Migrating Redis Data

15 Sep 2015 by Parker Selbert

Long lived applications invariably change up their infrastructure at some point. Perhaps the team has decided that it doesn’t want the devops overhead or worry of maintaining their own infrastructure. For some services, it is as simple as spinning up new hosts or pushing to a managed service. Other services have more blockers to migration, blockers like migrating a large set of important data.

This isn’t such a hypothetical situation. It happens with Redis servers often enough. Redis has a broad set of tools specifically to enable backing up, restoring, and migrating data. In most situations that set of tools is perfectly adequate. However, sometimes there are constraints imposed by various providers that make migration much trickier.

Think of this post as a verbose migration constraint solver. We’ll start with the ideal means of migration and gradually add constraints. Eventually, we’ll arrive at the last resort and see how to work with it.

Master/Slave Replication

The most reliable and preferred way to handle a migration is with master/slave replication using SLAVEOF. All that is required is configuring the destination instance as a slave of your current production instance. Once the new database has caught up to production your application’s Redis clients can be directed to the new database and the server can be promoted to master.


  1. Set the destination instance to be a slave of the source instance with SLAVEOF {SOURCE_HOST} {SOURCE_PORT}
  2. Verify replication is current with INFO replication, it will say that the link is up and the last sync was 0 or more milliseconds ago
  3. Change client URLs to point to the new server, reconnect clients
  4. Promote the destination server to master with SLAVEOF NO ONE


Use of SLAVEOF isn’t allowed.

Lacking access to SLAVEOF eliminates replication outright. Any managed Redis provider worth their salt supplies replication with failover, making it impossible for you to control the master/slave status of your instance. Master servers can’t be put into slave mode without killing the sentinels or other HA system controlling them, which certainly isn’t possible with a cloud service.

RDB Snapshot

When replication isn’t an option you can fall back to transferring the database dump persisted to disk. Even databases that aren’t configured for periodic RDB persistence can trigger a dump to disk with either SAVE or BGSAVE, the asynchronous version. With the database dumped to disk it can be transfered to the new server or uploaded with a data migration tool.

One major caveat around snapshot loading is the exact timing that is used. Snapshots must be loaded when the Redis server is shut down. On reboot the Redis instance will load data from the dump. Any server that is configured to perform RDB persistence will automatically save to disk on shutdown. That means you must shut down the destination instance before overwriting the snapshot or it will be overwritten automatically during shutdown.


  1. Save the source database to disk with BGSAVE
  2. Transfer the database to the target host with scp or rsync
  3. Stop the destination Redis process
  4. Overwrite the RDB dump, typically found at /var/lib/redis/dump.rdb
  5. Start the process back up


No access or mechanism to upload a RDB snapshot.

They don’t have any way to upload a RDB dump file. Understandable, any cloud service provider doesn’t want to grant you access to the underlying file-system. Still, some providers have interfaces that allow you to load a dump into the new instance.

Key by Key Migration

When bulk loading and replication aren’t available you can try to migrate one key at a time with MIGRATE. The MIGRATE command atomically transfers a key from a source database to a destination database. By default this is a destructive action unless the COPY option is passed, it must be handled with care when pointed at a production instance.

Migration isn’t automatic, it requires a script to iterate over all of the keys and call MIGRATE for every individual key.


  1. Connect to the source instance
  2. Iterate over all keys with SCAN, or KEYS for smaller data sets
  3. Run MIGRATE host port key destination-db timeout COPY for each key


Authorization is enabled (thankfully), preventing MIGRATE.

MIGRATE can’t be used if the target server requires authorization. There have been pull requests to allow authorization for MIGRATE commands, but it hasn’t been merged in.

What Does That Leave?

Maybe one could use DUMP/RESTORE - why would one want to do that though?

— HippieLogLog (@itamarhaber)

When you eliminate all of the traditional, supported, recommended means of performing data migration what are you left with? The answer is DUMP and RESTORE. Internally MIGRATE uses DUMP on the source database and RESTORE on the target database. Those commands are available to any Redis client, no special privileges are necessary.


  1. Connect to the source instance
  2. Connect to the destination instance
  3. Iterate over all source keys with SCAN, or KEYS for smaller data sets
  4. Call DUMP for each source key, which returns a serialized representation of the value
  5. Call TTL for each source key to record the expiration
  6. Call RESTORE key ttl value on the destination for each key

All of the same scripting work necessary to use MIGRATE is needed for dumping and restoring, along with the additional work of setting the TTL for each key that is restored. This should be your absolute last resort, let’s hope you’re never in the situation to actually use it.