MIGRATE

MIGRATE host port <nil> |  destination-db timeout [COPY] [REPLACE] [AUTH | AUTH2] [KEYS]

Available since: 2.6.0
Time complexity: This command actually executes a DUMP+DEL in the source instance, and a RESTORE in the target instance. See the pages of these commands for time complexity. Also an O(N) data transfer between the two instances is performed.
ACL categories: @keyspace, @write, @slow, @dangerous

Atomically transfer a key from a source Redict instance to a destination Redict instance. On success the key is deleted from the original instance and is guaranteed to exist in the target instance.

The command is atomic and blocks the two instances for the time required to transfer the key, at any given time the key will appear to exist in a given instance or in the other instance, unless a timeout error occurs. In 3.2 and above, multiple keys can be pipelined in a single call to MIGRATE by passing the empty string ("") as key and adding the !KEYS clause.

The command internally uses DUMP to generate the serialized version of the key value, and RESTORE in order to synthesize the key in the target instance. The source instance acts as a client for the target instance. If the target instance returns OK to the RESTORE command, the source instance deletes the key using DEL.

The timeout specifies the maximum idle time in any moment of the communication with the destination instance in milliseconds. This means that the operation does not need to be completed within the specified amount of milliseconds, but that the transfer should make progresses without blocking for more than the specified amount of milliseconds.

MIGRATE needs to perform I/O operations and to honor the specified timeout. When there is an I/O error during the transfer or if the timeout is reached the operation is aborted and the special error - IOERR returned. When this happens the following two cases are possible:

The key may be on both the instances.
The key may be only in the source instance.

It is not possible for the key to get lost in the event of a timeout, but the client calling MIGRATE, in the event of a timeout error, should check if the key is also present in the target instance and act accordingly.

When any other error is returned (starting with ERR) MIGRATE guarantees that the key is still only present in the originating instance (unless a key with the same name was also already present on the target instance).

If there are no keys to migrate in the source instance NOKEY is returned. Because missing keys are possible in normal conditions, from expiry for example, NOKEY isn’t an error.

Migrating multiple keys with a single command call #

Starting with Redict 3.0.6 MIGRATE supports a new bulk-migration mode that uses pipelining in order to migrate multiple keys between instances without incurring in the round trip time latency and other overheads that there are when moving each key with a single MIGRATE call.

In order to enable this form, the !KEYS option is used, and the normal key argument is set to an empty string. The actual key names will be provided after the !KEYS argument itself, like in the following example:

MIGRATE 192.168.1.34 6379 "" 0 5000 KEYS key1 key2 key3

When this form is used the NOKEY status code is only returned when none of the keys is present in the instance, otherwise the command is executed, even if just a single key exists.

Options #

!COPY – Do not remove the key from the local instance.
REPLACE – Replace existing key on the remote instance.
!KEYS – If the key argument is an empty string, the command will instead migrate all the keys that follow the !KEYS option (see the above section for more info).
!AUTH – Authenticate with the given password to the remote instance.
AUTH2 – Authenticate with the given username and password pair (Redict 6 or greater ACL auth style).

History #

3.0.0: Added the `COPY` and `REPLACE` options.