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From "Mike Rhodes" <couc...@dx13.co.uk>
Subject Re: Design doc index switching
Date Wed, 22 May 2019 16:03:00 GMT
>From all the messages in this thread, it sounds like there are two basic forms of change
to indexes here:

- Backwards compatible change (e.g., adding a field to the emitted value where the consumer
is agnostic to new fields).
- Backwards incompatible changes.

I think the automatic replace works well in the first case, but badly in the second. Because
of the fact that any backwards incompatible change requires an application deployment by definition,
and so having auto-flip of the output of the index happening would be a road to either over
complicated application code to handle both cases or just the application being down for a
time.

TBH if I were doing the second case, I'd be building entirely new indexes and just switching
my code to use them, then dropping the old indexes. That is, creating a new ddoc, letting
that index build, moving my application to that new index. The new ddoc is because of view
groups, of course.

I wonder whether this question is simpler if viewgroups are removed as a concept, such that
if I change an index's definition, only that index is rebuilt as opposed to other indexes
that I put in the same viewgroup. Certainly, the manual/auto-background-rebuild thing seems
to be mostly motivated by people making changes to one index (or adding a new index) in a
view group and not wanting all the other indexes to be out of action during the new index's
build process.

I'm mostly wondering whether this is a workaround to a more fundamental design decision?

-- 
Mike.

On Fri, 17 May 2019, at 14:26, Robert Samuel Newson wrote:
> If the consuming code can’t handle the switchover, the user should not 
> be asking couchdb to build an incompatible index in the first place, 
> but certainly shouldn’t ask couchdb to replace an index automatically 
> (i.e, they should not use our proposed new handling at all).
> 
> On replication, the doc would replicate as normal, causing the new 
> index to build wherever it lands. Each couchdb instance would do the 
> swap over independently. To address conflicts, we could block edits to 
> any design doc with a _replaces item if the view is still building. 
> Like for replication docs, and similarly we’d only allow a delete, 
> which would delete the new index also.
> 
> B.
> 
> > On 17 May 2019, at 10:59, Jan Lehnardt <jan@apache.org> wrote:
> > 
> > 
> > 
> >> On 16. May 2019, at 23:03, Robert Samuel Newson <rnewson@apache.org> wrote:
> >> 
> >> I suggest an alternative; the new design document could include the _id of design
document it’s replacing (“_replaces”:”_design/foo”). On completion of the view build
of the new design document, CouchDB itself updates the named _id to the same content as the
new design document (strictly, only the parts needed to make the view sig match) (perhaps
it also deletes the new document).
> >> 
> >> The advantage to this is that queries to the original design document continue
to work throughout and at no point is there a discrepancy between the design documents contents
(the map and reduce functions, etc) and the results you get from it.
> > 
> > 
> > As Stefan points out, this only works if the consuming code knows how to handle
both index result formats. If not, we need a way for either CouchDB to signal the build being
ready or a way to confirm the swap to coincide with a code deploy.
> > 
> > 
> >> 
> >> B.
> >> 
> >>> On 16 May 2019, at 14:55, Jan Lehnardt <jan@apache.org> wrote:
> >>> 
> >>> +1 on solving this for all users, and same caveats as Stefan raises :)
> >>> 
> >>>> On 16. May 2019, at 09:38, Stefan du Fresne <stefan@medicmobile.org>
wrote:
> >>>> 
> >>>> Hey Garren,
> >>>> 
> >>>> Having this a native part of CouchDB seems like a really cool idea:
we have automated the manual dance you're talking about with our deployment tooling, but it
would be really nice not to have to!
> >>>> 
> >>>> I'm not clear how it would work though, at least in terms of coherent
deployments. View changes are, like SQL migrations, an often non-backwards compatible change
that has to occur as your new code deploys.
> >>>> 
> >>>> Currently the naive approach is you deploy your new code alongside design
doc changes, which then block view queries on first request until they're ready to go.
> >>>> 
> >>>> The better approach is what you describe, which is what we do now, where
we extract our design documents out of our deployment bundle and place them in a "staging"
location to allow them to warm, then rename them and do the actual code deployment once that's
complete (managed by an external deployment service we built). This importantly lets us split
the "warming" bit from the deployment bit: we only deploy new code once the design documents
that are shipped with that code is ready to go.
> >>>> 
> >>>> How would you foresee this kind of flow happening here? Would there
be a way to query the design doc to know if it had flipped to the new version yet? Would you
be able to control when this flip occurs? Or would the expectation be that your code handles
both versions gracefully?
> >>>> 
> >>>> As an example to mull over, let's say you have design doc v1, which
has view a. You push design doc v2, which has added view b, but has also changed view a in
some backwards incompatible way. While v2 is still building and is not yet the active doc:
> >>>> - If you queried view a you'd get the v1 version, that's clear
> >>>> - If you queried view b you'd get... a 404? Some other custom code?
> >>>> - If you GET the design document what doc would you see? Presumably
v2?
> >>>> - Could you query something to determine which version is currently
active? Or perhaps just whether there is a background version building at all?
> >>>> 
> >>>> Cheers,
> >>>> Stefan
> >>>> 
> >>>>> On 16 May 2019, at 07:51, Garren Smith <garren@apache.org>
wrote:
> >>>>> 
> >>>>> Hi Everyone,
> >>>>> 
> >>>>> A common pattern we see for updating large indexes that can take
a few days
> >>>>> to build, is create a new design docs with the new updated views.
Then once
> >>>>> the new design doc is built, a user changes the new design doc’s
id to the
> >>>>> old design doc. That way the CouchDB url for the views remain the
same and
> >>>>> any requests to the design doc url automatically get the latest
views only
> >>>>> once they built.
> >>>>> 
> >>>>> This is an effective way of managing building large indexes, but
the
> >>>>> process is quite complicated and often users get it wrong. I would
like to
> >>>>> propose that we move this process into CouchDB and let CouchDB handle
the
> >>>>> actual process. From a users perspective, they would add a field
to the
> >>>>> options of a design document that lets CouchDB know, that this build
needs
> >>>>> to be built in the background and only replace the current index
once its
> >>>>> built:
> >>>>> 
> >>>>> ```
> >>>>> {
> >>>>> "_id": "_design/design-doc-id",
> >>>>> "_rev": "2-8d361a23b4cb8e213f0868ea3d2742c2",
> >>>>> "views": {
> >>>>> "map-view": {
> >>>>>  "map": "function (doc) {\n  emit(doc._id, 1);\n}"
> >>>>> }
> >>>>> },
> >>>>> "language": "javascript",
> >>>>> "options": {
> >>>>>    "build_and_replace": true
> >>>>> }
> >>>>> }
> >>>>> ```
> >>>>> 
> >>>>> I think this is something we could build quite effectively once
we have
> >>>>> CouchDB running on top of FoundationDB. I don’t want to implement
it for
> >>>>> version 1 of CouchDB on FDB, but it would be nice to keep this in
mind as
> >>>>> we build out the map/reduce indexes.
> >>>>> 
> >>>>> What do you think? Any issues we might have by doing this internally?
> >>>>> 
> >>>>> Cheers
> >>>>> Garren
> >>>> 
> >>> 
> >>> -- 
> >>> Professional Support for Apache CouchDB:
> >>> https://neighbourhood.ie/couchdb-support/
> >>> 
> >> 
> > 
> > -- 
> > Professional Support for Apache CouchDB:
> > https://neighbourhood.ie/couchdb-support/
> 
>

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