A server uses timestamp ordering for local concurrency control. What changes must be made to
adapt it for use with distributed transactions? Under what conditions could it be argued that the
two-phase commit protocol is redundant with timestamp ordering?
What will be an ideal response?
Timestamps for local concurrency control are just local counters. But for distributed transactions, timestamps
at different servers must have an agreed global ordering. For example, they can be generated as (local
timestamp, server-id) to make them different. The local timestamps must be roughly synchronized between
servers.
With timestamp ordering, a transaction may be aborted early at one of the servers by the read or write rule, in
which case the abort result is returned to the client. If a server crashes before the client has done all its actions
at that server, the client will realise that the transaction has failed. In both of these cases the client should the
send an abortTransaction to the coordinator.
When the client request to commit arrives, the servers should all be able to commit, provided they have not
crashed after their last operation in the transaction.
The two-phase commit protocol can be considered redundant under the conditions that (i) servers are assumed
to make their changes persistent before replying to the client after each successful action and (ii) the client does
not attempt to commit transactions that have failed.
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