Read Preference

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Overview

Configure the Read Preference at the Deployment Level
Configure the Read Preference at the Database or Collection Level
Configure the Read Concern at the Deployment Level
Configure the Read Concern at the Database or Collection Level
Combine Read Concern, Read Preference, and Write Concern
Additional Information

Overview

To choose which member of a replica set to read from, you can use the Java Reactive Streams driver to configure read preferences. You can additionally control the consistency and isolation properties of the data read from replica sets and sharded clusters by configuring read concerns. In this guide, you can learn how to use the MongoDB Java Reactive Streams Driver to configure read preferences and read concerns.

You can configure the read preference and read concern at the following levels:

For your MongoDB deployment
For your database
For your collection

Configure the Read Preference at the Deployment Level

You can configure the read preference at the deployment level in the following ways:

By creating a MongoClientSettings instance, as shown in the following code:

MongoClient mongoClient = MongoClients.create(MongoClientSettings.builder()
    .applyConnectionString(new ConnectionString("mongodb://host1,host2"))
    .readPreference(ReadPreference.secondary())
    .build());

By creating a ConnectionString instance, as shown in the following code:
```
MongoClient mongoClient =
MongoClients.create("mongodb://host1:27017,host2:27017/?readPreference=secondary");
```

Configure the Read Preference at the Database or Collection Level

You can configure the read preference at the database or collection level in the following ways:

In a MongoDatabase by using the withReadPreference() method, as shown in the following code:
```
MongoDatabase database = mongoClient.getDatabase("test")
    .withReadPreference(ReadPreference.secondary());
```
In a MongoCollection by using the withReadPreference() method, as shown in the following code:
```
MongoCollection<Document> collection = database.getCollection("restaurants")
    .withReadPreference(ReadPreference.secondary());
```

MongoDatabase and MongoCollection instances are immutable. Calling withReadPreference() on an existing MongoDatabase or MongoCollection instance returns a new instance and does not affect the instance on which the method is called.

In the following example, the collectionWithReadPref instance has the read preference of primaryPreferred while the read preference of the collection is unaffected:

MongoCollection<Document> collectionWithReadPref = collection.withReadPreference(ReadPreference.primaryPreferred());

Configure the Read Concern at the Deployment Level

You can configure the read concern at the deployment level in the following ways:

By creating a MongoClientSettings instance, as shown in the following code:
```
MongoClient mongoClient = MongoClients.create(MongoClientSettings.builder()
    .applyConnectionString(new ConnectionString("mongodb://host1,host2"))
    .readConcern(ReadConcern.MAJORITY)
    .build());
```
By creating a ConnectionString instance, as shown in the following code:
```
MongoClient mongoClient =
MongoClients.create("mongodb://host1:27017,host2:27017/?readConcernLevel=majority");
```

Configure the Read Concern at the Database or Collection Level

You can configure the read concern at the database or collection level in the following ways:

In a MongoDatabase by using the withReadConcern() method, as shown in the following code:
```
MongoDatabase database = mongoClient.getDatabase("test")
    .withReadConcern(ReadConcern.MAJORITY);
```
In a MongoCollection by using the withReadConcern() method, as shown in the following code:
```
MongoCollection<Document> collection = database.getCollection("restaurants")
    .withReadConcern(ReadConcern.MAJORITY);
```

MongoDatabase and MongoCollection instances are immutable. Calling withReadConcern() on an existing MongoDatabase or MongoCollection instance returns a new instance and does not affect the instance on which the method is called.

In the following example, the collWithReadConcern instance has an AVAILABLE read concern whereas the read concern of the collection is unaffected:

MongoCollection<Document> collWithReadConcern =
collection.withReadConcern(ReadConcern.AVAILABLE);

Combine Read Concern, Read Preference, and Write Concern

You can build MongoClientSettings, MongoDatabase, or MongoCollection instances to include combinations of read concerns, read preferences, and write concerns.

For example, the following code sets all three at the collection level:

collection = database.getCollection("restaurants")
    .withReadPreference(ReadPreference.primary())
    .withReadConcern(ReadConcern.MAJORITY)
    .withWriteConcern(WriteConcern.MAJORITY);

Additional Information

To learn more about read preference, see the Read Preference guide in the MongoDB Server manual. To learn more about read concern, see the Read Concern guide in the MongoDB Server manual.

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