jar selenium-server-<version>.jar node --publish-events tcp://<event-bus-ip>:4442 --subscribe-events tcp://<event-bus-ip>:4443

Metadata in tests

Add metadata to your tests and consume it via GraphQL or visualize parts of it (like se:name) through the Selenium Grid UI.

Metadata can be added by prefixing a capability with se:. Here is a quick example in Java showing that.

ChromeOptions chromeOptions = new ChromeOptions();
chromeOptions.setCapability("browserVersion", "100");
chromeOptions.setCapability("platformName", "Windows");
// Showing a test name instead of the session id in the Grid UI
chromeOptions.setCapability("se:name", "My simple test"); 
// Other type of metadata can be seen in the Grid UI by clicking on the 
// session info or via GraphQL
chromeOptions.setCapability("se:sampleMetadata", "Sample metadata value"); 
WebDriver driver = new RemoteWebDriver(new URL("http://gridUrl:4444"), chromeOptions);
driver.get("http://www.google.com");
driver.quit();

Querying Selenium Grid

After starting a Grid, there are mainly two ways of querying its status, through the Grid UI or via an API call.

The Grid UI can be reached by opening your preferred browser and heading to http://localhost:4444.

API calls can be done through the http://localhost:4444/status endpoint or using GraphQL

Using the Java 11 HTTP Client

Selenium v4.5

By default, Grid will use AsyncHttpClient. AsyncHttpClient is an open-source library built on top of Netty. It allows the execution of HTTP requests and responses asynchronously. Additionally it also provides WebSocket support. Hence it is a good fit.

However, AsyncHttpClient is not been actively maintained since June 2021. It coincides with the fact that Java 11+ provides a built-in HTTP and WebSocket client. Currently, Selenium has plans to upgrade the minimum version supported to Java 11. However, it is a sizeable effort. Aligning it with major releases and accompanied announcements is crucial to ensure the user experience is intact.

To do use the Java 11 client, you will need to download the selenium-http-jdk-client jar file and use the --ext flag to make it available in the Grid jar’s classpath.

The jar file can be downloaded directly from repo1.maven.org and then start the Grid in the following way:

java -Dwebdriver.http.factory=jdk-http-client -jar selenium-server-<version>.jar -—ext selenium-http-jdk-client-<version>.jar standalone

An alternative to downloading the selenium-http-jdk-client jar file is to use Coursier.

java -Dwebdriver.http.factory=jdk-http-client -jar selenium-server-<version>.jar —-ext $(coursier fetch -p org.seleniumhq.selenium:selenium-http-jdk-client:<version>) standalone

If you are using the Hub/Node(s) mode or the Distributed mode, setting the -Dwebdriver.http.factory=jdk-http-client and —-ext flags needs to be done for each one of the components.

Grid sizes

Choosing a Grid role depends on what operating systems and browsers need to be supported, how many parallel sessions need to be executed, the amount of available machines, and how powerful (CPU, RAM) those machines are.

Creating sessions concurrently relies on the available processors to the Distributor. For example, if a machine has 4 CPUs, the Distributor will only be able to create up to 4 sessions concurrently.

By default, the maximum amount of concurrent sessions a Node supports is limited by the number of CPUs available. For example, if the Node machine has 8CPUs, it can run up to 8 concurrent browser sessions (with the exception of Safari, which is always one). Additionally, it is expected that each browser session should use around 1GB RAM.

In general, it is a recommended to have Nodes as small as possible. Instead of having a machine with 32CPUs and 32GB RAM to run 32 concurrent browser sessions, it is better to have 32 small Nodes in order to better isolate processes. With this, if a Node fails, it will do it in an isolated way. Docker is a good tool to achieve this approach.

Note that the default values (1CPU/1GB RAM per browser) are a recommendation and they could not apply to your context. It is recommended to use them as a reference, but measuring performance continuously will help to determine the ideal values for your environment.

Grid sizes are relative to the amount of supported concurrent sessions and amount of Nodes, and there is no “one size fits all”. Sizes mentioned below are rough estimations thay can vary between different environments. For example a Hub/Node with 120 Nodes might work well when the Hub has enough resources. Values below are not set on stone, and feedback is welcomed!

Small

Standalone or Hub/Node with 5 or less Nodes.

Middle

Hub/Node between 6 and 60 Nodes.

Large

Hub/Node between 60 and 100 Nodes. Distributed with over 100 Nodes.

Warning

Selenium Grid must be protected from external access using appropriate firewall permissions.

Failure to protect your Grid could result in one or more of the following occurring:

• You provide open access to your Grid infrastructure
• You allow third parties to access internal web applications and files
• You allow third parties to run custom binaries

See this blog post on Detectify, which gives a good overview of how a publicly exposed Grid could be misused: Don’t Leave your Grid Wide Open

Further reading

• Components: learn how Grid’s internal components relate to each other.
• Configuration: customize your Grid setup.
• Architecture: understand key concepts in Grid.
• Advanced Features: explore more possibilities through Grid’s features.

2 - When to Use Grid

When would you use a Selenium Grid?

• To run your tests in parallel, against different browser types, browser versions, operating systems
• To reduce the time needed to execute a test suite

Selenium Grid runs test suites in parallel against multiple machines (called Nodes). For large and long-running test suites, this can save minutes, hours, or perhaps days. This shortens the turnaround time for test results as your application under test (AUT) changes.

Grid can run tests (in parallel) against multiple different browsers, and it can run against multiple instances of the same browser. As an example, let’s imagine a Grid with six Nodes. The first machine has Firefox’s latest version, the second has Firefox “latest minus one”, the third gets the latest Chrome, and the remaining three machines are Mac Minis, which allows for three tests to run in parallel on the latest version of Safari.

Execution time can be expressed as a simple formula:

Number of Tests * Average Test Time / Number of Nodes = Total Execution Time

   15      *       45s        /        1        =      11m 15s   // Without Grid
   15      *       45s        /        5        =      2m 15s    // Grid with 5 Nodes
   15      *       45s        /        15       =      45s       // Grid with 15 Nodes
  100      *       120s       /        15       =      13m 20s   // Would take over 3 hours without Grid

As the test suite is executing, the Grid allocates the tests to run against these browsers as configured in the tests.

A configuration such as this can greatly speed up the execution time of even the largest Selenium test suites.

Selenium Grid is a completely native part of the Selenium project, and is maintained in parallel by the same team of committers who work in the core Selenium development. Recognizing the importance of test execution speed, Grid has been a critical part of the Selenium project since the earliest days.

3 - Selenium Grid Components

Selenium Grid 4 is a ground-up rewrite from previous versions. In addition to a comprehensive set of improvements to performance and standards compliance, the different functions of the grid were broken out to reflect a more modern age of computing and software development. Purpose-build for containerization and cloud-distributed scalability, Selenium Grid 4 is a wholly new solution for the modern era.

Router

The Router is the entry point of the Grid, receiving all external requests, and forwards them to the correct component.

If the Router receives a new session request, it will be forwarded to the New Session Queue.

If the request belongs to an existing session, the Router will query the Session Map to get the Node ID where the session is running, and then the request will be forwarded directly to the Node.

The Router balances the load in the Grid by sending the requests to the component that is able to handle them better, without overloading any component that is not needed in the process.

Distributor

The Distributor has two main responsibilities:

Register and keep track of all Nodes and their capabilities

A Node registers to the Distributor by sending a Node registration event through the Event Bus. The Distributor reads it, and then tries to reach the Node via HTTP to confirm its existance. If the request is successfull, the Distributor registers the Node and keeps track of all Nodes capabilities through the GridModel.

Query the New Session Queue and process any pending new session requests

When a new session request is sent to the Router, it gets forwarded to the New Session Queue, where it will wait in the queue. The Distributor will poll the New Session Queue for pending new session requests, and then finds a suitable Node where the session can be created. After the session has been created, the Distributor stores in the Session Map the relation between the session id and Node where the session is being executed.

Session Map

The Session Map is a data store that keeps the relationship between the session id and the Node where the session is running. It supports the Router in the process of forwarding a request to the Node. The Router will ask the Session Map for the Node associated to a session id.

New Session Queue

The New Session Queue holds all the new session requests in a FIFO order. It has configurable parameters for setting the request timeout and request retry interval (how often the timeout will be checked).

The Router adds the new session request to the New Session Queue and waits for the response. The New Session Queue regularly checks if any request in the queue has timed out, if so the request is rejected and removed immediately.

The Distributor regularly checks if a slot is available. If so, the Distributor polls the New Session Queue for the first matching request. The Distributor then attempts to create a new session.

Once the requested capabilities match the capabilities of any of the free Node slots, the Distributor attempts to get the available slot. If all the slots are busy, the Distributor will send the request back to the queue. If request times out while retrying or adding to the front of the queue, it will be rejected.

After a session is created successfully, the Distributor sends the session information to the New Session Queue, which then gets sent back to the Router, and finally to the client.

Node

A Grid can contain multiple Nodes. Each Node manages the slots for the available browsers of the machine where it is running.

The Node registers itself to the Distributor through the Event Bus, and its configuration is sent as part of the registration message.

By default, the Node auto-registers all browser drivers available on the path of the machine where it runs. It also creates one slot per available CPU for Chromium based browsers and Firefox. For Safari, only one slot is created. Through a specific configuration, it can run sessions in Docker containers or relay commands.

A Node only executes the received commands, it does not evaluate, make judgments, or control anything other than the flow of commands and responses. The machines where the Node is running does not need to have the same operating system as the other components. For example, A Windows Node might have the capability of offering IE Mode on Edge as a browser option, whereas this would not be possible on Linux or Mac, and a Grid can have multiple Nodes configured with Windows, Mac, or Linux.

Event Bus

The Event Bus serves as a communication path between the Nodes, Distributor, New Session Queue, and Session Map. The Grid does most of its internal communication through messages, avoiding expensive HTTP calls. When starting the Grid in its fully distributed mode, the Event Bus is the first component that should be started.

4 - Configuration of Components

4.1 - Configuration help

Info Command

The info command provides detailed docs on the following topics:

• Configuring Selenium
• Security
• Session Map setup
• Tracing

Config help

Quick config help and overview is provided by running:

java -jar selenium-server-<version>.jar info config

Security

To get details on setting up the Grid servers for secure communication and node registration:

java -jar selenium-server-<version>.jar info security

Session Map setup

By default, Grid uses a local session map to store session information. Grid supports additional storage options like Redis and JDBC - SQL supported databases. To set up different session storage, use the following command to get setup steps:

java -jar selenium-server-<version>.jar info sessionmap

Setting up tracing with OpenTelemetry and Jaeger

By default, tracing is enabled. To export traces and visualize them via Jaeger, use the following command for instructions:

java -jar selenium-server-<version>.jar info tracing

List the Selenium Grid commands

java -jar selenium-server-<version>.jar --config-help

It will show all the available commands and description for each one.

Component help commands

Pass –help config option after the Selenium role to get component-specific config information.

Standalone

java -jar selenium-server-<version>.jar standalone --help

Hub

java -jar selenium-server-<version>.jar hub --help

Sessions

java -jar selenium-server-<version>.jar sessions --help

New Session Queue

java -jar selenium-server-<version>.jar sessionqueue --help

Distributor

java -jar selenium-server-<version>.jar distributor --help

Router

java -jar selenium-server-<version>.jar router --help

Node

java -jar selenium-server-<version>.jar node --help

4.2 - CLI options in the Selenium Grid

Different sections are available to configure a Grid. Each section has options can be configured through command line arguments.

A complete description of the component to section mapping can be seen below.

Sections

Distributor

Docker

Events

Logging

Network

Node

Relay

Router

Server

SessionQueue

Sessions

Configuration examples

All the options mentioned above can be used when starting the Grid components. They are a good way of exploring the Grid options, and trying out values to find a suitable configuration.

Command-line flags

To pass config options as command-line flags, identify the valid options for the component and follow the template below.

java -jar selenium-server-<version>.jar <component> --<option> value

Standalone, setting max sessions and main port

java -jar selenium-server-<version>.jar standalone --max-sessions 4 --port 4444

Hub, setting a new session request timeout, a main port, and disabling tracing

java -jar selenium-server-<version>.jar hub --session-request-timeout 500 --port 3333 --tracing false

Node, with 4 max sessions, with debug(fine) log, 7777 as port, and only with Firefox and Edge

java -jar selenium-server-<version>.jar node --max-sessions 4 --log-level "fine" --port 7777 --driver-implementation "firefox" --driver-implementation "edge"

Distributor, setting Session Map server url, Session Queue server url, and disabling bus

java -jar selenium-server-<version>.jar distributor --sessions http://localhost:5556 --sessionqueue http://localhost:5559 --bind-bus false

Setting custom capabilities for matching specific Nodes

Important: Custom capabilities need to be set in the configuration in all Nodes. They also need to be included always in every session request.

Start the Hub

java -jar selenium-server-<version>.jar hub

Start the Node A with custom cap set to true

java -jar selenium-server-<version>.jar node --detect-drivers false --driver-configuration display-name="Chrome (custom capability true)" max-sessions=1 stereotype='{"browserName":"chrome","gsg:customcap":true}' --port 6161

Start the Node B with custom cap set to false

java -jar selenium-server-<version>.jar node --detect-drivers false --driver-configuration display-name="Chrome (custom capability true)" max-sessions=1 stereotype='{"browserName":"chrome","gsg:customcap":false}' --port 6262

Matching Node A

ChromeOptions options = new ChromeOptions();
options.setCapability("gsg:customcap", true);
WebDriver driver = new RemoteWebDriver(new URL("http://localhost:4444"), options);
driver.get("https://selenium.dev");
driver.quit();

Set the custom capability to false in order to match the Node B.

4.3 - TOML configuration options

All the options shown in CLI options can be configured through a TOML file. This page shows configuration examples for the different Grid components.

Overview

Selenium Grid uses TOML format for config files. The config file consists of sections and each section has options and its respective value(s).

Refer to the TOML documentation for detailed usage guidance. In case of parsing errors, validate the config using TOML linter.

The general configuration structure has the following pattern:

[section1]
option1="value"

[section2]
option2=["value1","value2"]
option3=true

Below are some examples of Grid components configured with a Toml file, the component can be started in the following way:

java -jar selenium-server-<version>.jar <component> --config /path/to/file/<file-name>.toml

Standalone

A Standalone server, running on port 4449, and a new session request timeout of 500 seconds.

[server]
port = 4449

[sessionqueue]
session-request-timeout = 500

Specific browsers and a limit of max sessions

A Standalone server or a Node which only has Firefox and Chrome enabled by default.

[node]
drivers = ["chrome", "firefox"]
max-sessions = 3

Configuring and customising drivers

Standalone or Node server with customised drivers, which allows things like having Firefox Beta or Nightly, and having different browser versions.

[node]
detect-drivers = false
[[node.driver-configuration]]
max-sessions = 100
display-name = "Firefox Nightly"
stereotype = "{\"browserName\": \"firefox\", \"browserVersion\": \"93\", \"platformName\": \"MAC\", \"moz:firefoxOptions\": {\"binary\": \"/Applications/Firefox Nightly.app/Contents/MacOS/firefox-bin\"}}"
[[node.driver-configuration]]
display-name = "Chrome Beta"
stereotype = "{\"browserName\": \"chrome\", \"browserVersion\": \"94\", \"platformName\": \"MAC\", \"goog:chromeOptions\": {\"binary\": \"/Applications/Google Chrome Beta.app/Contents/MacOS/Google Chrome Beta\"}}"
[[node.driver-configuration]]
display-name = "Chrome Dev"
stereotype = "{\"browserName\": \"chrome\", \"browserVersion\": \"95\", \"platformName\": \"MAC\", \"goog:chromeOptions\": {\"binary\": \"/Applications/Google Chrome Dev.app/Contents/MacOS/Google Chrome Dev\"}}"
webdriver-executable = '/path/to/chromedriver/95/chromedriver'

Standalone or Node with Docker

A Standalone or Node server that is able to run each new session in a Docker container. Disabling drivers detection, having maximum 2 concurrent sessions. Stereotypes configured need to be mapped to a Docker image, and the Docker daemon needs to be exposed via http/tcp. In addition, it is possible to define which device files, accessible on the host, will be available in containers through the devices property. Refer to the docker documentation for more information about how docker device mapping works.

[node]
detect-drivers = false
max-sessions = 2

[docker]
configs = [
    "selenium/standalone-chrome:93.0", "{\"browserName\": \"chrome\", \"browserVersion\": \"91\"}", 
    "selenium/standalone-firefox:92.0", "{\"browserName\": \"firefox\", \"browserVersion\": \"92\"}"
]
#Optionally define all device files that should be mapped to docker containers
#devices = [
#    "/dev/kvm:/dev/kvm"
#]
url = "http://localhost:2375"
video-image = "selenium/video:latest"

Relaying commands to a service endpoint that supports WebDriver

It is useful to connect an external service that supports WebDriver to Selenium Grid. An example of such service could be a cloud provider or an Appium server. In this way, Grid can enable more coverage to platforms and versions not present locally.

The following is an en example of connecting an Appium server to Grid.

[node]
detect-drivers = false

[relay]
# Default Appium/Cloud server endpoint
url = "http://localhost:4723/wd/hub"
status-endpoint = "/status"
# Stereotypes supported by the service. The initial number is "max-sessions", and will allocate 
# that many test slots to that particular configuration
configs = [
  "5", "{\"browserName\": \"chrome\", \"platformName\": \"android\", \"appium:platformVersion\": \"11\"}"
]

Basic auth enabled

It is possible to protect a Grid with basic auth by configuring the Router/Hub/Standalone with a username and password. This user/password combination will be needed when loading the Grid UI or starting a new session.

[router]
username = "admin"
password = "myStrongPassword"

Here is a Java example showing how to start a session using the configured user and password.

URL gridUrl = new URL("http://admin:myStrongPassword@localhost:4444");
RemoteWebDriver webDriver = new RemoteWebDriver(gridUrl, new ChromeOptions());

Setting custom capabilities for matching specific Nodes

Important: Custom capabilities need to be set in the configuration in all Nodes. They also need to be included always in every session request.

[node]
detect-drivers = false

[[node.driver-configuration]]
display-name = "firefox"
stereotype = '{"browserName": "firefox", "platformName": "macOS", "browserVersion":"96", "networkname:applicationName":"node_1", "nodename:applicationName":"app_1" }'
max-sessions = 5

Here is a Java example showing how to match that Node

FirefoxOptions options = new FirefoxOptions();
options.setCapability("networkname:applicationName", "node_1");
options.setCapability("nodename:applicationName", "app_1");
options.setBrowserVersion("96");
options.setPlatformName("macOS");
WebDriver driver = new RemoteWebDriver(new URL("http://localhost:4444"), options);
driver.get("https://selenium.dev");
driver.quit();

5 - Grid architecture

The Grid is designed as a set of components that all fulfill a role in maintaining the Grid. It can seem quite complicated, but hopefully this document can help clear up any confusion.

The Key Components

The main components of the Grid are:

Event Bus

Used for sending messages which may be received asynchronously between the other components.

New Session Queue

Maintains a list of incoming sessions which have yet to be assigned to a Node by the Distributor.

Distributor

Responsible for maintaining a model of the available locations in the Grid where a session may run (known as "slots") and taking any incoming new session requests and assigning them to a slot.

Node

Runs a WebDriver session. Each session is assigned to a slot, and each node has one or more slots.

Session Map

Maintains a mapping between the session ID and the address of the Node the session is running on.

Router

Acts as the front-end of the Grid. This is the only part of the Grid which may be exposed to the wider Web (though we strongly caution against it). This routes incoming requests to either the New Session Queue or the Node on which the session is running.

While discussing the Grid, there are some other useful concepts to keep in mind:

• A slot is the place where a session can run.
• Each slot has a stereotype. This is the minimal set of capabilities that a new session session request must match before the Distributor will send that request to the Node owning the slot.
• The Grid Model is how the Distributor tracks the state of the Grid. As the name suggests, this may sometimes fall out of sync with reality (perhaps because the Distributor has only just started). It is used in preference to querying each Node so that the Distributor can quickly assign a slot to a New Session request.

Synchronous and Asynchronous Calls

There are two main communication mechanisms used within the Grid:

1. Synchronous “REST-ish” JSON over HTTP requests.
2. Asynchronous events sent to the Event Bus.

How do we pick which communication mechanism to use? After all, we could model the entire Grid in an event-based way, and it would work out just fine.

The answer is that if the action being performed is synchronous (eg. most WebDriver calls), or if missing the response would be problematic, the Grid uses a synchronous call. If, instead, we want to broadcast information to anyone who’s interested, or if missing the response doesn’t matter, then we prefer to use the event bus.

One interesting thing to note is that the async calls are more decoupled from their listeners than the synchronous calls are.

Start Up Sequence and Dependencies Between Components

Although the Grid is designed to allow components to start up in any order, conceptually the order in which components starts is:

1. The Event Bus and Session Map start first. These have no other dependencies, not even on each other, and so are safe to start in parallel.
2. The Session Queue starts next.
3. It is now possible to start the Distributor. This will periodically connect to the Session Queue and poll for jobs, though this polling might be initiated either by an event (that a New Session has been added to the queue) or at regular intervals.
4. The Router(s) can be started. New Session requests will be directed to the Session Queue, and the Distributor will attempt to find a slot to run the session on.
5. We are now able to start a Node. See below for details about how the Node is registered with the Grid. Once registration is complete, the Grid is ready to serve traffic.

You can picture the dependencies between components this way, where a “✅” indicates that there is a synchronous dependency between the components.

Node Registration

The process of registering a new Node to the Grid is lightweight.

1. When the Node starts, it should emit a “heart beat” event on a regular basis. This heartbeat contains the node status.
2. The Distributor listens for the heart beat events. When it sees one, it attempts to GET the /status endpoint of the Node. It is from this information that the Grid is set up.

The Distributor will use the same /status endpoint to check the Node on a regular basis, but the Node should continue sending heart beat events even after started so that a Distributor without a persistent store of the Grid state can be restarted and will (eventually) be up to date and correct.

The Node Status Object

The Node Status is a JSON blob with the following fields:

It is recommended to put values in all fields.

The Slot Object

The Slot object represents a single slot within a Node. A “slot” is where a single session may be run. It is possible that a Node will have more slots than it can run concurrently. For example, a node may be able to run up 10 sessions, but they could be any combination of Chrome, Edge, or Firefox; in this case, the Node would indicate a “max session count” of 10, and then also say it has 10 slots for Chrome, 10 for Edge, and 10 for Firefox.

The Session Object

This represents a running session within a slot

6 - Advanced features of Selenium

6.1 - Observability in Selenium Grid

Table of Contents

• Selenium Grid
• Observability
  • Distributed tracing
  • Event logging
• Grid Observability
  • Visualizing Traces
  • Leveraging event logs
• References

Selenium Grid

Grid aids in scaling and distributing tests by executing tests on various browser and operating system combinations.

Observability

Observability has three pillars: traces, metrics and logs. Since Selenium Grid 4 is designed to be fully distributed, observability will make it easier to understand and debug the internals.

Distributed tracing

A single request or transaction spans multiple services and components. Tracing tracks the request lifecycle as each service executes the request. It is useful in debugging in an error scenario. Some key terms used in tracing context are:

Trace Tracing allows one to trace a request through multiple services, starting from its origin to its final destination. This request’s journey helps in debugging, monitoring the end-to-end flow, and identifying failures. A trace depicts the end-to-end request flow. Each trace has a unique id as its identifier.

Span Each trace is made up of timed operations called spans. A span has a start and end time and it represents operations done by a service. The granularity of span depends on how it is instrumented. Each span has a unique identifier. All spans within a trace have the same trace id.

Span Attributes Span attributes are key-value pairs which provide additional information about each span.

Events Events are timed-stamped logs within a span. They provide additional context to the existing spans. Events also contain key-value pairs as event attributes.

Event logging

Logging is essential to debug an application. Logging is often done in a human-readable format. But for machines to search and analyze the logs, it has to have a well-defined format. Structured logging is a common practice of recording logs consistently in a fixed format. It commonly contains fields like:

• Timestamp
• Logging level
• Logger class
• Log message (This is further broken down into fields relevant to the operation where the log was recorded)

Logs and events are closely related. Events encapsulate all the possible information available to do a single unit of work. Logs are essentially subsets of an event. At the crux, both aid in debugging. Refer following resources for detailed understanding:

1. https://www.honeycomb.io/blog/how-are-structured-logs-different-from-events/
2. https://charity.wtf/2019/02/05/logs-vs-structured-events/

Grid Observability

Selenium server is instrumented with tracing using OpenTelemetry. Every request to the server is traced from start to end. Each trace consists of a series of spans as a request is executed within the server. Most spans in the Selenium server consist of two events:

1. Normal event - records all information about a unit of work and marks successful completion of the work.
2. Error event - records all information till the error occurs and then records the error information. Marks an exception event.

Running Selenium server

1. Standalone
2. Hub and Node
3. Fully Distributed
4. Docker

Visualizing Traces

All spans, events and their respective attributes are part of a trace. Tracing works while running the server in all of the above-mentioned modes.

By default, tracing is enabled in the Selenium server. Selenium server exports the traces via two exporters:

1. Console - Logs all traces and their included spans at FINE level. By default, Selenium server prints logs at INFO level and above. The log-level flag can be used to pass a logging level of choice while running the Selenium Grid jar/s.

java -jar selenium-server-4.0.0-<selenium-version>.jar standalone --log-level FINE

2. Jaeger UI - OpenTelemetry provides the APIs and SDKs to instrument traces in the code. Whereas Jaeger is a tracing backend, that aids in collecting the tracing telemetry data and providing querying, filtering and visualizing features for the data.

Detailed instructions of visualizing traces using Jaeger UI can be obtained by running the command :

java -jar selenium-server-4.0.0-<selenium-version>.jar info tracing

A very good example and scripts to run the server and send traces to Jaeger

Leveraging event logs

Tracing has to be enabled for event logging as well, even if one does not wish to export traces to visualize them.
By default, tracing is enabled. No additional parameters need to be passed to see logs on the console. All events within a span are logged at FINE level. Error events are logged at WARN level.

All event logs have the following fields :

Sample log

FINE [LoggingOptions$1.lambda$export$1] - {
  "traceId": "fc8aef1d44b3cc8bc09eb8e581c4a8eb",
  "spanId": "b7d3b9865d3ddd45",
  "spanKind": "INTERNAL",
  "eventTime": 1597819675128886121,
  "eventName": "Session request execution complete",
  "attributes": {
    "http.status_code": 200,
    "http.handler_class": "org.openqa.selenium.grid.router.HandleSession",
    "http.url": "\u002fsession\u002fdd35257f104bb43fdfb06242953f4c85",
    "http.method": "DELETE",
    "session.id": "dd35257f104bb43fdfb06242953f4c85"
  }
}

In addition to the above fields, based on OpenTelemetry specification error logs consist of :

Sample error log

WARN [LoggingOptions$1.lambda$export$1] - {
  "traceId": "7efa5ea57e02f89cdf8de586fe09f564",
  "spanId": "914df6bc9a1f6e2b",
  "spanKind": "INTERNAL",
  "eventTime": 1597820253450580272,
  "eventName": "exception",
  "attributes": {
    "exception.type": "org.openqa.selenium.ScriptTimeoutException",
    "exception.message": "Unable to execute request: java.sql.SQLSyntaxErrorException: Table 'mysql.sessions_mappa' doesn't exist ..." (full message will be printed),
    "exception.stacktrace": "org.openqa.selenium.ScriptTimeoutException: java.sql.SQLSyntaxErrorException: Table 'mysql.sessions_mappa' doesn't exist\nBuild info: version: '4.0.0-alpha-7', revision: 'Unknown'\nSystem info: host: 'XYZ-MacBook-Pro.local', ip: 'fe80:0:0:0:10d5:b63a:bdc6:1aff%en0', os.name: 'Mac OS X', os.arch: 'x86_64', os.version: '10.13.6', java.version: '11.0.7'\nDriver info: driver.version: unknown ...." (full stack will be printed),
    "http.handler_class": "org.openqa.selenium.grid.distributor.remote.RemoteDistributor",
    "http.url": "\u002fsession",
    "http.method": "POST"
  }
}

Note: Logs are pretty printed above for readability. Pretty printing for logs is turned off in Selenium server.

The steps above should set you up for seeing traces and logs.

References

1. Understanding Tracing
2. OpenTelemetry Tracing API Specification
3. Selenium Wiki
4. Structured logs vs events
5. Jaeger framework

6.2 - GraphQL query support

GraphQL is a query language for APIs and a runtime for fulfilling those queries with your existing data. It gives users the power to ask for exactly what they need and nothing more.

Enums

Enums represent possible sets of values for a field.

For example, the Node object has a field called status. The state is an enum (specifically, of type Status) because it may be UP , DRAINING or UNAVAILABLE.

Scalars

Scalars are primitive values: Int, Float, String, Boolean, or ID.

When calling the GraphQL API, you must specify nested subfield until you return only scalars.

Structure of the Schema

The structure of grid schema is as follows:

{
    session(id: "<session-id>") : {
        id,
        capabilities,
        startTime,
        uri,
        nodeId,
        nodeUri,
        sessionDurationMillis
        slot : {
            id,
            stereotype,
            lastStarted
        }
    }
    grid: {
        uri,
        totalSlots,
        nodeCount,
        maxSession,
        sessionCount,
        version,
        sessionQueueSize
    }
    sessionsInfo: {
        sessionQueueRequests,
        sessions: [
            {
                id,
                capabilities,
                startTime,
                uri,
                nodeId,
                nodeUri,
                sessionDurationMillis
                slot : {
                    id,
                    stereotype,
                    lastStarted
                }
            }
        ]
    }
    nodesInfo: {
        nodes : [
            {
                id,
                uri,
                status,
                maxSession,
                slotCount,
                sessions: [
                    {
                        id,
                        capabilities,
                        startTime,
                        uri,
                        nodeId,
                        nodeUri,
                        sessionDurationMillis
                        slot : {
                            id,
                            stereotype,
                            lastStarted
                        }
                    }
                ],
                sessionCount,
                stereotypes,
                version,
                osInfo: {
                    arch,
                    name,
                    version
                }
            }
        ]
    }
}

Querying GraphQL

The best way to query GraphQL is by using curl requests. The query is interpreted as JSON. Ensure double quotes are properly escaped to avoid unexpected errors. GraphQL allows you to fetch only the data that you want, nothing more nothing less.

Some of the example GraphQL queries are given below. You can build your own queries as you like.

Querying the number of maxSession and sessionCount in the grid :

curl -X POST -H "Content-Type: application/json" --data '{"query": "{ grid { maxSession, sessionCount } }"}' -s <LINK_TO_GRAPHQL_ENDPOINT>

Generally on local machine the <LINK_TO_GRAPHQL_ENDPOINT> would be http://localhost:4444/graphql

Querying all details for session, node and the Grid :

curl -X POST -H "Content-Type: application/json" --data '{"query":"{ grid { uri, maxSession, sessionCount }, nodesInfo { nodes { id, uri, status, sessions { id, capabilities, startTime, uri, nodeId, nodeUri, sessionDurationMillis, slot { id, stereotype, lastStarted } }, slotCount, sessionCount }} }"}' -s <LINK_TO_GRAPHQL_ENDPOINT>

Query for getting the current session count in the Grid :

curl -X POST -H "Content-Type: application/json" --data '{"query":"{ grid { sessionCount } }"}' -s <LINK_TO_GRAPHQL_ENDPOINT>

Query for getting the max session count in the Grid :

curl -X POST -H "Content-Type: application/json" --data '{"query":"{ grid { maxSession } }"}' -s <LINK_TO_GRAPHQL_ENDPOINT>

Query for getting all session details for all nodes in the Grid :

curl -X POST -H "Content-Type: application/json" --data '{"query":"{ sessionsInfo { sessions { id, capabilities, startTime, uri, nodeId, nodeId, sessionDurationMillis } } }"}' -s <LINK_TO_GRAPHQL_ENDPOINT>

Query to get slot information for all sessions in each Node in the Grid :

curl -X POST -H "Content-Type: application/json" --data '{"query":"{ sessionsInfo { sessions { id, slot { id, stereotype, lastStarted } } } }"}' -s <LINK_TO_GRAPHQL_ENDPOINT>

Query to get session information for a given session:

curl -X POST -H "Content-Type: application/json" --data '{"query":"{ session (id: \"<session-id>\") { id, capabilities, startTime, uri, nodeId, nodeUri, sessionDurationMillis, slot { id, stereotype, lastStarted } } } "}' -s <LINK_TO_GRAPHQL_ENDPOINT>

Querying the capabilities of each node in the grid :

curl -X POST -H "Content-Type: application/json" --data '{"query": "{ nodesInfo { nodes { stereotypes } } }"}' -s <LINK_TO_GRAPHQL_ENDPOINT>

Querying the status of each node in the grid :

curl -X POST -H "Content-Type: application/json" --data '{"query": "{ nodesInfo { nodes { status } } }"}' -s <LINK_TO_GRAPHQL_ENDPOINT>

Querying the URI of each node and the grid :

curl -X POST -H "Content-Type: application/json" --data '{"query": "{ nodesInfo { nodes { uri } } }"}' -s <LINK_TO_GRAPHQL_ENDPOINT>

Query for getting the current requests in the New Session Queue:

curl -X POST -H "Content-Type: application/json" --data '{"query":"{ sessionsInfo { sessionQueueRequests } }"}' -s <LINK_TO_GRAPHQL_ENDPOINT>

Query for getting the New Session Queue size :

curl -X POST -H "Content-Type: application/json" --data '{"query":"{ grid { sessionQueueSize } }"}' -s <LINK_TO_GRAPHQL_ENDPOINT>

6.3 - Grid endpoints

Grid

Grid Status

Grid status provides the current state of the Grid. It consists of details about every registered Node. For every Node, the status includes information regarding Node availability, sessions, and slots.

cURL GET 'http://localhost:4444/status'

In the Standalone mode, the Grid URL is the Standalone server address.

In the Hub-Node mode, the Grid URL is the Hub server address.

In the fully distributed mode, the Grid URL is the Router server address.

Default URL for all the above modes is http://localhost:4444.

Distributor

Remove Node

To remove the Node from the Grid, use the cURL command enlisted below. It does not stop any ongoing session running on that Node. The Node continues running as it is unless explicitly killed. The Distributor is no longer aware of the Node and hence any matching new session request will not be forwarded to that Node.

In the Standalone mode, the Distributor URL is the Standalone server address.

In the Hub-Node mode, the Distributor URL is the Hub server address.

cURL --request DELETE 'http://localhost:4444/se/grid/distributor/node/<node-id>' --header 'X-REGISTRATION-SECRET: <secret> '

In the fully distributed mode, the URL is the Distributor server address.

cURL --request DELETE 'http://localhost:5553/se/grid/distributor/node/<node-id>' --header 'X-REGISTRATION-SECRET: <secret>'

If no registration secret has been configured while setting up the Grid, then use

cURL --request DELETE 'http://<Distributor-URL>/se/grid/distributor/node/<node-id>' --header 'X-REGISTRATION-SECRET;'

Drain Node

Node drain command is for graceful node shutdown. Draining a Node stops the Node after all the ongoing sessions are complete. However, it does not accept any new session requests.

In the Standalone mode, the Distributor URL is the Standalone server address.

In the Hub-Node mode, the Distributor URL is the Hub server address.

cURL --request POST 'http://localhost:4444/se/grid/distributor/node/<node-id>/drain' --header 'X-REGISTRATION-SECRET: <secret> '

In the fully distributed mode, the URL is the Distributor server address.

cURL --request POST 'http://localhost:5553/se/grid/distributor/node/<node-id>/drain' --header 'X-REGISTRATION-SECRET: <secret>'

If no registration secret has been configured while setting up the Grid, then use

cURL --request POST 'http://<Distributor-URL>/se/grid/distributor/node/<node-id>/drain' --header 'X-REGISTRATION-SECRET;'

Node

The endpoints in this section are applicable for Hub-Node mode and fully distributed Grid mode where the Node runs independently. The default Node URL is http://localhost:5555 in case of one Node. In case of multiple Nodes, use Grid status to get all Node details and locate the Node address.

Status

The Node status is essentially a health-check for the Node. Distributor pings the node status are regular intervals and updates the Grid Model accordingly. The status includes information regarding availability, sessions, and slots.

cURL --request GET 'http://localhost:5555/status'

Drain

Distributor passes the drain command to the appropriate node identified by the node-id. To drain the Node directly, use the cuRL command enlisted below. Both endpoints are valid and produce the same result. Drain finishes the ongoing sessions before stopping the Node.

cURL --request POST 'http://localhost:5555/se/grid/node/drain' --header 'X-REGISTRATION-SECRET: <secret>'

If no registration secret has been configured while setting up the Grid, then use

cURL --request POST 'http://<node-URL>/se/grid/node/drain' --header 'X-REGISTRATION-SECRET;'

Check session owner

To check if a session belongs to a Node, use the cURL command enlisted below.

cURL --request GET 'http://localhost:5555/se/grid/node/owner/<session-id>' --header 'X-REGISTRATION-SECRET: <secret>'

If no registration secret has been configured while setting up the Grid, then use

cURL --request GET 'http://<node-URL>/se/grid/node/owner/<session-id>' --header 'X-REGISTRATION-SECRET;'

It will return true if the session belongs to the Node else it will return false.

Delete session

Deleting the session terminates the WebDriver session, quits the driver and removes it from the active sessions map. Any request using the removed session-id or reusing the driver instance will throw an error.

cURL --request DELETE 'http://localhost:5555/se/grid/node/session/<session-id>' --header 'X-REGISTRATION-SECRET: <secret>'

If no registration secret has been configured while setting up the Grid, then use

cURL --request DELETE 'http://<node-URL>/se/grid/node/session/<session-id>' --header 'X-REGISTRATION-SECRET;'

New Session Queue

Clear New Session Queue

New Session Request Queue holds the new session requests. To clear the queue, use the cURL command enlisted below. Clearing the queue rejects all the requests in the queue. For each such request, the server returns an error response to the respective client. The result of the clear command is the total number of deleted requests.

In the Standalone mode, the Queue URL is the Standalone server address.

In the Hub-Node mode, the Queue URL is the Hub server address.

cURL --request DELETE 'http://localhost:4444/se/grid/newsessionqueue/queue' --header 'X-REGISTRATION-SECRET: <secret>'

In the fully distributed mode, the Queue URL is New Session Queue server address.

cURL --request DELETE 'http://localhost:5559/se/grid/newsessionqueue/queue' --header 'X-REGISTRATION-SECRET: <secret>'

If no registration secret has been configured while setting up the Grid, then use

cURL --request DELETE 'http://<URL>/se/grid/newsessionqueue/queue' --header 'X-REGISTRATION-SECRET;'

Get New Session Queue Requests

New Session Request Queue holds the new session requests. To get the current requests in the queue, use the cURL command enlisted below. The response returns the total number of requests in the queue and the request payloads.

In the Standalone mode, the Queue URL is the Standalone server address.

In the Hub-Node mode, the Queue URL is the Hub server address.

cURL --request GET 'http://localhost:4444/se/grid/newsessionqueue/queue'

In the fully distributed mode, the Queue URL is New Session Queue server address.

cURL --request GET 'http://localhost:5559/se/grid/newsessionqueue/queue'

6.4 - Customizing a Node

How to customize a Node

There are times when we would like a Node to be customized to our needs.

For e.g., we may like to do some additional setup before a session begins execution and some clean-up after a session runs to completion.

Following steps can be followed for this:

• Create a class that extends org.openqa.selenium.grid.node.Node

• Add a static method (this will be our factory method) to the newly created class whose signature looks like this:

  public static Node create(Config config). Here:

  • Node is of type org.openqa.selenium.grid.node.Node
  • Config is of type org.openqa.selenium.grid.config.Config

• Within this factory method, include logic for creating your new Class.

• To wire in this new customized logic into the hub, start the node and pass in the fully qualified class name of the above class to the argument --node-implementation

Let’s see an example of all this:

Custom Node as an uber jar

1. Create a sample project using your favourite build tool (Maven|Gradle).
2. Add the below dependency to your sample project.
   • org.seleniumhq.selenium/selenium-grid
3. Add your customized Node to the project.
4. Build an uber jar to be able to start the Node using java -jar command.
5. Now start the Node using the command:

java -jar custom_node-server.jar node \
--node-implementation org.seleniumhq.samples.DecoratedLoggingNode

Note: If you are using Maven as a build tool, please prefer using maven-shade-plugin instead of maven-assembly-plugin because maven-assembly plugin seems to have issues with being able to merge multiple Service Provider Interface files (META-INF/services)

Custom Node as a regular jar

1. Create a sample project using your favourite build tool (Maven|Gradle).
2. Add the below dependency to your sample project.
   • org.seleniumhq.selenium/selenium-grid
3. Add your customized Node to the project.
4. Build a jar of your project using your build tool.
5. Now start the Node using the command:

java -jar selenium-server-4.6.0.jar \
--ext custom_node-1.0-SNAPSHOT.jar node \
--node-implementation org.seleniumhq.samples.DecoratedLoggingNode

Below is a sample that just prints some messages on to the console whenever there’s an activity of interest (session created, session deleted, a webdriver command executed etc.,) on the Node.

package org.seleniumhq.samples;

import java.net.URI;
import java.util.UUID;
import org.openqa.selenium.Capabilities;
import org.openqa.selenium.NoSuchSessionException;
import org.openqa.selenium.WebDriverException;
import org.openqa.selenium.grid.config.Config;
import org.openqa.selenium.grid.data.CreateSessionRequest;
import org.openqa.selenium.grid.data.CreateSessionResponse;
import org.openqa.selenium.grid.data.NodeId;
import org.openqa.selenium.grid.data.NodeStatus;
import org.openqa.selenium.grid.data.Session;
import org.openqa.selenium.grid.log.LoggingOptions;
import org.openqa.selenium.grid.node.HealthCheck;
import org.openqa.selenium.grid.node.Node;
import org.openqa.selenium.grid.node.local.LocalNodeFactory;
import org.openqa.selenium.grid.security.Secret;
import org.openqa.selenium.grid.security.SecretOptions;
import org.openqa.selenium.grid.server.BaseServerOptions;
import org.openqa.selenium.internal.Either;
import org.openqa.selenium.remote.SessionId;
import org.openqa.selenium.remote.http.HttpRequest;
import org.openqa.selenium.remote.http.HttpResponse;
import org.openqa.selenium.remote.tracing.Tracer;

public class DecoratedLoggingNode extends Node {

  private Node node;

  protected DecoratedLoggingNode(Tracer tracer, URI uri, Secret registrationSecret) {
    super(tracer, new NodeId(UUID.randomUUID()), uri, registrationSecret);
  }

  public static Node create(Config config) {
    LoggingOptions loggingOptions = new LoggingOptions(config);
    BaseServerOptions serverOptions = new BaseServerOptions(config);
    URI uri = serverOptions.getExternalUri();
    SecretOptions secretOptions = new SecretOptions(config);

    // Refer to the foot notes for additional context on this line.
    Node node = LocalNodeFactory.create(config);

    DecoratedLoggingNode wrapper = new DecoratedLoggingNode(loggingOptions.getTracer(),
        uri, secretOptions.getRegistrationSecret());
    wrapper.node = node;
    return wrapper;
  }

  @Override
  public Either<WebDriverException, CreateSessionResponse> newSession(
      CreateSessionRequest sessionRequest) {
    System.out.println("Before newSession()");
    try {
      return this.node.newSession(sessionRequest);
    } finally {
      System.out.println("After newSession()");
    }
  }

  @Override
  public HttpResponse executeWebDriverCommand(HttpRequest req) {
    try {
      System.out.println("Before executeWebDriverCommand(): " + req.getUri());
      return node.executeWebDriverCommand(req);
    } finally {
      System.out.println("After executeWebDriverCommand()");
    }
  }

  @Override
  public Session getSession(SessionId id) throws NoSuchSessionException {
    try {
      System.out.println("Before getSession()");
      return node.getSession(id);
    } finally {
      System.out.println("After getSession()");
    }
  }

  @Override
  public HttpResponse uploadFile(HttpRequest req, SessionId id) {
    try {
      System.out.println("Before uploadFile()");
      return node.uploadFile(req, id);
    } finally {
      System.out.println("After uploadFile()");
    }
  }

  @Override
  public void stop(SessionId id) throws NoSuchSessionException {
    try {
      System.out.println("Before stop()");
      node.stop(id);
    } finally {
      System.out.println("After stop()");
    }
  }

  @Override
  public boolean isSessionOwner(SessionId id) {
    try {
      System.out.println("Before isSessionOwner()");
      return node.isSessionOwner(id);
    } finally {
      System.out.println("After isSessionOwner()");
    }
  }

  @Override
  public boolean isSupporting(Capabilities capabilities) {
    try {
      System.out.println("Before isSupporting");
      return node.isSupporting(capabilities);
    } finally {
      System.out.println("After isSupporting()");
    }
  }

  @Override
  public NodeStatus getStatus() {
    try {
      System.out.println("Before getStatus()");
      return node.getStatus();
    } finally {
      System.out.println("After getStatus()");
    }
  }

  @Override
  public HealthCheck getHealthCheck() {
    try {
      System.out.println("Before getHealthCheck()");
      return node.getHealthCheck();
    } finally {
      System.out.println("After getHealthCheck()");
    }
  }

  @Override
  public void drain() {
    try {
      System.out.println("Before drain()");
      node.drain();
    } finally {
      System.out.println("After drain()");
    }

  }

  @Override
  public boolean isReady() {
    try {
      System.out.println("Before isReady()");
      return node.isReady();
    } finally {
      System.out.println("After isReady()");
    }
  }
}

Foot Notes:

In the above example, the line Node node = LocalNodeFactory.create(config); explicitly creates a LocalNode.

There are basically 2 types of user facing implementations of org.openqa.selenium.grid.node.Node available.

These classes are good starting points to learn how to build a custom Node and also to learn the internals of a Node.

• org.openqa.selenium.grid.node.local.LocalNode - Used to represent a long running Node and is the default implementation that gets wired in when you start a node.
  • It can be created by calling LocalNodeFactory.create(config);, where:
    • LocalNodeFactory belongs to org.openqa.selenium.grid.node.local
    • Config belongs to org.openqa.selenium.grid.config
• org.openqa.selenium.grid.node.k8s.OneShotNode - This is a special reference implementation wherein the Node gracefully shuts itself down after servicing one test session. This class is currently not available as part of any pre-built maven artifact.
  • You can refer to the source code here to understand its internals.
  • To build it locally refer here.
  • It can be created by calling OneShotNode.create(config), where:
    • OneShotNode belongs to org.openqa.selenium.grid.node.k8s
    • Config belongs to org.openqa.selenium.grid.config

6.5 - External datastore

Table of Contents

• Introduction
• Setup
• Database backed Session Map
  • Steps
• Redis backed Session Map
  • Steps

Introduction

Selenium Grid allows you to persist information related to currently running sessions into an external data store. The external data store could be backed by your favourite database (or) Redis Cache system.

Setup

• Coursier - As a dependency resolver, so that we can download maven artifacts on the fly and make them available in our classpath
• Docker - To manage our PostGreSQL/Redis docker containers.

Database backed Session Map

For the sake of this illustration, we are going to work with PostGreSQL database.

We will spin off a PostGreSQL database as a docker container using a docker compose file.

Steps

You can skip this step if you already have a PostGreSQL database instance available at your disposal.

• Create a sql file named init.sql with the below contents:

CREATE TABLE IF NOT EXISTS sessions_map(
    session_ids varchar(256),
    session_caps text,
    session_uri varchar(256),
    session_stereotype text,
    session_start varchar(256)
 );

• In the same directory as the init.sql, create a file named docker-compose.yml with its contents as below:

version: '3.8'
services:
  db:
    image: postgres:9.6-bullseye
    restart: always
    environment:
      - POSTGRES_USER=seluser
      - POSTGRES_PASSWORD=seluser
      - POSTGRES_DB=selenium_sessions
    ports:
      - "5432:5432"
    volumes:
    - ./init.sql:/docker-entrypoint-initdb.d/init.sql

We can now start our database container by running:

docker-compose up -d

Our database name is selenium_sessions with its username and password set to seluser

If you are working with an already running PostGreSQL DB instance, then you just need to create a database named selenium_sessions and the table sessions_map using the above mentioned SQL statement.

• Create a Selenium Grid configuration file named sessions.toml with the below contents:

[sessions]
implementation = "org.openqa.selenium.grid.sessionmap.jdbc.JdbcBackedSessionMap"
jdbc-url = "jdbc:postgresql://localhost:5432/selenium_sessions"
jdbc-user = "seluser"
jdbc-password = "seluser"

Note: If you plan to use an existing PostGreSQL DB instance, then replace localhost:5432 with the actual host and port number of your instance.

• Below is a simple shell script (let’s call it distributed.sh) that we will use to bring up our distributed Grid.

SE_VERSION=<current_selenium_version>
JAR_NAME=selenium-server-${SE_VERSION}.jar
PUBLISH="--publish-events tcp://localhost:4442"
SUBSCRIBE="--subscribe-events tcp://localhost:4443"
SESSIONS="--sessions http://localhost:5556"
SESSIONS_QUEUE="--sessionqueue http://localhost:5559"
echo 'Starting Event Bus'
java -jar $JAR_NAME event-bus $PUBLISH $SUBSCRIBE --port 5557 &
echo 'Starting New Session Queue'
java -jar $JAR_NAME sessionqueue --port 5559 &
echo 'Starting Sessions Map'
java -jar $JAR_NAME \
--ext $(coursier fetch -p org.seleniumhq.selenium:selenium-session-map-jdbc:${SE_VERSION} org.postgresql:postgresql:42.3.1) \
sessions $PUBLISH $SUBSCRIBE --port 5556 --config sessions.toml &
echo 'Starting Distributor'
java -jar $JAR_NAME  distributor $PUBLISH $SUBSCRIBE $SESSIONS $SESSIONS_QUEUE --port 5553 --bind-bus false &
echo 'Starting Router'
java -jar $JAR_NAME router $SESSIONS --distributor http://localhost:5553 $SESSIONS_QUEUE --port 4444 &
echo 'Starting Node'
java -jar $JAR_NAME node $PUBLISH $SUBSCRIBE &

• At this point the current directory should contain the following files:

  • docker-compose.yml
  • init.sql
  • sessions.toml
  • distributed.sh

• You can now spawn the Grid by running distributed.sh shell script and quickly run a test. You will notice that the Grid now stores session information into the PostGreSQL database.

In the line which spawns a SessionMap on a machine:

export SE_VERSION=<current_selenium_version>
java -jar selenium-server-${SE_VERSION}.jar \
--ext $(coursier fetch -p org.seleniumhq.selenium:selenium-session-map-jdbc:${SE_VERSION} org.postgresql:postgresql:42.3.1) \
sessions --publish-events tcp://localhost:4442 \
--subscribe-events tcp://localhost:4443 \
--port 5556 --config sessions.toml 

• The variable names from the above script have been replaced with their actual values for clarity.
• Remember to substitute localhost with the actual hostname of the machine where your Event-Bus is running.
• The arguments being passed to coursier are basically the GAV (Group Artifact Version) Maven co-ordinates of:
  • selenium-session-map-jdbc which is needed to help us store sessions information in database
  • postgresql which is needed to help us talk PostGreSQL database.
• sessions.toml is the configuration file that we created earlier.

Redis backed Session Map

We will spin off a Redis Cache docker container using a docker compose file.

Steps

You can skip this step if you already have a Redis Cache instance available at your disposal.

• Create a file named docker-compose.yml with its contents as below:

version: '3.8'
services:
  redis:
    image: redis:bullseye
    restart: always
    ports:
      - "6379:6379"

We can now start our Redis container by running:

docker-compose up -d

• Create a Selenium Grid configuration file named sessions.toml with the below contents:

[sessions]
scheme = "redis"
implementation = "org.openqa.selenium.grid.sessionmap.redis.RedisBackedSessionMap"
hostname = "localhost"
port = 6379

Note: If you plan to use an existing Redis Cache instance, then replace localhost and 6379 with the actual host and port number of your instance.

• Below is a simple shell script (let’s call it distributed.sh) that we will use to bring up our distributed grid.

SE_VERSION=<current_selenium_version>
JAR_NAME=selenium-server-${SE_VERSION}.jar
PUBLISH="--publish-events tcp://localhost:4442"
SUBSCRIBE="--subscribe-events tcp://localhost:4443"
SESSIONS="--sessions http://localhost:5556"
SESSIONS_QUEUE="--sessionqueue http://localhost:5559"
echo 'Starting Event Bus'
java -jar $JAR_NAME event-bus $PUBLISH $SUBSCRIBE --port 5557 &
echo 'Starting New Session Queue'
java -jar $JAR_NAME sessionqueue --port 5559 &
echo 'Starting Session Map'
java -jar $JAR_NAME \
--ext $(coursier fetch -p org.seleniumhq.selenium:selenium-session-map-redis:${SE_VERSION}) \
sessions $PUBLISH $SUBSCRIBE --port 5556 --config sessions.toml &
echo 'Starting Distributor'
java -jar $JAR_NAME  distributor $PUBLISH $SUBSCRIBE $SESSIONS $SESSIONS_QUEUE --port 5553 --bind-bus false &
echo 'Starting Router'
java -jar $JAR_NAME router $SESSIONS --distributor http://localhost:5553 $SESSIONS_QUEUE --port 4444 &
echo 'Starting Node'
java -jar $JAR_NAME node $PUBLISH $SUBSCRIBE &

• At this point the current directory should contain the following files:

  • docker-compose.yml
  • sessions.toml
  • distributed.sh

• You can now spawn the Grid by running distributed.sh shell script and quickly run a test. You will notice that the Grid now stores session information into the Redis instance. You can perhaps make use of a Redis GUI such as TablePlus to see them (Make sure that you have setup a debug point in your test, because the values will get deleted as soon as the test runs to completion).

In the line which spawns a SessionMap on a machine:

export SE_VERSION=<current_selenium_version>
java -jar selenium-server-${SE_VERSION}.jar \
--ext $(coursier fetch -p org.seleniumhq.selenium:selenium-session-map-redis:${SE_VERSION}) \
sessions --publish-events tcp://localhost:4442 \
--subscribe-events tcp://localhost:4443 \
--port 5556 --config sessions.toml 

• The variable names from the above script have been replaced with their actual values for clarity.
• Remember to substitute localhost with the actual hostname of the machine where your Event-Bus is running.
• The arguments being passed to coursier are basically the GAV (Group Artifact Version) Maven co-ordinates of:
  • selenium-session-map-redis which is needed to help us store sessions information in Redis Cache.
• sessions.toml is the configuration file that we created earlier.