> For the complete MongoDB documentation index, see www.mongodb.com/docs/llms.txt

# Tuning Your Connection Pool Settings

| Problem | Solution |
| --- | --- |
| Slow application-side operation times that are not reflected in the database [server logs](https://www.mongodb.com/docs/manual/reference/log-messages.md#std-label-log-messages-ref) or the real time panel. | Use [`connectTimeoutMS`](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.connectTimeoutMS) to ensure the driver does not wait indefinitely during the connection phase. Set [`connectTimeoutMS`](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.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`](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.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`](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.socketTimeoutMS) to ensure that sockets are always closed. Set [`socketTimeoutMS`](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.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`.](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.minPoolSize) Set [minPoolSize](https://www.mongodb.com/docs/manual/administration/connection-pool-overview.md#std-label-minpoolsize-cp-setting) 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](https://www.mongodb.com/docs/manual/administration/connection-pool-overview.md#std-label-maxpoolsize-cp-setting), 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](https://www.mongodb.com/docs/manual/administration/connection-pool-overview.md#std-label-maxpoolsize-cp-setting) or reduce the number of threads in your application. This can reduce load and response times. |

**Warning:**

Do not use [`socketTimeoutMS`](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.socketTimeoutMS) to prevent long-running server operations. Instead, use [`maxTimeMS()`](https://www.mongodb.com/docs/manual/reference/method/cursor.maxtimems.md#mongodb-method-cursor.maxTimeMS) with queries so that the server can cancel long-running operations.

## Calculate Maximum Number of Connections

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`](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.maxPoolSize) by the number of members.

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

**100** ([`maxPoolSize`](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.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`](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.maxPoolSize) default `100`) x **4** (application servers) = **400** (incoming connections to each mongod).