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Configure one Ubuntu Redis server as the primary and a second as its replica by setting replicaof on the replica, supplying matching authentication, restarting or reloading both services, and checking INFO replication and ROLE. Redis replication is asynchronous and keeps a copy current, but it does not elect a replacement primary; automatic failover requires Sentinel or another high-availability system.
What Redis replication does
Redis uses a primary-replica model. Clients normally send writes to the primary, which streams the resulting writes, key expirations, and evictions to one or more replicas. Replicas reconnect automatically after a network interruption and can request a partial resynchronization when the primary still has the required history in its replication backlog. If that history is unavailable, Redis performs a full synchronization instead.
The default mode is asynchronous: a successful write on the primary does not mean every replica has already applied it. Replication therefore improves read scale and reduces the impact of a single-node failure, but it is not a backup, a consistency guarantee, or an automatic failover mechanism. Keep independent RDB or AOF persistence and a tested restore process for recovery from deletion, corruption, or operator error.
Plan the two Ubuntu nodes first
Choose stable private addresses and decide which networks may reach Redis. Permit TCP port 6379 only from trusted application, administration, and replication networks. Do not expose an unauthenticated Redis listener to the public internet. The exact service name, configuration-file path, and firewall command vary with the Ubuntu release and Redis package, so confirm those details on each host before changing anything.
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| Node | Purpose | Typical settings | Client behavior |
|---|---|---|---|
| Primary | Accepts writes and serves reads | Private bind address, protected mode, authentication, persistence as required |
Applications write here unless an intentional read-routing design says otherwise |
| Replica | Applies the primary’s stream and serves read-only queries | replicaof, matching authentication, read-only mode |
Use for reads or promotion; do not treat it as an independent writable database |
Record the primary’s private address, the replica’s private address, the Redis port, the selected authentication model, and the firewall rules. Resolve hostnames from both machines before editing Redis; a name that works on one host may not resolve on the other.
Configure the primary Redis server
Back up and locate the active configuration
Make a copy of the configuration file before editing it. Use the file loaded by the running service, not an unused example file. You can identify the process and unit with the package’s documented Ubuntu service workflow and inspect the service status before making changes. Store secrets outside shell history and source control.
Bind Redis to a private interface
Set bind to the primary’s private address and loopback. Replace the address below with the interface reachable by the replica:
bind <primary-private-address> 127.0.0.1
protected-mode yesKeep protected-mode yes unless you have a deliberately controlled, authenticated network design that requires another choice. Binding to all interfaces while relying only on an application firewall makes mistakes much more costly.
Choose authentication
For legacy password authentication, set a long random secret on the primary:
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requirepass <long-primary-secret>Redis 6 and later support ACLs and are usually better served by named users with narrowly scoped permissions. Create a dedicated replication identity rather than sharing an application administrator account. The account must be enabled, have a secret, and be allowed to run the replication-related commands required by your installed Redis version. Redis ACL syntax and command categories should be checked against that version; do not assume a permission set from another release.
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When using ACL authentication, keep the default user and application users separate from the replication user. Give application accounts only the commands and key patterns they need. The replica will use masteruser and its secret to authenticate this connection.
Configure the replica
Point the replica at the primary
Edit the replica’s active Redis configuration and add the primary’s private address and port:
replicaof <primary-private-address> 6379
masterauth <long-primary-secret>
replica-read-only yesmasterauth is required when the primary expects a password. With ACLs, add the replication identity as well:
masteruser <replication-user>
masterauth <replication-user-secret>Do not put a different password in masterauth than the one configured for the selected primary user. Keep replica-read-only yes. Redis warns that writable replicas can accept changes that never came from the primary, creating divergence when the primary later synchronizes or the replica is promoted.
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Understand an existing dataset
Pointing a replica at a primary may replace the replica’s current dataset during synchronization. Treat the replica as disposable until you have verified the source data and your persistence policy. If the host already contains important keys, take an appropriate backup and schedule the initial synchronization for a maintenance window.
Apply the configuration on Ubuntu
Use the service-management procedure documented by the Redis package installed on the host. On many Ubuntu packages the unit is named redis-server, but verify the unit name instead of assuming it:
- Check the unit and current health with the package’s service-status command.
- Restart or reload the primary after saving its configuration, then confirm it returns to an active state.
- Restart or reload the replica after saving its configuration and watch its service log while it attempts the connection.
- Record the final configuration in the file so a later restart does not discard a runtime-only change.
Redis also supports changing replication at runtime. From an authenticated administrative client connected to the replica, run:
REPLICAOF <primary-private-address> 6379The command changes the replica’s relationship immediately. Persist the same values in redis.conf; otherwise the next restart can return the node to its previous role. Runtime changes such as CONFIG SET are useful for controlled operations, but they should not become the only record of production configuration.
Verify that synchronization is working
Inspect both roles
Connect with an authenticated administrative client on each node and run:
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INFO replication
ROLEOn the primary, ROLE should identify a primary and list connected replicas. On the replica, it should identify a replica and show the expected primary address and port. The replication section also exposes the link state and offsets. A healthy connected replica normally reports an up link; offsets should advance as the primary receives writes.
Run a disposable end-to-end test
- Connect to the primary and write a key whose name clearly marks it as a test, such as
replication-check-2026-10-15. - Read that key from the replica. Allow for a short delay because replication is asynchronous.
- Remove the test key from the primary after confirming the result.
- Run
INFO replicationagain and retain the relevant status output in your change record.
Do not use a production credential, customer value, or unbounded key scan for this check. A successful test proves the path, authentication, and basic synchronization; it does not establish a maximum lag, durability guarantee, or failover time.
What to record
- The primary and replica addresses and Redis versions.
- The primary’s connected-replica count and the replica’s reported primary address.
- The replica link status, offsets, and any synchronization state shown by
INFO replication. - The exact configuration-management or package-service action used to apply the change.
Troubleshoot a replica stuck in CONNECTING
| Symptom | Likely causes | Checks and corrective action |
|---|---|---|
| Authentication errors in the replica log | Wrong password, wrong ACL user, disabled user, or insufficient permissions | Match masterauth to the primary’s requirepass or replication user. If using ACLs, verify the user is enabled and permitted to run the replication commands required by this Redis version. |
| Replica cannot establish a TCP connection | Incorrect address or port, primary bound only to loopback, firewall or network ACL denial | Test reachability from the replica to the primary’s private address and port. Check the primary’s bind setting, host firewall, cloud network rules, and the Redis service status. |
| Protected-mode or access-denied messages | Cross-host access without an appropriate private bind and authentication design | Keep protected mode enabled where possible, bind only to trusted interfaces, and provide authentication. Do not solve the error by exposing an unauthenticated listener. |
| Replica is stale after a brief outage | Unstable link or a backlog too small to cover the interruption | Review link stability and the primary’s replication backlog configuration. Redis may be attempting partial resynchronization; if the required history is gone, it will need a full synchronization. |
| Writes appear to succeed on the replica | replica-read-only was disabled or the command was sent to a promoted node |
Check CONFIG GET replica-read-only and ROLE. Re-enable read-only mode unless writable-replica behavior is an explicitly justified design, then investigate any divergent keys. |
| Operators expect a new primary automatically | Basic replication has no leader election | Use a monitored high-availability layer such as Redis Sentinel or another orchestration system. Replication alone cannot promote a node after failure. |
When logs are inconclusive, reduce the problem to four tests: can the replica reach the primary’s address and port, does the primary accept the selected credentials, does the primary recognize the replica connection, and are the reported offsets moving? Check one condition at a time so a firewall change is not confused with an authentication fix.
Authentication and network security
A password by itself does not encrypt Redis traffic. Redis authentication is sent as a normal command; without TLS, anyone able to observe the connection could potentially capture it. On a trusted isolated network this may be an accepted risk, but cross-host replication across shared or untrusted networks should use TLS.
Use TLS for replication when required
Configure certificates, private keys, trusted certificate authorities, and identity or hostname checks consistently on both nodes. On the replica, tls-replication yes makes its outgoing replication connection use TLS. The primary must have its TLS listener and certificate policy configured as well, and clients must use the matching protocol and port. Test certificate expiry, trust-chain validation, and hostname mismatches before production cutover.
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Limit exposure
- Bind only to interfaces needed by clients and replication.
- Allow port 6379 only from known application, administration, and replication networks.
- Use ACL users with least privilege where Redis 6 or later is available.
- Keep secrets out of shell history, tickets, and public configuration repositories.
- Monitor failed authentication, unexpected clients, and configuration changes.
Replication recovery and backlog behavior
After a short disconnection, a replica can often continue from the primary’s in-memory replication backlog instead of copying the entire dataset. This partial resynchronization reduces recovery work, but it is not guaranteed. A longer outage, a backlog that has wrapped, a restarted primary without the needed history, or incompatible state can force a full synchronization.
Choose backlog capacity and retention with your outage patterns, write rate, memory budget, and Redis version in mind. There is no universal backlog size or recovery-time number: the right value depends on workload and network conditions. Observe reconnect frequency, synchronization duration, and offset gaps in your own environment, then test the chosen settings by interrupting the replication path in a controlled window.
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Promote a replica manually
If the primary is unavailable and you have decided that the replica contains the dataset to serve, connect to the replica and run:
REPLICAOF NO ONEThis stops replication and leaves the replica with its current dataset as an independent primary. Before directing clients to it, fence or isolate the old primary so both nodes cannot accept writes at the same time. Update the application endpoint or service discovery record, verify writes, and document the promotion time and observed replication state.
When the old primary returns, do not simply point it at clients. Decide which dataset is authoritative, back up anything needed for investigation, and reconfigure the recovered node as a replica of the chosen primary. A manual promotion is an operational procedure, not a conflict-resolution system.
Add monitored failover
Redis Sentinel can monitor instances, detect a failed primary, promote a suitable replica, and inform clients. Sentinel itself must be configured with authentication and network security, and every Redis instance must use compatible credentials and access rules. Test quorum behavior, client reconnection, fencing, DNS or endpoint updates, and the case where the network is partitioned. If your availability requirements exceed what you can operate safely, evaluate a managed Redis service instead of treating a second Ubuntu VM as a complete high-availability design.
Choose an operating model
| Model | Operational effort | Failover behavior | Security and control | Best fit |
|---|---|---|---|---|
| Manual primary plus replica | Lowest initial complexity; operators handle checks, promotion, and reconfiguration | Manual REPLICAOF NO ONE promotion |
Full control of Ubuntu networking, Redis configuration, ACLs, and TLS | Development, secondary read capacity, or teams with a documented runbook |
| Primary, replicas, and Sentinel | Higher design and testing effort; monitor the monitors and client behavior | Sentinel can perform monitored promotion when quorum and safety conditions are met | You control credentials, topology, certificates, and failure policy | Self-managed production systems that need automated failover |
| Managed Redis service | Less host maintenance; provider-specific limits and procedures apply | Provider-controlled or configurable automatic failover | Less infrastructure control, with provider security and networking features | Teams that prefer service operations over maintaining Ubuntu nodes |
Replication, Sentinel, and managed operation solve different problems. Replication copies state; Sentinel coordinates promotion; a managed service additionally removes much of the operating-system and infrastructure work. Select the smallest model that meets your recovery, security, and staffing requirements, then test it under realistic failure conditions.
Quick Recap
Production checklist
- Primary and replica use private, reachable addresses and restricted firewall rules.
- The primary’s
bind, protected mode, and authentication settings are intentional. - The replica has matching
masterauthand, for ACLs, the correctmasteruserpermissions. replica-read-only yesremains enabled unless a documented exception exists.- Final settings are persisted in the active configuration file, not only changed at runtime.
INFO replicationandROLEshow the intended roles, link state, and advancing offsets.- A disposable write/read test succeeds without exposing production data.
- Persistence, backups, restore tests, replication backlog, and monitoring are handled separately from replication setup.
- The team has a manual promotion runbook or has deployed and tested Sentinel or another high-availability layer.
- TLS is enabled for replication whenever the network trust boundary requires encryption.
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