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From Andy Wenk <andyw...@apache.org>
Subject Re: [DISCUSS] Rewriting the CouchDB HTTP Layer
Date Wed, 20 Aug 2014 12:00:34 GMT
Hey Russel,

I have read your blog post about "Rewriting the CouchDB HTTP Layer". Thanks
a lot for that!

As a note from a non-core-CouchDB-dev - this sounds great and very
reasonable. Making the code easier to test, removing unnecessary code
duplication, organising the code even better and making it easier to write
plugins are things, that will lead to better code and will make it easier
for devs to contribute. So all thumbs up! Great work! I hope the discussion
will lead to a good decision :)

Cheers

Andy


On 19 August 2014 02:27, Russell Branca <russell@chewbranca.com> wrote:

> On Aug 17, 2014 8:15 PM, "Jason Smith" <jason.h.smith@gmail.com> wrote:
> >
> > Hi, Russell. This is okay for a starting point but it is a bit vague.
> Could
> > you perhaps flesh out the plan and make it more comprehensive?
> >
> > ^^ That is a joke!
> >
> > Seriously, thank you very much for this analysis and plan. This is very
> > exciting! (Not least because the http codebase is the part I know best
> and
> > I can get excited about.)
> >
>
> Thanks!
>
> > One quick question that I don't see from your writeup: What version of
> > CouchDB are you thinking of targeting? 2.0? 2.1? 3.0? Is this completely
> an
> > internal change, or does it affect users?
> >
>
> I think this is outside the scope of 2.0 given how close we are on that.
> There's a fair bit of legwork involved in doing the rewrite, so I wouldn't
> want to block 2.0. So I think the question is 2.x or 3.0. If we go the web
> machine route we should rework the api and definitely introduce backwards
> incompatible changes, so we would want to do a 3.0 release there. If we
> used cowboy we could mimic the current api and release 2.x.
>
> > For me, I am not so interested in an internal rewrite with zero advantage
> > (besides "it's cleaner"), however I am am very interested to use the
> > rewrite for a better opportunity to explore plugin opportunities or other
> > extensibility features.
> >
> >
>
> This rewrite needs to happen. The internals of the http layer need some
> serious love and there's a lot of duplication to remove. I think the big
> win here is that this would get the ball rolling on taking a closer look at
> the various internal applications and figuring out what needs to be
> restructured. For instance, the next logical step after reworking the http
> later is to standardize the clustered and local api modules, and there was
> some great discussion in the Dev channel today about that.
>
> The more we can decouple the various apps, the more easily we can extend
> CouchDB with plugins and new functionality.
>
> -Russell
>
> >
> >
> > On Mon, Aug 18, 2014 at 1:41 AM, Russell Branca <chewbranca@apache.org>
> > wrote:
> >
> > > # Rewriting the CouchDB HTTP Layer
> > >
> > > With the light at the end of tunnel on the BigCouch merge, I thought
> > > it was time to get the conversation going on cleaning up the current
> > > HTTP stack duality. We've got a good opportunity to do some major
> > > cleanup, remove duplication, and really start more clearly separating
> > > the various components of CouchDB.
> > >
> > >
> > > ## Primary objectives
> > >
> > >     * Consolidate down to one HTTP layer
> > >     * Isolate HTTP functionality
> > >     * Separate HTTP server from HTTP resources
> > >     * Easy plugin integration
> > >     * Build clustered/local API
> > >
> > >
> > > ### Consolidate down to one HTTP layer
> > >
> > > We currently have two HTTP layers, `couch_httpd` and `chttpd`. This
> > > was a useful construct when BigCouch was a separate application where
> > > isolating the clustered layer from the local layer was necessary, and
> > > quite useful.
> > >
> > > This is no longer the case, and we can significantly reduce code
> > > duplication by consolidating down to one http layer. There are a
> > > number of places in the two apps where the code is nearly identical,
> > > except one calls out to `fabric` and the other calls out for
> > > `couch_*`. For instance, compare `couch_httpd_db:couch_doc_open/4` [1]
> > > with `chttpd_db:couch_doc_open/4` [2]. These are completely identical
> > > aside from whether it goes through the clustered layer, `fabric`, or
> > > through the local layer `couch_db`.
> > >
> > > There are plenty of other places with similar duplication. This is
> > > obviously ripe with opportunity to refactor and introduce some higher
> > > level abstractions to make the HTTP layer function independently of the
> > > document/database level APIs.
> > >
> > >
> > > ### Isolate HTTP functionality
> > >
> > > I don't think `couch_doc_open/4` has any business existing in
> > > the HTTP layer, we should move all non HTTP logic out. IMO the HTTP
> > > layer should only concern itself with:
> > >
> > >     1. Receiving the HTTP requests
> > >     2. Extracting out the request data into a standard data structure
> > >     3. Dispatch requests to the appropriate internal APIs
> > >     4. Forward the response
> > >
> > > Anything that doesn't fit into those four steps should be ripped out
> > > and moved elsewhere. For instance, the primary logic for determining
> the
> > > database redundancy and shard values is done in `chttpd_db` [3]. I
> > > would greatly prefer to see this logic in a database API.
> > >
> > > The more we can isolate HTTP logic from database logic the
> > > better. Once they are fully decoupled, then the HTTP layer is merely
> > > one particular client interface on top of the core database. We also
> > > get all the benefits of isolation for testing and what not.
> > >
> > > Along these lines, I think we greatly overuse the #http{} record for
> > > passing around request data, and instead you extract the body, and
> > > then combine all of the user supplied headers and query string params
> > > into a standard options list. This we can we completely separate
> > > making database requests from the representation of the client
> > > request.
> > >
> > >
> > > ### Separate HTTP server from HTTP resources.
> > >
> > > I think everything I've said so far is pretty clear cut in terms of
> > > it's _the_ logical thing to do, but separating the HTTP server from
> > > the HTTP endpoints is less clearly defined. However, we do have
> > > precedence for this and there are a number of solid benefits.
> > >
> > > First, let me explain what I mean here. There are two pieces to an
> > > HTTP stack, first there's the core HTTP engine that handles receiving
> > > and responding to requests and other things along those lines, and
> > > second there's the places where you supply your business logic and
> > > figure what content to send to the user.
> > >
> > > CouchDB has a handful of places using this aproach, where instead of
> > > defining all the logic in the HTTP stack directly, we have auxilary
> > > modules defined within the appropriate applications that specify how
> > > any HTTP requests for that application are handled. A good clean
> > > example of this approach is `couch_mrview_http` [4].
> > >
> > >
> > > ### Easy plugin integration
> > >
> > > One big advantage of the above separation of HTTP resources is that it
> > > provides a standard way of plugins hooking in new HTTP endpoints. The
> > > more we can treat the "core" CouchDB applications as plugins, the more
> > > easily it is to isolate and replace various parts of the stack.
> > >
> > >
> > > ### Build clustered/local API
> > >
> > > The above example of `couch_doc_open/4` is a clear cut case where
> > > we want to abstract the process of loading a document. Not all places
> > > are as easily abstractable, but this is a great example of why I think
> > > we should have a standard API on top of clustered and local layers,
> > > where deciding which to use is based on a local/clustered flag, or
> > > some other heuristic.
> > >
> > > I've been toying around with the idea of making a request object of
> > > some sort, is something like `couch_req:make(ReqBody, ReqOptions)`
> > > that you can then pass to `couch_doc_api` or some such, but I don't
> > > have any strong opinions on this.
> > >
> > >
> > > ## Where I've gotten so far: chttpd2, a proof of concept
> > >
> > > I've hacked out an experimental WebMachine [5] based rewrite of the
> > > HTTP stack called `chttpd2` [6]. This PoC follows the same ideas I've
> > > outlined above, so I'll run back through the previous outlined items
> > > and explain how `chttpd2` handles it.
> > >
> > >
> > > ### Consolidate down to one HTTP layer
> > >
> > > Right now I'm not doing anything special here, I still think building
> > > an API layer that handles deciding whether to make a clustered or
> > > local request is the proper approach, so I've not included any logic
> > > in the HTTP stack for doing so.
> > >
> > >
> > > ### Isolate HTTP functionality
> > >
> > > I've got a solid separation of functionality in `chttpd2`. If you
> > > notice the current codebase in [6], there is zero logic for actually
> > > handling any particular CouchDB requests. Rather those are self
> > > contained within the appropriate sub applications. I've started this
> > > for `couchdb-couch` [7] and `couchdb-config` [8]. Here's a simple
> > > example of the new welcome resource [9].
> > >
> > > As you can see, there is zero database logic in the welcome request
> > > module. In fact, I started moving all the random logic in the current
> > > HTTP layer to a temporary module I'm calling `couch_api` [10]. As you
> > > can see from that module, it removes all the logic that was previously
> > > nested in `couch_httpd_misc_handlers` [11]. More complicated examples
> > > for creating a database and viewing database info are in [12], and an
> > > all dbs example is in [13]. Also I've done similar things for
> > > `couchdb-couch` as mentioned above in [8].
> > >
> > >
> > > ### Easy plugin integration
> > >
> > > As I mentioned above, by making it easy to plugin in new HTTP
> > > endpoints, we also make it easier for plugins to do the same. On that
> > > front I've made it so each application can optionally declare a
> > > `couch_dispatch` function describing what endpoints it can handle, and
> > > then `chttpd2` will go and find all of those to figure out how to
> > > dispatch requests [14]. And for example, here's how the
> > > `couchdb-couch` endpoints are declared [15].
> > >
> > >
> > > ### Build clustered/local API
> > >
> > > I have not started on this front, and have only built these endpoints
> > > for interacting with the clustered layer for simplicity as this is
> > > just a proof of concept I hacked together. However, as I mentioned
> > > above I've started moving all the logic out of the HTTP layer into
> > > more appropriate places. I've made similar changes to `couch-config`
> > > by moving all of the logic from [16] into the `couch-config`
> > > application itself.
> > >
> > >
> > > ### Why WebMachine?
> > >
> > > I find WebMachine [5] to be one of the more interesting HTTP stacks for
> > > building webapps. In particular I like how they have a specific flow
> > > chart [17] and coordinate point corresponds to a particular definition
> > > of the `webmachine_decision_core:decision/1` function.
> > >
> > > That said I think Cowboy [19] has more momentum and might be a better
> > > long term project to tie ourselves too.
> > >
> > > Also, if we decide to go the WebMachine route, we'll need to
> > > restructure a fair bit of the current HTTP layer, making a number of
> > > breaking changes. I'm a strong -1 for coercing WebMachine into the
> > > current haphazard CouchDB API. WebMachine is very opinionated on how
> > > you structure your API (for good reason!) and I think going against
> > > that is a mistake.
> > >
> > > So if we wanted to just do a drop in replacement of the current
> > > CouchDB API, then Cowboy is the way to go. Although one of these days
> > > we should clean up the HTTP API.
> > >
> > >
> > > # Conclusion
> > >
> > > I hope this can start a good discussion on a game plan for the HTTP
> > > layer. Like I said, this is a proof of concept that I hacked out, so
> > > I'm not attached to the code or the use of WebMachine, but I do think
> > > it's a good representation of the ideas outlined above.
> > >
> > > Looking forward to hearing your thoughts and comments!
> > >
> > >
> > >
> > > #### Footnotes
> > >
> > > [1]
> > >
>
> https://github.com/apache/couchdb-couch/blob/master/src/couch_httpd_db.erl#L805-L823
> > >
> > > [2]
> > >
>
> https://github.com/apache/couchdb-chttpd/blob/master/src/chttpd_db.erl#L886-L904
> > >
> > > [3]
> > >
>
> https://github.com/apache/couchdb-chttpd/blob/master/src/chttpd_db.erl#L203-L205
> > >
> > > [4]
> > >
>
> https://github.com/apache/couchdb-couch-mrview/blob/master/src/couch_mrview_http.erl
> > >
> > >
> > > [5] https://github.com/basho/webmachine
> > >
> > > [6] https://github.com/chewbranca/chttpd2/tree/initial-branch
> > >
> > > [7]
> > >
>
> https://github.com/apache/couchdb-couch/tree/2073-feature-webmachine-http-engine
> > >
> > > [8]
> > >
>
> https://github.com/apache/couchdb-config/tree/2073-feature-webmachine-http-engine
> > >
> > > [9]
> > >
>
> https://github.com/apache/couchdb-couch/blob/2073-feature-webmachine-http-engine/src/couch_httpr_welcome.erl
> > >
> > > [10]
> > >
> > >
>
> https://github.com/apache/couchdb-couch/blob/2073-feature-webmachine-http-engine/src/couch_api.erl
> > >
> > > [11]
> > >
>
> https://github.com/apache/couchdb-couch/blob/master/src/couch_httpd_misc_handlers.erl#L32-L45
> > >
> > > [12]
> > >
>
> https://github.com/apache/couchdb-couch/blob/2073-feature-webmachine-http-engine/src/couch_httpr_db.erl
> > >
> > > [13]
> > >
>
> https://github.com/apache/couchdb-couch/blob/2073-feature-webmachine-http-engine/src/couch_httpr_dbs.erl
> > >
> > > [14]
> > >
>
> https://github.com/chewbranca/chttpd2/blob/initial-branch/src/chttpd2_config.erl#L26-L33
> > >
> > > [15]
> > >
>
> https://github.com/apache/couchdb-couch/blob/2073-feature-webmachine-http-engine/src/couch.erl#L68-L73
> > >
> > > [16]
> > >
>
> https://github.com/apache/couchdb-couch/blob/master/src/couch_httpd_misc_handlers.erl#L155-L249
> > >
> > >
> > > [17]
> > >
>
> https://raw.githubusercontent.com/basho/webmachine/develop/docs/http-headers-status-v3.png
> > >
> > > [18]
> > >
>
> https://github.com/basho/webmachine/blob/develop/src/webmachine_decision_core.erl#L158-L595
> > >
> > > [19] https://github.com/ninenines/cowboy
> > >
> > >
> > > P.S. I've decided to stop using gists.github.com for posting content,
> > > as I can never find my posts again and the comments there are a black
> > > hole. I've instead posted this at:
> > >
> http://www.chewbranca.com/tech/2014/08/17/rewriting-the-couchdb-http-layer/
> > >
>



-- 
Andy Wenk
Hamburg - Germany
RockIt!

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