Understanding Redis TTL Inconsistencies

Redis allows users to set expiration times for keys using EXPIRE or SETEX. However, due to Redis eviction policies, replication delays, and improper client implementations, TTLs may behave unexpectedly.

Common Causes of TTL Inconsistencies

  • Key expiration overridden: Subsequent commands modify the key, unintentionally removing TTL.
  • Replication lag: Delays between master and replica nodes cause discrepancies.
  • Eviction policies: Keys expire prematurely due to Redis memory policies.
  • Key persistence conflicts: Some keys persist despite TTL due to snapshot and AOF restore inconsistencies.

Diagnosing TTL Issues

Checking TTL for a Key

Verify expiration time:

TTL myKey

Monitoring Eviction Policies

Check configured memory eviction settings:

CONFIG GET maxmemory-policy

Analyzing Replication Delays

Check replication lag between primary and replicas:

INFO replication

Fixing Redis TTL Issues

Ensuring TTL is Not Overwritten

Use the PERSIST command cautiously and ensure TTL persists after updates:

EXPIRE myKey 3600
SET myKey "newValue" EX 3600

Configuring Proper Eviction Policies

Use a suitable eviction strategy:

CONFIG SET maxmemory-policy allkeys-lru

Handling Replication Lag

Reduce replication delay by adjusting replica buffer size:

CONFIG SET repl-backlog-size 104857600

Ensuring Persistence Consistency

Verify that AOF and RDB properly save TTL:

CONFIG SET appendonly yes

Preventing Future TTL Inconsistencies

  • Always set TTL explicitly after modifying keys.
  • Monitor INFO memory for eviction triggers.
  • Ensure replication is properly synchronized.

Conclusion

Redis TTL inconsistencies can cause unexpected data persistence or premature expiration. By ensuring proper TTL usage, optimizing eviction policies, and monitoring replication health, developers can maintain reliable Redis expiration behavior.

FAQs

1. Why does my Redis key expire earlier than expected?

It may be evicted due to memory policies or TTL removal by overwriting operations.

2. How do I check if my key has a TTL?

Use the TTL keyname command to inspect expiration time.

3. Can Redis restore keys with TTL after a restart?

Yes, if using AOF persistence, but RDB snapshots may not always save TTLs correctly.

4. Why do some keys persist despite having an expiration?

If a key is modified, its TTL may be reset unless explicitly re-applied.

5. How can I prevent replication delays from affecting TTLs?

Optimize network latency and increase the Redis backlog buffer size.