Redis replication.

Primary / replica

Replica config:

replicaof primary.host 6379
masterauth password
replica-read-only yes

redis-cli REPLICAOF NO ONE       # promote
redis-cli REPLICAOF host 6379    # demote

Async replication

Default. Eventual consistency. Primary doesn’t wait for replicas.

Replica reads

replica-read-only yes

Spread read load across replicas. Tolerate stale data.

WAIT command

WAIT 2 1000      # wait up to 1s for 2 replicas to ack

Gives “best effort” durability. Not synchronous.

INFO replication

INFO replication
# role:master
# connected_slaves:2
# slave0:ip=...,port=...,state=online,offset=...,lag=0

Sentinel (HA)

3+ Sentinel processes monitor primary; elect new primary on failure.

# sentinel.conf
sentinel monitor mymaster 10.0.0.1 6379 2
sentinel down-after-milliseconds mymaster 5000
sentinel failover-timeout mymaster 10000
sentinel parallel-syncs mymaster 1

redis-sentinel /etc/redis/sentinel.conf --port 26379

Clients discover primary via Sentinel:

from redis.sentinel import Sentinel

sentinel = Sentinel([("s1", 26379), ("s2", 26379), ("s3", 26379)])
master = sentinel.master_for("mymaster")
replica = sentinel.slave_for("mymaster")

Cluster (sharding + HA)

For horizontal scale + HA:

redis-cli --cluster create node1:6379 node2:6379 node3:6379 \
    node4:6379 node5:6379 node6:6379 --cluster-replicas 1

16384 hash slots distributed across primaries. Each primary has a replica.

from redis.cluster import RedisCluster
client = RedisCluster(host="node1", port=6379)

Hash tags

SET {user:1}:profile ...
SET {user:1}:sessions ...

Same {...} → same slot → multi-key ops work.

Cross-region

Async replication across regions: stale reads possible. For strong consistency: avoid.

Common mistakes

  • Treating replica as backup.
  • No fencing → split brain.
  • Sentinel running on same node as Redis it monitors.
  • Replica drift from primary (lag).
  • Cluster without hash tags then surprised multi-key ops fail.

Read this next

If you want my Sentinel + Cluster setup, it’s at rajpoot.dev .

Building something AI-, backend-, or data-heavy and want a second pair of eyes? I do consulting and freelance work — see my projects and ways to reach me at rajpoot.dev .