In a MemSQL cluster, an aggregator is a node that functions as a cluster-aware query router. An aggregator stores only metadata and acts as a gateway into the distributed system. A MemSQL cluster may have one or many aggregators depending on query volume.

An aggregator is responsible for:

• querying the leaves
• aggregating the results
• returning the results to the client

The Master Aggregator is a specialized aggregator responsible for cluster monitoring and failover.

Aggregator Commands

• SHOW AGGREGATORS
• AGGREGATOR SET AS MASTER

Starting an Aggregator

To add a child aggregator to a MemSQL cluster, pick a node in the cluster that will serve as a child aggregator and append the line master-aggregator = <master aggregator ip address> to the end its memsql.cnf. Then start MemSQL using the following command:

$ sudo service memsql start

If you’re running the MemSQL binary directly, start the child aggregator with the following command:

./memsqld --master-aggregator=host:port

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