Sticky routing keeps a client on its previously assigned backend; it does not make that assignment permanent by design. If inactive clients’ rows never expire, however, a routing table can keep counting old assignments as current. A node may then appear busier than it is and influence later placement decisions. The fix is to give assignments an inactivity lifetime and check expiry in the routing path rather than relying only on background deletion.
What sticky routing preserves
In a common design, the router stores a mapping from a client or session ID to a backend. On a later request, it looks up that mapping and sends the client to the same backend instead of making a fresh choice. The purpose is affinity: continuity across requests, not durable ownership of a backend forever.
For example, a router might consult its assignment table before selecting the least-loaded node. If a client has no valid assignment, the router chooses a node and records the mapping. If the client already has a valid mapping, it reuses it. This works only if the system’s definition of a valid assignment reflects whether the client is still active.
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Suppose a client stops making requests but its mapping remains in the table. If the router counts every row as a current assignment, that idle client continues to add to the apparent load of its former node. The node can look fuller than it is, affecting where later clients are placed. The row may still accurately record the client’s last backend; what has become misleading is treating that historical association as a current load signal.
This is a possible failure pattern, not a measured claim about how often it occurs or how large the imbalance will be. Its impact depends on how the router uses its table, what it counts as load, and how many inactive assignments accumulate.
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Use an inactivity TTL to release idle assignments
A time-to-live (TTL) can give each assignment an inactivity window. Refresh the expiry after a successful event that demonstrates the client is still using the route. If the client does not return before that window ends, the assignment becomes expired; a later request can be treated as unassigned and routed anew.
There is no universal routing TTL established by the available documentation. Choose one based on the application’s expected gaps between legitimate requests and the cost of keeping an affinity versus releasing it. Make the refresh event explicit: a successful request, a particular session action, or another meaningful signal may be appropriate, but merely receiving unrelated traffic should not silently extend an assignment.
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When reviewing the design, ask: when does this stop being true? In other words, what evidence shows that an old client-to-backend relationship should still govern its next request?
Why disconnect events are not enough
A delete triggered by a clean disconnect can remove an assignment promptly, but clients do not always disconnect cleanly. A process may crash, a browser tab may close, a connection may be dropped, or a network partition may prevent a final message from reaching the service. In those cases the router may never receive the event needed to delete the row.
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TTL-based expiry covers these silent departures: the client must return within the inactivity window to keep its assignment alive. A disconnect handler can still provide faster cleanup when it runs, but it should not be the only path for reclaiming stale assignments.
Keep routing correctness separate from physical TTL cleanup
Datastore TTL is often an asynchronous cleanup mechanism, not a guarantee that expired records disappear immediately. For DynamoDB, AWS says, “DynamoDB typically deletes expired items within two days of expiration.” AWS qualifies that timing as workload-dependent and notes that expired items awaiting deletion can remain visible in reads, queries, and scans. The figure describes DynamoDB’s item-deletion timing, not routing performance or the prevalence of routing imbalance. AWS DynamoDB TTL developer guide and AWS DynamoDB API reference.
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Therefore, do not make correctness depend on a background sweep having physically removed an expired row. On the routing read path, evaluate the stored expiry time and treat an expired assignment as invalid. Where expired DynamoDB items are invalid for a read, AWS documents filtering them; the router’s own logic should likewise enforce expiry before counting or using an assignment. The sweep can reclaim storage later.
Affinity is not a guarantee of backend permanence
Routing systems implement affinity differently. NVIDIA Dynamo v1.2.0 documents a StickySessionRouter with an in-memory mapping from session_id to (worker_id, dp_rank) and a sliding-window TTL. Its bind action establishes router-only affinity, and subsequent requests with that session ID go to the pinned worker and rank. This is a version-specific example, not a default lifetime for other systems. NVIDIA Dynamo router guide.
TrueFoundry describes sticky routing based on a client-provided session header and consistent-hash routing, while warning that Kubernetes pods and nodes are transient. Deletion, eviction, or scaling can result in reassignment, so affinity is best effort. Its guidance cautions against putting critical stateful logic in Services. A routing pin is not durable storage, nor does it promise that a client will stay on one backend indefinitely. TrueFoundry sticky routing documentation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Questions to settle before choosing an expiry policy
- Fixed or sliding expiry? A fixed lifetime expires at a set time; a sliding window moves forward when qualifying activity occurs.
- What refreshes the TTL? Identify the successful request or session event that proves the assignment is still useful.
- What if the pinned backend fails? Decide whether to reassign immediately, retry, or fail the request; affinity must not prevent recovery from an unavailable worker.
- Does the read path check expiry? Treat an expired mapping as invalid even if the datastore has not yet swept it.
- Do other session or resource timeouts align? A routing assignment that outlives the local session or resource it was meant to support may preserve affinity without preserving useful state.
These decisions depend on the application and routing system. The documentation cited here does not establish a single TTL value suitable for all workloads.
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