[🐛 Bug]: maxSessions config value larger then CPU count is ignored

[Zlaman]

What happened?

My VM has 4 CPU's. I am starting selenium/hub and selenium/node-docker with below commands.
config.toml has override-max-sessions = true and max-sessions = 10
As per the logs from selenium/node-docker (see them below) - Max sessions set to 10, but another line is Mapping Capabilities {browserName: chrome, platformName: linux, se:downloadsEnabled: true} to docker image selenium/standalone-chrome:latest **4 times**
It results in "maxSessions": 4 while expected is 10 as per config.toml

curl localhost:4444/wd/hub/status

{
  "value": {
    "ready": true,
    "message": "Selenium Grid ready.",
    "nodes": [
      {
        "id": "0f578f41-ac9c-484a-b06d-6777a8384a4a",
        "uri": "http:\u002f\u002f172.18.0.3:5555",
        "maxSessions": 4,
        "sessionTimeout": 300000,
        "osInfo": {
          "arch": "amd64",
          "name": "Linux",
          "version": "5.15.167.4-microsoft-standard-WSL2"
        },
        "heartbeatPeriod": 60000,
        "availability": "UP",
        "version": "4.27.0 (revision d6e718d)",
        "slots": [
          {
            "id": {
              "hostId": "0f578f41-ac9c-484a-b06d-6777a8384a4a",
              "id": "9ecdd10f-ac80-4a06-b1c2-cc1ced9232d4"
            },
            "lastStarted": "1970-01-01T00:00:00Z",
            "session": null,
            "stereotype": {
              "browserName": "chrome",
              "platformName": "linux",
              "se:downloadsEnabled": true
            }
          },
          {
            "id": {
              "hostId": "0f578f41-ac9c-484a-b06d-6777a8384a4a",
              "id": "a58f5da3-83ce-422d-b390-e4bd63203cc2"
            },
            "lastStarted": "1970-01-01T00:00:00Z",
            "session": null,
            "stereotype": {
              "browserName": "chrome",
              "platformName": "linux",
              "se:downloadsEnabled": true
            }
          },
          {
            "id": {
              "hostId": "0f578f41-ac9c-484a-b06d-6777a8384a4a",
              "id": "592f3a21-3c70-4258-b4eb-13a92125a6dd"
            },
            "lastStarted": "1970-01-01T00:00:00Z",
            "session": null,
            "stereotype": {
              "browserName": "chrome",
              "platformName": "linux",
              "se:downloadsEnabled": true
            }
          },
          {
            "id": {
              "hostId": "0f578f41-ac9c-484a-b06d-6777a8384a4a",
              "id": "9a7759c7-0d04-4b0f-a420-5f08d2f7d58d"
            },
            "lastStarted": "1970-01-01T00:00:00Z",
            "session": null,
            "stereotype": {
              "browserName": "chrome",
              "platformName": "linux",
              "se:downloadsEnabled": true
            }
          }
        ]
      }
    ]
  }
}

I have also tried to start selenium-node-docker with extra environment variables
-e SE_NODE_MAX_SESSIONS=10
-e SE_NODE_OVERRIDE_MAX_SESSIONS=true
but it does not help as well.

Command used to start Selenium Grid with Docker (or Kubernetes)

docker network create grid
docker run -d -p 4442-4444:4442-4444 --network grid --name selenium-hub selenium/hub:latest
docker run -d --network grid --name selenium-node-docker \
    -e SE_EVENT_BUS_HOST=selenium-hub \
    -e SE_EVENT_BUS_PUBLISH_PORT=4442 \
    -e SE_EVENT_BUS_SUBSCRIBE_PORT=4443 \
    -e SE_BROWSER_ARGS_DISABLE_DSHM=--disable-dev-shm-usage \
    -e SE_START_VNC=false \
    -e SE_START_NO_VNC=false \
    -e SE_ENABLE_TRACING=false \
    -e SE_NODE_DOCKER_CONFIG_FILENAME=docker.toml \
    -v /home/user/selenium/config.toml:/opt/selenium/docker.toml:ro \
    -v /home/user/selenium/assets:/opt/selenium/assets \
    -v /home/user/selenium/downloads:/home/seluser/Downloads \
    -v /var/run/docker.sock:/var/run/docker.sock \
    selenium/node-docker:latest

# config.toml
[docker]
configs = [
    "selenium/standalone-chrome:latest", '{"browserName": "chrome"}'
]
host-config-keys = ["Binds"]
url = "http://127.0.0.1:2375"
video-image = "selenium/video:latest"

[node]
detect-drivers = false
enable-managed-downloads = true
override-max-sessions = true
max-sessions = 10

Relevant log output

selenium/node-docker
Starting Selenium Grid Node Docker...
2025-01-23 04:21:37,253 INFO success: socat entered RUNNING state, process has stayed up for > than 0 seconds (startsecs)
2025-01-23 04:21:37,253 INFO success: selenium-grid-docker entered RUNNING state, process has stayed up for > than 0 seconds (startsecs)
Appending Selenium option: --log-level INFO
Appending Selenium option: --http-logs false
Appending Selenium option: --structured-logs false
Appending Selenium option: --tracing false
Tracing is disabled
04:21:37.483 INFO [LoggingOptions.configureLogEncoding] - Using the system default encoding
04:21:37.485 INFO [LoggingOptions.getTracer] - Using null tracer
04:21:37.507 INFO [UnboundZmqEventBus.<init>] - Connecting to tcp://selenium-hub:4442 and tcp://selenium-hub:4443
04:21:37.528 INFO [UnboundZmqEventBus.<init>] - Sockets created
04:21:38.529 INFO [UnboundZmqEventBus.<init>] - Event bus ready
04:21:38.578 INFO [NodeServer.createHandlers] - Reporting self as: http://172.18.0.3:5555
04:21:38.580 INFO [LoggingOptions.getTracer] - Using null tracer
04:21:38.592 INFO [NodeOptions.getSessionFactories] - Detected 4 available processors
04:21:38.593 WARN [NodeOptions.getSessionFactories] - Overriding max recommended number of 4 concurrent sessions. Session stability and reliability might suffer!
04:21:38.594 WARN [NodeOptions.getSessionFactories] - One browser session is recommended per available processor. Safari is always limited to 1 session per host.
04:21:38.594 WARN [NodeOptions.getSessionFactories] - Overriding this value for Internet Explorer is not recommended. Issues related to parallel testing with Internet Explored won't be accepted.
04:21:38.594 WARN [NodeOptions.getSessionFactories] - Double check if enabling 'override-max-sessions' is really needed
04:21:38.594 WARN [NodeOptions.getSessionFactories] - Max sessions set to 10
04:21:38.814 INFO [V141Docker.isContainerPresent] - Checking if container is present: e67e0b9cbd42
04:21:38.923 INFO [Docker.getImage] - Obtaining image: selenium/standalone-chrome:latest
04:21:38.926 INFO [V141Docker.getImage] - Listing local images: Reference{domain='docker.io', name='selenium/standalone-chrome', tag='latest', digest='null'}
04:21:38.935 INFO [Docker.getImage] - Obtaining image: selenium/video:latest
04:21:38.935 INFO [V141Docker.getImage] - Listing local images: Reference{domain='docker.io', name='selenium/video', tag='latest', digest='null'}
04:21:38.938 INFO [Docker.getImage] - Obtaining image: selenium/video:latest
04:21:38.938 INFO [V141Docker.getImage] - Listing local images: Reference{domain='docker.io', name='selenium/video', tag='latest', digest='null'}
04:21:38.942 INFO [Docker.getImage] - Obtaining image: selenium/standalone-chrome:latest
04:21:38.942 INFO [V141Docker.getImage] - Listing local images: Reference{domain='docker.io', name='selenium/standalone-chrome', tag='latest', digest='null'}
04:21:38.947 INFO [DockerOptions.lambda$getDockerSessionFactories$2] - Mapping Capabilities {browserName: chrome, platformName: linux, se:downloadsEnabled: true} to docker image selenium/standalone-chrome:latest 4 times
04:21:38.954 INFO [Node.<init>] - Binding additional locator mechanisms: relative
04:21:39.025 INFO [NodeServer$2.start] - Starting registration process for Node http://172.18.0.3:5555
04:21:39.026 INFO [NodeServer.execute] - Started Selenium node 4.27.0 (revision d6e718d): http://172.18.0.3:5555
04:21:39.031 INFO [NodeServer$2.lambda$start$1] - Sending registration event...
04:21:39.193 INFO [NodeServer.lambda$createHandlers$2] - Node has been added

Operating System

Ubuntu

Docker Selenium version (image tag)

selenium/hub:4.28.0-20250120 selenium/node-docker:4.28.0-20250120

Selenium Grid chart version (chart version)

No response

[github-actions]

@Zlaman, thank you for creating this issue. We will troubleshoot it as soon as we can.

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Thank you!

[VietND96]

Ok, looks like the TOML config applies for the node-docker but doesn't apply for the browser node defined inside.
In this case, can you set ENV vars when starting node-docker SE_NODE_OVERRIDE_MAX_SESSIONS=true and SE_NODE_MAX_SESSIONS=10

[Zlaman]

Thanks, I tried starting selenium/node-docker with below command, but it still results in "maxSessions": 4 from wd/hub/status and same log output from docker logs selenium-node-docker command

docker run -d --network grid --name selenium-node-docker \
    -e SE_EVENT_BUS_HOST=selenium-hub \
    -e SE_EVENT_BUS_PUBLISH_PORT=4442 \
    -e SE_EVENT_BUS_SUBSCRIBE_PORT=4443 \
    -e SE_BROWSER_ARGS_DISABLE_DSHM=--disable-dev-shm-usage \
    -e SE_START_VNC=false \
    -e SE_START_NO_VNC=false \
    -e SE_ENABLE_TRACING=false \
    -e SE_NODE_MAX_SESSIONS=10 \
    -e SE_NODE_OVERRIDE_MAX_SESSIONS=true \
    -e SE_NODE_DOCKER_CONFIG_FILENAME=docker.toml \
    -v /home/zalman/selenium/config.toml:/opt/selenium/docker.toml:ro \
    -v /home/zalman/selenium/assets:/opt/selenium/assets \
    -v /home/zalman/selenium/downloads:/home/seluser/Downloads \
    -v /var/run/docker.sock:/var/run/docker.sock \
    selenium/node-docker:latest

[VietND96]

This one let me check again.
However, you are using dynamic grid, where max-sessions doesn't matter. Although there are 4 sessions per Node, but the number of Node is dynamic. For example you submitted 10 requests, expected that there are 3 Nodes with 12 sessions available to pickup those.

[Zlaman]

Thank you. I am using only one node, I guess as the max-sessions parameter is under [node] section, this is a setting per node, not the global setting per whole hub. Node logs also state Max sessions set to 10 but then Mapping Capabilities {browserName: chrome, platformName: linux, se:downloadsEnabled: true} to docker image selenium/standalone-chrome:latest **4 times**

[VietND96]

With your requirement, I think just a standalone image is suitable for you.

docker run -p 4444:4444 -e SE_NODE_OVERRIDE_MAX_SESSIONS=true -e SE_NODE_MAX_SESSIONS=10 selenium/standalone-chrome:latest

No need to have node-docker layer.

[Zlaman]

I need an ability to record videos from every session, not sure it is possible to use selenium/standalone-chrome for that, it will only record video from one of the sessions that is currently visible on screen

[VietND96]

Currently, multiple sessions are opening the browsers on the same screen, and recording only listens on that screen.
We can create multiple VNC displays. However, we have to control a session that can open the browser on a unique display and then notify the recorder, which displays the port for recording. It would be a complex process that requires WebDriver and tools to work together.
That is why in Dynamic Grid (node-docker), we keep the browser node spawned up, and each node takes a single session, where the session can be recorded correctly.

[diemol]

This is a duplicate of #1817

[Zlaman]

Thanks @VietND96 , that's why I am using Dynamic Grid.

Thanks @diemol I see that #1817 is closed without resolution. I think that the situation I am facing is still a bug because it does not correspond to this documentation https://github.com/SeleniumHQ/docker-selenium?tab=readme-ov-file#increasing-session-concurrency-per-container