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Date: Wed, 20 Aug 2014 17:45:58 -0700
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Subject: Re: [DISCUSS] Rewriting the CouchDB HTTP Layer
From: Russell Branca <russell@chewbranca.com>
To: dev@couchdb.apache.org, andywenk@apache.org
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Thanks Andy! The next step I want to take is build another prototype in
Cowboy and compare it with the web machine  implementation. Hopefully will
have some time for that over the weekend.

-Russell
On Aug 20, 2014 5:01 AM, "Andy Wenk" <andywenk@apache.org> wrote:

> 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!
>
> GPG fingerprint: C044 8322 9E12 1483 4FEC 9452 B65D 6BE3 9ED3 9588
>
>  https://people.apache.org/keys/committer/andywenk.asc
>

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