[vwnc] Subsystem activation ordering

We just ran into a very interesting problem in our system related to Subsystem activation ordering.  When starting our system, we use -filein to file in a script that loads all of our parcels and launches our system.  One of the parcels we load is CairoGraphics, which has its own Subsystem class.  That Subsystem class will, when activated, flush all of the known handles to Cairo objects.  This is a good thing.

However, given the way we're starting our system, the Cairo system doesn't activate until well after our UI is up and running (and displaying various objects that were rendered with Cairo).  We have been caching some ImageSurfaces, for example (which we may or may not need to do), and these blow up because their handles get flushed out from under them.

I'm not sure if this is really a bug in VW itself, but I wanted to raise the issue in case other people run into similar problems.  Even splitting our startup into two phases, a -pcl to load the code and a -filein to launch the system, didn't resolve the issue (probably because both -pcl and -filein are processed by ImageConfigurationSystem).  Instead, we introduced our own Subsystem that does the same thing as -filein, but that pre-reqs CairoSystem.  So, the -pcl to load the code loads our Subsystem, and then it (and Cairo) gets activated before processing its own command-line args.  This solves our problem.

However, the more general question about Subsystem activation timing and ordering remains.

Thoughts?

Randy
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Randy Coulman
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When you're loading code that contains a subsystem, that subsystem will activate after loading. Which, if you've got a subsystem that does those sorts of things, is probably inappropriate.

The thing that occurs to me off the top of my head is to put a check in the subsystem activation not to do it if we're past, say, early system installation, by checking if some subsystem that you'd normally expect to be after you has run. Finalization is probably a reasonable choice.

At 01:14 PM 4/15/2008, Randy Coulman wrote:

We just ran into a very interesting problem in our system related to Subsystem activation ordering.  When starting our system, we use -filein to file in a script that loads all of our parcels and launches our system.  One of the parcels we load is CairoGraphics, which has its own Subsystem class.  That Subsystem class will, when activated, flush all of the known handles to Cairo objects.  This is a good thing.

However, given the way we're starting our system, the Cairo system doesn't activate until well after our UI is up and running (and displaying various objects that were rendered with Cairo).  We have been caching some ImageSurfaces, for example (which we may or may not need to do), and these blow up because their handles get flushed out from under them.

I'm not sure if this is really a bug in VW itself, but I wanted to raise the issue in case other people run into similar problems.  Even splitting our startup into two phases, a -pcl to load the code and a -filein to launch the system, didn't resolve the issue (probably because both -pcl and -filein are processed by ImageConfigurationSystem).  Instead, we introduced our own Subsystem that does the same thing as -filein, but that pre-reqs CairoSystem.  So, the -pcl to load the code loads our Subsystem, and then it (and Cairo) gets activated before processing its own command-line args.  This solves our problem.

However, the more general question about Subsystem activation timing and ordering remains.

Thoughts?

Randy
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Alan Knight [|], Engineering Manager, Cincom Smalltalk
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I wanted to follow one thread you raised (see below)...

On Wed, Apr 16, 2008 at 3:15 PM, Alan Knight <[hidden email]> wrote:

When you're loading code that contains a subsystem, that subsystem will activate after loading. Which, if you've got a subsystem that does those sorts of things, is probably inappropriate.

You say here that the subsystem will "activate after loading".  How soon after loading?  Immediately?

What I was seeing with Cairo here is that the CairoSystem wasn't being activated until after the -filein was completely finished.  Even when I split the load off into a -pcl followed by the -filein to do the rest of the work, CairoSystem wasn't being activated until after the -filein.

It seems to me that the activation of newly-loaded Subsystems isn't actually happening until the currently activating Subsystem (ImageConfigurationSystem in this case) is finished.  I can't tell if this is correct behavior or not.