What does each of the service do? documentation

[Jabir Ahmed]

            org.apache.oozie.service.SchedulerService,
            org.apache.oozie.service.InstrumentationService,
            org.apache.oozie.service.CallableQueueService,
            org.apache.oozie.service.UUIDService,
            org.apache.oozie.service.ELService,
            org.apache.oozie.service.AuthorizationService,
            org.apache.oozie.service.HadoopAccessorService,
            org.apache.oozie.service.MemoryLocksService,
            org.apache.oozie.service.DagXLogInfoService,
            org.apache.oozie.service.SchemaService,
            org.apache.oozie.service.LiteWorkflowAppService,
            org.apache.oozie.service.JPAService,
            org.apache.oozie.service.StoreService,
            org.apache.oozie.service.CoordinatorStoreService,
            org.apache.oozie.service.SLAStoreService,
            org.apache.oozie.service.DBLiteWorkflowStoreService,
            org.apache.oozie.service.CallbackService,
            org.apache.oozie.service.ActionService,
            org.apache.oozie.service.ActionCheckerService,
            org.apache.oozie.service.RecoveryService,
            org.apache.oozie.service.PurgeService,
            org.apache.oozie.service.CoordinatorEngineService,
            org.apache.oozie.service.BundleEngineService,
            org.apache.oozie.service.DagEngineService,
            org.apache.oozie.service.CoordMaterializeTriggerService,
            org.apache.oozie.service.StatusTransitService,
            org.apache.oozie.service.PauseTransitService,
            org.apache.oozie.service.GroupsService,
            org.apache.oozie.service.ProxyUserService

Where do i get documentation on what each service does,

also when i only add PurgeService it doesnt work unless i add the
rest? whats the dependency?

Thanks

Jabir