Automatic failover in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What triggers automatic failover in Redis Sentinel?

In Redis Sentinel, automatic failover happens when the master node is considered down. What condition causes Sentinel to start failover?

A When a slave node sends a manual failover request to Sentinel

B When the master node stops responding to PINGs from Sentinel for a configured period

C When the client application disconnects from the master node

D When the master node reaches maximum memory usage

💡 Hint

Think about how Sentinel detects a master failure automatically.

Sentinel monitors the master by sending periodic PING commands. If the master does not respond within a configured timeout, Sentinel marks it as subjectively down and then, after agreement from other Sentinels, as objectively down, triggering failover.

❓ query_result

intermediate

What is the output of the SENTINEL FAILOVER command?

Consider the Redis Sentinel command SENTINEL FAILOVER <master-name>. What is the immediate output after running this command?

A "ERROR: No such master"

B "FAILOVER_STARTED"

C "OK"

D "FAILOVER_ABORTED"

💡 Hint

Check the Redis Sentinel documentation for the command response.

The SENTINEL FAILOVER command returns "OK" immediately after starting the failover process. It does not wait for failover completion.

📝 Syntax

advanced

Which Redis Sentinel configuration enables automatic failover?

Which configuration directive in sentinel.conf enables Redis Sentinel to perform automatic failover?

A sentinel monitor <master-name> <ip> <port> <quorum>

B sentinel failover-timeout <master-name> <milliseconds>

C sentinel down-after-milliseconds <master-name> <milliseconds>

D sentinel parallel-syncs <master-name> <number>

💡 Hint

Which directive tells Sentinel which master to watch?

The sentinel monitor directive tells Sentinel to watch a master and enables automatic failover. Other directives configure timing and behavior but do not enable monitoring by themselves.

❓ optimization

advanced

How to reduce failover time in Redis Sentinel?

You want to reduce the time Redis Sentinel takes to detect a master failure and complete failover. Which combination of settings helps achieve this?

A Set <code>down-after-milliseconds</code> to a very high value and decrease <code>failover-timeout</code>

B Increase <code>parallel-syncs</code> and <code>quorum</code> values

C Disable <code>sentinel monitor</code> to avoid delays

D Decrease <code>down-after-milliseconds</code> and <code>failover-timeout</code> values

💡 Hint

Think about how Sentinel detects failure and how long it waits before failover.

Lowering down-after-milliseconds makes Sentinel detect failure faster. Reducing failover-timeout shortens the failover process. Increasing quorum or disabling monitoring would delay or prevent failover.

🔧 Debug

expert

Why does Redis Sentinel failover not happen automatically?

You have a Redis Sentinel setup monitoring a master and its slaves. The master crashes, but Sentinel does not perform failover automatically. Which is the most likely cause?

A The quorum value is set higher than the number of Sentinels available

B The master node is still responding to PINGs from Sentinel

C The slave nodes are configured with <code>slave-read-only no</code>

D The <code>parallel-syncs</code> value is set to zero

💡 Hint

Consider how Sentinel decides a master is objectively down.

If the quorum (number of Sentinels that must agree the master is down) is higher than the number of Sentinels running, failover will not start because Sentinel never reaches consensus.