Improve efficiency of readonly transactions by reusing the same read ts #2604
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…d make readonly queries more efficient by utilizing the same readonly timestamp, instead of having to lease and propagate a new MaxAssigned each time.
…Also, avoid streaming MaxAssigned if nothing new was assigned.
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Jul 19, 2019
…ts (dgraph-io#2604) Currently, every transaction gets a new timestamp from Zero, which then has to propagate the allocated timestamp to every Alpha, so that it knows when to service that transaction. It would apply all the updates up until that ts, so the txn always returns consistent results. If a user is only doing read-only transactions, then this system causes a lot of unnecessary work, because each txn gets a new ts from Zero. This PR changes that by allocating a new read-only timestamp, which can be reused across many read-only transactions, if no RW txns are going on in the system. This speeds up reads, because it avoids the wait for ts propagation from Zero to Alpha leader to followers. Commits: * Logic to retrieve read only timestamps from Dgraph cluster. This would make readonly queries more efficient by utilizing the same readonly timestamp, instead of having to lease and propagate a new MaxAssigned each time. * Working code for ReadOnly Txns. * Add a test to ensure that all read-only txns get the same timestamp. Also, avoid streaming MaxAssigned if nothing new was assigned.
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Currently, every transaction gets a new timestamp from Zero, which then has to propagate the allocated timestamp to every Alpha, so that it knows when to service that transaction. It would apply all the updates up until that ts, so the txn always returns consistent results.
If a user is only doing read-only transactions, then this system causes a lot of unnecessary work, because each txn gets a new ts from Zero. This changes that by allocating a new read-only timestamp, which can be reused across many read-only transactions, if no RW txns are going on in the system. This speeds up reads, because it avoids the wait for ts propagation from Zero to Alpha leader to followers.
This change is