Atlas Replica Set Tags¶

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Replica Set Tag Descriptions

Use Cases and Examples
Built-In Custom Write Concerns

Note

Feature unavailable in Free and Shared-Tier Clusters

This feature is not available for M0 free clusters, M2, and M5 clusters. To learn more about which features are unavailable, see Atlas M0 (Free Cluster), M2, and M5 Limitations.

Atlas clusters are configured with pre-defined replica set tags for different member types in the cluster. You can utilize these pre-defined replica set tags to direct queries from specific applications to desired node types and regions. These tags sets allow you to customize read preferences for a replica set, thus improving cluster performance and reliability.

To use replica set tags in your connection string and direct queries to desired nodes, set the tag in your the readPreferenceTags connection string option. For examples, see Use Cases and Examples.

Replica Set Tag Descriptions¶

The following table describes the pre-defined replica set tags Atlas implements.

Tag Name	Description	Example
Provider	Cloud provider on which the node is provisioned. Possible values are: `AWS` `GCP` `AZURE`	`{"provider" : "AWS"}`
Region	Cloud region in which the node resides. For a complete list of possible `region` values for each cloud provider, refer to the cloud provider's reference page: AWS GCP Azure	`{"region" : "US_EAST_2"}`
Node Type	Node type. Possible values are: `ELECTABLE` `READ_ONLY` `ANALYTICS` For more information, see Node Types.	`{"nodeType" : "ANALYTICS"}`
Workload Type	Tag to distribute your workload evenly among your non-analytics (electable or read-only) nodes. Possible values are: `OPERATIONAL`	`{"workloadType" : "OPERATIONAL"}`

Node Types¶

The following table describes the possible nodeType values in your replica set tags.

Node Type	Description
`ELECTABLE`	Read from nodes eligible to be elected primary. `ELECTABLE` nodes correspond to Electable nodes for high availability in the cluster creation UI.
`READ_ONLY`	Read from read-only nodes. `READ_ONLY` nodes correspond to Read-only nodes for optimal local reads in the cluster creation UI.
`ANALYTICS`	Read from read-only analytics nodes. `ANALYTICS` nodes correspond to Analytics nodes for workload isolation in the cluster creation UI.

To learn how to configure electable, read-only, and analytics nodes for your cluster, see High Availability and Workload Isolation Options.

Tip

Use Cases and Examples¶

Consider the following scenarios where utilizing pre-defined replica set tags would be beneficial, and see the corresponding sample connection strings.

Note

Each of the following example connection strings employ the readConcernLevel=local connection string option. Specifying a read concern of local ensures that secondary reads on sharded clusters do not return orphaned documents. You do not need to specify this option when connecting to non-sharded replica sets.

Use Analytics Nodes to Isolate Workloads¶

If an application performs complex or long-running operations, such as ETL or reporting, you may want to isolate the application's queries from the rest of your operational workload by connecting exclusively to analytics nodes.

Consider the following connection string:

mongodb+srv://<USERNAME>:<PASSWORD>@foo-q8x1v.mycluster.com/test?readPreference=secondary&readPreferenceTags=nodeType:ANALYTICS&readConcernLevel=local

The connection string options appear in the following order:

readPreference=secondary
readPreferenceTags=nodeType:ANALYTICS
readConcernLevel=local

The readPreference option of secondary and readPreferenceTag option of { nodeType : ANALYTICS } limit the application connections to analytic nodes.

Isolate Normal Application Secondary Reads from Analytics Nodes¶

You may want to isolate regular application reads from the workload on analytics nodes.

Consider the following connection string:

mongodb+srv://<USERNAME>:<PASSWORD>@foo-q8x1v.mycluster.com/test?readPreference=secondary&readPreferenceTags=workloadType:OPERATIONAL&readConcernLevel=local

The connection string options appear in the following order:

readPreference=secondary
readPreferenceTags=workloadType:OPERATIONAL
readConcernLevel=local

The specified options prevent your application from reading from analytics nodes. The application must read from operational, or non-analytics, nodes.

Target Local Reads for Geographically-Distributed Applications¶

You can utilize Atlas replica set tags to target local reads to specific regions for globally distributed applications. Prior to the introduction of these tags, local reads for globally distributed applications relied on correctly calculating the nearest read preference. With Atlas replica set tags, specifying appropriate geographic tags in combination with a read preference mode of nearest provides more consistent behavior.

The following connection string prioritizes connections to the AWS US_EAST_1 region, followed by the US_EAST_2 region:

mongodb+srv://<USERNAME>:<PASSWORD>@foo-q8x1v.mycluster.com/test?readPreference=nearest&readPreferenceTags=provider:AWS,region:US_EAST_1&readPreferenceTags=provider:AWS,region:US_EAST_2&readPreferenceTags=&readConcernLevel=local

The connection string options appear in the following order:

readPreference=nearest
readPreferenceTags=provider:AWS,region:US_EAST_1
readPreferenceTags=provider:AWS,region:US_EAST_2
readPreferenceTags=
readConcernLevel=local

Atlas considers each read preference tag in the order you specify them. After Atlas matches a node to a tag, it finds all eligible nodes that match that tag. Atlas then ignores any following readPreferenceTags.

In this example, the application first tries to connect to a node in AWS region US_EAST_1. If there no nodes in that region are available, the application tries to connect to a node in AWS region US_EAST_2.

The final (empty) readPreferenceTags= provides a fallback option. With an empty readPreferenceTags= option, the application can connect to any available node regardless of provider or region.

These options help ensure that the application connects to the closest geographic region for reduced latency and improved performance.

Tip

Built-In Custom Write Concerns¶

Atlas provides built-in custom write concerns for multi-region clusters. Write concern describes the level of acknowledgment requested from MongoDB for write operations to a cluster.

Atlas's built-in custom write concerns can help improve data consistency by ensuring your operations are propagated to a set number of regions to succeed.

To use a custom write concern, specify the write concern in the write concern document of your operation.

Atlas provides the following custom write concerns for multi-region clusters:

Write Concern	Tags	Description
`twoRegions`	`{ region: 2 }`	Write operations must be acknowledged by at least two regions in your cluster.
`threeRegions`	`{ region: 3 }`	Write operations must be acknowledged by at least three regions in your cluster.
`twoProviders`	`{ provider: 2 }`	Write operations must be acknowledged by at least two regions in your cluster with distinct cloud providers.

Example

Consider a multi-region cluster across three regions: us-east-1, us-east-2, and us-west-1. You want to have write operations propagate to all three regions in your cluster before Atlas accepts them.

The following operation inserts a document and requires that the operation be propagated to all three regions due to the { w: "threeRegions" } write concern object:

db.employees.insertOne(
  { name: "Bob Smith", company: "MongoDB" },
  { writeConcern: { w: "threeRegions" } }
)

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