write to the same file from multiple processes at the same time?

[Jp Calderone]

On 27 May 2005 06:43:04 -0700, Paul Rubin <"http://phr.cx"@nospam.invalid> wrote:
>Jp Calderone <exarkun at divmod.com> writes:
>> >But they haven't.  They depend on messy things like server processes
>> >constantly running, which goes against the idea of a cgi that only
>> >runs when someone calls it.
>>
>> SQLite is an in-process dbm.
>
>http://www.sqlite.org/faq.html#q7
>
>    (7) Can multiple applications or multiple instances of the same
>    application access a single database file at the same time?
>
>    Multiple processes can have the same database open at the same
>    time. Multiple processes can be doing a SELECT at the same
>    time. But only one process can be making changes to the database
>    at once.
>
>But multiple processes changing the database simultaneously is
>precisely what the OP wants to do.

Er, no.  The OP precisely wants exactly one process to be able to write at a time.  If he was happy with multiple processes writing simultaneously, he wouldn't need any locking mechanism at all >:)

If you keep reading that FAQ entry, you discover that SQLite implements its own locking mechanism internally, allowing different processes to *interleave* writes to the database, and preventing any data corruption which might arise from simultaneous writes.

That said, I think an RDBM is a ridiculously complex solution to this simple problem.  A filesystem lock, preferably using the directory or symlink trick (but flock() is fun too, if you're into that sort of thing), is clearly the solution to go with here.

Jp