ReQL command: rebalance

Command syntax

table.rebalance() → object
database.rebalance() → object

Description

Rebalances the shards of a table. When called on a database, all the tables in that database will be rebalanced.

The rebalance command operates by measuring the distribution of primary keys within a table and picking split points that will give each shard approximately the same number of documents. It won’t change the number of shards within a table, or change any other configuration aspect for the table or the database.

A table will lose availability temporarily after rebalance is called; use the wait command to wait for the table to become available again, or status to check if the table is available for writing.

RethinkDB automatically rebalances tables when the number of shards are increased, and as long as your documents have evenly distributed primary keys—such as the default UUIDs—it is rarely necessary to call rebalance manually. Cases where rebalance may need to be called include:

• Tables with unevenly distributed primary keys, such as incrementing integers
• Changing a table’s primary key type
• Increasing the number of shards on an empty table, then using non-UUID primary keys in that table

The web UI (and the info command) can be used to tell you when a table’s shards need to be rebalanced.

The return value of rebalance is an object with two fields:

• rebalanced: the number of tables rebalanced.
• status_changes: a list of new and old table status values. Each element of the list will be an object with two fields:
  • old_val: The table’s status value before rebalance was executed.
  • new_val: The table’s status value after rebalance was executed. (This value will almost always indicate the table is unavailable.)

See the status command for an explanation of the objects returned in the old_val and new_val fields.

Example: Rebalance a table.

r.table('superheroes').rebalance().run(conn)

Example return:

{
  "rebalanced": 1,
  "status_changes": [
    {
      "old_val": {
        "db": "database",
        "id": "5cb35225-81b2-4cec-9eef-bfad15481265",
        "name": "superheroes",
        "shards": [
          {
            "primary_replica": "jeeves",
            "replicas": [
              {
                "server": "jeeves",
                "state": "ready"
              }
            ]
          },
          {
            "primary_replica": "jeeves",
            "replicas": [
              {
                "server": "jeeves",
                "state": "ready"
              }
            ]
          }
        ],
        "status": {
          "all_replicas_ready": True,
          "ready_for_outdated_reads": True,
          "ready_for_reads": True,
          "ready_for_writes": True
        }
      },
      "new_val": {
        "db": "database",
        "id": "5cb35225-81b2-4cec-9eef-bfad15481265",
        "name": "superheroes",
        "shards": [
          {
            "primary_replica": "jeeves",
            "replicas": [
              {
                "server": "jeeves",
                "state": "transitioning"
              }
            ]
          },
          {
            "primary_replica": "jeeves",
            "replicas": [
              {
                "server": "jeeves",
                "state": "transitioning"
              }
            ]
          }
        ],
        "status": {
          "all_replicas_ready": False,
          "ready_for_outdated_reads": False,
          "ready_for_reads": False,
          "ready_for_writes": False
        }
      }

    }
  ]
}

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