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What is the difference between Selenium Grid's Hub-Node model and the new Router-Distributor model?

Selenium 3's Grid used a single centralized Hub that both accepted incoming test requests and directly tracked every registered Node's capacity - one component doing both routing and capacity management.

Hub-Node (Grid 3)Router-Distributor (Grid 4)
Single Hub handles routing and capacity tracking togetherRouter and Distributor are separate, independently scalable components
Hub is a single point of failure/bottleneckComponents can be scaled or run independently
No built-in session queueingDistributor includes a session queue for when Nodes are full

Splitting routing (Router) from capacity/assignment decisions (Distributor) is what makes Grid 4 more resilient to a single component becoming a bottleneck, and it's also why Grid 4 can run in a fully distributed mode across containers or Kubernetes pods, with each component scaled to its own actual load rather than everything funneling through one Hub process.

In Grid 3's Hub-Node model, routing and capacity tracking were handled by:
A practical benefit of splitting Router and Distributor in Grid 4 is:

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