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Rust Driver

Performance Considerations

On this page

  • Overview
  • Client Lifecycle
  • Connection Pool
  • Configure Maximum Pool Size
  • Configure Concurrent Connection Options
  • Configure Maximum Idle Time
  • Parallelism
  • Runtime
  • Additional Information
  • API Documentation

In this guide, you can learn how to optimize the performance of the Rust driver. To connect to MongoDB, you must create a Client instance. Your Client instance automatically handles most aspects of connection, such as discovering server topology, monitoring your connection, and maintaining an internal connection pool. This guide describes best practices for configuring and using your Client instance.

This guide includes the following sections:

  • Client Lifecycle describes best practices for creating and managing a Client instance

  • Connection Pool describes how connection pooling works in the driver

  • Parallelism provides sample code for running parallel, asynchronous tasks

  • Runtime describes how to manage runtimes by using functionalities of the tokio and async_std crates

  • Additional Information provides links to resources and API documentation for types and methods mentioned in this guide

We recommend that you reuse your client across sessions and operations. You can use the same Client instance to perform multiple tasks, as the Client type is safe for concurrent use by multiple threads. Creating a new Client instance for each request results in slower performance.

The following code creates a method that accepts a pointer to an existing Client instance, which allows you to perform many requests by using the same client:

// ... Create a client earlier in your code
async fn make_request(client: &Client) -> Result<(), Box<dyn Error>> {
// Use the client to perform operations

Every Client instance has a built-in connection pool for each server in your MongoDB topology. Connection pools open sockets on demand to support concurrent requests to MongoDB in your application.

The default configuration for a Client works for most applications. The following code shows how to create a client with default connection settings:

let client = Client::with_uri_str("<connection string>").await?;

Alternatively, you can tune the connection pool to best fit the needs of your application and optimize performance. For more information on how to customize your connection settings, see the following subsections of this guide:


To learn more about configuring a connection pool, see Tuning Your Connection Pool Settings in the Server manual.

The maximum size of each connection pool is set by the max_pool_size option, which defaults to 10. If the number of in-use connections to a server reaches the value of max_pool_size, the next request to that server waits until a connection becomes available.

In addition to the sockets needed to support your application's requests, each Client instance opens two more sockets per server in your MongoDB topology for monitoring the server's state. For example, a client connected to a three-node replica set opens six monitoring sockets. If the application uses the default setting for max_pool_size and only queries the primary (default) node, then there can be at most 16 total connections in the connection pool. If the application uses a read preference to query the secondary nodes, those connection pools grow and there can be 36 total connections.

To support high numbers of concurrent MongoDB requests within one process, you can increase the value of the max_pool_size option. The following code demonstrates how to specify a value for max_pool_size when instantiating a Client:

let mut client_options = ClientOptions::parse_async("<connection string>").await?;
client_options.max_pool_size = Some(20);
let client = Client::with_options(client_options)?;

Connection pools are rate-limited. The max_connecting option determines the number of connections that the pool can create in parallel at any time. For example, if the value of max_connecting is 2, the default value, the third request that attempts to concurrently check out a connection succeeds only when one of the following cases occurs:

  • The connection pool finishes creating a connection and the number of connections in the pool is less than or equal to the value of max_pool_size.

  • An existing connection is checked back into the pool.

  • The driver's ability to reuse existing connections improves due to rate limits on connection creation.

You can set the minimum number of concurrent connections to each server with the min_pool_size option, which defaults to 0. The driver initializes the connection pool with this number of sockets. If sockets are closed and the total number of sockets, both in use and idle, drops below the minimum, the connection pool opens more sockets until the minimum is reached.

The following code sets the max_connecting and min_pool_size options when instantiating a Client:

let mut client_options = ClientOptions::parse_async("<connection string>").await?;
client_options.max_connecting = Some(3);
client_options.min_pool_size = Some(1);
let client = Client::with_options(client_options)?;

You can set the maximum amount of time that a connection can remain idle in the pool by setting the max_idle_time option. Once a connection has been idle for the duration specified in max_idle_time, the connection pool removes and replaces that connection. This option defaults to 0, or no limit.

When the Client::shutdown() method is called at any point in your application, the driver closes all idle sockets and closes all sockets that are in use as they are returned to the pool. Calling Client::shutdown() closes only inactive sockets, so you cannot interrupt or terminate any ongoing operations by using this method. The driver closes these sockets only when the process completes.

The following code sets the value of the max_idle_time option to 90 seconds when instantiating a Client:

let mut client_options = ClientOptions::parse_async("<connection string>").await?;
client_options.max_idle_time = Some(Duration::new(90, 0));
let client = Client::with_options(client_options)?;

If you can run parallel data operations, you can optimize performance by running asynchronous, concurrent tasks. The following code uses the spawn() method from the tokio::task module to create separate, concurrent tasks to perform insert operations:

let client = Client::with_uri_str("<connection string>").await?;
let data = doc! { "title": "1984", "author": "George Orwell" };
for i in 0..5 {
let client_ref = client.clone();
let data_ref = data.clone();
task::spawn(async move {
let collection = client_ref
.collection::<Document>(&format!("coll{}", i));
collection.insert_one(data_ref, None).await

A Client instance is bound to the instance of the tokio or async-std runtime in which you created it. If you use a Client instance to perform operations on a different runtime, you might experience unexpected behavior or failures.

If use the test helper macro from the tokio or async_std crate to test your application, you might accidentally run operations on a different runtime than intended. This is because these helper macros create a new runtime for each test. However, you can use one of the following strategies to avoid this issue:

  • Attach the runtime to the Client instance without using the test helper macros.

  • Create a new Client instance for every async test.

This example follows the first strategy and creates a global runtime used only for testing. In the following code, the test_list_dbs() method uses a client that manually connects to this runtime to list databases in the deployment:

use tokio::runtime::Runtime;
use once_cell::sync::Lazy;
static CLIENT_RUNTIME: Lazy<(Client, Runtime)> = Lazy::new(|| {
let rt = Runtime::new().unwrap();
let client = rt.block_on(async {
Client::with_uri_str("<connection string>").await.unwrap()
(client, rt)
fn test_list_dbs() -> Result<(), Box<dyn Error>> {
let (client, rt) = &*CLIENT_RUNTIME;
rt.block_on(async {
client.list_database_names(None, None).await

Implementing the second strategy, the following code creates a new Client instance for each test run with tokio::test, ensuring that there are no unintended interactions between runtimes:

async fn test_list_dbs() -> Result<(), Box<dyn Error>> {
let client = Client::with_uri_str("<connection string>").await?;
client.list_database_names(None, None).await?;

To learn more about connecting to MongoDB, see the Connection Guide.

To learn more about the available runtimes for the Rust driver, see the guide on Asynchronous and Synchronous APIs.

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