Customize Server Selection

Overview

All MongoDB drivers follow a defined algorithm when selecting a server to read or write from. By using the ClusterSettings property of a MongoClientSettings object, you can customize this algorithm to choose the server that works best for your application.

Server Selection Algorithm

When the Kotlin Sync driver executes a read operation, it performs the following steps, in order, to select a MongoDB deployment:

1. Selects all servers from the list of known servers suitable for the requested operation, barring those that the driver considers unavailable or problematic

   1. For reads, selects all servers that match the active read preference

   2. For writes, selects all writeable servers

2. Calls the user-defined server-selector function, if the user provides one, and passes in the list from the previous step

3. Applies the localThreshold connection setting to the list of servers returned from the function

4. Selects at most two random servers from the list of servers that match the preceding criteria, then selects the server with fewer outstanding concurrent operations

When the Kotlin Sync driver executes a write operation, it begins by selecting all writeable servers from the list of known servers, not just those that match the active read preference. The remaining steps are identical to the preceding list.

To learn more about the MongoDB server selection algorithm, see Server Selection Algorithm in the MongoDB Server manual.

Implement Custom Server Selection Logic

You can implement your own custom server selection logic by creating a class that implements the ServerSelector interface. The following example shows a simple custom server selection class that selects servers with a type value of ServerType.REPLICA_SET_SECONDARY:

class CustomServerSelector : ServerSelector {
    override fun select(cluster: ClusterDescription): List<ServerDescription> {
        return cluster.serverDescriptions.filter { it.type == ServerType.REPLICA_SET_SECONDARY }
    }
}

Use the applyToClusterSettings() method to pass an instance of this class to your MongoClientSettings. The following example shows how to create a MongoClient with an instance of the custom server selector class from the preceding example:

val settings = MongoClientSettings.builder()
    .applyConnectionString(ConnectionString("<connection URI>"))
    .applyToClusterSettings { builder ->
        builder.serverSelector(CustomServerSelector())
    }
    .build()
val mongoClient = MongoClient.create(settings)

Use Settings to Configure Server Selection

You can specify the following server selection settings in your MongoClient object or in your connection URI:

readPreference=primaryPreferred
&maxStalenessSeconds=90
&readPreferenceTags=dc:ny,rack:1

API Documentation

To learn more about the classes and methods used in this guide, see the following API documentation:

• MongoClient

• MongoClientSettings

• ServerSelector

• ReadPreference