Cluster failover in Redis - Step-by-Step Execution

Concept Flow - Cluster failover

Primary Node Active

↓

Primary Node Fails?

No→Continue Serving

Yes↓

Detect Failure

↓

Promote Replica to Primary

↓

Redirect Clients to New Primary

↓

Cluster Continues Operating

The cluster monitors the primary node; if it fails, a replica is promoted to primary and clients are redirected to maintain service.

Execution Sample

Redis

1. Primary node fails
2. Cluster detects failure
3. Replica promoted to primary
4. Clients redirected
5. Service continues

This sequence shows how a Redis cluster handles a primary node failure by promoting a replica to keep the cluster running.

Execution Table

Step	Event	Action Taken	Node Role Changes	Client Impact
1	Primary node is active	No action	Primary active	Clients connected to primary
2	Primary node fails	Failure detected	Primary down	Clients lose connection
3	Replica detects failure	Replica promoted	Replica becomes new primary	Clients must reconnect
4	Clients redirected	Clients connect to new primary	New primary active	Service restored
5	Cluster stable	Monitoring continues	New primary active	Normal operation
6	No further failure	No action	New primary active	Clients connected

💡 Cluster stabilizes after replica promotion and client reconnection

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
Primary Node Status	Active	Failed	Down	Down	Down
Replica Node Status	Replica	Replica	Promoted to Primary	Primary	Primary
Client Connection	Connected to Primary	Disconnected	Disconnected	Connected to New Primary	Connected

Key Moments - 3 Insights

Why do clients lose connection immediately after the primary node fails?

Because the primary node is down (see execution_table step 2), clients lose their connection until the cluster promotes a replica and redirects them.

How does the cluster decide which replica to promote?

The cluster uses internal consensus and health checks to select a healthy replica to promote, ensuring minimal downtime (implied in step 3).

What happens if clients do not reconnect to the new primary?

Clients will not be able to perform writes or reads that require the primary, causing service disruption until reconnection (see execution_table step 4).

Visual Quiz - 3 Questions

Look at the execution_table, what is the status of the primary node at step 3?

A Active

B Failed

C Down

D Promoted

💡 Hint

Check the 'Node Role Changes' column at step 3 in the execution_table.

At which step do clients reconnect to the new primary node?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Client Impact' column for when clients connect to the new primary.

If the replica fails to promote, what would likely happen to the cluster?

A Cluster remains down until manual intervention

B Clients continue as normal

C Another replica is promoted automatically

D Primary node recovers automatically

💡 Hint

Consider the failover steps and what happens if promotion does not occur (refer to key_moments).

Concept Snapshot

Cluster failover in Redis:
- Primary node failure triggers detection
- Replica promoted to primary
- Clients redirected to new primary
- Cluster continues serving without manual restart
- Ensures high availability and minimal downtime

Full Transcript

In Redis cluster failover, the primary node is monitored continuously. When it fails, the cluster detects this failure and promotes a replica node to become the new primary. Clients lose connection briefly but are redirected to the new primary to continue operations. This process ensures the cluster remains available and operational with minimal downtime. Variables such as node status and client connections change step-by-step during failover, as shown in the execution table. Understanding these steps helps grasp how Redis maintains high availability.

Practice

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

easy

A. To backup data to an external storage

B. To clear all keys from the database

C. To increase the number of replicas manually

D. To switch a replica to master if the current master fails

Solution

1. Step 1: Understand cluster failover concept

   Cluster failover automatically promotes a replica to master when the current master fails.

2. Step 2: Identify the main purpose

   This process ensures Redis availability without manual intervention.

3. Final Answer:

   To switch a replica to master if the current master fails -> Option D

4. Quick Check:

   Failover = switch replica to master [OK]

Hint: Failover means replica becomes master on failure [OK]

Common Mistakes:

• Confusing failover with backup
• Thinking failover increases replicas
• Assuming failover clears data

2. Which Redis command syntax correctly triggers a manual failover on a replica node?

easy

A. CLUSTER FAILOVER

B. FAILOVER CLUSTER

C. CLUSTER FAILOVER NOW

D. FAILOVER REPLICA

Solution

1. Step 1: Recall Redis failover command syntax

   The correct command to trigger failover is CLUSTER FAILOVER.

2. Step 2: Check options for syntax correctness

   Options B, C, and D are invalid Redis commands or incorrect order.

3. Final Answer:

   CLUSTER FAILOVER -> Option A

4. Quick Check:

   Correct command = CLUSTER FAILOVER [OK]

Hint: Remember command starts with CLUSTER then FAILOVER [OK]

Common Mistakes:

• Swapping command words order
• Adding invalid keywords
• Using non-existent commands

3. Given a Redis cluster where the master node fails, what will happen if you run CLUSTER FAILOVER FORCE on a replica?

medium

A. The replica deletes all its data before failing over

B. The replica waits for the master to recover before failing over

C. The replica immediately becomes master regardless of cluster state

D. The command returns an error because FORCE is invalid

Solution

1. Step 1: Understand FORCE option in failover

   FORCE forces the replica to become master immediately, ignoring cluster state.

2. Step 2: Analyze other options

   Waiting for master recovery is default without FORCE; FORCE does not cause errors or data deletion.

3. Final Answer:

   The replica immediately becomes master regardless of cluster state -> Option C

4. Quick Check:

   FORCE = immediate failover [OK]

Hint: FORCE triggers instant failover ignoring cluster state [OK]

Common Mistakes:

• Thinking FORCE waits for master
• Assuming FORCE causes errors
• Believing FORCE deletes data

4. You tried running CLUSTER FAILOVER TAKEOVER on a replica but got an error. What is the most likely cause?

medium

A. The TAKEOVER option is not supported in Redis

B. The replica is not connected to the cluster

C. The replica is already the master

D. The command syntax is incorrect

Solution

1. Step 1: Understand TAKEOVER option requirements

   TAKEOVER forces failover even if the master is alive but requires cluster connectivity.

2. Step 2: Identify error cause

   If the replica is disconnected, the command fails with an error.

3. Final Answer:

   The replica is not connected to the cluster -> Option B

4. Quick Check:

   Disconnected replica causes TAKEOVER error [OK]

Hint: Replica must be connected for TAKEOVER to work [OK]

Common Mistakes:

• Assuming TAKEOVER is unsupported
• Ignoring replica connection status
• Thinking syntax is wrong

5. In a Redis cluster with multiple replicas, which failover option should you use to safely promote a replica without risking data loss, and why?

hard

A. Use CLUSTER FAILOVER without options to allow safe automatic failover

B. Use CLUSTER FAILOVER FORCE to force immediate promotion regardless of state

C. Use CLUSTER FAILOVER TAKEOVER to take over even if master is alive

D. Manually disconnect the master before failover

Solution

1. Step 1: Review failover options and safety

   Default CLUSTER FAILOVER waits for safe conditions, avoiding data loss.

2. Step 2: Compare with FORCE and TAKEOVER risks

   FORCE and TAKEOVER can cause data loss by ignoring cluster state or master status.

3. Step 3: Consider manual disconnection risks

   Manually disconnecting master is risky and not recommended for safe failover.

4. Final Answer:

   Use CLUSTER FAILOVER without options to allow safe automatic failover -> Option A

5. Quick Check:

   Safe failover = default CLUSTER FAILOVER [OK]

Hint: Default failover is safest to avoid data loss [OK]

Common Mistakes:

• Using FORCE or TAKEOVER without understanding risks
• Manually disconnecting master unnecessarily
• Assuming all failover options are equally safe