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Tuning Your Connection Pool Settings

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  • Calculate Maximum Number of Connections
Slow application-side operation times that are not reflected in the database server logs or the real time panel.

Use connectTimeoutMS to ensure the driver does not wait indefinitely during the connection phase.

Set connectTimeoutMS to a value greater than the longest network latency you have to a member of the set.

For example: if a member has a latency of 10000 milliseconds, setting connectTimeoutMS to 5000 (milliseconds) prevents the driver from connecting to that member.

A misconfigured firewall closes a socket connection incorrectly and the driver cannot detect that the connection closed improperly.

Use socketTimeoutMS to ensure that sockets are always closed.

Set socketTimeoutMS to two or three times the length of the slowest operation that the driver runs.

The server logs or real time panel show that the application spends too much time creating new connections.

Not enough connections are available at startup. Allocate connections in the pool by setting minPoolSize.

Set minPoolSize to the number of connections you want to be available at startup.

The MongoClient instance ensures that number of connections exists at all times.

The load on the database is low and there's a small number of active connections at any time. Application performs fewer operations at any one time than expected.
Increase maxPoolSize, or increase the number of active threads in your application or the framework you are using.
Database CPU usage is higher than expected. The server logs or real time panel show more connection attempts than expected.
Decrease the maxPoolSize or reduce the number of threads in your application. This can reduce load and response times.


Do not use socketTimeoutMS to prevent long-running server operations. Instead, use maxTimeMS() with queries so that the server can cancel long-running operations.

Calculate usage to find the number of operations running for each connection.

Consider four application servers connecting to a replica set with three members. In this scenario, each of the four application servers creates a connection pool for each replica set member.

Calculate the maximum number of connections that are opened by each application server by multiplying maxPoolSize by the number of members.

Calculate outgoing connections from an application to a three-member replica set:

100 (maxPoolSize default 100) x 3 (replica set members) = 300 (outgoing connections from the application).

Calculate incoming connections from four application servers to a replica set:

100 (maxPoolSize default 100) x 4 (application servers) = 400 (incoming connections to each mongod).

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