Manifest Refactor

Martin von Zweigbergk martinvonz at google.com
Wed Jul 13 02:52:14 EDT 2016


On Tue, Jul 12, 2016 at 7:08 PM, Gregory Szorc <gregory.szorc at gmail.com> wrote:
> On Tue, Jul 12, 2016 at 3:03 PM, Durham Goode <durham at fb.com> wrote:
>>
>> We'll be looking at moving to tree manifests as our source of truth over
>> the next few months, and one problem area is the fact that the manifest
>> class is not well factored for this usecase. This one class is the
>> collection of all manifests, the accessor for information about individual
>> manifests, and the storage format (revlog).
>>
>> Before I do a bunch of work, I wanted to run my proposal for breaking up
>> the manifest by you guys:
>>
>> 1. Add a "manifestlog" class that represents the collection of all
>> root-level manifests (i.e. what commits point to; not any sub-trees)  It's
>> basically what repo.manifest would return, and mainly consist of "get" and
>> "add" apis that return and accept manifest instances.  It would be
>> responsible for caching recently used manifests, and potentially serving up
>> the right kind of manifest when demanded (ex: during our transition from
>> flat manifests to tree manifests, we may want to allow loading both, and
>> this class would multiplex them). It would have no, or very little,
>> knowledge about revlogs/storage.
>>
>> 2. Make the "manifest" class represent a single instance of a manifest (it
>> would point at other instances of "manifest" for sub-trees).  From a
>> consumers point of view, when they do 'repo.manifest.get(node)' they will
>> receive a manifest instance and they should be able to not care how it's
>> implemented.  It would expose apis like 'children', 'walk',
>> 'get(fileordirname)', 'parents', 'linkrev', etc.
>>
>> The specific implementation of the manifest instance could use whatever
>> storage scheme it wants.  For example, in the normal vanilla manifest, it
>> would look much like manifestdict does today, with no knowledge of revlogs
>> (you just pass text to the constructor). In a tree world, each instance in
>> the tree could have knowledge of its own backing revlog, and be able to
>> construct new instances as someone recurses down.
>>
>> 3. Add a "manifestrevlog" class that inherits from revlog. This is the
>> actual ondisk storage.  Ideally "manifest" instances would just call simple
>> read and write apis (and not depend on revlog implementation details), so we
>> could in theory replace the revlog storage with something else (packed
>> revlogs, lookaside to memcache, whatever) without having to rewrite the
>> actual manifest business logic.
>>
>>
>> Breaking the manifest into these three parts (collection, instance,
>> storage) should make it easier to mix and match manifest implementations and
>> storage schemes, without rewriting lots of logic.
>>
>> For thing that do take heavy dependencies on it being a revlog (like
>> push/pull/changegroup), they will be able to reach around the abstractions
>> and talk directly to the revlog when necessary. And future storage
>> implementations will either have to do the same or find a common API that
>> can allow changegroups to be created/received for both storages.
>>
>>
>> Thoughts? Concerns? Is renaming the collection class (which is the primary
>> interface for how the rest of mercurial interacts with the manifest) from
>> manifest to manifestlog a bad idea?  I could rename the instance concept to
>> manifestctx or something instead.
>
>
> I still owe a detailed reply but first...

So do I. I'll try to find time tomorrow.

>
> Does this change anything with regards to the availability of narrow clone
> in the official repo/distribution? Or is the plan to still rely on Google's
> out-of-repo narrowhg extension?

At the sprint, we talked about moving narrowhg into the hg repo. I
still hope (for someone) to move the non-ellipsis parts
(ellipsis=faked squashed revisions) of narrowhg into core somewhat
soon, either before or after moving narrowhg into the hg repo. I just
haven't had time to work on that lately. I know it would still mean a
clone of > 500MB for the main Firefox repo (full changelog and
manifest log), but I'm more comfortable with enabling the non-ellipsis
stuff separately from the ellipsis stuff as a first step. I'm sure
we'll have plenty of bugs to work on with just the non-ellipsis pieces
enabled. Specifically narrow, trees and remotefilelog has needed work
to get along. It will be nice to have narrow in core so it doesn't
have to know about remotefilelog and it becomes clear in which
extension to monkey patch a function (i.e. in remotefilelog, if narrow
is in core).

> Also, how does this work relate to
> Facebook's "fastmanifest" and "cfastmanifest" extensions? (I've seen lots of
> activity on these extensions and am curious what the purpose and plans for
> them are. There seems to be some cool experiments going on!)
>
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