DBS-C01 Monitoring and Troubleshooting Practice Question

A developer is troubleshooting an application that writes to an Amazon ElastiCache for Redis cluster. The application occasionally fails with 'OOM command not allowed when used memory > maxmemory'. What is the most likely cause?

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

The cluster's maxmemory-policy is set to noeviction.

The error 'OOM command not allowed when used memory > maxmemory' occurs when Redis has reached its configured memory limit and the eviction policy is set to 'noeviction', which prevents any further writes. Option B is correct because 'noeviction' causes Redis to return an error instead of evicting keys. Option A is incorrect because the 'maxclients' limit produces a different error ('max number of clients reached'). Option C is incorrect because keyspace notifications do not affect memory limits. Option D is incorrect because cross-slot commands relate to cluster mode, not memory errors.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
The cluster's maxclients limit has been reached.
Why it's wrong here
The 'maxclients' limit would produce an error like 'max number of clients reached', not an OOM error.
✓
The cluster's maxmemory-policy is set to noeviction.
Why this is correct
Correct. Setting 'maxmemory-policy' to 'noeviction' prevents eviction and causes Redis to reject write commands when memory is full.
✗
The cluster has too many keyspace notifications enabled.
Why it's wrong here
Keyspace notifications are unrelated to memory limits and do not cause OOM errors.
✗
The cluster is in cluster mode and cross-slot commands are used.
Why it's wrong here
Cross-slot commands are a cluster mode restriction and do not directly cause OOM errors.

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Same concept, more angles

1 more way this is tested on DBS-C01

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. An application uses Amazon ElastiCache for Redis as a session store. Users report that sessions are being lost intermittently. The ElastiCache cluster has replication enabled with one replica. CloudWatch metrics show 'Evictions' spiking during peak hours. What is the MOST likely cause?

hard

A.Replication lag between primary and replica is causing read failures
B.Encryption in transit is enabled and causes decryption errors
✓ C.The cache's memory is full and the eviction policy is removing keys
D.The cluster is performing automatic snapshots that block writes

Why C: A spike in 'Evictions' CloudWatch metric indicates that the cache's memory is full, and ElastiCache is evicting keys based on the configured eviction policy (e.g., allkeys-lru). This removal of keys directly causes session data to be lost. Option A is incorrect: replication lag can cause stale reads but not evictions or loss of keys. Option B is incorrect: encryption in transit does not affect memory usage or evictions. Option D is incorrect: automatic snapshots may cause a brief latency spike but do not result in evictions.

Last reviewed: Jun 20, 2026

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