What is Thread Safety in timer jobs on SharePoint?

Question

Hi All,
What is Thread Safety in timer jobs on SharePoint?
Thanks in advance!

Answer 1

hi
thread safety in timer jobs means the same as in other code: it should be possible to run multiple instances of the same job simultaneously. If job uses some shared resource, access to this resource should be synchronized. But the tricky moment is that it
is not enough to just use standard .Net thread synchronization mechanisms here (e.g. lock), because in most cases Sharepoint runs on the server farm and the same job instance may be executed on different servers while standard synchronization mechanisms work
within single process memory space (it is possible to guarantee that jobs are running on the same server by assigning preferrable server for timer jobs in Central administration > Content databases > content database, but often it is left to Sharepoint
to decide on what server jobs are executed). In case of timer jobs you need to store some flag that job is started in some shared storage, e.g. in SPWebApplication.Properties:
lock(obj)
try
if ((bool)web.AllProperties["jobstarted"])
return;
web.SetProperty("jobstarted", true);
web.Update();
finally
web.AllProperties.Remove("jobstarted");
web.Update();
It is just idea and there is still minor possibility that 2 jobs instances may run at the same time (if 2nd job instances set jobstarted flar after 1st jobs checked it in AllProperties, but before 1st jobs set it), but it will solve most of the problems in
reality. In order to make the code more secure you may use timestamps instead of flag.
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