Binds a hash slot to a node.
CLUSTER
SETSLOT
slot <IMPORTING
node-id | MIGRATING
node-id | NODE
node-id | STABLE
> [TIMEOUT
timeout]
CLUSTER SETSLOT
is responsible of changing the state of a hash slot in the receiving node in different ways. It can, depending on the subcommand used:
MIGRATING
subcommand: Set a hash slot in migrating state.IMPORTING
subcommand: Set a hash slot in importing state.STABLE
subcommand: Clear any importing / migrating state from hash slot.NODE
subcommand: Bind the hash slot to a different node.The command with its set of subcommands is useful in order to start and end cluster live resharding operations, which are accomplished by setting a hash slot in migrating state in the source node, and importing state in the destination node.
Each subcommand is documented below. At the end you’ll find a description of how live resharding is performed using this command and other related commands.
<slot>
MIGRATING <destination-node-id>
This subcommand sets a slot to migrating state. In order to set a slot in this state, the node receiving the command must be the hash slot owner, otherwise an error is returned.
When a slot is set in migrating state, the node changes behavior in the following way:
ASK
redirection is emitted by the node, asking the client to retry only that specific query into destination-node
. In this case the client should not update its hash slot to node mapping.TRYAGAIN
error in order for the keys interested to finish being migrated to the target node, so that the multi keys command can be executed.<slot>
IMPORTING <source-node-id>
This subcommand is the reverse of MIGRATING
, and prepares the destination node to import keys from the specified source node. The command only works if the node is not already owner of the specified hash slot.
When a slot is set in importing state, the node changes behavior in the following way:
MOVED
redirection is generated as usually, but in the case the command follows an ASKING
command, in this case the command is executed.In this way when a node in migrating state generates an ASK
redirection, the client contacts the target node, sends ASKING
, and immediately after sends the command. This way commands about non-existing keys in the old node or keys already migrated to the target node are executed in the target node, so that:
ASKING
the behavior is the same as usually. This guarantees that clients with a broken hash slots mapping will not write for error in the target node, creating a new version of a key that has yet to be migrated.<slot>
STABLEThis subcommand just clears migrating / importing state from the slot. It is mainly used to fix a cluster stuck in a wrong state by valkey-cli --cluster fix
. Normally the two states are cleared automatically at the end of the migration using the SETSLOT ... NODE ...
subcommand as explained in the next section.
<slot>
NODE <node-id>
The NODE
subcommand is the one with the most complex semantics. It associates the hash slot with the specified node, however the command works only in specific situations and has different side effects depending on the slot state. The following is the set of pre-conditions and side effects of the command:
It is important to note that step 3 is the only time when a Valkey Cluster node will create a new config epoch without agreement from other nodes. This only happens when a manual configuration is operated. However it is impossible that this creates a non-transient setup where two nodes have the same config epoch, since Valkey Cluster uses a config epoch collision resolution algorithm.
The CLUSTER SETSLOT
command is an important piece used by Valkey Cluster in order to migrate all the keys contained in one hash slot from one node to another. This is how the migration is orchestrated, with the help of other commands as well. We’ll call the node that has the current ownership of the hash slot the source
node, and the node where we want to migrate the destination
node.
CLUSTER SETSLOT <slot> IMPORTING <source-node-id>
.CLUSTER SETSLOT <slot> MIGRATING <destination-node-id>
.CLUSTER GETKEYSINSLOT
command and move them into the destination node using the MIGRATE
command.CLUSTER SETSLOT <slot> NODE <destination-node-id>
to the destination node.CLUSTER SETSLOT <slot> NODE <destination-node-id>
to the source node.CLUSTER SETSLOT <slot> NODE <destination-node-id>
to the other primary nodes (optional).Notes:
ASK
redirections when the source node is configured to redirect.SETSLOT
to the nodes not involved in the resharding, is not technically necessary since the configuration will eventually propagate itself. However, it is a good idea to do so in order to stop nodes from pointing to the wrong node for the hash slot moved as soon as possible, resulting in less redirections to find the right node.Starting from Valkey 8.0, CLUSTER SETSLOT
is synchronously replicated to all healthy replicas running Valkey version 8.0+. By default, this synchronous replication must complete within 2 seconds. If the replication fails, the primary does not execute the command, and the client receives a NOREPLICAS Not enough good replicas to write
error. Operators can retry the command or customize the timeout using the TIMEOUT
parameter to further increase the reliability of live reconfiguration:
CLUSTER SETSLOT slot [MIGRATING|IMPORTING|NODE] node-id [TIMEOUT timeout]
Here, timeout
is measured in seconds, with 0 meaning to wait indefinitely.
Simple string reply: all the sub-commands return OK
if the command was successful. Otherwise an error is returned.
O(1)
@admin @dangerous @slow
TIMEOUT
option.ASKING, CLUSTER, CLUSTER ADDSLOTS, CLUSTER ADDSLOTSRANGE, CLUSTER BUMPEPOCH, CLUSTER COUNT-FAILURE-REPORTS, CLUSTER COUNTKEYSINSLOT, CLUSTER DELSLOTS, CLUSTER DELSLOTSRANGE, CLUSTER FAILOVER, CLUSTER FLUSHSLOTS, CLUSTER FORGET, CLUSTER GETKEYSINSLOT, CLUSTER HELP, CLUSTER INFO, CLUSTER KEYSLOT, CLUSTER LINKS, CLUSTER MEET, CLUSTER MYID, CLUSTER MYSHARDID, CLUSTER NODES, CLUSTER REPLICAS, CLUSTER REPLICATE, CLUSTER RESET, CLUSTER SAVECONFIG, CLUSTER SET-CONFIG-EPOCH, CLUSTER SHARDS, CLUSTER SLOT-STATS, CLUSTER SLOTS, READONLY, READWRITE.