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

# Connection Pool Overview

This document describes how to use a connection pool to manage connections between applications and MongoDB instances.

## What is a Connection Pool?

### Definition

A connection pool is a cache of open, ready-to-use database connections maintained by the [driver](https://www.mongodb.com/docs/drivers/). Your application can get connections from the pool, perform operations, and return connections to the pool. Connection pools are thread-safe.

### Benefits of a Connection Pool

A connection pool reduces application latency and the number of new connections created. The pool creates connections at startup, and connections return to the pool automatically — applications do not need to return them manually. Some connections are active and some are available. If your application requests a connection and one is available in the pool, a new connection does not need to be created.

## Create and Use a Connection Pool

### Use an Instance of Your Driver's `MongoClient` Object

Most [drivers](https://www.mongodb.com/docs/drivers/) provide an object of type `MongoClient`.

Use one `MongoClient` instance per application unless the application is connecting to many separate clusters. Each `MongoClient` instance manages its own connection pool to the MongoDB cluster or node specified when the `MongoClient` is created. `MongoClient` objects are thread-safe in most drivers.

**Note:**

Store your `MongoClient` instance in a place that is globally available to your application.

### Authentication

To use a connection pool with LDAP, see [LDAP Connection Pool Behavior.](https://www.mongodb.com/docs/manual/core/security-ldap-external.md#std-label-ldap-connection-pool-behavior)

## Sharded Cluster Connection Pooling

[`mongos`](https://www.mongodb.com/docs/manual/reference/program/mongos.md#mongodb-binary-bin.mongos) routers have connection pools for each node in the cluster. The availability of connections to individual nodes within a sharded cluster affects latency. Operations must wait for a connection to be established.

## Connection Pool Configuration Settings

You can specify connection pool settings in these locations:

- The [MongoDB URI](https://www.mongodb.com/docs/manual/reference/connection-string.md#std-label-mongodb-uri)

- Your application's `MongoClient` instance

- Your application framework's configuration files

### Settings

| Setting | Description |
| --- | --- |
| [`connectTimeoutMS`](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.connectTimeoutMS) | Most drivers default to never time out. Some versions of the Java drivers (for example, version 3.7) default to `10`. *Default:* `0` for most drivers. See your [driver](https://www.mongodb.com/docs/drivers/) documentation. |
| [`maxConnecting`](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.maxConnecting) | Maximum number of connections a pool may be establishing concurrently. `maxConnecting` is supported for all drivers **except** the [Rust Driver.](https://www.mongodb.com/docs/drivers/rust/current/) Raising the value of `maxConnecting` allows the client to establish connection to the server faster, but increases the chance of [connection storms](https://www.mongodb.com/docs/manual/reference/glossary.md#std-term-connection-storm). If the value of `maxConnecting` is too low, your connection pool may experience heavy throttling and increased tail latency for clients checking out connections. *Default:* `2` |
| [`maxIdleTimeMS`](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.maxIdleTimeMS) | The maximum number of milliseconds that a connection can remain idle in the pool before being removed and closed. *Default:* See your [driver](https://www.mongodb.com/docs/drivers/) documentation. |
| [`maxPoolSize`](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.maxPoolSize) | Maximum number of connections opened in the pool. When the connection pool reaches the maximum number of connections, new connections wait up to the value of [`waitQueueTimeoutMS`.](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.waitQueueTimeoutMS) *Default:* `100` |
| [`minPoolSize`](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.minPoolSize) | Minimum number of connections opened in the pool. The value of [`minPoolSize`](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.minPoolSize) must be less than the value of [`maxPoolSize`.](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.maxPoolSize) *Default*: `0` |
| [`socketTimeoutMS`](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.socketTimeoutMS) | Number of milliseconds to wait before timeout on a TCP connection. Do *not* use [`socketTimeoutMS`](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.socketTimeoutMS) as a mechanism for preventing long-running server operations. Setting low socket timeouts may result in operations that error before the server responds. *Default*: `0`, which means no timeout. |
| [`waitQueueTimeoutMS`](https://www.mongodb.com/docs/manual/reference/connection-string-options.md#mongodb-urioption-urioption.waitQueueTimeoutMS) | Maximum wait time in milliseconds that a thread can wait for a connection to become available. A value of `0` means there is no limit. *Default*: `0` |