The HStoreCoordinator is how each HStoreSite communicates with other HStoreSites in the database cluster. It uses Google’s Protocol Buffers with H-Store’s asynchronous ProtoRPC System event loop for sending and recieving messages. The HStoreCoordinator’s RPC interface and message data structures are defined in the hstore.proto file. All of the operations are non-blocking.

The main operations of the HStoreCoordinator are:

1. TransactionInit:
   Send a request to all partitions to notify them that a transaction needs to access them as part of a distributed transaction. The HStoreCoordinator will not release the transaction back to the HStoreSite until all of the partitions acknowledge that they are blocked for that transaction.
2. TransactionWork:
   The transaction is requesting that a partition on another HStoreSite execute a PlanFragment of a query. This is only invoked for remote partitions (i.e., not managed by the same HStoreSite as the transaction’s base partition).
3. TransactionPrepare:
   Perform the PREPARE phase of the two-phase commit protocol for a distributed transaction. This signals to a partition that a distributed transaction is finishd with that partition and will not be returning to it to execute further queries. Note that this step is skipped for aborted transactions.
4. TransactionFinish:
   The final FINISH phase of the two-phase commit protocol for distributed transactions. This is always invoked for each distributed regardless of whether it completed successfully or aborted.