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Rebalance shards in cluster in Azure Cosmos DB for PostgreSQL

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APPLIES TO: Azure Cosmos DB for PostgreSQL (powered by the Citus database extension to PostgreSQL)

To take advantage of newly added nodes, rebalance distributed table shards. Rebalancing moves shards from existing nodes to the new ones. Azure Cosmos DB for PostgreSQL offers zero-downtime rebalancing, meaning queries continue without interruption during shard rebalancing.

Determine if the cluster is balanced

The Azure portal shows whether data is distributed equally between worker nodes in a cluster or not. From the Cluster management menu, select Shard rebalancer.

  • If data is skewed between workers: You'll see the message, Rebalancing is recommended and a list of the size of each node.

  • If data is balanced: You'll see the message, Rebalancing is not recommended at this time.

Run the Shard rebalancer

To start the Shard rebalancer, connect to the coordinator node of the cluster and then run the rebalance_table_shards SQL function on distributed tables.

The function rebalances all tables in the colocation group of the table named in its argument. You don't have to call the function for every distributed table. Instead, call it on a representative table from each colocation group.

SELECT rebalance_table_shards('distributed_table_name');

Monitor rebalance progress

You can view the rebalance progress from the Azure portal. From the Cluster management menu, select Shard rebalancer . The message Rebalancing is underway displays with two tables:

  • The first table shows the number of shards moving into or out of a node. For example, "6 of 24 moved in."
  • The second table shows progress per database table: name, shard count affected, data size affected, and rebalancing status.

Select Refresh to update the page. When rebalancing is complete, you'll see the message Rebalancing is not recommended at this time.

Next steps