Description

Switch all three proxy transport constructors (streamable, HTTP-SSE, and transparent) to use session.NewManagerWithRedis, enabling session-sticky routing across multiple proxyrunner replicas (RC-12).

Note: RC-13 (LRU cache-size wiring) has been dropped. RedisStorage.Load uses GETEX, which atomically returns session data and refreshes the TTL in a single round-trip, making a local LRU cache layer unnecessary. See #4294 for the rationale.

Parent epic: stacklok/stacklok-epics#263

Dependencies: #4236 (RC-7 — NewManagerWithRedis constructor), #4239 (TASK-001 — RunConfig fields)
Blocks: none (leaf node in the DAG)

Acceptance Criteria

RC-12 — Session-aware routing

• NewHTTPProxy (streamable): constructs session manager via session.NewManagerWithRedis; accepts a Redis storage override via WithSessionStorage option (defaults to session.NewLocalStorage() when not provided)
• NewHTTPSSEProxy: same
• NewTransparentProxyWithOptions: same
• Transparent proxy Rewrite reads "backend_url" metadata key from session and overrides pr.Out.URL; falls back to static targetURL when absent or unparseable
• Non-initialize requests with unknown session ID return HTTP 400 in transparent proxy handler
• Existing single-replica behavior preserved when no WithSessionStorage is provided (defaults to LocalStorage)

General

• All existing tests in pkg/transport/proxy/streamable/, pkg/transport/proxy/httpsse/, and pkg/transport/proxy/transparent/ pass
• go vet ./pkg/transport/proxy/... reports no issues
• Tests for transparent proxy Rewrite: (a) backend_url in metadata overrides target; (b) no backend_url falls back to static target; (c) unknown session ID on non-initialize returns HTTP 400
• Code reviewed and approved

Technical Approach

RC-12 — Storage wiring

Add WithSessionStorage(storage session.Storage) Option to each proxy constructor. When not provided, default to session.NewLocalStorage(). Pass the storage to session.NewManagerWithRedis:

// At call sites in pkg/transport/http.go, when Redis is configured:
mgr := session.NewManagerWithRedis(redisClient)
// Otherwise fall back to local storage (single-replica default)

Extend the transparent proxy Rewrite:

if sid := pr.In.Header.Get("Mcp-Session-Id"); sid != "" {
    if sess, ok := p.sessionManager.Get(sid); ok {
        if backendURL, _ := sess.GetMetadataValue("backend_url"); backendURL != "" {
            if parsed, err := url.Parse(backendURL); err == nil {
                pr.Out.URL = parsed
            }
        }
    }
}

Code Pointers

• pkg/transport/proxy/streamable/streamable_proxy.go:72–98 — NewHTTPProxy; session.NewManager → session.NewManagerWithRedis
• pkg/transport/proxy/streamable/streamable_proxy.go:617–621 — resolveSessionForBatch; pattern for HTTP 400 on unknown session
• pkg/transport/proxy/httpsse/http_proxy.go:93–120 — NewHTTPSSEProxy; same replacement
• pkg/transport/proxy/transparent/transparent_proxy.go:253–310 — NewTransparentProxyWithOptions; session manager wiring
• pkg/transport/proxy/transparent/transparent_proxy.go:487–513 — Rewrite closure; backend_url override injected here
• pkg/transport/http.go — call sites for both proxy constructors

Out of Scope

• RoutingStorage / LRU cache layer (dropped — see #4294)
• RunConfig.SessionCacheSize wiring (no longer needed)
• Backend pod selection / round-robin on initialize
• StatefulSet replica count wiring (TASK-002) or graceful shutdown (TASK-003)

References

• Epic #263
• RC-7 #4236: NewManagerWithRedis constructor
• RFC THV-0047