@startuml
'https://plantuml.com/sequence-diagram

autonumber

title KCL Shard Syncing Main Loop (Abridged)

participant PeriodShardSyncManager as PSS
participant ShardSyncTask as SST
participant ShardDetector as SD
participant HierarchicalShardSyncer as HSS
participant LeaseRefresher as LR
participant LeaseSynchronizer as LS
participant "Lease Table\n(DynamoDB)" as DDB

loop every leasesRecoveryAuditorExecutionFrequencyMillis
  opt if worker is not leader
    PSS->PSS: go back to sleep
  end

  PSS->PSS: runShardSync()
  opt if not required to sync shards
    PSS->PSS: go back to sleep
  end

  PSS->SST: call()
  SST->HSS: checkAndCreateLeasesForNewShards(\n  shardDetector, initialPosition, ...)
  opt if lease table is empty
    HSS->HSS: getShardListAtInitialPosition(\n  shardDetector, initialPosition)
    HSS->SD: listShardsWithFilter(initialPositionFilter)
  else lease table is non-empty
    HSS->HSS: getShardList(shardDetector)
    HSS->SD: listShards(...)
    note over SD
      ShardDetector invokes the
      Kinesis Data Streams ListShards API.
    end note
  end
  HSS->HSS: checkAndCreateLeaseForNewShards(\n  shardDetector, leaseRefresher,\n  initialPosition, ...)
  HSS->LR: listLeases()
  LR->DDB: scan(:streamName=streamName)
  DDB->LR: leases from table
  LR->HSS: leases from table
  HSS->LS: determine leases to create
  LS->HSS: leases that are eligible for processing
  loop every lease to create
    HSS->LR: createLeaseIfNotExists(lease)
    LR->DDB: putItem(lease)
  end
end

@enduml