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.