Ohad Shacham created OMID-90:
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Summary: Reducing begin/commit latency by distributing the write
to the commit table
Key: OMID-90
URL: https://issues.apache.org/jira/browse/OMID-90
Project: Apache Omid
Issue Type: New Feature
Reporter: Ohad Shacham
Today, Omid's commits are done by the transaction manager. In order to
efficiently write to the commit table, the transaction manager batches these
writes. This optimization, even thought reduces the write time to HBase,
significantly increases the begin and commit latency. The commit latency
increases since a commit operation returns only after its commit timestamp was
persisted in the commit table. And the begin latency increases since begin
returns a transaction id that is also used by the transaction to identify its
snapshot and therefore, begin returns only after all commits with commit id
smaller than the begin id was persisted in the commit table. This is crucial,
since a snapshot change during a transaction run may violate snapshot
isolation.
The idea of this feature is to distribute the commit by moving the write to the
commit table from the server to the client. The transaction manager does
conflict analysis and returns a commit timestamp. While the client atomically
persists this commit in the commit table.
This significantly reduces the begin and commit latency, since batching is not
required anymore. A begin operation can immediately returns and a commit
operation returns after conflict detection.
This can introduce snapshot isolation violation since a slow client can commit
and change other transaction's snapsho. Therefore, we use an invalidation
technique which is similar to the one Omid uses today to maintain snapshot
isolation in high availability mode.
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