ALTER INDEX

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    The ALTER INDEX statement moves the placement of an existing index or replica among different GSI nodes.

    Purpose

    You can use the ALTER INDEX statement to change the placement of an existing index or replica among different GSI nodes, to increase or decrease the number of replicas, or to drop a specified index replica temporarily. You can also use it to perform the same alterations to a partitioned index and any replica partitions. You may use this statement when you encounter any of the following situations:

    • An imbalance occurs due to a particular index growing faster than expected and is needed on a different node.

    • An imbalance occurs due to a cluster of indexes being dropped on a single node.

    • A machine is scheduled for removal, so its indexes need to move off its current node.

    • The automated process of rebalancing does not give the expected results.

    • Other types of scaling up or scaling down are needed.

    For example, if a node fails and you need to move it from node 172.23.130.24 to node 172.23.130.25, then you can simply execute the following code with your own index names and addresses:

    CREATE INDEX idx1 ON `travel-sample`(id, airline) WHERE type="route";
    
    ALTER INDEX `travel-sample`.idx1
    WITH {"action": "move", "nodes": ["172.23.120.25:8091"]}
    The ALTER INDEX move operation is asynchronous. As soon as the move alter index command is executed, the command returns. If there is no error in the input, the move operation can be tracked through the console UI and any error can be found in the Console logs and Indexer logs.

    If a node goes down while an ALTER INDEX operation is in progress, then the index would rollback to its original node (not affecting queries) and a notification would appear.

    It is not possible to move an index or index replica and change the number of index replicas at the same time.

    Prerequisites

    Only users with the RBAC role of Administrator are allowed to run the ALTER INDEX directive.

    This directive is applicable only for Standard GSI (Plasma) and MOI Indexes; and hence supported only for Enterprise Edition nodes. (Not applicable for Forest DB.)

    This statement is only supported only in Couchbase Server 5.5 and later; if the cluster is a mix of Couchbase Server 5.5 and previous versions, then this statement will not work.

    In addition, altering the number of replicas or deleting an index replica is only supported in Couchbase Server 6.5 and later; if the cluster is a mix of Couchbase Server 6.5 and previous versions, then these operations will not work.

    ALTER INDEX will not work while the cluster is undergoing a rebalance.

    Syntax

    alter-index ::= ALTER INDEX key-expr [ USING GSI ] WITH options
    'ALTER' 'INDEX' key-expr ( 'USING' 'GSI' )? 'WITH' options

    Key Expression

    A string representing the corresponding named keyspace reference, with an optional index name, for example named_keyspace_ref.index_name.

    USING GSI

    In Couchbase Server 6.5 and later, the index type for a secondary index must be Global Secondary Index (GSI). The USING GSI keywords are optional and may be omitted.

    Options

    An object with the following properties:

    action

    [Required] A string denoting the operation to be performed. The possible values are:

    move

    Moves an index (or its replicas) to a different node while not making any changes to the index topology — for example, the number of replicas remains the same. You must use the nodes property to specify the target node or nodes.

    replica_count

    Alters the number of replicas. You must use the num_replica property to specify the required number of replicas. You can use the nodes property to restrict the placement of index replicas to the specified nodes. The planner decides where to place any new index replicas on the available index nodes, based on the server load.

    drop_replica

    Drops a specified replica temporarily; for example, to repair a replica. You must use the replicaId property to specify the replica to drop.

    num_replica

    [Required if action is set to replica_count] An integer specifying the number of replicas of the index. The index service will automatically distribute these indexes amongst the index nodes in the cluster for load balancing and high availability purposes. The index service attempts to distribute the replicas based on the server groups in use in the cluster where possible. (You can restrict the number of index nodes available for index and index replica placement using the nodes property, described below.)

    nodes

    [Required if action is set to move; optional if action is set to replica_count] An array of strings, specifying a list of nodes. If action is set to move, the node list determines the new destination nodes for the index and its replicas. If action is set to replica_count and you are increasing the number of replicas, the node list restricts the set of nodes available for placement of the index and its replicas. However, if action is set to replica_count and you are decreasing the number of replicas, the nodes property is ignored.

    replicaId

    [Required if action is set to drop_replica] An integer, specifying a replica ID.

    Usage

    Moving an Index or Index Replicas

    When moving an index or index replicas, the number of destination nodes must be the same as the number of nodes on which the index and any replicas are currently placed. You must specify the full node list, even if only one replica needs to be moved.

    Likewise, when moving a partitioned index, the number of destination nodes must be the same as the number of nodes on which the index partitions and any replicas are currently placed. You cannot use this statement to repartition an index across a different number of nodes.

    The source and destination node ranges may overlap, for example you may move a partitioned index from ["192.168.0.15:9000", "27.0.0.1:9001"] to ["192.168.0.15:9000", "127.0.0.1:9002"].

    Changing the Replica Count

    When changing the number of replicas, the specified number of replicas must be less than the number of index nodes available for placement. If the specified number of replicas is greater than or equal to the number of index nodes available for placement, then the operation will fail.

    If you specify a node list when changing the number of replicas, the specified nodes must include all of the nodes on which the index or index partitions and any index replicas are currently placed.

    When increasing the number of replicas, whether you specify a node list or not, no single index node will host more than one replica of the same index, or the same partition of the same index. Replicas are distributed across the available server groups.

    When reducing the number of replicas, the index service will first drop unhealthy replicas, where an unhealthy replica is a replica with missing partitions. After all unhealthy replicas are dropped, the index service will if necessary drop replicas with the highest replica ID. An unhealthy replica may not have the highest replica ID, so after an index reduction there may be "gaps" in the sequence of replica IDs — for example, 1, 2, 4, where replica ID 3 was dropped.

    Dropping a Specific Replica

    When dropping a replica, the index topology does not change. The indexing service remembers the number of partitions and replicas specified for this index. Given sufficient capacity, the dropped replica is rebuilt after the next rebalance — although it may be placed on a different index node, depending on the resource usage statistics of the available nodes.

    To find the ID of an index replica and see which node it is placed on, you can use the Indexes screen in the Couchbase Web Console or the Index service REST API.

    When dropping a replica, it is possible to leave a server group with no replica. For a partitioned index, run a rebalance to move a replica into the vacant server group.

    Return Value

    If the ALTER INDEX succeeds, then:

    • The Query Workbench shows { Results: [] }

    • The index progress is visible on the UI.

    • After the movement is complete, the new indexes begin to service query scans.

    • The command line displays the new index nodes.

    If the ALTER INDEX fails, then:

    • The original indexes continue to service query scans.

    • The UI Log and Query Workbench has the appropriate error message.

    • Some common errors include:

      Error Message Possible Cause

      GSI index xxxxxxxx not found

      • Mistyped an index name

      Missing Node Information For Move Index

      • Mistyped "node" instead of "nodes"

      • Mistyped punctuation or other item

      No Index Movement Required for Specified Destination List

      • Entered the current node instead of the target node

      syntax error - at \",\"

      • Missed a double-quote mark (")

      Unable to find Index service for destination xxx.xxx.xxx.xxx:8091 or destination is not part of the cluster

      • Address doesn’t exist or was mistyped

      • Node isn’t running

      • Node not properly added to the cluster

      Unsupported action value

      • Mistyped the "action"

    Examples

    When using the below examples, make sure that an up-to-date version of Couchbase Server Enterprise Edition is already running on the named nodes.

    Example 1. Move the def_faa index from one node to another

    Create a cluster of 3 nodes and then go to Settings  Sample buckets to install the travel-sample bucket. The indexes are then installed in a round-robin fashion and distributed over the 3 nodes. Then move the def_faa index from the first node (192.168.10.10 in the screenshot) to the second node (192.168.10.11 in the screenshot).

    alter index servers step1
    ALTER INDEX `travel-sample`.def_faa
    WITH {"action": "move", "nodes": ["192.168.10.11:8091"]}

    You should see:

    {
      "results": []
    }
    alter index servers step2
    Example 2. Create and move an index replica from one node to another

    Create an index on node 192.168.10.10 with a replica on node 192.168.10.11, then move its replica from node 192.168.10.11 to 192.168.10.12.

    CREATE INDEX country_idx ON `travel-sample`(country, city)
           WHERE type="route" USING GSI
           WITH {"nodes": ["192.168.10.10:8091", "192.168.10.11:8091"]};
    
    ALTER INDEX `travel-sample`.country_idx
    WITH {"action": "move", "nodes": ["192.168.10.10:8091", "192.168.10.12:8091"]}
    Example 3. Moving multiple replicas

    Create an index on node 192.168.10.10 with replicas on nodes 192.168.10.11 and 192.168.10.12, then move the replicas to nodes 192.168.10.13 and 192.168.10.14.

    CREATE INDEX country_idx ON `travel-sample`(country, city)
    WITH {"nodes": ["192.168.10.10:8091", "192.168.10.11:8091", "192.168.10.12:8091"]}
    
    ALTER INDEX `travel-sample`.country_idx
    WITH {"action": "move", "nodes":
          ["192.168.10.10:8091", "192.168.10.13:8091", "192.168.10.14:8091"]}
    Example 4. Increasing the number of replicas

    Create an index on node 192.168.10.10 with replicas on nodes 192.168.10.11 and 192.168.10.12, then increase the number of replicas to 4 and specify that new replicas may be placed on any available index nodes in the cluster.

    CREATE INDEX country_idx ON `travel-sample`(country, city)
    WITH {"nodes": ["192.168.10.10:8091", "192.168.10.11:8091", "192.168.10.12:8091"]}
    
    ALTER INDEX `travel-sample`.country_idx
    WITH {"action": "replica_count", "num_replica": 4}
    Example 5. Increasing the number of replicas and restricting the nodes

    Create an index on node 192.168.10.10 with replicas on nodes 192.168.10.11 and 192.168.10.12, then increase the number of replicas to 4, and specify that replicas may now also be placed on nodes 192.168.10.13 and 192.168.10.14.

    CREATE INDEX country_idx ON `travel-sample`(country, city)
    WITH {"nodes": ["192.168.10.10:8091", "192.168.10.11:8091", "192.168.10.12:8091"]}
    
    ALTER INDEX `travel-sample`.country_idx
    WITH {"action": "replica_count", "num_replica": 4, "nodes": ["192.168.10.10:8091", "192.168.10.11:8091", "192.168.10.12:8091", "192.168.10.13:8091", "192.168.10.14:8091"]}
    Example 6. Decreasing the number of replicas

    Create an index on node 192.168.10.10 with replicas on nodes 192.168.10.11 and 192.168.10.12, then decrease the number of replicas to 1.

    CREATE INDEX country_idx ON `travel-sample`(country, city)
    WITH {"nodes": ["192.168.10.10:8091", "192.168.10.11:8091", "192.168.10.12:8091"]}
    
    ALTER INDEX `travel-sample`.country_idx
    WITH {"action": "replica_count", "num_replica": 1}
    Example 7. Dropping a specific replica

    Create an index with two replicas, and specify that nodes 192.168.10.10, 192.168.10.11, 192.168.10.12, and 192.168.10.13 should be available for index and replica placement. Then delete replica 2.

    CREATE INDEX country_idx ON `travel-sample`(country, city)
    WHERE type="route" USING GSI
    WITH {"num_replica": 2, "nodes": ["192.168.10.10:8091", "192.168.10.11:8091", "192.168.10.12:8091", "192.168.10.13:8091"]};
    
    ALTER INDEX `travel-sample`.country_idx WITH {"action": "drop_replica", "replicaId": 2};