Forking SocketServer daemon -- updating state

[Nick Craig-Wood]

Reid Priedhorsky <reid at umn.edu> wrote:
>  I am implementing a forking SocketServer daemon that maintains significant
>  internal state (a graph that takes ~30s to build by fetching from a SQL
>  database, and eventually further state that may take up to an hour to
>  build).
> 
>  I would like to be able to notify the daemon that it needs to update its
>  state. Because it forks for each new request, a request handler can't
>  update the state because then only the child would have the new state.
> 
>  One idea I had was to use signals. Is it safe to write a signal handler
>  that does extensive work (several seconds)? Seems like even so, it might
>  be tricky to do this without race conditions.

In general no it isn't safe.

However due to the way python handles its interrupts (it sets a flag
in its own signal handler which the bytecode interpreter examines and
acts upon when it is safe to do so) you won't corrupt python if you do
this.

I'd still recommend the set a flag and do it in your mainloop approach
though if you can.

>  Another possibility is that the signal handler simply sets a
>  needs_update flag, which I could check for in a handle_request()
>  loop. The disadvantage here is that the update wouldn't happen
>  until after the next request is handled, and I would like the state
>  to be available for that next request.  A solution might be to send
>  a signal followed by a dummy request, which seems a bit awkward.

Can't you get SocketServer to timeout and return you control
regularly?

-- 
Nick Craig-Wood <nick at craig-wood.com> -- http://www.craig-wood.com/nick