On Fri, 3 May 2013, David Lang wrote:
On Fri, 3 May 2013, Rainer Gerhards wrote:
tracking state between messages will require a major set of changes to
rsyslog internals (think of of problems like having multiple threads
processing messages and the need for them to start sharing state
information (potentially a LOT of state information)
Let me jump on that one (waited for the discussion). I think global
variables are not that hard, I just did not do them as I questioned if
there is any use case. In essence it just needs to have some separate
namespace.
Do you see use cases where this would make sense?
Setting variables that get used for other messages sounds useful, I can think
of several ways to use this (a simple one would be to tag if messages happen
during business hours or not, triggered by special log messages.
My main concern is around the locking needed to update such global variables
when there are lots of threads (and lots of global variables) around. Since
we are talking about variable length strings there is no way to do atomic
updates, so you will need to have reader and writer locks or some form of rcu
to manage the updates. Just the overhead of grabbing and releasing the read
lock for each message is enough overhead to be worried about, even when no
updates happen.
Now, as always, I may be missing some trick that eliminates this worry, if so
I'd love to hear about it.
A secondary concern I have is the misunderstandings that will arise around the
use of global variables. Given that various things can cause logs to be
re-ordered (network issues, queuing issues, thread scheduling, relay delays,
etc), expectations that you can see one log before another and rely on global
variables being set for the first one before the second is processed will
usually, but not always work.
David Lang
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