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Document sessionAffinity in MCPServer scaling #956

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Document sessionAffinity in MCPServer scaling
danbarr Jun 18, 2026
eb1d287
Clarify source-IP basis of sessionAffinity
danbarr Jun 18, 2026
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29 changes: 29 additions & 0 deletions docs/toolhive/guides-k8s/run-mcp-k8s.mdx
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Expand Up @@ -639,6 +639,35 @@ backends.

:::

### Session affinity for proxy replicas

Clients connect to the proxy runner through its Service. When you run more than
one proxy replica (`spec.replicas > 1`), the `spec.sessionAffinity` field
controls whether that Service routes repeated connections from the same source
IP to the same proxy pod:

- `ClientIP` (default) - routes connections from the same source IP to the same
proxy pod. Because MCP transports (SSE and Streamable HTTP) are stateful, this
keeps a client's connections on the replica that holds its in-memory session.
- `None` - load-balances each connection freely across proxy pods.

```yaml title="MCPServer resource"
spec:
replicas: 2
# highlight-next-line
sessionAffinity: None # default is ClientIP
```

Affinity only influences routing; it does not move session state between pods.
`ClientIP` is also unreliable behind NAT or shared egress IPs, where many
clients share a source address. For reliable scaling, configure
[Redis session storage](./redis-session-storage.mdx#horizontal-scaling-session-storage)
so any proxy pod can serve any client rather than depending on affinity alone.

This client-facing affinity is separate from the backend session routing
described next, which controls how the proxy reaches backend pods when
`spec.backendReplicas > 1`.

### Session routing for backend replicas

MCP connections are stateful: once a client establishes a session with a
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