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From Russell Branca <chewbra...@apache.org>
Subject [DISCUSS] Rewriting the CouchDB HTTP Layer
Date Sun, 17 Aug 2014 18:41:03 GMT
# 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/

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