Synchronization process in Redis - Step-by-Step Execution

Concept Flow - Synchronization process

Client connects to Redis server

↓

Server sends RDB snapshot

↓

Client loads RDB snapshot

↓

Server sends command stream

↓

Client applies commands to sync data

↓

Synchronization complete

↓

Client ready

The Redis synchronization process starts with the client connecting, receiving a snapshot, loading it, then applying live commands until fully synced.

Execution Sample

Redis

CLIENT CONNECT
SERVER SENDS RDB
CLIENT LOADS RDB
SERVER SENDS COMMANDS
CLIENT APPLIES COMMANDS
SYNC COMPLETE

Shows the step-by-step flow of Redis synchronization from connection to completion.

Execution Table

Step	Action	Data Sent/Received	Client State	Server State
1	Client connects to server	Connection request	Connecting	Waiting for client
2	Server sends RDB snapshot	RDB snapshot data	Receiving snapshot	Sending snapshot
3	Client loads RDB snapshot	None	Loading data	Snapshot sent
4	Server sends command stream	Commands since snapshot	Applying commands	Sending commands
5	Client applies commands	None	Syncing data	Streaming commands
6	Synchronization complete	None	Ready	Sync complete

💡 Synchronization ends when client has fully loaded snapshot and applied all commands.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5	Final
Client State	Disconnected	Receiving snapshot	Loading data	Syncing data	Ready
Server State	Idle	Sending snapshot	Snapshot sent	Streaming commands	Sync complete

Key Moments - 2 Insights

Why does the client first load an RDB snapshot before applying commands?

The client loads the RDB snapshot to get a full copy of the database at a point in time, then applies commands to catch up with changes after that snapshot (see execution_table rows 2 and 4).

What happens if the client misses commands after loading the snapshot?

If commands are missed, the client data will be out of sync. The command stream ensures the client applies all changes after the snapshot to stay consistent (see execution_table rows 4 and 5).

Visual Quiz - 3 Questions

Look at the execution_table, what is the client state after step 3?

A Loading data

B Ready

C Syncing data

D Disconnected

💡 Hint

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

At which step does the server start sending commands to the client?

A Step 2

B Step 4

C Step 5

D Step 6

💡 Hint

Look at the 'Action' and 'Data Sent/Received' columns in execution_table.

If the client never applies commands after loading the snapshot, what will be the final client state?

A Ready

B Syncing data

C Loading data

D Disconnected

💡 Hint

Refer to variable_tracker for client state changes after loading snapshot and applying commands.

Concept Snapshot

Redis Synchronization Process:
1. Client connects to server.
2. Server sends RDB snapshot (full data).
3. Client loads snapshot.
4. Server streams commands since snapshot.
5. Client applies commands to sync.
6. Client ready when fully synced.

Full Transcript

The Redis synchronization process begins when the client connects to the Redis server. The server sends a full snapshot of the database called an RDB snapshot. The client loads this snapshot to get a complete copy of the data at that moment. After loading, the server sends a stream of commands that happened after the snapshot. The client applies these commands to update its data and stay in sync. When all commands are applied, synchronization is complete and the client is ready to use. This process ensures the client has an accurate and up-to-date copy of the Redis data.

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