Redis Persistence

12. Redis Persistence

Why Persistence Matters

Redis stores data in RAM, which is volatile — a server restart means all data is gone. Redis provides two persistence mechanisms to save data to disk.

RDB (Redis Database) Snapshots

RDB creates a point-in-time snapshot of the entire dataset and saves it to a binary file (dump.rdb).

How it works:

At configured intervals, Redis forks a child process
The child writes the entire dataset to a temporary file
The temporary file replaces the old dump.rdb

Configure in redis.conf:

save 900 1     # Save if 1 key changed in 900 seconds
save 300 10    # Save if 10 keys changed in 300 seconds
save 60 10000  # Save if 10000 keys changed in 60 seconds

Manual snapshot:

# Synchronous — blocks Redis until done (avoid in production)
SAVE

# Asynchronous — Redis continues serving clients
BGSAVE

AOF (Append Only File) Logs

AOF logs every write command to a file (appendonly.aof). On restart, Redis replays all commands to rebuild the dataset.

Configure in redis.conf:

appendonly yes
appendfilename "appendonly.aof"

# How often to sync to disk:
# always   — every command (safest, slowest)
# everysec — every second (good balance)
# no       — let OS decide (fastest, least safe)
appendfsync everysec

RDB vs AOF Comparison

Feature	RDB	AOF
Data safety	May lose up to minutes of data	Can lose at most 1 second of data
File size	Compact binary file	Larger (all commands logged)
Performance impact	Low (periodic fork)	Slightly higher (every write)
Recovery speed	Fast	Slower (replay all commands)
Use case	Backups, disaster recovery	High durability requirements

Best practice: Use both RDB and AOF together. RDB for fast recovery, AOF for minimal data loss.

Persistence Flow Diagram

flowchart TB
    Redis[Redis Server<br/>Data in RAM] --> RDB[RDB Snapshot
Point-in-time dump.rdb]
    Redis --> AOF[AOF Append-Only Log
Every write command logged]

    RDB --> RDBHow["Saves periodically<br/>save 900 1 / save 300 10<br/>Compact binary file<br/>Fast recovery ⚡"]
    AOF --> AOFHow["Logs every command<br/>appendfsync everysec<br/>Larger file, slower recovery
Minimal data loss 🛡️"]

    RDB --> Best1["Best for: Backups,
    disaster recovery"]
    AOF --> Best2["Best for: High durability,
    crash recovery"]

    Both["✅ Best Practice: Use BOTH<br/>RDB for fast restart + AOF for safety"]
    RDBHow --> Both
    AOFHow --> Both

    style Redis fill:#7c3aed,color:#fff
    style RDB fill:#3b82f6,color:#fff
    style AOF fill:#059669,color:#fff
    style RDBHow fill:#3b82f6,color:#fff
    style AOFHow fill:#059669,color:#fff
    style Best1 fill:#3b82f6,color:#fff
    style Best2 fill:#059669,color:#fff
    style Both fill:#f59e0b,color:#fff

One-liner: RDB takes photos of your data at set times. AOF records every single action. Use both for best results.