RDB Persistence — Snapshot to Disk

Redis is an in-memory database, but it needs to survive restarts. RDB (Redis Database) snapshots are point-in-time dumps of all data.

How Real Redis Does It

1. BGSAVE forks the process (copy-on-write)
2. Child process serializes all keys to a binary .rdb file
3. On startup, Redis loads the .rdb file to restore state

The RDB format is compact and fast to load — Redis can restore millions of keys per second.

Trade-offs

• Pro: Compact file, fast restore, great for backups
• Con: You can lose data between snapshots (last save to crash)
• Real Redis also supports AOF for durability (next lesson)

Our Simplified Format

We use a text-based format (real RDB is binary):

KEY string name Alice
KEY list mylist a,b,c
KEY hash user name=Alice,age=30
KEY set myset a,b,c

What You'll Build

• SAVE — serialize all data, output the snapshot
• RESTORE — load a snapshot back into memory
• This is the foundation of Redis's durability story