How to handle failed COMMIT

[Håvar Nøvik <havar@novik.email>]

I've been wondering what the behavior of postgres is when the server process stops, for whatever reason, at certain critical points in the execution flow.

In the following example the client will only regard the data as stored until the COMMIT command is successfully executed. But the the server, client or network may fail at any point during the execution and therefore the server and client may not be in sync of what the current state is.

BEGIN;
INSERT INTO ....;
COMMIT;

To experiment with this I inserted a stupid if statement (see patch) which will make the server process exit(1) if the client sends a COMMIT command, but only after the COMMIT command has been processed on the server and just before the server send the close commend (wire protocol). I.e. the server has COMMITed the transaction, but the client just experiences that the connection has been closed for some reason:

server closed the connection unexpectedly
	This probably means the server terminated abnormally
	before or while processing the request.
The connection to the server was lost. Attempting reset: Succeeded.

So my question is, how should the client handle these situations? I think most systems will have some variation of the following:

try
   execute transactional sql
catch (commit failed)
   // regard data as not stored

But, this doesn't seem to be enough, so I guess you would have to do something like:

try
   execute transactional sql
catch (commit failed)
   if (data is not stored)
     // regard data as not stored

Thanks in advance.

/Håvar Nøvik

Attachment: 0001-exit-before-close-on-commit-command.patch
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