Configure Auto-Scaling

Scaling Up a Cluster Tier

If the next highest cluster tier is within your Maximum Cluster Size range, Atlas scales the cluster up to the next tier if one of the following criteria is true for any node in the cluster:

• The Average CPU Utilization has exceeded 75% for the past hour.
• The Memory Utilization has exceeded 75% for the past hour.

Atlas scales a cluster to the next tier when the cluster hasn't been scaled up in the past hour.

Scaling Down a Cluster Tier

If the next lowest cluster tier is within your Minimum Cluster Size range, Atlas scales the cluster down to the next lowest tier if both of the following criteria are true for all nodes in the cluster:

• The average CPU Utilization and Memory Utilization over the past 24 hours is below 50%.
• The cluster hasn't been scaled down (manually or automatically) in the past 24 hours.

To learn more about downward auto-scaling behavior, see Considerations for Downward Auto-Scaling of Cluster Tier and Storage.

Scaling a Sharded Cluster

Atlas auto-scales the cluster tier for sharded clusters using the same criteria as replica sets. Atlas applies the following rules:

• Auto-scaling applies to all shards in the sharded cluster. You can't apply auto-scaling for some shards and not for others within the same cluster.
• If any shard meets the criteria to auto-scale its cluster tier up, all shards will scale their cluster tier up.
• All shards in the cluster must meet the criteria before Atlas will auto-scale the cluster tier down.
• The Config server replica set doesn't auto-scale.

Cluster Tier and Cluster Storage Might Scale in Parallel

When Atlas attempts to automatically scale your cluster storage capacity, it might need to scale your storage outside of the bounds supported that your current cluster tier supports. To help ensure that your cluster doesn't experience any downtime, Atlas will scale your cluster tier (in addition to cluster storage) to accommodate the new storage capacity.

In the event that your specified maximum cluster tier can't support the new storage capacity, Atlas:

1. Raises your maximum cluster tier to the next lowest tier that can accommodate the new storage capacity.
2. Scales your cluster tier to that new maximum tier.

If Atlas attempts to scale your cluster tier down and the target tier can't support your current disk capacity, provisioned IOPS, or both, Atlas doesn't scale your cluster down. In this scenario, Atlas updates your auto-scaling settings based on the relationship between your current cluster tier and the configured maximum cluster tier:

• If the cluster is currently at the configured maximum cluster tier, Atlas disables the cluster from automatically scaling down because all smaller tiers wouldn't be able to accommodate the necessary storage settings. If you want to re-enable downward auto-scaling, you must do so manually from your Cluster Settings.
• If the cluster isn't currently at the configured maximum cluster tier, Atlas raises the minimum cluster tier to the current cluster tier. In this case, Atlas doesn't disable downward auto-scaling.

This auto-scaling logic is designed to reduce the downtime incurred by your storage settings not matching your workload.

When an auto-scaling event modifies your configured minimum or maximum cluster tier, Atlas sends an email to all project owners about the new cluster tier and any modified minimum or maximum tier bounds.

Oplog Considerations

When you increase the storage capacity of a cluster, Atlas increases the cluster's oplog size. Atlas scales the oplog to 5% of the cluster capacity, not to exceed 50 GB. NVMe storage requires an oplog which is 10% of the storage capacity. Atlas doesn't change the oplog size if it exceeds 5% of the new storage capacity (10% in the case of NVMe storage).

As cluster storage capacity decreases, Atlas doesn't change the oplog size unless it exceeds a certain maximum determined according to MongoDB best practices.

Auto-Scaling Enabled by Default

When you create a new cluster, Atlas enables auto-scaling for cluster tier and cluster storage. You don't need to explicitly enable auto-scaling. If you prefer, you can opt out for cluster tier and cluster storage.

With auto-scaling enabled, your cluster can automatically:

• Scale up to increase capability with a higher cluster tier.
• Decrease the current cluster tier to a lower cluster tier.

In the Cluster tier section of the Auto-scale options, you can specify the Maximum Cluster Size and Minimum Cluster Size values that your cluster can automatically scale to. Atlas sets these values as follows:

• The Maximum Cluster Size is set to one tier above your current cluster tier.
• The Minimum Cluster Size is set to the current cluster tier.

Review the Cluster Tier Auto-Scaling Options

To review the enabled auto-scaling options for cluster tier and storage:

1. In the selected Auto-Scale checkbox, review the Maximum Cluster Size and Minimum Cluster Size values, and adjust them if needed.
2. Review the Allow cluster to be scaled down option that is checked by default when you create a new cluster.
3. Review the options under the Storage Scaling checkbox that is checked by default.

Opt Out of Cluster Tier and Storage Auto-Scaling

To opt out of cluster auto-scaling (increasing the cluster tier), when creating a new cluster, navigate to the Cluster Tier menu, and un-check the Cluster Tier Scaling checkbox in the Auto-scale section.

To opt out of cluster auto-scaling (decreasing the cluster tier), when creating a new cluster, navigate to Cluster Tier menu, and un-check the Allow cluster to be scaled down checkbox in the Auto-scale section.

To opt out of cluster storage scaling, un-check the Storage Scaling checkbox in the Auto-scale section.