Thomas, thanks for your input... But I'm not quite getting the answer I need....
> But what precisely is the algorithm and timing involved with streaming > WALs? > > > > Is it: > > * client issues COMMIT > > * master receives commit > > * master processes transaction internally > > * master creates WAL record > > | master appends WAL to local WAL file, flushes disk > > | master sends WAL record to all streaming clients > > * master sends OK to client > > * master applies WAL > > > > So is this correct? Is it correct to say: PG async guarantees that the > WAL > > is *sent* to the receivers, but not that they are received, before the > > client receives acknowledgement? > > Async replication doesn't guarantee anything at all about receivers, > or even that there is one right at this moment. Did you mean to write > "synchronous" instead of "asynchronous"? I'm only concerned with async (for this thread). In asynchronous replication, > the primary writes to the WAL and flushes the disk. Then, for any > standbys that happen to be connected, a WAL sender process trundles > along behind feeding new WAL doesn the socket as soon as it can, but > it can be running arbitrarily far behind or not running at all (the > network could be down or saturated, the standby could be temporarily > down or up but not reading the stream fast enough, etc etc). This is the *process* I want more detail about. The question is the same as above: > (is it true that) PG async guarantees that the WAL > is *sent* to the receivers, but not that they are received, before the > client receives acknowledgement? But I will refine what I mean by "sent"... does PostgreSQL write the WAL to the socket and flush the socket before acknowledging the transaction to the client? Does it *always* do this? Or does it make a best effort? Or does the write to the socket and return to client happen asynchronously? I realize that the data might not be *seen* at the client, i realize network buffers may take time to reach the network, I realize various levels of synchronous replication provide higher guarantees. But For the purposes of this topic, I'm interest to know what PG actually does. I can't tell that from the documentation (because it is not clearly stated and because it is self contradictory). -- Otheus otheus.u...@gmail.com otheus.shell...@uibk.ac.at
