Rebalance

During the Data Service rebalance stage, vBuckets are moved in phases. The phases — which differ, depending on whether the vBucket is an active or a replica vBucket — are described below.

Since vBucket moves are highly resource-intensive, Couchbase Server allows the concurrency of such moves to be limited: a setting is provided that determines the maximum number of concurrent vBucket moves permitted on any node. The minimum value for the setting is 1, the maximum 64, the default 4.

A move counts toward this restriction only when in the backfill phase, as described above, in Data Service Rebalance Phases. The move may be of either an active or a replica vBucket. A node’s participation in the move may be as either a source or a target.

For example, if a node is at a given time the source for two moves in backfill phase, and is the target for two additional moves in backfill phase, and the setting stands at 4, the node may participate in the backfill phase of no additional moves, until at least one of its current moves has completed its backfill phase.

The setting may be established by means of the Couchbase Web Console, the Couchbase CLI, or the REST API.

A higher setting may improve rebalance performance, at the cost of higher resource consumption; in terms of CPU, memory, disk, and bandwidth. Conversely, a lower setting may degrade rebalance performance, while freeing up such resources. Note, however, that rebalance performance can be affected by many additional factors; and that in consequence, changing this parameter may not always have the expected effects. Note also that a higher setting, due to its additional consumption of resources, may degrade the performance of other systems, including the Data Service.

Couchbase Server creates a report on every rebalance that occurs. The report contains a JSON document, which can be inspected in any browser or editor. The document provides summaries of the concluded rebalance activity, as well as details for each of the vBuckets affected: in consequence, the report may be of considerable length.

On conclusion of a rebalance, its report can be accessed in any of the following ways:

A complete account of the report-content is provided in the Rebalance Reference.

The Index Service maintains a cluster-wide set of index definitions and metadata, which allows the redistribution of indexes and index replicas during a rebalance.

The rebalance process takes into account the nodes' CPU, RAM, and disk bandwidth to limit its effect on database performance.

The Search Service automatically partitions its indexes across all Search nodes in the cluster, ensuring optimal distribution, following rebalance.

To achieve this, in versions of Couchbase Server prior to 7.1, by default, partitions needing to be newly created were entirely built, on their newly assigned nodes. In 7.1+, by default, new partitions are instead created by the transfer of partition files from old nodes to new nodes: this significantly enhances performance. This is an Enterprise-only feature, which requires all Search Service nodes either to be running 7.1 or later; or to be running 7.0.2, with the feature explicitly switched on.

Community Edition clusters that are upgraded to Enterprise Edition 7.1+ thus gain this feature in its default setting. Community Edition clusters that are upgraded to Enterprise Edition 7.0.2 can have this feature switched on, subsequent to upgrade.

During file transfer, should an unresolvable error occur, file transfer is automatically abandoned, and partition build is used instead.

The file-transfer feature can be enabled and disabled by means of the REST API. See Rebalance Based on File Transfer.

When a node is removed and rebalanced, the Query Service will allow existing queries and transactions to complete before shutting down, which may result in the rebalancing operation taking longer to complete. The Query Service diagnostic log on the node(s) being removed will contain messages indicating how many transactions and queries are still running. Any new connection attempts to nodes that are shutting down will receive error 1180 (E_SERVICE_SHUTTING_DOWN), and may receive error 1181 (E_SERVICE_SHUT_DOWN) in the brief period between the completion of the last statement or transaction and the service exiting. Such rejected requests will have HTTP status code 503 (service unavailable) set.

When an Eventing Service node has been added or removed, rebalance causes the mutation (vBucket processing ownership) and timer event processing workload to be redistributed among available Eventing Service nodes. The Eventing Service continues to process mutations both during and after rebalance. Checkpoint information ensures that no mutations are lost.

The Analytics Service uses shadow data, which is a copy of all or some of the data maintained by the Data Service. By default, the shadow data is not replicated; however, it may be partitioned across all cluster nodes that run the Analytics Service. Starting with Couchbase Server 7.1, the shadow data and its partitions may be replicated up to 3 times. Each replica resides on an Analytics node: a given Analytics node can host a replica partition, or the active partition on which replicas are based.

If there are no Analytics replicas, and an Analytics node fails over, the Analytics Service stops working cluster-wide: ingestion of shadow data stops and no Analytics operations can be run. In this case:

If there are Analytics replicas, and an Analytics node fails over, the Analytics Service continues to work: one of the replicas is promoted to serve the shadow data that was stored on the failed over node. The Analytics Service only needs to rebuild any shadow data that isn’t already ingested from the Data Service, depending on the state of the promoted replica. In this case:

If no Analytics Service node has been removed or replaced, shadow data is not affected by rebalance. In consequence of rebalance, the Analytics Service receives an updated cluster map, and continues to work with the modified vBucket-topology.

A rebalance causes the scheduler for the Backup Service to stop running. This means that no new backup tasks are triggered until the rebalance has concluded; at which point, the scheduler restarts, and reconstructs the task schedule. Then, the triggering of Backup Service tasks is resumed.

Note that a rebalance has the effect of restarting the Backup Service whenever the service has previously been stopped, due to loss of its leader: for information, see Backup-Service Architecture.