Hi Glenn,
On Tue, 13 Jul 2010, Glenn Lagasse wrote:
* Alok Aggarwal ([email protected]) wrote:
Karen raised an important point during her
review of the DC design spec.
The spec proposes that the manifest-parser be run
as a checkpoint mainly to provide the ability to
pause at that step (and obviously resume from it).
manifest-parser is highly likely to be the very
first checkpoint that gets run.
This presents us with two problems -
a) A chicken-and-egg problem. manifest-parser can't
be executed until the manifest has been parsed.
The manifest can't be parsed until the manifest-parser
has been executed.
b) If DC is resumed from one of the checkpoints, say,
"ba-init", manifest parser still needs to get
executed prior to resuming from "ba-init". If manifest
parser is executed as a checkpoint and it is one of
the checkpoints that is listed prior to "ba-init",
it won't even get executed.
These problems could concievably solved by having
manifest-parser not be a checkpoint at all. It can't
be resumed from anyway so it would not be a huge deal;
the manifest data is represented in the volatile tree
that isn't snapshotted. We do however lose the observability
that comes with being able to stop at the manifest parsing
step.
What do people think about this?
Can we teach the execution engine to allow us to stop before running the
first checkpoint (which isn't the manifest-parser) which would allow us
to observe what manifest-parser has done before we move on to the real
work?
We may be able to work around this by making
manifest-parser an internal-to-DC checkpoint
that always gets executed but also gets appended
to the 'distro_const -l' output.
Maybe it isn't as bad as I think it is ..
Alok
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