Support Backend Cluster Based Pinning

[Peiyingy]

Background

Currently, routing rules in Trino Gateway only support directing traffic to a routing group. In environments where a routing group contains multiple backend clusters, there’s no straightforward way to route queries to a specific backend when needed (for example, for testing, debugging, or gradual rollout purposes). Currently, the only workaround would be registering a cluster multiple times under different routing groups, which is really hacky and complicates configuration management.

Supporting direct routing to a backend cluster would provide finer control and flexibility in query distribution without requiring redundant backend registrations or workarounds.

Proposal

Extend the routing rule framework to allow rules to specify either:

• routingGroup or
• backendCluster

We can still rely on the priority mechanism to determine which setting takes precedence when both are defined. This feature would enable targeted query routing to a specific backend cluster while maintaining compatibility with the existing routing group based design.

---

Example YAML rule

---
name: "user1_pinning_rule"
description: "Pin user1 directly to adhoc01 backend"
priority: 1
condition: "request.getHeader(\"X-Trino-User\") == \"user1\""
actions:
  - "result.put(\"backendCluster\", \"adhoc01\")"

[mosabua]

This looks like a good idea to have as a feature, not necessarily for production use at scale but definitely useful.

My one suggestion is that we opted in the past to remove the term "backend cluster" and instead just use "Trino cluster" .. the renaming is not complete but we should not pile on more.

Also I think theoretically it would be sufficient to just allow naming a routing group and then users would just have to set up a specific group for the one cluster but that seems clumsy. So I vote for implementing both group and cluster.