Class claims compared to resource claims

There are two types of claim you can choose from when working with services on Tanzu Application Platform (commonly known as TAP). These are ClassClaim and ResourceClaim. This Services Toolkit topic explains the similarities and differences between the two and when using one is preferable over the other.

It is usually advisable to work with a ClassClaim where possible as they are easier to create and are more portable across multiple clusters. They are also used as the trigger mechanism for dynamic provisioning of service instances.

Similarities

• Both APIs express that you want to access to a service instance.
• Both APIs adhere to the ProvisionedService duck type. They both have the field .status.binding.name in their API. This means that you can target them using a ServiceBinding and, therefore, you can feed them into Cartographer’s Workload API.
• Both APIs ensure that mutual exclusivity of claims on service instances. After using either a ClassClaim or a ResourceClaim to claim a service instance, no other ClassClaim or a ResourceClaim can claim that same service instance.

Using a ResourceClaim

A ResourceClaim targets a specific resource in the Kubernetes cluster. To target that resource, the ResourceClaim needs the name, namespace, kind, and API version of the resource.

The specificity of the ResourceClaim means it is most useful when you must guarantee which service instance the application workload uses. For example, if the application must connect to the exact same database instance while it advances through development, test, and production environments. If you do not need this guarantee VMware recommends that you use the ClassClaim API instead.

Using a ClassClaim

A ClassClaim targets a ClusterInstanceClass in the Kubernetes cluster. To target this class, the ClassClaim only requires the name of the ClusterInstanceClass.

The ClusterInstanceClass can represent any set of service instances and therefore each time you create a new ClassClaim, you can claim any of the service instances represented by that ClusterInstanceClass. After a ClassClaim has claimed a service instance, it never looks for another. This is true even if the ClassClaim’s spec is updated, or the ClusterInstanceClass is updated. Therefore, the ClassClaim is performing a point-in-time lookup at its creation, using the ClusterInstanceClass for that lookup.

The loose coupling between the ClassClaim and the service instances means that a ClassClaim is best in situations where:

• You must inject different service instances into the application workload at different points in its advancement from development to production environments. For more information, see Abstracting Service Implementations Behind A Class Across Clusters.
• The ClassClaim, and also any workload referencing it, must be promoted from one environment to the next without changing their specification.

The ClassClaim is the only type of claim that you can use to dynamically provision service instances.