In JSR-352, a checkpoint algorithm is an algorithm which allows to customize the checkpoint policy for a chunk step. To achieve this, we can either specify the number of items to process per chunk when using the item checkpoint policy, or we can provide an implementation of interface javax.batch.api.chunk.CheckpointAlgorithm to benefit more customized checkpoint policy.

Current, we use the second approach: we provide an implementation of interface, called org.hibernate.search.jsr352.massindexing.impl.steps.lucene.CheckpointAlgorithm. However, this implementation is unnecessary, because our checkpoint policy is simple, the
checkpoint interval can be defined via attribute `item-count`, which accepts a Job Specification Language (JSL) expression, see Spec 1.0 - §8.2.1. Syntax:

So we can ask user to provide the value of checkpoint interval, and then fill it into the value of the example above using JSL. If user does not provide a value explicitly, then the default value will be applied.