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.

Practice

1. What is the main purpose of automatic failover in Redis?

easy

A. To backup data to an external storage

B. To increase the speed of Redis queries

C. To switch to a backup server if the main server fails

D. To encrypt data during transmission

Solution

1. Step 1: Understand the concept of failover

   Failover means switching to a backup system when the main one stops working.

2. Step 2: Apply this to Redis context

   Redis automatic failover switches to a backup Redis server if the main one fails.

3. Final Answer:

   To switch to a backup server if the main server fails -> Option C

4. Quick Check:

   Automatic failover = switch to backup server [OK]

Hint: Failover means backup server takes over on failure [OK]

Common Mistakes:

• Confusing failover with data backup
• Thinking failover speeds up queries
• Mixing failover with encryption

2. Which Redis component is responsible for managing automatic failover?

easy

A. Redis Cluster

B. Redis Sentinel

C. Redis Streams

D. Redis Modules

Solution

1. Step 1: Identify Redis components related to failover

   Redis Sentinel is designed to monitor Redis servers and manage failover.

2. Step 2: Confirm other options are unrelated

   Redis Cluster manages sharding, Streams handle data streams, Modules add features but don't manage failover.

3. Final Answer:

   Redis Sentinel -> Option B

4. Quick Check:

   Failover manager = Sentinel [OK]

Hint: Sentinel monitors and triggers failover in Redis [OK]

Common Mistakes:

• Confusing Sentinel with Cluster
• Thinking Streams handle failover
• Assuming Modules manage failover

3. Given a Redis setup with 3 Sentinel instances monitoring 1 master and 2 replicas, what happens if the master fails?

medium

A. Sentinels promote one replica to master automatically

B. All Sentinels shut down to prevent split-brain

C. Clients lose connection permanently

D. The master restarts automatically without promotion

Solution

1. Step 1: Understand Sentinel quorum and failover

   With multiple Sentinels, they agree on master failure and promote a replica.

2. Step 2: Analyze options

   Sentinels do not shut down; clients reconnect after failover; master does not restart automatically.

3. Final Answer:

   Sentinels promote one replica to master automatically -> Option A

4. Quick Check:

   Master failure triggers replica promotion [OK]

Hint: Sentinels elect new master when original fails [OK]

Common Mistakes:

• Thinking Sentinels shut down on failure
• Assuming clients lose connection forever
• Believing master restarts without failover

4. You configured Redis Sentinel with only one instance monitoring a master and replicas. What is a likely problem with automatic failover?

medium

A. Failover may not happen due to lack of quorum

B. Sentinel will promote multiple replicas simultaneously

C. Sentinel will delete all data on replicas

D. Failover will happen instantly without checks

Solution

1. Step 1: Understand Sentinel quorum requirement

   Sentinel needs multiple instances to agree (quorum) before failover.

2. Step 2: Analyze single Sentinel behavior

   With only one Sentinel, no quorum exists, so failover may not trigger reliably.

3. Final Answer:

   Failover may not happen due to lack of quorum -> Option A

4. Quick Check:

   Single Sentinel lacks quorum for failover [OK]

Hint: Multiple Sentinels needed for reliable failover [OK]

Common Mistakes:

• Thinking one Sentinel can safely failover
• Believing Sentinel deletes replica data
• Assuming failover is instant without checks

5. In a Redis master-replica setup with automatic failover, which combination ensures the highest availability?

hard

A. One master, no replicas, one Sentinel instance

B. One master, one replica, one Sentinel instance

C. Multiple masters, no replicas, multiple Sentinel instances

D. One master, multiple replicas, multiple Sentinel instances

Solution

1. Step 1: Identify components for high availability

   Multiple replicas provide backup data; multiple Sentinels provide reliable failover decisions.

2. Step 2: Evaluate each option

   One master, one replica, one Sentinel instance lacks multiple Sentinels; C has multiple masters causing conflicts; D lacks replicas and multiple Sentinels.

3. Final Answer:

   One master, multiple replicas, multiple Sentinel instances -> Option D

4. Quick Check:

   Replicas + multiple Sentinels = high availability [OK]

Hint: Use multiple replicas and Sentinels for best failover [OK]

Common Mistakes:

• Thinking one Sentinel is enough
• Confusing multiple masters with high availability
• Ignoring replicas in failover setup