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To move a live Redis dataset to a new server with minimal downtime, make the destination a replica of the current master, let Redis synchronize it, verify that replication has caught up, then pause writes briefly and redirect clients. Redis uses REPLICAOF to attach the new instance, PSYNC for incremental history when possible, and an RDB-based full synchronization when the required history is unavailable.
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This approach avoids copying a changing dataset by hand, but it is not zero-risk: replication is asynchronous, a replica can lag or serve stale data, and a large full sync can consume substantial network, disk, CPU and destination startup time. The procedure below gives you observable checkpoints, a controlled cutover and a rollback path.
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What master–replica migration does
Redis replication is a one-way stream from a source master to a destination replica. The source continues accepting writes while the destination receives and applies them. Once the destination has loaded the source dataset and processed the recent write stream, it can be promoted or configured as the new master and clients can be pointed to it.
Redis documentation describes asynchronous replication as the default because it provides low latency and high performance for most use cases. The source does not wait for every replica to process each write, so “caught up” is a measured operational state, not a guarantee that no write can be lost if the source fails during the transition.
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When this method fits
- You can keep the source online while the initial copy runs.
- The destination can run a compatible Redis version, command set, modules and data model.
- You need one-way synchronization until a planned cutover, rather than a permanently independent export.
- You can schedule a short write pause or application quiescence for the final handoff.
When to choose another method
An RDB export/import can be simpler when the source may be stopped, the transfer can happen offline, or the destination cannot connect to the source. It does not, by itself, keep up with writes made after the snapshot. Live replication is usually preferable when downtime tolerance is low and the destination must track ongoing writes.
Plan the migration before starting
Inventory the source
- Record the Redis version, enabled modules, persistence settings, ACL users and TLS requirements.
- Measure used memory, peak memory, dataset size on disk, key count, largest values, expiration rates and write throughput.
- Identify clients that hard-code the old host, connection pool behavior, sentinels, proxies, DNS records or service discovery names.
- Find writes that are not safe to pause, such as counters, queues, locks or sessions with strict expiration requirements.
Size the destination
Provide enough RAM for the dataset plus the destination’s overhead and headroom for normal peaks. A full synchronization also needs disk space for the incoming RDB and temporary files, network capacity for the snapshot and buffered writes, and CPU for parsing and loading. A destination that only barely fits the current dataset can fail as soon as normal growth or a burst of writes occurs.
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Protect the replication connection
Restrict the source’s replication port with firewall or private-network rules. Use TLS when traffic crosses an untrusted network and configure certificates and client settings on both instances. Authentication must match the source configuration: a password-protected source requires masterauth; Redis 6 and later ACL deployments may require a dedicated replication user configured with masteruser and its password, with permissions for synchronization and related replication commands.
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Attach the destination to the source
Set the relationship in the destination’s redis.conf so it survives a restart, or issue the equivalent command at runtime for a test or controlled change.
Persistent configuration
replicaof <source-host> <source-port>
masterauth <source-password>
masteruser <replication-acl-user>
Include only the authentication directives that your source requires. Store secrets using your deployment’s protected configuration mechanism rather than committing them to a repository or exposing them in shell history.
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redis-cli -h <destination-host> -p <destination-port>
REPLICAOF <source-host> <source-port>
If the source requires authentication, configure the destination’s replication credentials first, then run REPLICAOF. A successful command starts the connection process; it does not mean that the dataset is already synchronized.
What happens immediately
- The destination connects to the source and identifies the source’s replication ID and the offset it has processed.
- Redis attempts partial synchronization with
PSYNC. If the source’s replication backlog still contains every missing byte, only the missing write stream is sent. - If the replication ID is unknown or the required history has fallen out of the backlog, Redis falls back to a full synchronization.
- For a full synchronization, the source creates an RDB snapshot in the background, transfers it, and then sends writes buffered while the snapshot was being created.
- The destination loads the RDB and applies the buffered and subsequent commands. Initial loading happens in the main thread and can briefly block client connections to the destination.
PSYNC has been available since Redis Open Source 2.8.0. Redis 4.0 introduced replication-ID and offset behavior that lets promoted replicas retain the old history, improving the chance of partial resynchronization after a failover.
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Monitor synchronization and determine whether it is caught up
Use both INFO replication and ROLE on the destination. These commands expose state and offsets; they do not provide a single universal “migration complete” flag.
Inspect replication state
redis-cli -h <destination-host> -p <destination-port> INFO replication
redis-cli -h <destination-host> -p <destination-port> ROLE
For a healthy destination replica, check:
master_link_status:up— the destination currently has a working link to the source.master_sync_in_progress:0— an initial or recovery synchronization is not still loading.- The destination’s processed offset is advancing and remains close to the source’s current replication offset.
- The role reported by
ROLEisslave(Redis’s protocol output retains this historical term) while the destination is being synchronized. - There are no repeated connection, authentication, timeout or full-sync failures in the Redis log.
Interpret offsets correctly
During normal writes, the source offset will continue to move. A destination is operationally caught up when synchronization is complete, the link is up, and the offset gap is stable at zero or an acceptably small value for your workload. Measure the gap repeatedly rather than relying on one sample. A large or growing gap indicates network saturation, destination CPU or disk pressure, or a write rate that the destination cannot apply fast enough.
Do not treat master_link_status:up alone as proof of data parity. A link can be connected while the destination is still processing a backlog, and a recently reconnected replica may be serving an older dataset.
Validate data before switching clients
Redis supplies replication status, not an application-specific integrity test. Build validation around the data your application actually depends on.
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Technical checks
- Confirm the expected database indexes exist and that key counts are in the expected range.
- Sample representative strings, hashes, lists, sets, sorted sets, streams and module data types.
- Compare TTL behavior on sampled expiring keys; verify that sessions, caches and locks retain appropriate expiration semantics.
- Check ACL users, command permissions, modules and configuration values required by clients.
- Run application-level invariants, such as totals, queue depth constraints or reference relationships, rather than relying only on a key count.
Use a repeatable sample
Record a deterministic sample of important key names on the source, read the same keys from the destination, and compare type, value or a checksum plus remaining TTL. Avoid scanning the entire production keyspace during peak traffic. For very large datasets, combine random sampling with business-critical key lists and a separately scheduled full verification.
Perform a controlled cutover
Do not redirect clients while the source is still accepting uncontrolled writes. The final handoff should make the last source write observable and give the destination time to apply it.
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- Record the source state. Capture
INFO replicationand the current source replication offset. Confirm the destination link is up and no sync is in progress. - Wait for the final writes to arrive. Poll the destination’s processed offset until it has reached the recorded source offset. Continue checking that the link remains up and that the gap does not reopen.
- Repeat validation. Recheck critical keys, TTLs, ACL access and application invariants after the final offset has been processed.
- Redirect clients. Change the service-discovery record, proxy backend, configuration or connection string to the destination. Drain old connection pools and verify new connections reach the intended instance.
- Choose the destination’s role. If it will become the new master, remove the replication relationship with
REPLICAOF NO ONEafter clients are ready to use it. If it should remain a replica, leave the relationship in place and direct only the intended traffic to it. - Resume writes gradually. Re-enable workers and monitor errors, latency, memory, replication state and application metrics.
redis-cli -h <destination-host> -p <destination-port> REPLICAOF NO ONE
Run REPLICAOF NO ONE only when the destination is intentionally being promoted. Removing replication before the final offset is processed can leave the new master missing source writes.
Keep rollback available
Do not immediately destroy or repurpose the old source. Keep it isolated from new writes and retain its configuration, persistence files and access for the rollback period defined by your application. If post-cutover checks fail, stop writes to the destination, point clients back to the old source only if you can account for writes made after the switch, and reconcile or discard divergent data according to your recovery plan. A blind DNS reversal can lose acknowledged writes that reached only the new instance.
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Replication failure modes and fixes
The destination repeatedly performs full synchronization
A broken link triggers reconnection and an attempt at partial resynchronization. If the outage or write volume exceeds the source’s replication backlog, the required history is gone and a full sync is unavoidable. Full syncs increase network, disk and CPU load and can delay the destination’s availability.
Increase the backlog to cover the largest realistic outage and write burst, subject to memory limits. Redis Software documents a product default of 1% of database size divided by shard count, bounded between 1 MB and 250 MB per shard. That figure is a Redis Software product default, not a universal Redis Open Source default; size the backlog from your measured write rate and recovery objectives.
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The link is down or authentication fails
- Verify host, port, routing, firewall rules and TLS certificate settings.
- Check
masterauthfor password authentication. - For ACL-based setups, verify
masteruser, its password and permissions forPSYNCand required replication commands. - Read both Redis logs and
INFO replicationfor the exact failure and retry after correcting configuration.
The destination serves stale data
When disconnected, a replica can continue serving reads if replica-serve-stale-data is enabled; this is the upstream default. Set it to no if stale reads are unacceptable. With that setting, data commands return a MASTERDOWN error while the link is unavailable. This prevents silent staleness but requires clients to handle the error or fail over elsewhere.
Initial loading blocks destination clients
Loading a new RDB replaces the destination’s old dataset and occurs in the main thread. Expect a brief availability impact on that instance, especially with large datasets. Keep clients on the source until synchronization and validation are complete, and do not use the destination for latency-sensitive traffic during its initial load.
The destination runs out of memory or disk
Stop the migration, correct capacity or configuration, and restart synchronization only after confirming headroom. Check that persistence files, temporary RDB storage, logs and the in-memory dataset all fit within the destination’s limits. Reducing the source dataset during a live migration without a deliberate application plan can create confusing parity results.
Should you use WAIT before cutover?
WAIT can ask Redis to wait until a specified number of replicas acknowledge writes. It can reduce the chance that a recent write exists only on the source, but Redis explicitly states that WAIT does not turn a group of instances into a strongly consistent CP system. Acknowledged writes can still be lost during certain failovers. Use it only as an additional safeguard, not as a replacement for quiescing writes, checking offsets and validating the destination.
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| Decision factor | Live master–replica migration | RDB export/import |
|---|---|---|
| Downtime tolerance | Usually supports a short final write pause after the online sync. | Requires stopping writes or accepting that changes after export are absent. |
| Writes during transfer | Continues to stream writes made during the snapshot and afterward. | Writes made after the export must be replayed or otherwise reconciled. |
| Bandwidth pattern | Initial RDB transfer plus ongoing replication traffic; a backlog miss causes another full sync. | One planned snapshot transfer, followed by any separately designed delta process. |
| Dataset size | Large datasets are feasible but can impose sustained network, disk and destination load. | Large one-time files may be easier to move during an offline window. |
| Rollback | Old source can remain available until post-cutover checks pass. | Rollback depends on which instance received subsequent writes. |
| Destination compatibility | Requires compatible replication protocol, commands, modules, authentication and TLS settings. | Still requires a destination that can load and use the exported data. |
| Ongoing synchronization | Built in until you change or remove the replica relationship. | Not provided by the export itself. |
Operational checklist
- Destination capacity, Redis version, modules and persistence settings are approved.
- Network controls, TLS and replication authentication have been tested.
replicaof/REPLICAOFis configured and the destination reports the expected source.master_link_status:up,master_sync_in_progress:0and advancing offsets are observed.- Backlog capacity is based on measured write rate and outage duration, not an assumed universal default.
- Representative values, types, TTLs, ACLs and application invariants pass validation.
- Writes are paused for the final offset drain and client redirection.
- The old source is retained for rollback, and the team knows how to handle writes made after cutover.
Bottom line
Attach the new Redis instance with REPLICAOF, let PSYNC synchronize it (or allow the RDB full-sync path when necessary), and use INFO replication plus ROLE to observe completion. A safe migration is completed only after the final writes are quiesced, offsets have drained, application data has been validated and clients have switched with a rollback path still available. Asynchronous replication makes the process fast and low-latency, but it never removes the need to measure lag and plan for a failure window.
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