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AWS CloudHSM
User Guide

AWS CloudHSM Clusters

AWS CloudHSM provides hardware security modules (HSMs) in a cluster. A cluster is a collection of individual HSMs that AWS CloudHSM keeps in sync. You can think of a cluster as one logical HSM. When you perform a task or operation on one HSM in a cluster, the other HSMs in that cluster are automatically kept up to date.

You can create a cluster that has from 1 to 28 HSMs (the default limit is 6 HSMs per AWS account per AWS Region). You can place the HSMs in different Availability Zones in an AWS Region. Adding more HSMs to a cluster provides higher performance. Spreading clusters across Availability Zones provides redundancy and high availability.

Making individual HSMs work together in a synchronized, redundant, highly available cluster can be difficult, but AWS CloudHSM does some of the undifferentiated heavy lifting for you. You can add and remove HSMs in a cluster and let AWS CloudHSM keep the HSMs connected and in sync for you.

To create a cluster, see Getting Started.

For more information about clusters, see the following topics.

Cluster Architecture

When you create a cluster, you specify an Amazon Virtual Private Cloud (VPC) in your AWS account and one or more subnets in that VPC. We recommend that you create one subnet in each Availability Zone (AZ) in your chosen AWS Region. To learn how, see Create a Private Subnet.

Each time you create an HSM, you specify the cluster and Availability Zone for the HSM. By putting the HSMs in different Availability Zones, you achieve redundancy and high availability in case one Availability Zone is unavailable.

When you create an HSM, AWS CloudHSM puts an elastic network interface (ENI) in the specified subnet in your AWS account. The elastic network interface is the interface for interacting with the HSM. The HSM resides in a separate VPC in an AWS account that is owned by AWS CloudHSM. The HSM and its corresponding network interface are in the same Availability Zone.

To interact with the HSMs in a cluster, you need the AWS CloudHSM client software. Typically you install the client on Amazon EC2 instances, known as client instances, that reside in the same VPC as the HSM ENIs, as shown in the following figure. That's not technically required though; you can install the client on any compatible computer, as long as it can connect to the HSM ENIs. The client communicates with the individual HSMs in your cluster through their ENIs.

The following figure represents an AWS CloudHSM cluster with three HSMs, each in a different Availability Zone in the VPC.


        Architecture of an AWS CloudHSM cluster with three HSMs.

Cluster Synchronization

In an AWS CloudHSM cluster, AWS CloudHSM keeps the keys on the individual HSMs in sync. You don't need to do anything to synchronize the keys on your HSMs. To keep the users and policies on each HSM in sync, update the AWS CloudHSM client configuration file before you manage HSM users. For more information, see Keep HSM Users In Sync.

When you add a new HSM to a cluster, AWS CloudHSM makes a backup of all keys, users, and policies on an existing HSM. It then restores that backup onto the new HSM. This keeps the two HSMs in sync.

If the HSMs in a cluster fall out of synchronization, AWS CloudHSM automatically resynchronizes them. To enable this, AWS CloudHSM uses the credentials of the appliance user. This user exists on all HSMs provided by AWS CloudHSM and has limited permissions. It can get a hash of objects on the HSM and can extract and insert masked (encrypted) objects. AWS cannot view or modify your users or keys and cannot perform any cryptographic operations using those keys.

Cluster High Availability and Load Balancing

When you create an AWS CloudHSM cluster with more than one HSM, you automatically get load balancing. Load balancing means that the AWS CloudHSM client distributes cryptographic operations across all HSMs in the cluster based on each HSM's capacity for additional processing.

When you create the HSMs in different AWS Availability Zones, you automatically get high availability. High availability means that you get higher reliability because no individual HSM is a single point of failure. We recommend that you have a minimum of two HSMs in each cluster, with each HSM in different Availability Zones within an AWS Region.

For example, the following figure shows an Oracle database application that is distributed to two different Availability Zones. The database instances store their master keys in a cluster that includes an HSM in each Availability Zone. AWS CloudHSM automatically synchronizes the keys to both HSMs so that they are immediately accessible and redundant.


        An application and AWS CloudHSM cluster distributed to two Availability Zones for high
          availability.