When Should I Add CNs or Scale out a cluster?

Introduction to CN Concurrency

CN is short for Coordinator Node. A CN is an important component of GaussDB(DWS) and is closely related to users. It provides interfaces to external applications, optimizes global execution plans, distributes execution plans to DataNodes, and summarizes and processes execution results. A CN is an interface to external applications. The concurrency capability of the CN determines the service concurrency.

CN concurrency is determined by the following parameters:

• max_connections: specifies the maximum number of concurrent connections to the database. This parameter affects the concurrent processing capability of the cluster. The default value depends on the cluster specifications. For details, see Managing Database Connections.
• max_active_statements: specifies the maximum number of concurrent jobs. This parameter applies to all the jobs on one CN. The default value is 60, which indicates a maximum of 60 jobs can run at the same time. Other jobs will be queued.

Add CNs or Scale out a Cluster?

• Insufficient connections: When a cluster is created for the first time, the default number of CNs in the cluster is 3, which can meet the customer's basic connection requirements. If the cluster has a large number of concurrent requests and the number of connections to each CN is large, or the CPU usage of a CN exceeds its capacity, you are advised to add CNs. For details, see CNs.
• Insufficient storage capacity and performance: If your business grows and you have higher requirements on storage capacity and performance, or the CPU of your cluster is insufficient, you are advised to scale out your cluster. For details, see Scaling Out a Cluster.

With the expansion of cluster nodes, more CNs are needed to meet the distribution requirements of GaussDB(DWS). In short, adding CNs does not necessarily require cluster scale-out. However, after cluster scale-out, CNs may need to be added.