On 07/14/10 08:00 AM, Sarah Jelinek wrote:
Hi Alok,
On 07/13/10 05:04 PM, Alok Aggarwal 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.
We currently always re-read the manifest when we pause and resume
today with DC, right?
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.
I am not sure this is true. It's one thing to have the MP as a
declared checkpoint in the manifest it is another to have DC register
this checkpoint always. Which is the way I think it should be. We
always re-read the manifest today with DC(AFAIK).
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.
It can be a checkpoint, just not one exposed to the user. It has to be
run first. There is no other way around that.
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?
I think we leave it as a checkpoint and just not expose it as one to
the user via the manifest.
I agree with all that has been said... I see only one potential nit:
if an error message says that the Manifest Parser checkpoint failed (and
the MP will fail if the manifest doesn't validate) but there is no MP
checkpoint listed in the manifest, will that confuse the user? Error
messages should account for this.
Thanks,
Jack
thanks,
sarah
Alok
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