Specify How CRUD Operations Run on Replica Sets

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Overview

Write Concern
Example: Set the Write Concern for a Single Write Operation
Example: Retrieve and Apply an Existing Write Concern
Read Concern
Example: Set the Read Concern Level of an Aggregation
Example: Change the Read Concern of a Database
Read Preference
Example: Set Read Preference and Concerns for a Transaction
Example: Set the Read Preference of a Cluster in the Connection String
API Documentation

Overview

In this guide, you can learn how to use the write concern, read concern, and read preference configurations to modify the way that MongoDB runs create, read, update, and delete (CRUD) operations on replica sets.

You can set write concern, read concern, and read preference options at the following levels:

Client, which sets the default for all operation executions unless overridden
Session
Transaction
Database
Collection

This list also indicates the increasing order of precedence of the option settings. For example, if you set a read concern level for a transaction, it will override a read concern level set for the client.

These options allow you to customize the causal consistency and availability of the data in your replica sets.

Write Concern

The write concern specifies the level of acknowledgement requested from MongoDB for write operations, such as an insert or update, before the operation successfully returns. Operations that do not specify an explicit write concern inherit the global default write concern settings.

For more information, see Write Concern in the Server manual. For detailed API documentation, see the WriteConcern API documentation.

The following table describes the WriteConcern parameters:

Parameter	Type	Description
`w` (optional)	W	Requests acknowledgment that the write operation has propagated to a specified number of `mongod` instances or to `mongod` instances that are labelled specified tags
`wtimeoutMS` (optional)	number	Specifies a time limit to prevent write operations from blocking indefinitely
`journal` (optional)	boolean	Requests acknowledgment that the write operation has been written to the on-disk journal

Example: Set the Write Concern for a Single Write Operation

This code uses custom WriteConcern settings while creating new a document:

myDB.myCollection.insertOne(
   { name: "anotherDocumentName" },
   { writeConcern:
     { w: 2, wtimeoutMS: 5000 }
   }
);

Example: Retrieve and Apply an Existing Write Concern

This code uses the fromOptions() method to construct a WriteConcern from the options of an existing database reference, myDB. Note that myDB could be replaced with a reference to any entity that accepts a write concern option. Then the new write concern is applied to a document, myDoc.

const newWriteConcern = WriteConcern.fromOptions(myDB);
const myDoc = { name: "New Document" };
WriteConcern.apply(myDoc,newWriteConcern);

Read Concern

The read concern specifies the following behaviors:

Level of causal consistency across replica sets
Isolation guarantees maintained during a query

You can specify the read concern setting by using the level parameter. The default read concern level is local. This means that the client returns the data from the replica set member that the client is connected to, with no guarantee that the data has been written to all replica set members. Note that lower read concern level requirements may reduce latency.

For more information about read concerns or read concern levels, see Read Concern in the Server manual. For more detail on the ReadConcern type and definitions of the read concern levels, see the ReadConcern in the API documentation.

Example: Set the Read Concern Level of an Aggregation

This code sets the read concern level of an an aggregation to "majority":

const pipeline = [
   {"$match": {
     category: "KITCHENWARE",
   }},
   {"$unset": [
     "_id",
     "category",
   ]}
 ];
result = await myDB.collection("mycollection")
   .aggregate(
     pipeline,
     { readConcern:
       { level: "available" }
     }
   );

For more information about aggregates, see the Aggregation page.

Example: Change the Read Concern of a Database

This code changes the read concern level of a database to "local":

const options = { readConcern: { level: "local" } };
const myDB = client.db("mydb", options);

Read Preference

The read preference determines which member of a replica set MongoDB reads when running a query. You can also customize how the server evaluates members.

For more detailed API documentation, see the ReadPreference API documentation.

The following table describes the ReadPreference parameters:

Parameter	Type	Description
`mode`	ReadPreferenceMode	Specifies a requirement or preference for which replica set member the server reads from. The default mode, `primary`, specifies that operations read from the primary member of the replica set.
`tags` (optional)	TagSet List	Assigns tags to secondary replica set members to customize how the server evaluates them. Tags cannot be used with the `primary` read preference mode setting.
`options` (optional)	ReadPreferenceOptions	Sets various options, including hedge and maxStalenessSeconds that can be applied to your read preference.

Example: Set Read Preference and Concerns for a Transaction

This code sets the read preference, read concern, and write concern for the operations in a transaction:

const transactionOptions = {
   readPreference: "primary",
   readConcern: { level: "local" },
   writeConcern: { w: "majority" },
};
const session = client.startSession();
session.startTransaction(transactionOptions);
// ...
await session.commitTransaction();
await session.endSession();

For more information about transactions, see Transactions.

Example: Set the Read Preference of a Cluster in the Connection String

This code example creates a MongoClient that uses the "secondary" read preference mode when performing queries on a cluster:

const uri = "mongodb+srv://<user>:<password>@<cluster-url>?readPreference=secondary&maxStalenessSeconds=120";
const client = new MongoClient(uri);

This example also sets the maxStalenessSeconds option. For more information about connection string options, see the Connection String Options section in the manual.

API Documentation

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

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