NATS JetStream ↔ Application Cache: Managing Stale Data on Message Redelivery

Answer

To keep an application cache in sync with NATS JetStream despite at‑least‑once delivery and possible reordering, make each message carry a stable identifier and a monotonic version, then use that information to detect superseded or duplicate messages before updating the cache.

Confirmed facts

1. JetStream guarantees at‑least‑once delivery; a message is redelivered if the consumer does not acknowledge within the AckWait interval or if MaxDeliver is exceeded.
2. JetStream provides a Nats-Msg-Id header that is identical for every redelivery of the same message.
3. When the consumer processes a message and acknowledges it, JetStream stops further redeliveries.

Steps to detect superseded messages and bound staleness

1. Enrich the payload:
   • Include the JetStream‑provided Nats-Msg-Id (or a UUID generated by the publisher).
   • Add a monotonically increasing version or timestamp (e.g., a Unix nanosecond counter or a stream sequence number).
2. Store processed IDs:
   • Maintain a bounded set (LRU or TTL‑based) of message IDs that have been successfully processed.
   • Before handling a message, check if its ID is already in the set; if yes, skip processing and acknowledge immediately.
3. Compare versions:
   • If the ID is new, compare the message’s version/timestamp with the version stored alongside the cached entry.
   • If the incoming version is ≤ cached version, treat the message as stale or duplicate; acknowledge without updating the cache.
   • If the incoming version is greater, proceed to update the cache.
4. Update cache and acknowledge:
   • Apply the update to the cache.
   • Record the message ID in the processed‑ID store.
   • Send an acknowledgment to JetStream to stop further redeliveries.
5. Tune acknowledgment parameters:
   • Set AckWait to a value slightly larger than the worst‑case processing time (including cache update latency). This limits how long JetStream will wait before redelivering.
   • Configure MaxDeliver to a low number (e.g., 1 or 2) if you want to bound the number of redelivery attempts; otherwise rely on AckWait and idempotent handling.

Verification (scoped, safe)

# Observe the JetStream header on each message
nats sub --stream <stream> --msg-id

# Example log line when a duplicate is skipped
[INFO] Message ID <id> already processed – skipping (version 5 ≤ cached 5)

# After a test where AckWait is exceeded, confirm cache holds the highest version only

Ask for one missing diagnostic detail only if it would change the recommendation: what is the current AckWait setting on the consumer? Knowing this helps decide whether to increase it or rely more on idempotent checks.