Modify Execution of CRUD Operations

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Overview

Write Concern
Read Concern
Read Preference
Additional Information

Overview

In this guide, you can learn how to modify the way that the MongoDB Go Driver executes create, read, update, and delete (CRUD) operations using write concern, read concern, and read preference configurations for replica sets.

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

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

In the following sections, you can learn how to customize the consistency and availability of the data in your replica sets.

Write Concern

A write concern describes the number of data-bearing members in a replica set that must acknowledge a write operation, such as an insert or update, before the operation is returned as successful. By default, the write concern requires only the primary replica set member to acknowledge the write operation before the operation is deemed successful.

Options

The MongoDB Go Driver provides the writeconcern package, which lets you specify the write concern for a replica set. Set the write concern using the SetWriteConcern() method with a WriteConcern type. The WriteConcern type has the following methods to select common write concern specifications:

Method	Description
`Custom()`	The client requests acknowledgement that write operations propagate to tagged members of a `mongod` instance. For more information, see the Write Concern specification. Parameter: `string`
`Journaled()`	The client requests acknowledgement that write operations are written to the on-disk journal. For more information, see the Write Concern specification. Parameter: none
`Majority()`	The client requests acknowledgement that write operations propagate to the majority of data-bearing voting members. For more information, see the Write Concern specification. Parameter: none
`Unacknowledged()`	The client requests requests no acknowledgment of write operations. For more information, see the Write Concern specification for w: 0. Parameter: none
`W1()`	The client requests acknowledgement that write operations have been written to memory on one node, such as the standalone mongod or the primary in a replica set. For more information, see the Write Concern specification for w: 1. Parameter: none

If you require a more specialized write concern, you can define a custom WriteConcern struct literal. You can set the following fields in a WriteConcern struct:

Field	Description
`W`	Specifies the number of `mongod` instances or tagged members that write operations must propagate to for acknowledgement. Common values include `1`, `0`, and `"majority"`. Type: `string` or `int`
`Journal`	Specifies if write operations must be written to the on-disk journal for acknowledgement. Type: `bool`
`WTimeout`	Specifies a time limit for the write concern. This setting is applicable only for `W` values greater than 1. Specifying this setting and specifying a Timeout on the client at the same time results in undefined behavior. To learn more, see the Write Concern specification for wtimeout. Type: `time.Duration`

Tip

You can alternatively specify a write concern in your connection string. See the Server manual entry on Write Concern Options for more information.

Example

The following code shows how you can specify different write concerns at the client and collection level. The client-level write concern requests acknowledgement from two replica set members and sets journaling to false. The collection-level write concern requests acknowledgement from the majority of replica set members.

uri := "mongodb://<hostname>:<port>"
journal := false
cliWC := &writeconcern.WriteConcern{
    W: 2,
    Journal: &journal,
}
clOpts := options.Client().ApplyURI(uri).SetWriteConcern(cliWC)
client, err := mongo.Connect(context.TODO(), clOpts)
...
collWC := writeconcern.Majority()
collOpts := options.Collection().SetWriteConcern(collWC)
coll := client.Database("db").Collection("myColl", collOpts)

Read Concern

The read concern option allows you to determine which data the client returns from a query. The default read concern level is "local", meaning that the client returns the instance’s most recent data, with no guarantee that the data has been written to a majority of the replica set members.

Options

The MongoDB Go Driver provides the readconcern package, which lets you specify the read concern for a replica set. Set the read concern using the SetReadConcern() method with a ReadConcern type. The ReadConcern type has the following methods to specify the read concern:

Method	Description
`Available()`	The query returns data from the instance with no guarantee that the data has been written to a majority of the replica set members. For more information, see the Read Concern specification.
`Linearizable()`	The query returns data that reflects all successful writes issued with a write concern of `majority` and acknowledged prior to the start of the read operation. For more information, see the Read Concern specification.
`Local()`	The query returns the instance’s most recent data. For more information, see the Read Concern specification.
`Majority()`	The query returns the instance’s most recent data acknowledged as having been written to a majority of members in the replica set. For more information, see the Read Concern specification.
`Snapshot()`	The query returns a complete copy of the data in a `mongod` instance at a specific point in time. Only available for operations within multi-document transactions. For more information, see the Read Concern specification.

Example

The following code shows how you can specify a read concern of "majority". The code then selects a Collection with this option.

rc := readconcern.Majority()
opts := options.Collection().SetReadConcern(rc)
database := client.Database("db")
coll := database.Collection("myCollection", opts)

Read Preference

The read preference option specifies how the MongoDB client routes read operations to the members of a replica set. By default, an application directs its read operations to the primary member in a replica set.

Read preference consists of the read preference mode and, optionally, a tag set list, the maxStalenessSeconds option, and the hedged read option.

Options

The MongoDB Go Driver provides the readpref package, which lets you specify the read preference for a replica set. Set the read preference using the SetReadPreference() method with a ReadPref type. The ReadPref type has the following methods to specify the read preference:

Method	Description
`Nearest()`	The client reads from a random eligible replica set member, primary or secondary, based on a specified latency threshold. For more information, see the Read Preference Server manual entry.
`Primary()`	The client reads from the current replica set primary node. For more information, see the Read Preference Server manual entry.
`PrimaryPreferred()`	The client reads from the primary node in most situations. If the primary is unavailable, operations read from secondary members. For more information, see the Read Preference Server manual entry.
`Secondary()`	The client reads from the secondary members of the replica set. For more information, see the Read Preference Server manual entry.
`SecondaryPreferred()`	The client reads from the secondary nodes in most situations. If the secondaries are unavailable, operations read from the primary member. For more information, see the Read Preference Server manual entry.

Tip

You can alternatively specify a read preference in your connection string. See the Server manual entry on Read Preference Options for more information.

Example

The following code shows how you can specify a read preference to read from secondary nodes. The code then selects a Database with this option.

rp := readpref.Secondary()
opts := options.Database().SetReadPreference(rp)
database := client.Database("db", opts)

Additional Information

For more information about the concepts in this guide, see the following Server documentation:

API Documentation

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