Manage Connections with Connection Pools使用连接池管理连接

maxPoolSize

In addition to the sockets needed to support your application's requests, each MongoClient instance opens up to two connections per server in your MongoDB topology for monitoring the server's state.除了支持应用程序请求所需的套接字外，每个MongoClient实例在MongoDB拓扑中每个服务器最多打开两个连接，用于监控服务器的状态。

For example, a client connected to a three-node replica set opens six monitoring sockets. 例如，连接到三节点副本集的客户端打开六个监控套接字。If the application uses the default setting for maxPoolSize and only queries the primary (default) node, then there can be at most 106 open sockets and 100 connections in the connection pool. 如果应用程序使用maxPoolSize的默认设置并且只查询主（默认）节点，则连接池中最多只能有106个打开的套接字和100个连接。If the application uses a read preference to query the secondary nodes, those connection pools grow and there can be 306 total connections including the open monitoring sockets.如果应用程序使用读取首选项来查询辅助节点，则这些连接池会增长，包括打开的监控套接字在内，总共有306个连接。

To support high numbers of concurrent MongoDB requests within one process, you can increase maxPoolSize.为了在一个进程中支持大量并发MongoDB请求，您可以增加maxPoolSize。

The following code creates a MongoClient instance with a maximum connection pool size of 200 by specifying the maxPoolSize option in the options object:以下代码通过在options对象中指定maxPoolSize选项，创建了一个最大连接池大小为200的MongoClient实例：

const { MongoClient } = require('mongodb');

const uri = '<connection-string>';
const client = new MongoClient(uri, {
  maxPoolSize: 200
});

maxConnecting

Connection pools rate-limit connection establishment. The maxConnecting option determines the number of connections that the pool can create in parallel at any time. 连接池速率限制连接建立。maxConnecting选项决定池在任何时候可以并行创建的连接数。For example, if the value of maxConnecting is 2, the third request that attempts to concurrently check out a connection succeeds only when one the following cases occurs:例如，如果maxConnecting的值为2，则尝试并发签出连接的第三个请求仅在发生以下情况之一时成功：

• The connection pool finishes creating a connection and there are fewer than maxPoolSize connections in the pool.连接池已完成创建连接，池中的连接数小于maxPoolSize。
• An existing connection is checked back into the pool.将现有连接检回池中。

The following code creates a MongoClient instance with a maximum number of 2 connections to be established concurrently per pool by specifying the maxConnecting option in the options object:以下代码通过在options对象中指定maxConnecting选项，创建了一个MongoClient实例，每个池最多可同时建立2个连接：

const { MongoClient } = require('mongodb');

const uri = '<connection-string>';
const client = new MongoClient(uri, {
   maxConnecting: 2
});

minPoolSize

You can set the minimum number of connections to each server with the minPoolSize option. The driver ensures that there are always at least the number of connections set by the minPoolSize option in the connection pool. 您可以使用minPoolSize选项设置每个服务器的最小连接数。驱动程序确保连接池中始终至少有minPoolSize选项设置的连接数。If sockets are closed, causing the total number of sockets (both in use and idle) to drop below the minimum, more sockets are opened until the minimum is reached.如果套接字关闭，导致套接字总数（使用中和空闲）降至最低值以下，则会打开更多套接字，直到达到最低值。

The following code creates a MongoClient instance with a minimum connnection pool size of 10 by specifying the minPoolSize option in the options object:以下代码通过在options对象中指定minPoolSize选项，创建了一个最小连接池大小为10的MongoClient实例：

const { MongoClient } = require('mongodb');

const uri = '<connection-string>';
const client = new MongoClient(uri, {
  minPoolSize: 10
});

maxIdleTimeMS

You can set the maximum number of milliseconds that a connection can remain idle in the pool by setting the maxIdleTimeMS option. Once a connection has been idle for maxIdleTimeMS, the connection pool removes and replaces it. This option defaults to 0 (no limit).通过设置maxIdleTimeMS选项，可以设置连接在池中保持空闲的最大毫秒数。一旦连接在maxIdleTimeMS中处于空闲状态，连接池就会删除并替换它。此选项默认为0（无限制）。

The following code creates a MongoClient instance with a maximum idle time of 10000 milliseconds (10 seconds) by specifying the maxIdleTimeMS setting in the options object:以下代码通过在options对象中指定maxIdleTimeMS设置，创建了一个最大空闲时间为10000毫秒（10秒）的MongoClient实例：

const { MongoClient } = require('mongodb');

const uri = '<connection-string>';
const client = new MongoClient(uri, {
  maxIdleTimeMS: 10000
});

waitQueueTimeoutMS

MongoClient supports multiple concurrent requests. For each process, create a client and reuse it for all operations in a process. This practice is more efficient than creating a client for each request.支持多个并发请求。对于每个流程，创建一个客户端并将其重用于流程中的所有操作。这种做法比为每个请求创建一个客户端更有效。

The driver does not limit the number of requests that can wait for sockets to become available, and it is the application's responsibility to limit the size of its pool to bound queuing during a load spike. Requests wait for the amount of time specified in the waitQueueTimeoutMS option, which defaults to 0 (no limit).驱动程序不限制可以等待套接字可用的请求数量，应用程序有责任在负载高峰期间将其池的大小限制在绑定队列中。请求等待waitQueueTimeoutMS选项中指定的时间，默认为0（无限制）。

A request that waits more than the length of time defined by waitQueueTimeoutMS for a socket raises a connection error. Use this option if it is more important to bound the duration of operations during a load spike than it is to complete every operation.等待时间超过waitQueueTimeoutMS为套接字定义的时间长度的请求会引发连接错误。如果在负载高峰期间限制操作持续时间比完成每个操作更重要，请使用此选项。

The following code creates a MongoClient instance with a maximum wait queue timeout of 10000 milliseconds (10 seconds) by declaring it in the options object:以下代码通过在options对象中声明来创建最大等待队列超时为10000毫秒（10秒）的MongoClient实例：

const { MongoClient } = require('mongodb');

const uri = '<connection-string>';
const client = new MongoClient(uri, {
  waitQueueTimeoutMS: 10000
});