Principle:SeleniumHQ Selenium Grid Health Monitoring

Knowledge Sources	Selenium Grid Docs Selenium
Domains	Distributed_Testing, Observability, Selenium_Grid
Last Updated	2026-02-11 00:00 GMT

Overview

Mechanism for observing Grid health, node availability, session counts, and component status through REST endpoints, GraphQL API, and web UI.

Description

Selenium Grid provides multiple monitoring interfaces:

• /status REST endpoint: Powered by GridStatusHandler, which queries Distributor.getStatus() with a 2-second timeout. Returns JSON with readiness state (true if any node is UP with available capacity), node information (id, uri, maxSessions, sessionTimeout, osInfo, heartbeatPeriod, availability, version, slots), and a human-readable message.
• GraphQL API at /graphql: Served by GraphqlHandler, which uses a schema defined in selenium-grid-schema.graphqls and wired via GridData and SessionData data fetchers. Supports queries for grid metadata, node information, and session details. Uses a Caffeine cache (5% of heap, 30-minute TTL) for parsed query documents.
• Grid UI at /ui: Served by GridUiRoute, a React-based web application loaded from classpath resources. Can be disabled via RouterOptions.disableUi().
• /readyz endpoint: A Kubernetes-compatible liveness probe that bypasses authentication.

Usage

Use /status for simple health checks and load balancer probes. Use GraphQL for detailed programmatic monitoring and alerting. Use the Grid UI for visual inspection during debugging or demonstrations. Use /readyz for Kubernetes liveness probes when authentication is enabled.

Theoretical Basis

# Monitoring Endpoints
GET /status  -> GridStatusHandler -> Distributor.getStatus() (2s timeout)
               -> { "value": { "ready": boolean, "message": string, "nodes": [...] } }
               -> ready = any node is UP AND has capacity

POST /graphql -> GraphqlHandler -> GraphQL schema execution
               -> { "data": { "grid": {...}, "nodesInfo": {...}, "sessionsInfo": {...} } }

GET /ui      -> GridUiRoute -> React web application (classpath resource)

GET /readyz  -> Readiness check (bypasses BasicAuthenticationFilter)
               -> "Standalone is true" or "Router is true"

# Status Node Fields
{
  "id": NodeId,
  "uri": URI,
  "maxSessions": int,
  "sessionTimeout": long (millis),
  "osInfo": Map,
  "heartbeatPeriod": long (millis),
  "availability": "UP"|"DOWN"|"DRAINING",
  "version": String,
  "slots": [...]
}

Related Pages

Implemented By

• Implementation:SeleniumHQ_Selenium_Grid_Status_Endpoint