Why replication provides redundancy in Redis - Visual Breakdown

Concept Flow - Why replication provides redundancy

Primary Server receives data

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Primary writes data to its storage

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Primary sends data to Replica

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Replica receives and stores data

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If Primary fails

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Replica can serve data

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Redundancy achieved

Data is copied from the primary server to replicas, so if the primary fails, replicas have the data to keep the system running.

Execution Sample

Redis

PRIMARY: SET key "value"
REPLICA: RECEIVE key "value"
REPLICA: STORE key "value"
PRIMARY: FAILS
REPLICA: SERVE key "value"

Shows how data set on primary is copied to replica, which can serve data if primary fails.

Execution Table

Step	Action	Primary State	Replica State	Result
1	Primary sets key to 'value'	key='value'	empty	Data stored on primary
2	Primary sends data to replica	key='value'	receiving 'value'	Replica receives data
3	Replica stores key='value'	key='value'	key='value'	Replica stores data
4	Primary fails	down	key='value'	Primary unavailable
5	Replica serves key	down	key='value'	Data served from replica

💡 Primary failure triggers replica to serve data, showing redundancy

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5
Primary State	empty	key='value'	key='value'	key='value'	down	down
Replica State	empty	empty	receiving 'value'	key='value'	key='value'	key='value'

Key Moments - 2 Insights

Why does the replica have the data even after the primary fails?

Because in step 3 the replica stores the data sent from the primary before the primary fails in step 4, as shown in the execution_table.

What happens if the primary fails before sending data to the replica?

The replica will not have the latest data, so redundancy is not complete. This is why replication timing matters, as seen between steps 2 and 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the replica state at step 3?

A empty

B key='value'

C receiving 'value'

D down

💡 Hint

Check the 'Replica State' column at step 3 in the execution_table.

At which step does the primary server fail?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look for 'Primary fails' action in the execution_table.

If the primary never sends data to the replica, what will the replica state be at step 5?

A key='value'

B receiving 'value'

C empty

D down

💡 Hint

Refer to variable_tracker and consider what happens if step 2 is skipped.

Concept Snapshot

Replication copies data from primary to replicas.
Replicas store data to provide backup.
If primary fails, replicas serve data.
This creates redundancy and improves availability.
Replication timing is key for up-to-date data.

Full Transcript

Replication in Redis means copying data from the primary server to one or more replicas. When the primary server receives data, it stores it and sends a copy to the replicas. The replicas store this data as well. If the primary server fails, the replicas still have the data and can serve it, providing redundancy. This process ensures the system keeps working even if one server goes down. The timing of replication is important because if the primary fails before sending data, the replicas won't have the latest information.