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efb02fa
initial sticky sessions work
thushan 44999cf
fix ttlcache == 0
thushan b4d2bf9
Merge main into feature/sticky-sessions
thushan 17d167f
lint test
thushan df441b3
- OLLA-284: move SetPurgeDeadEndpoints registration from app.go wirin…
thushan 7e426e9
Fix latent rarce on purgedead & replace TTL sleep with poll loop
thushan d15a63d
doc updates
thushan 4bbe95b
reference updates
thushan ed1609a
fix: address CodegRabbit review issues
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| Original file line number | Diff line number | Diff line change |
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| # Sticky Sessions | ||
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| > :memo: **Default Configuration** | ||
| > ```yaml | ||
| > proxy: | ||
| > sticky_sessions: | ||
| > enabled: false | ||
| > ``` | ||
| > Sticky sessions are **opt-in**. Set `enabled: true` under `proxy.sticky_sessions` to activate KV-cache affinity routing. | ||
| > | ||
| > **Environment Variable**: `OLLA_PROXY_STICKY_SESSIONS_ENABLED` | ||
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| Sticky sessions route repeat turns in a multi-turn conversation to the same backend endpoint, maximising KV-cache reuse across turns. The feature wraps the configured load balancer as a decorator. The underlying strategy (priority, round-robin, least-connections) is unchanged for new sessions and fallback cases. | ||
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| ## Why sticky sessions | ||
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| Modern LLM backends maintain a KV-cache for the token sequence they have already processed. When the next turn of a conversation lands on the **same** backend, the backend can skip re-ingesting the full context and jump straight to generating new tokens. For long conversations this produces a substantial reduction in both time-to-first-token and compute cost; the benefit scales with context length. | ||
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| Without affinity, a load balancer may distribute turn N and turn N+1 to different backends. The receiving backend for turn N+1 has a cold cache and must process the entire prompt from scratch. For workloads with short prompts or single-turn completions this overhead is negligible; for chat-style applications with growing context it compounds with every turn. | ||
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| ## How it works | ||
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| On each request, Olla computes a **session key** from one of the configured key sources (see [Key sources](#key-sources) below). The key is looked up in an in-memory LRU/TTL store: | ||
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| - **Hit**: the pinned backend is still routable; the request is sent there and the TTL is refreshed. | ||
| - **Miss**: no entry exists; the request is forwarded to the underlying balancer, and the selected backend is stored. | ||
| - **Repin**: an entry exists but the pinned backend is no longer routable; the underlying balancer selects a new backend and the entry is overwritten. | ||
| - **Disabled**: no key source produced a usable key (e.g. no `X-Olla-Session-ID` header and no other sources configured); the request is passed through to the underlying balancer without recording anything. | ||
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| ```mermaid | ||
| sequenceDiagram | ||
| participant C as Client | ||
| participant O as Olla | ||
| participant Store as Session Store | ||
| participant B as Backend | ||
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| C->>O: POST /olla/proxy/... (turn N+1) | ||
| O->>O: Derive session key | ||
| O->>Store: Lookup key | ||
| alt Cache hit (backend routable) | ||
| Store-->>O: Pinned backend URL | ||
| O->>B: Forward to pinned backend | ||
| B-->>O: Response | ||
| O-->>C: Response + X-Olla-Sticky-Session: hit | ||
| else Cache miss / repin | ||
| Store-->>O: No entry (or dead backend) | ||
| O->>O: Delegate to underlying balancer | ||
| O->>Store: Store key → selected backend | ||
| O->>B: Forward to selected backend | ||
| B-->>O: Response | ||
| O-->>C: Response + X-Olla-Sticky-Session: miss | ||
| end | ||
| ``` | ||
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| ## Key sources | ||
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| The `key_sources` list is evaluated in order; the first source that produces a non-empty value wins. All keys are scoped to the model name so the same client talking to different models maintains independent session state. | ||
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| | Source | How the key is derived | When to prefer it | Caveats | | ||
| |---|---|---|---| | ||
| | `session_header` | FNV-64a hash of the `X-Olla-Session-ID` request header | Explicit client opt-in; most reliable | Client must send the header consistently | | ||
| | `prefix_hash` | FNV-64a hash of the first `prefix_hash_bytes` bytes of the `messages` JSON field | No client changes needed; best cache locality | Two conversations with identical opening messages share a session | | ||
| | `auth_header` | FNV-64a hash of the `Authorization` header value | Per-user affinity without client changes | Breaks if the token rotates mid-conversation; unreliable with shared tokens | | ||
| | `ip` | Client IP address (extracted via `net.SplitHostPort`) | Simple deployments with no NAT | Unreliable behind NAT, load balancers, or Docker networking | | ||
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| All header and token values are hashed before storage; plaintext secrets are never written to the session store. | ||
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| The default configuration enables `session_header`, `prefix_hash`, and `auth_header` (in that order) and comments out `ip` because it is unreliable behind typical container networking. Adjust the list to suit your deployment. | ||
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| ## Session lifecycle and eviction | ||
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| Sessions do not live forever. Three mechanisms remove them: | ||
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| **Sliding TTL**: every cache hit refreshes the expiry timer. A session that goes idle for longer than `idle_ttl_seconds` is expired automatically. Active conversations are never interrupted mid-session. | ||
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| **LRU eviction**: when the store reaches `max_sessions`, the least-recently-used entry is evicted to make room. Under normal load this should never occur; it acts as a safety cap to bound memory usage. | ||
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| **Health-based purge**: when the health checker transitions a backend to an unhealthy state, Olla immediately calls `PurgeDeadEndpoints` with the current routable set. Any session entry pointing to the now-dead backend is deleted without waiting for TTL. The next request for that session falls through to the underlying balancer and receives a `repin`. | ||
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| !!! note "Busy endpoints are not purged" | ||
| A backend in the **Busy** state is still considered routable (`IsRoutable() == true`). Sticky sessions are preserved through Busy transitions; the backend is overloaded but still serving. Only transitions to Unhealthy, Offline, or Unknown trigger a purge. | ||
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| ```mermaid | ||
| stateDiagram-v2 | ||
| [*] --> Active: First request (miss) | ||
| Active --> Active: Subsequent requests (hit, TTL refreshed) | ||
| Active --> Expired: Idle longer than idle_ttl_seconds | ||
| Active --> Purged: Backend becomes unhealthy | ||
| Active --> Evicted: LRU cap (max_sessions) reached | ||
| Expired --> [*] | ||
| Purged --> [*] | ||
| Evicted --> [*] | ||
| ``` | ||
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| ## Response headers | ||
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| Olla writes three response headers so clients and operators can observe affinity decisions: | ||
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| | Header | Values | Meaning | | ||
| |---|---|---| | ||
| | `X-Olla-Sticky-Session` | `hit` / `miss` / `repin` / `disabled` | Outcome of the affinity lookup for this request | | ||
| | `X-Olla-Sticky-Key-Source` | `session_header` / `prefix_hash` / `auth_header` / `ip` / `none` | Which key source was used (absent when outcome is `disabled`) | | ||
| | `X-Olla-Session-ID` | _(echoed from request)_ | Present in the response only when the client sent `X-Olla-Session-ID`; lets stateless clients confirm the header was received | | ||
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| Example: first request (miss), client provides explicit session ID: | ||
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| ```bash | ||
| curl -i -X POST http://localhost:40114/olla/proxy/api/chat \ | ||
| -H "X-Olla-Session-ID: conv-abc123" \ | ||
| -H "Content-Type: application/json" \ | ||
| -d '{"model":"llama3.2","messages":[{"role":"user","content":"Hello"}]}' | ||
| ``` | ||
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| ```http | ||
| HTTP/1.1 200 OK | ||
| X-Olla-Endpoint: gpu-server-1 | ||
| X-Olla-Sticky-Session: miss | ||
| X-Olla-Sticky-Key-Source: session_header | ||
| X-Olla-Session-ID: conv-abc123 | ||
| ``` | ||
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| Subsequent request (hit): | ||
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| ```http | ||
| HTTP/1.1 200 OK | ||
| X-Olla-Endpoint: gpu-server-1 | ||
| X-Olla-Sticky-Session: hit | ||
| X-Olla-Sticky-Key-Source: session_header | ||
| X-Olla-Session-ID: conv-abc123 | ||
| ``` | ||
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| ## Configuration | ||
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| All fields live under `proxy.sticky_sessions`: | ||
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| ```yaml | ||
| proxy: | ||
| sticky_sessions: | ||
| enabled: false # opt-in: set true to activate affinity routing | ||
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| idle_ttl_seconds: 600 # sliding TTL in seconds; 0 = sessions never expire by TTL | ||
| # (not recommended, sessions accumulate until LRU eviction) | ||
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| max_sessions: 10000 # LRU capacity; oldest entries are evicted when full | ||
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| key_sources: # ordered cascade, first match wins | ||
| - "session_header" # X-Olla-Session-ID header (explicit client opt-in) | ||
| - "prefix_hash" # hash of first N bytes of messages JSON | ||
| - "auth_header" # hash of Authorization header (per-user affinity) | ||
| # - "ip" # client IP, opt-in; unreliable behind NAT/Docker | ||
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| prefix_hash_bytes: 512 # bytes of the messages field to hash for prefix_hash source; | ||
| # larger values reduce false collisions at a small CPU cost | ||
| ``` | ||
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| The only env var exposed for this feature is `OLLA_PROXY_STICKY_SESSIONS_ENABLED` (boolean). The remaining fields are configuration-file only. | ||
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| ## Observability | ||
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| ### Stats endpoint | ||
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| ```bash | ||
| curl http://localhost:40114/internal/stats/sticky | ||
| ``` | ||
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| When sticky sessions are **enabled**: | ||
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| ```json | ||
| { | ||
| "enabled": true, | ||
| "active_sessions": 142, | ||
| "insertions": 1500, | ||
| "hits": 9231, | ||
| "misses": 1500, | ||
| "evictions": 0, | ||
| "max_sessions": 10000, | ||
| "idle_ttl_seconds": 600 | ||
| } | ||
| ``` | ||
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| When sticky sessions are **disabled** (stable shape for scripting): | ||
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| ```json | ||
| { | ||
| "enabled": false | ||
| } | ||
| ``` | ||
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| Scripts should branch on the `enabled` field; the endpoint always returns `200 OK` regardless of whether the feature is active. | ||
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| ## When NOT to use sticky sessions | ||
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| Sticky sessions trade load distribution for cache locality. They are not always the right choice: | ||
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| - **Stateless or single-turn workloads**: embeddings, one-shot completions, and batch jobs gain nothing from affinity; use the plain load balancer. | ||
| - **Model-routing-dominated traffic**: if requests are already hard-routed to specific endpoints by model routing, the sticky wrapper adds overhead with no benefit. | ||
| - **Very small deployments**: two endpoints with priority load balancing already behave predictably; adding stickiness is unnecessary complexity. | ||
| - **Homogeneous short-prompt workloads**: when prompts are short and vary widely, KV-cache hit rates on the backend are already low; affinity provides little gain and reduces load distribution. | ||
| - **Deployments with aggressive autoscaling**: if backends are added and removed frequently, sessions will repin often and the affinity benefit is diluted. | ||
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| ## Developer notes | ||
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| **Decorator pattern**: `StickySessionWrapper` in `internal/adapter/balancer/sticky.go` wraps any `domain.EndpointSelector` implementation. No factory or registry changes are needed to add a new inner balancer; the wrapper is applied in `ProxyServiceWrapper.applyStickySessions()` inside `internal/app/services/proxy.go`. | ||
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| **Hashing**: FNV-64a (`hash/fnv`) is used for all key derivation. It is non-cryptographic and intended only as a compact routing hint; collisions are acceptable (two different sessions occasionally land on the same backend). Do not use these keys as security tokens. | ||
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| **Import cycle avoidance**: `StickyOutcome` is defined in `internal/core/domain/routing.go` rather than in `internal/adapter/balancer/sticky.go`. This allows `internal/adapter/proxy/core` (the proxy engine shared layer) to read the outcome and write response headers without importing the balancer package, which would create a cycle. | ||
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| **Purge wiring order**: `applyStickySessions()` assigns `s.stickyWrapper` and then immediately calls `s.discoverySvc.SetPurgeDeadEndpointsFn(s.PurgeDeadEndpoints)`. This ensures the write of `stickyWrapper` happens-before the health-checker goroutine can observe it via the purge callback. The registration happens inside `ProxyServiceWrapper.Start()`, not at construction time, to respect service startup ordering. | ||
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| **Session store**: backed by `github.com/jellydator/ttlcache/v3` with `WithCapacity` (LRU) and `WithTTL` (sliding expiry). The `ttlcache.Get` call inside `Select` refreshes the TTL automatically on every hit. | ||
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| Relevant source files: `internal/adapter/balancer/sticky.go`, `internal/core/domain/routing.go`, `internal/app/services/proxy.go`, `internal/app/services/discovery.go`, `internal/app/handlers/handler_proxy.go`, `internal/app/handlers/handler_stats_sticky.go`. | ||
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| ## See also | ||
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| - [Load Balancing](load-balancing.md): underlying strategies that sticky sessions wrap | ||
| - [Health Checking](health-checking.md): health states and the routable concept | ||
| - [Configuration Reference](../configuration/reference.md#sticky-sessions): complete field reference |
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