[HIVE-14778] document threading model of Streaming API

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.14.0
• Fix Version/s: 2.3.0
• Component/s: HCatalog, Transactions
• Target Version/s: 2.2.0
• Assignee: Eugene Koifman
• Reporter: Eugene Koifman
• Created: 16/Sep/16 18:14
• Updated: 21/Jul/17 18:12
• Resolved: 29/Sep/16 17:44

Description

The model is not obvious and needs to be documented properly.

A StreamingConnection internally maintains 2 MetaStoreClient objects (each has 1 Thrift client for actual RPC). Let's call them "primary" and "heartbeat". Each TransactionBatch created from a given StreamingConnection, gets a reference to both of these MetaStoreClients.
So the model is that there is at most 1 outstanding (not closed) TransactionBatch for any given StreamingConnection and for any given TransactionBatch there can be at most 2 threads accessing it concurrently. 1 thread calling TransactionBatch.heartbeat() (and nothing else) and the other calling all other methods.