AOF (Append Only File)

AOF (Append Only File) is Redis’s other persistence mode. Every write command — SET, DEL, LPUSH, you name it — gets appended to a log file on disk. To restore, Redis replays the log from scratch.

In simple language — think of it like a bank ledger. RDB takes a photo of your account balance once a day. AOF writes down every single deposit and withdrawal. If something goes wrong, you can replay the ledger and reconstruct the exact balance.

Why AOF?

• Durability — depending on fsync policy, you can lose at most 1 second of data (sometimes zero).
• Human-readable — the file is just Redis commands in RESP protocol. You can cat it.
• Repairable — if the file gets truncated, redis-check-aof --fix can salvage what’s left.

The cost — bigger files, slower restarts (replay every command), and more disk I/O during normal operation.

The fsync policies

Here’s where it gets interesting. Writing to a file goes through the OS buffer cache. To actually flush to disk you need fsync(). AOF lets you pick how often that happens.

everysec is the sweet spot. You’d only pick always if you absolutely cannot lose a single write (financial logs maybe), and no if Redis is just a cache and you don’t really care.

# redis.conf
appendonly yes
appendfsync everysec   # always | everysec | no

The rewrite problem

Here’s the issue with an append-only log — it grows forever. If you INCR counter a million times, the log has a million lines. But the final state is just counter = 1000000.

Redis fixes this with BGREWRITEAOF. It forks a child process (same COW trick as RDB), walks the current dataset, and writes the minimum set of commands needed to reconstruct it. Then it swaps the new file in.

BGREWRITEAOF       # manual trigger

Auto-rewrite config:

auto-aof-rewrite-percentage 100   # rewrite when AOF is 100% bigger than last rewrite
auto-aof-rewrite-min-size 64mb    # but only if it's at least 64mb

What’s in the file?

Just RESP-encoded commands. Open it up:

*2
$6
SELECT
$1
0
*3
$3
SET
$3
foo
$3
bar

That’s SELECT 0 followed by SET foo bar. Replay this top to bottom and you get the dataset back.

Tradeoffs at a glance

For most production setups, you’d run AOF alongside RDB — see Hybrid Persistence for the recommended combo.