Feature requestED: monitoring Nginx from the outside
grzegorz.nosek at gmail.com
Thu May 1 22:35:24 MSD 2008
On Thu, May 01, 2008 at 11:39:50AM +0200, Manlio Perillo wrote:
> Each variable will be correct, but since you are writing the value to a
> shared memory location not atomically, the variable as read by the
> client will not be correct.
Why? Reading several variables at once will not be atomic (unless
protected by a lock) but individual reads will, as long as the variables
in shm will fit into a sig_atomic_t each (e.g. 4 bytes on x86).
> It can happen that when the client is reading the shared memory, Nginx
> concurrently writes this memory.
The worst you can expect is that e.g. "requests ok" + "requests failed"
won't be equal to "total requests". A sig_atomic_t is effectively atomic
on most architectures (don't know whether POSIX guarantees it
> Moreover it really does not make sense to write in a shared memory if
> you want to support monitoring tools!
> Just write a module like the stub_status module!
... which has to get its data from somewhere so it's going to face
essentially the same problem.
> Finally there is one last problem.
> Nginx uses only a few shared variables (the one handled the the
> stub_status module).
> All other variables are private to each worker process.
> This means that probably your shared memory location will contain values
> not very useful.
You could always add more information to a shared segment (like
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