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The safest low-downtime Redis migration is to make the new server a replica of the current master, let Redis synchronize it, verify that it has caught up, then pause writes briefly and redirect clients. Redis uses incremental PSYNC when the required history is still in the master’s replication backlog; otherwise it performs a full RDB synchronization. Because replication is asynchronous, a replica that appears current is not an absolute guarantee against data loss, so keep the old master available for rollback until application checks pass.
What this migration method does
Master-replica replication creates a continuously updated copy while the existing Redis instance continues serving traffic. The destination starts with the source dataset and receives subsequent writes over the replication connection. After synchronization, you can promote or detach the destination and point applications at it.
The control point is REPLICAOF (written as replicaof in redis.conf). Redis replication is asynchronous by default: the master accepts a write without waiting for every replica to process it. That design keeps latency low, but it leaves a lag window during failures or a final cutover.
When live replication is the right choice
Use a live replica when you need the destination to follow ongoing writes, the source and destination can communicate for the duration of the transfer, and you can schedule a short write pause for the final switch. It is particularly useful for a large dataset that would make a long export/import outage unacceptable.
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An RDB export/import can be simpler when the dataset is static, the destination cannot connect to the source, or you can tolerate downtime. It does not provide ongoing synchronization, so all writes made after the export must be handled separately.
| Decision factor | Live replica migration | RDB export/import |
|---|---|---|
| Downtime | Usually limited to the final write pause and client switch; destination loading during initial sync can affect the destination. | Requires an outage or a separate write-free period while the export is produced and imported. |
| Writes during transfer | Source continues accepting writes; they are streamed after the snapshot and during catch-up. | Writes after the export are not included unless you arrange another synchronization method. |
| Network use | Transfers an RDB snapshot plus buffered writes, then ongoing replication traffic. | Transfers the exported file once, unless another copy mechanism is added. |
| Dataset size | Large full syncs consume source CPU, destination CPU, disk, and network capacity. | Large files also require storage and transfer capacity, but the work can be scheduled offline. |
| Rollback | Old master can remain intact until post-cutover validation; reverse replication requires a deliberate design. | Rollback means restoring the old instance or another backup and accounting for writes made on the new instance. |
| Compatibility | Destination must support compatible commands, data types, modules, persistence behavior, and authentication. | Import tooling and Redis versions still need compatibility, but the source need not maintain a live link. |
| Ongoing one-way synchronization | Built in until you detach or promote the replica. | Not provided by the export itself. |
Prepare the destination
Size memory, disk, and network capacity
Provision enough RAM for the dataset plus Redis overhead, allocator fragmentation, connection buffers, replication buffers, and the workload’s normal headroom. Reserve disk space for the incoming RDB file and any persistence files created by the destination. A full synchronization can temporarily require more disk and network throughput than steady-state operation.
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Measure the source’s dataset size and write rate, then check that the link can transfer an RDB of that size within your migration window while also carrying live writes. If the destination is a different machine class, test command latency and memory behavior before production cutover.
Match software behavior
Use a Redis version and module set that can load the source data and execute the commands your application uses. Confirm module versions, eviction policy, persistence settings, database numbering, keyspace notifications, ACL definitions, and client protocol settings. A replica can synchronize bytes successfully while an application still fails because a module or command is missing.
Protect the replication connection
Restrict the replication port with network controls and use TLS where your deployment supports it. Authentication must match the source. A password-authenticated source uses masterauth; Redis 6 and later ACL deployments can use a dedicated replication user configured with masteruser and its password. Give that user the permissions required for PSYNC and related replication commands, and test the credentials from the destination before starting a production sync.
Attach the destination to the source
You can make the relationship persistent in redis.conf or apply it at runtime. Replace the placeholders with routable hostnames or addresses and the source’s Redis port.
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- On the destination, add
replicaof <source-host> <source-port>toredis.conf. Configuremasterauth <source-password>when the source requires password authentication. For ACL authentication, also setmasteruser <replication-user>and the correspondingmasterauthvalue. - Restart the destination if you changed the configuration file, or issue
REPLICAOF <source-host> <source-port>inredis-clifor a running instance. - Confirm that the destination can resolve and reach the source, negotiate TLS if enabled, and authenticate with the configured account.
- Do not write application data to the destination while it is a replica. A synchronization replaces the destination’s existing dataset.
To remove the relationship later, use REPLICAOF NO ONE. Do this only when your cutover design calls for the destination to operate independently; detaching it before validation stops the normal stream of source updates.
What Redis does during synchronization
Partial synchronization with PSYNC
When a replica connects, it sends the source replication ID it knows and the replication offset it has processed. Redis’s PSYNC protocol, available since Redis Open Source 2.8.0, can then send only the missing commands if the source still has that history in its replication backlog. Redis documentation describes this as the normal recovery path after a short disconnection.
A partial sync is not possible when the replication ID is unknown or the backlog has already discarded the required range. Redis then falls back to a full synchronization. After failover, Redis 4.0 and later can retain the old replication ID and offset on a promoted replica, which can make a subsequent partial resynchronization possible when the history is still available.
Full synchronization with an RDB snapshot
For a full sync, the source creates an RDB snapshot in the background, transfers it to the destination, and buffers writes made while the snapshot is being produced. The destination loads the snapshot and then applies the buffered writes before continuing with the live stream. Loading the new dataset occurs in the destination’s main thread and can briefly block client connections, especially for a large dataset, so treat initial loading as an availability event for the new server.
During the snapshot and transfer, watch source CPU, disk I/O, free disk space, network saturation, and latency. On the destination, watch memory pressure, load time, and client errors. A full sync that repeatedly restarts can consume substantially more resources than one uninterrupted transfer.
Monitor progress and decide when it has caught up
Use both INFO replication and ROLE on the source and destination. The exact command output varies by role, but the following signals are the ones needed for a cutover decision:
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master_link_status:upon the destination indicates that its link to the source is currently established.master_sync_in_progress:0indicates that the destination is not still performing its initial synchronization.- Replication offsets reported by the two instances should converge. Compare the source’s current replication offset with the destination’s processed offset and observe the difference over several checks rather than trusting one sample.
ROLEshould identify the destination as a replica of the expected source and show a healthy offset.- Repeated connection resets, authentication failures, or sync restarts are reasons to postpone cutover even if one status sample looks healthy.
“Caught up” means the measured offset gap is zero or acceptably small for your application’s write rate and cutover pause, the link remains healthy, and no synchronization is in progress. It does not mean the two servers form a strongly consistent pair.
Validate the copy before switching clients
Redis exposes replication state, not a universal application-data validation command. Build checks that reflect your data model and run them while the destination is still a replica.
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- Check representative keys from every important data type, namespace, and database number. Compare values, set or sorted-set membership, hashes, lists, streams, and binary payloads where applicable.
- Compare key counts or carefully designed samples. A full key-by-key comparison may be too expensive for a busy production dataset.
- Check TTLs and expiration behavior, including keys close to expiry. A matching value with a wrong remaining lifetime can still break an application.
- Verify ACL users, command permissions, modules, scripts, functions, and configuration that the application requires.
- Run application-level invariants, such as matching order totals, session references, queue sequence expectations, or cache namespace rules.
- Test reads through the same client libraries, TLS settings, DNS names, and connection pools that production will use after the switch.
Record the validation result and the replication offset at which it was performed. If checks fail, leave the destination attached and investigate rather than detaching it and creating a second divergence.
Perform a controlled cutover
Plan a maintenance window even when the expected interruption is short. The final sequence must prevent new writes from landing on the old master after you decide the destination is authoritative.
- Announce the window and identify every writer: application servers, workers, scheduled jobs, admin scripts, and data pipelines.
- Quiesce writes using the application’s maintenance mode, a write gate, or an equivalent control. Do not rely on client traffic merely becoming quiet.
- Keep read traffic only if your application can tolerate reads from the old master during the transition. Otherwise stop or drain clients cleanly.
- Wait for the destination to process the final source writes. Recheck
master_link_status,master_sync_in_progress,ROLE, and the offset gap until it is zero or within the explicitly approved threshold. - Repeat the critical key and invariant checks against the quiesced dataset.
- Redirect clients by changing the service endpoint, configuration, DNS, service-discovery record, or connection pool target. Use a low DNS TTL only if your operational environment honors it; existing persistent connections may still need to be drained.
- If the destination must become a standalone master, issue
REPLICAOF NO ONEonly after clients are directed to it and your design requires detachment. If it should remain a replica, leave the relationship in place. - Re-enable writes on the new authoritative endpoint and watch errors, latency, key expirations, and application invariants.
- Keep the old master stopped or protected from writes and retain it for rollback until post-cutover checks have passed. Document exactly which writes would be lost if you reverted.
Understand data-loss and consistency limits
Redis states that its default asynchronous replication is chosen for low latency and high performance. A master can acknowledge a write before a replica has received or processed it. If the master fails during that interval, the acknowledged write may be absent from the destination.
The WAIT command can ask Redis to wait for a specified number of replicas to acknowledge writes. It does not turn the deployment into a strongly consistent system; acknowledged writes can still be lost during failover. Use it only when its latency and availability trade-off fits the application, not as proof that a migration has zero loss risk.
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Replication backlog and reconnection behavior
When the link breaks, the destination reconnects and requests a partial resynchronization. The source can satisfy that request only while the missing commands remain in its replication backlog. A small backlog combined with a long outage or high write volume forces a full sync, increasing disk, CPU, and network load.
Redis Software documents a product default backlog sized at 1% of database size divided by shard count, bounded between 1 MB and 250 MB per shard. That figure is a Redis Software default, not a universal Redis Open Source rule. Size the backlog for the maximum expected disconnection duration and write rate, then add operational margin. A useful planning calculation is: required backlog bytes are at least the peak replication-stream bytes per second multiplied by the longest outage you intend to recover from, plus headroom.
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With the upstream default, replica-serve-stale-data is enabled, so a disconnected replica may continue serving data that is no longer current. Setting replica-serve-stale-data no makes data commands return a MASTERDOWN error while the link is down. The safer choice depends on whether stale reads are worse than temporary read unavailability for your application. Whichever policy you choose, monitor link state and ensure clients handle the resulting errors.
Common failure modes and fixes
Authentication or ACL errors
If master_link_status never becomes up and logs show authentication failures, verify the source password, the masterauth value, the ACL username in masteruser, and that the user can run replication-related commands. Check that TLS certificates and server names also match when encrypted transport is used.
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Repeated full syncs usually indicate that the replica is offline longer than the backlog can cover, that the backlog is undersized for the write rate, or that the replication ID no longer matches. Increase backlog capacity where supported, reduce outage duration, investigate network resets, and ensure the destination has enough disk and memory to complete one uninterrupted load.
Destination runs out of disk or memory
Stop and resize before retrying. A full sync needs room for the incoming RDB and normal runtime files, while loading needs memory for the dataset and overhead. Do not delete source persistence files as a shortcut; preserve a recoverable copy.
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Clients receive stale data
Confirm whether the destination is disconnected and whether replica-serve-stale-data is enabled. Route reads to a known-current endpoint, disable stale serving if the application cannot tolerate old values, and fix the replication link before attempting a cutover.
Cutover exposes missing writes
Check that every writer was quiesced and that the final offset was processed. Background jobs and administrative scripts are common sources of writes that application maintenance mode does not stop. If a write was accepted by the old master after the final synchronization point, preserve the old instance, identify the affected records, and reconcile them before deciding whether to continue or roll back.
Rollback planning
Rollback is easiest when the old master remains untouched and clients can be redirected back. Define the rollback trigger before the switch: for example, failed invariant checks, unacceptable error rates, or missing keys. If clients have already written to the new master, reverting can create a fork and lose those new writes. Either reconcile them explicitly or use a tested reverse-replication and recovery design; never assume that changing the endpoint automatically merges the two histories.
After the observation period, take a fresh backup of the new master, remove obsolete replication credentials and network access, and decommission the old server only after retention and compliance requirements are satisfied.
Migration checklist
- Destination has compatible Redis and module versions, sufficient RAM, disk, and network capacity.
- Replication traffic is protected, and password or ACL authentication has been tested.
replicaofis configured persistently or the runtimeREPLICAOFcommand has been issued.- Initial synchronization completed without repeated failures.
INFO replicationandROLEshow the expected source, healthy link, no sync in progress, and a converged offset.- Representative data, TTLs, modules, ACLs, and application invariants passed validation.
- All writers are covered by the quiesce plan, and the client redirection method is tested.
- Old-master retention and rollback criteria are documented.
Bottom line
Attach the new Redis instance with REPLICAOF, let Redis use PSYNC or complete an RDB full sync, and measure convergence with INFO replication and ROLE. A brief, controlled write pause plus application-level validation makes the switch safer, but asynchronous replication and backlog limits mean that no status flag alone guarantees zero data loss. Keep the old master available until the new endpoint has passed real production checks.
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