Synchronization process in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the synchronization process in Redis?

The synchronization process in Redis ensures that data between the master and replica servers is consistent and up to date.

intermediate

What happens during a full synchronization in Redis replication?

During full synchronization, the replica requests a complete copy of the master's dataset, which is sent as an RDB file, and then applies any new commands received after the snapshot.

intermediate

What is partial synchronization in Redis and when is it used?

Partial synchronization is used when the replica reconnects and the master's replication backlog still contains the missing data, allowing the replica to catch up without a full data transfer.

intermediate

What is the role of the replication backlog in Redis synchronization?

The replication backlog is a fixed-size buffer on the master that stores recent write commands to help replicas perform partial synchronization without needing a full resync.

beginner

Why is synchronization important for Redis replicas?

Synchronization keeps replicas updated with the master's data, ensuring high availability, fault tolerance, and load distribution in Redis setups.

What triggers a full synchronization in Redis replication?

A Replica connects for the first time or backlog is missing

B Replica has all data up to date

C Master restarts without data loss

D Replica disconnects briefly

A full synchronization happens when a replica connects for the first time or when the master's backlog does not contain the missing data.

What does Redis send to the replica during full synchronization?

A Only the latest commands

B A snapshot of the dataset as an RDB file

C A list of keys to delete

D Nothing, replica fetches data itself

During full sync, Redis sends the replica a snapshot of the dataset as an RDB file.

What is the replication backlog used for in Redis?

A To backup the entire database

B To store user passwords

C To store recent write commands for partial sync

D To log errors

The replication backlog stores recent write commands to help replicas catch up during partial synchronization.

Which synchronization type is faster when possible in Redis replication?

A Partial synchronization

B Full synchronization

C Manual synchronization

D No synchronization

Partial synchronization is faster because it only sends missing commands instead of the full dataset.

What happens if the replication backlog is too small in Redis?

A Replication stops completely

B Data is lost permanently

C Master crashes

D Replicas may need full synchronization more often

If the backlog is too small, replicas might miss data and require full synchronization more frequently.

Explain the difference between full synchronization and partial synchronization in Redis replication.

Think about how replicas catch up with the master after disconnects.

Describe the role of the replication backlog in Redis synchronization and why it is important.

Consider how Redis avoids sending full data every time a replica reconnects.

Practice

1. What is the main purpose of the Redis synchronization process between master and replica?

easy

A. To keep data the same between master and replica

B. To delete old data from the master

C. To speed up client requests

D. To backup data to an external server

Solution

1. Step 1: Understand synchronization goal

   Synchronization in Redis ensures that the replica has the same data as the master.

2. Step 2: Compare options with synchronization purpose

   Only To keep data the same between master and replica matches the goal of keeping data consistent between master and replica.

3. Final Answer:

   To keep data the same between master and replica -> Option A

4. Quick Check:

   Synchronization = Data consistency [OK]

Hint: Synchronization means data stays the same on master and replica [OK]

Common Mistakes:

• Thinking synchronization deletes data
• Confusing synchronization with backup
• Assuming synchronization speeds up queries

2. Which Redis command initiates a full synchronization from master to replica?

easy

A. REPLCONF

B. SYNC

C. PSYNC

D. SAVE

Solution

1. Step 1: Identify command for full sync

   The SYNC command copies all data from master to replica for full synchronization.

2. Step 2: Differentiate from other commands

   REPLCONF is for replication config, PSYNC is partial sync, SAVE is for persistence, so SYNC is correct.

3. Final Answer:

   SYNC -> Option B

4. Quick Check:

   Full sync command = SYNC [OK]

Hint: SYNC command copies all data from master to replica [OK]

Common Mistakes:

• Confusing PSYNC with full sync
• Using SAVE instead of SYNC
• Thinking REPLCONF triggers data copy

3. Given a Redis master and replica setup, what happens after the replica sends the SYNC command to the master?

medium

A. The replica deletes its data and waits

B. The replica becomes the new master

C. The master stops accepting writes temporarily

D. The master sends all its data to the replica

Solution

1. Step 1: Understand SYNC command effect

   When replica sends SYNC, master sends all its data to replica to copy.

2. Step 2: Check other options for correctness

   Replica does not delete data first, master does not stop writes, and replica does not become master here.

3. Final Answer:

   The master sends all its data to the replica -> Option D

4. Quick Check:

   SYNC triggers full data send from master [OK]

Hint: SYNC means master sends full data to replica [OK]

Common Mistakes:

• Thinking replica deletes data before sync
• Assuming master stops writes during sync
• Confusing roles of master and replica

4. You set up a Redis replica but it never completes synchronization. Which of these is a likely cause?

medium

A. The replica's SYNC command is misspelled

B. The replica is running a different Redis version

C. Network issues prevent data transfer

D. The master has no data to send

Solution

1. Step 1: Analyze synchronization failure reasons

   Network problems can block data transfer, causing sync to never complete.

2. Step 2: Evaluate other options

   SYNC command is fixed in Redis protocol, master always sends data if present, version mismatch usually causes errors but not infinite sync.

3. Final Answer:

   Network issues prevent data transfer -> Option C

4. Quick Check:

   Network problems block sync completion [OK]

Hint: Check network if sync never finishes [OK]

Common Mistakes:

• Assuming command spelling causes sync failure
• Ignoring network connectivity
• Blaming version mismatch without error logs

5. In a Redis cluster, how does the synchronization process handle data consistency when a replica reconnects after a network failure?

hard

A. It performs a partial resynchronization using PSYNC if possible

B. It always performs a full SYNC command regardless of previous state

C. It ignores missed commands and continues normally

D. It requires manual data copy from master to replica

Solution

1. Step 1: Understand reconnection sync in Redis

   Redis tries partial resynchronization (PSYNC) to send only missing data after failure.

2. Step 2: Compare with other options

   Full SYNC is fallback if PSYNC fails, ignoring missed commands causes inconsistency, manual copy is not needed.

3. Final Answer:

   It performs a partial resynchronization using PSYNC if possible -> Option A

4. Quick Check:

   Partial sync (PSYNC) handles reconnection efficiently [OK]

Hint: PSYNC tries partial sync after reconnect to save data transfer [OK]

Common Mistakes:

• Thinking full SYNC always runs after reconnect
• Ignoring partial sync optimization
• Assuming manual copy is required