Hacker News Reader

Comment by niccl | Hacker News Reader

yes, but postmaster is still running to roll back the transaction. If you crash a single multi-threaded process, you may lose postmaster as well and then sadness would ensue

View on HN

The threaded design wouldn't necessarily be single-process, it would just not have 1 process for every connection. Things like crash detection could still be handled in a separate process. The reason to use threading in most cases is to reduce communication and switching overhead, but for low-traffic backends like a crash handler the overhead of it being a process is quite limited - when it gets triggered context switching overhead is the least of your problems.

Seconded. For instance, Firefox' crash reporter has always been a separate process, even at the time Firefox was mostly single-process, single-threaded. Last time I checked, this was still the case.

If you read the thread you’d see the discussion includes still having e.g. postmaster as a separate process.

PostgreSQL can recover from abruptly aborted transactions (think "pulled the power cord") by replaying the journal. This is not going to change anyway.

Transaction roll back is a part of the WAL. Databases write to the disk an intent to change things, what should be changed, and a "commit" of the change when finished so that all changes happen as a unit. If the DB process is interrupted during that log write then all changes associated with that transaction are rolled back.

Threaded vs process won't affect that.

Running the whole DBMS as a bunch of threads in single process changes how fast is the recovery from some kind of temporary inconsistency. In the ideal world, this should not happen, but in reality it does and you do not want to bring the whole thing down because of some superficial data corruption.

On the other hand, all cases of fixable corrupted data in PostgreSQL I have seen were result of somebody doing something totally dumb (rsyncing live cluster, even between architectures), while on InnoDB it seems to happen somewhat randomly without any obvious reason of somebody doing stupid things.

We would still have a separate process doing that part of postmaster's work.

You can still have a master control process separate from the client connections.

Restart on crash doesn't sound that difficult to do.

Reply on news.ycombinator.com