I was trying to understand (and then perhaps mimic) how pg_standby does a fast failover.
My current understanding is that when a secondary db is in standby mode, it will exhaust all the archive log to be replayed from the primary and then start streaming. It is at this point that xlog.c checks for the existence of a trigger file to promote the secondary. This was been a cause of some irritation for some of our customers who do not really care about catching up all the way. I want to achieve the exact semantics of pg_standby's fast failover option. I manipulated the restore command to return 'failure' when the word "fast" is present in the trigger file (see below), hoping that when I want a secondary database to come out fast, I can just echo the word "fast" into the trigger file thereby simulating pg_standby's fast failover behavior. However, that did not work. Techically, I did not truncate the trigger file like how pg_standby. <New restore_command> = ! fgrep -qsi fast <trigger_file> && <Old restore_command> And that is where I have a question. I noticed that in pg_standby.c when we detect the word "fast" in the trigger file we truncate the file. https://github.com/postgres/postgres/blob/REL9_1_11/contrib/pg_standby/pg_standby.c#L456 There is also a comment above it about not "upsetting" the server. https://github.com/postgres/postgres/blob/REL9_1_11/contrib/pg_standby/pg_standby.c#L454 What is the purpose of truncating the file? To do a smart failover once you come out of standby? But, when I look at xlog.c, when we come out of standby due to a failure returned by restore_command, we call CheckForStandbyTrigger() here: https://github.com/postgres/postgres/blob/REL9_1_11/src/backend/access/transam/xlog.c#L10441 Now, CheckForStandbyTrigger() unlinks the trigger file. I noticed through the debugger that the unlinking happens before xlog.c makes a call to the next restore_command. So, what is the reason for truncating the "fast" word from the trigger file if the file is going to be deleted soon after it is discovered? How will we "upset" the server if we don't? Assuming this question is answered and I get a better understanding, I have a follow up question. If truncation is indeed necessary, can I simulate the truncation by manipulating restore_command and achieve the same effect as a fast failover in pg_standby? Thanks in advance for the help. Neil
