Allan Wind wrote:
How do you backup a metakit database?
The cold case is obvious as usual, ensure that no one else has the database file open prior to making a copy of the data with a file level tools (cp, tar etc).
What are the options for hot (i.e. open with an active writer) backups? I noticed the information in the python api reference for doing this from the writer thread/process, but are there any options for doing it externally? If the file is open commit-extend, can you use the same trick if you open the datbase read-only in a 2nd process? If using commit-aside, is it then safe to just low-level copy the "main" database?
The race is during a commit (from when it starts to until it completes), because that is when MK writes to file. You will need to stay out of that time span if you wish to have a solid backup. It seems to me that it could be done on a not-too-active DB simply as follows:
- determine clock time T
- wait until at least one sec has passed since T
(actually: the time resolution of the underlying filesystem)
- copy entire datafile
- check mod date of orig, must still be <= T
- rinse and repeat if this test failed
On Windows, I am not sure this will work: if the O/S does not update modtimes right away then the above will not be reliable.
The other way to do it is with support from the committing app so independent readers have a way of telling whether there was a commit, say by incrementing a revision number of a separate info file.
Is journaling planned? I.e. point in time recoverability between backups.
There is a first cut at this via the commit-aside mode. There have also been simple-but-working tricks in the Tcl wrapper to intercept all calls (to do remoting, as well as creating transcripts of all requests for debugging/replay).
With custom viewers, one could write a view layer which intercepts all changes at the C++ level, but that requires more work and discipline during use. I've been hesitant to implement such thing only because I am trying to improve the raw core of MK before building more on top. They are definitely good ideas and *very* worthwhile, IMO.
-jcw
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