FAQ

What Is the Difference Between "connectTimeoutMS", "socketTimeoutMS" and "maxTimeMS"?

To specify the optional settings for your MongoClient, declare one or more available settings in the options object of the constructor as follows:

const client = new MongoClient(uri, {
  connectTimeoutMS: <integer value>,
  socketTimeoutMS: <integer value>
});

To see all the available settings, see the MongoClientOptions API Documentation.

To specify maxTimeMS, chain the maxTimeMS() method with a timeout specification to an operation that returns a Cursor:

const cursor = collection.find({}).maxTimeMS(50);

How Can I Prevent the Driver From Hanging During Connection or From Spending Too Long Trying to Reach Unreachable Replica Sets?

To prevent the driver from hanging during connection or to prevent the driver from spending too long trying to reach unreachable replica sets, you can set the connectTimeoutMS option of your connection options. Generally, you should ensure that the connectTimeoutMS setting is not lower than the longest network latency you have to a member of the set. If one of the secondary members is on the other side of the planet and has a latency of 10000 milliseconds, setting the connectTimeoutMS to anything lower will prevent the driver from ever connecting to that member.

Should I Use "socketTimeoutMS" as a Way of Preventing Long-Running Operations From Slowing Down the Server?

No, you should not use socketTimeoutMS to end operations that may run for too long and slow down the application. Attempting to do so may not achieve the intended result.

Closing the socket causes a reconnect of the driver's connection pool, introducing latency to any other queued up operations. Chronically slow operations will, therefore, cause a large number of reconnect requests, negatively impacting throughput and performance.

Also, closing the socket does not terminate the operation; it will continue to run on the MongoDB server, which could cause data inconsistencies if the application retries the operation on failure.

However, there are important use cases for socketTimeoutMS - consider the following cases:

• A MongoDB process erroring out

• A misconfigured firewall causing a socket connection without sending a FIN packet

In those cases, there is no way to detect that the connection has died. Setting the socketTimeoutMS is essential to ensure that the sockets are closed correctly. A good rule of thumb is to set socketTimeoutMS to two to three times the length of the slowest operation that runs through the driver.

How Can I Prevent Sockets From Timing out Before They Become Active?

Having a large connection pool does not always reduce reconnection requests. Consider the following example:

An application has a connection pool size of 5 sockets and has the socketTimeoutMS option set to 5000 milliseconds. Operations occur, on average, every 3000 milliseconds, and reconnection requests are frequent. Each socket times out after 5000 milliseconds, which means that all sockets must do something during those 5000 milliseconds to avoid closing.

One message every 3000 milliseconds is not enough to keep the sockets active, so several of the sockets will time out after 5000 milliseconds. Reduce the poolSize in the connection settings to fix the problem.

To specify the optional poolSize setting for your MongoClient, declare it in the options object of the constructor as follows:

const client = new MongoClient(uri, {
  poolSize: <integer value>,
});

What Does a Value of "0" mean for "connectTimeoutMS" and "socketTimeoutMS"?

If you set the value of connectTimeoutMS or socketTimeoutMS to 0, your application will use the operating system's default socket timeout value.

How Can I Prevent Long-Running Operations From Slowing Down the Server?

You can prevent long-running operations from slowing down the server by specifying a timeout value. You can chain the maxTimeMS() method to an operation that returns a Cursor to set a timeout on a specific action.

The following example shows how you can chain the maxTimeMS() method to an operation that returns a Cursor:

// Execute a find command
await collection
  .find({ $where: "sleep(100) || true" })
  .maxTimeMS(50)
  .count((err, count) => {});

What Does the "keepAlive" Setting Do?

keepAlive is a connection-setting that sets the number of milliseconds to wait before initiating a TLS keepAlive on a TCP Socket. The keepAlive option will keep a socket alive by sending periodic probes to MongoDB. However, this only works if the operating system supports SO_KEEPALIVE .

What Can I Do If I'm Experiencing Unexpected Network Behavior?

Internal firewalls that exist between application servers and MongoDB are often misconfigured and are overly aggressive in their removal of socket connections.

If you experience unexpected network behavior, here are some things to check:

1. The firewall should send a FIN packet when closing a socket,allowing the driver to detect that the socket is closed.

2. The firewall should allow keepAlive probes.

What Can I Do If I'm Getting "ECONNRESET" When Calling "client.connect()"?

In most operating systems, each connection is associated with a file descriptor. There is typically a limit set by the operating system on the number of file descriptors used by a single process. An ECONNRESET error can occur if the connection pool size surpasses the limit of file descriptors.

Consider the following operation:

1
const uri = "mongodb://localhost:27017/test?maxPoolSize=5000";
2
// create a new MongoClient
3
const client = new MongoClient(uri);
4
5
await client.connect(err => {
6
  // connection
7
});

If this operation causes an ECONNRESET error, you may have run into the file descriptor limit for your Node.js process. In that case you must increase the number of file descriptors for the Node.js process. On MacOS and Linux you do this with the ulimit shell command.

ulimit -n 6000

This sets the maximum number of file descriptors for the process to 6000, allowing Node.js to connect with a pool size of 5000 sockets.

How Can I Prevent a Slow Operation From Delaying Other Operations?

A slow operation may delay your other operations that occur after it, if the poolSize has not been set in the connection options. MongoDB is synchronous and uses a single execution thread per socket, meaning that MongoDB will execute one single operation per socket at any point in time. Any other operation sent to that socket will have to wait until the current operation is finished. If you have a slow-running operation that holds up other operations, the best solution is to create a separate connection pool for the slow operation, isolating it from other, faster operations.

To create a separate connection pool, instantiate another MongoClient call the connect() method on it. See the following example for the syntax you can use to create two clients, each with its own connection pool:

const clientA = new MongoClient(uri, options);
clientA.connect(); // any method calls on clientA use clientA's connection pool
const clientB = new MongoClient(uri, options);
clientB.connect(); // any method calls on clientB use clientB's connection pool

How Can I Ensure My Connection String Is Valid for a Replica Set?

The connection string passed to the driver must use exact hostnames for the servers as set in the Replica Set Config. Given the following configuration settings for your Replica Set, in order for the Replica Set discovery and failover to work the driver should be able to reach server1, server2, and server3.

{
  "_id": "testSet",
  "version": 1,
  "protocolVersion": 1,
  "members": [
    {
      "_id": 1,
      "host": "server1:31000"
    },
    {
      "_id": 2,
      "host": "server2:31001"
    },
    {
      "_id": 3,
      "host": "server3:31002"
    }
  ]
}

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