Re: Sync Rep: First Thoughts on Code

> If it's guaranteed to be visible on the standby after it's committed on
> the master, and you don't have any way to make it actually simultaneous,
> then that implies that it's visible on the slave for some brief period
> of time before it's committed on the master.
>
> That situation is still asymmetric, so why is that a better use of the
> term "synchronous"?

Because that happens anyway. If I request a commit on a single,
unreplicated server, the server makes the commit visible to new
transactions and then sends me a message informing me that the commit
has completed. Since the message takes some finite time to reach me,
there is a window of time after the commit has completed and before I
know that the commit has been completed.

Suppose for the sake of argument that the single, unreplicated server
did these two tasks in the opposite order - namely, first, it sent a
message to the process requesting the commit stating that the commit
had completed, and only then made the transaction visible. This would
create a race condition: the process requesting the commit might
receive the commit and begin a new transaction before the previous
transaction had been made visible, and would therefore not be able to
see the results of its own previous actions. I think it's fair to say
that this behavior would be judged totally intolerable.

Therefore, there can't possibly be any applications out there which
are depending on the fact that commits don't become visible until they
are acknowledged, but there very well could be some applications which
depend on the fact that one commits are acknowledged, they are
visible. If replication is synchronous in this sense, then I can open
a connection to the master, write some data, close the connection,
open a new connection to the master or the slave (not caring which),
and read back the data that I just wrote (assuming no one else has
modified it in the mean time). If it isn't, then I can't. Some
people will not care about this, but some will.

The point here is that synchronous replication, at least to some
people, is going to imply that the user-visible states of the two
copies are consistent. To other people, it is going to imply that
committed transactions will never be lost even in the event of a
catastropic loss of the primary 1 picosecond after the commit is
acknowledged. We need to choose some word that implies that we are
guaranteeing the latter of these two things but not the former.
Otherwise, we will have confused users, and terminological confusion
when and if we ever implement the former as well.

...Robert