Delaying a job by less than a second

Learn an easy way to poll a file.

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While regularly polling a data queue or an MQ queue is easy, it is not always practical to use such a queue. Sometimes, a file must be used. There are ways to wait for the latest record to be added, for instance, using end of file delay (OVRDBF with EOFDLY), but this is equivalent to having a delayed job should no record be found, and try to read it again. It is also possible to couple a data queue to a file and send a message to the...

data queue every time a record is added to the file. This in turn will "wake up" the batch job and make it read the file.

The easiest way to poll a file would be to reposition to the start and read a record every time the end of file is met. But this is not a good solution as jobs continually polling a file in this way will take far too much CPU and slow the system down. Delay must be introduced. The simplest way is to add a delay job (DLYJOB) every time an end of file condition is met. But DLYJOB is not perfect. The minimum time you can delay a job with it is one second. You can delay a job by only a number of seconds, not a fraction of a second.

One second is fine in most cases, but sometimes, you can't afford to wait for one second and you can't afford not to wait. This is where a C function comes in handy. "pthread_delay_np" delays a thread for a number of nanoseconds! Well, not quite. The IBM manual mentions that it's more useful when you need to delay your thread by thousands of a second rather than nanoseconds, but still, it is a godsend. With it, you may decide to poll your file every tenth of a second. In most cases, this should not be too taxing for your system.

This API is written in C and therefore expects parameters in a specific format. Reproducing the timespec format in RPG ILE, I obtained:

 

D  timeSpec       ds                                 
D   seconds                     10i 0                
D   nanoseconds                 10i 0 

I declared the API as follows:

D  delay          pr             5i 0 extProc('pthread_delay_np')    
d                                 *   value

The API expects a pointer to the timespec definition. It also returns a non-zero value if a problem occurred (which is unlikely if you are passing a valid timespec). Nevertheless, I added monitoring just in case to avoid the system doing no delay whatsoever.

  

c                   eval      seconds = 0                
c                   eval      nanoseconds = 10000000        
C                   eval      return = delay(%addr(timeSpec))   
c                   if        return <> 0                       
c                   callP (e) system('DLYJOB 1')                
c                   endIf                        
 
  

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This was first published in March 2005

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