7. CouchDB Externals API

Author:Paul Joseph Davis
Date:2010-09-26
Source:http://davispj.com/2010/09/26/new-couchdb-externals-api.html

For a bit of background, CouchDB has had an API for managing external OS processes that are capable of handling HTTP requests for a given URL prefix. These OS processes communicate with CouchDB using JSON over stdio. They’re dead simple to write and provide CouchDB users an easy way to extend CouchDB functionality.

Even though they’re dead simple to write, there are a few issues. The implementation in CouchDB does not provide fancy pooling semantics. The current API is explicitly synchronous which prevents people from writing event driven code in an external handler. In the end, they may be simple, but their simplicity is also quite limiting.

During CouchCamp a few weeks ago I had multiple discussions with various people that wanted to see the _externals API modified in slight ways that weren’t mutually compatible. After having multiple discussions with multiple people we formed a general consensus on what a new API could look like.

7.1. The New Hotness

So the first idea for improving the _external API was to make CouchDB act as a reverse proxy. This would allow people to write an HTTP server that was as simple or as complicated as they wanted. It will allow people to change their networking configuration more easily and also allow for external processes to be hosted on nodes other than the one running CouchDB. Bottom line, it not only allows us to have similar semantics as _externals, it provides a lot more fringe benefits as well. I’m always a fan of extra awesomeness.

After hitting on the idea of adding a reverse proxy, people quickly pointed out that it would require users to start manually managing their external processes using something like Runit or Supervisord. After some more discussions I ran into people that wanted something like _externals that didn’t handle HTTP requests. After that it was easy to see that adding a second feature that managed OS processes was the way to go.

I spent this weekend implementing both of these features. Both are at the stage of working but requiring more testing. In the case of the HTTP proxy I have no tests because I can’t decide how to test the thing. If you have ideas, I’d sure like to hear them.

[Update]: I woke up the other morning realizing that I was being an idiot and that Erlang is awesome. There’s no reason that I can’t have an HTTP client, proxy, and server all hosted in the same process. So that’s what I did. It turns out to be a fairly nice way of configuring matching assertions between the client and the server to test the proxy transmissions.

7.2. How does it work? - HTTP Proxying

To configure a proxy handler, edit your local.ini and add a section like such:

[httpd_global_handlers]
_fti = {couch_httpd_proxy, handle_proxy_req, <<"http://127.0.0.1:5985">>}

This would be approximately what you’d need to do to get CouchDB-Lucene handled through this interface. The URL you use to access a query would be:

A couple things to note here. Anything in the path after the configured proxy name (“_fti” in this case) will be appended to the configured destination URL (“http://127.0.0.1:5985” in this case). The query string and any associated body will also be proxied transparently.

Also, of note is that there’s nothing that limits on what resources can be proxied. You’re free to choose any destination that the CouchDB node is capable of communicating with.

7.3. How does it work? - OS Daemons

The second part of the new API gives CouchDB simple OS process management. When CouchDB boots it will start each configured OS daemon. If one of these daemons fails at some point, it will be restarted. If one of these daemons fails too often, CouchDB will stop attempting to start it.

OS daemons are one-to-one. For each daemon, CouchDB will make sure that exactly one instance of it is alive. If you have something where you want multiple processes, you need to either tell CouchDB about each one, or have a main process that forks off the required sub-processes.

To configure an OS daemon, add this to your local.ini:

[os_daemons]
my_daemon = /path/to/command -with args

7.3.1. Configuration API

As an added benefit, because stdio is now free, I implemented a simple API that OS daemons can use to read the configuration of their CouchDB host. This way you can have them store their configuration inside CouchDB’s config system if you desire. Or they can peek at things like the httpd/bind_address and httpd/port that CouchDB is using.

A request for a config section looks like this:

["get", "os_daemons"]\n

And the response:

{"my_daemon": "/path/to/command -with args"}\n

Or to get a specific key:

["get", "os_daemons", "my_daemon"]\n

And the response:

"/path/to/command -with args"\n

All requests and responses are terminated with a newline (indicated by \n).

7.3.2. Logging API

There’s also an API for adding messages to CouchDB’s logs. Its simply:

["log", $MESG]\n

Where $MESG is any arbitrary JSON. There is no response from this command. As with the config API, the trailing \n represents a newline byte.

7.3.3. Dynamic Daemons

The OS daemons react in real time to changes to the configuration system. If you set or delete keys in the os_daemons section, the corresponding daemons will be started or killed as appropriate.

7.4. Neat. But So What?

It was suggested that a good first demo would be a Node.js handler. So, I present to you a “Hello, World” Node.js handler. Also, remember that this currently relies on code in my fork on GitHub.

File node-hello-world.js:

var http = require('http');
var sys = require('sys');

// Send a log message to be included in CouchDB's
// log files.

var log = function(mesg) {
    console.log(JSON.stringify(["log", mesg]));
}

// The Node.js example HTTP server

var server = http.createServer(function (req, resp) {
    resp.writeHead(200, {'Content-Type': 'text/plain'});
    resp.end('Hello World\n');
    log(req.method + " " + req.url);
})

// We use stdin in a couple ways. First, we
// listen for data that will be the requested
// port information. We also listen for it
// to close which indicates that CouchDB has
// exited and that means its time for us to
// exit as well.

var stdin = process.openStdin();

stdin.on('data', function(d) {
    server.listen(parseInt(JSON.parse(d)));
});

stdin.on('end', function () {
    process.exit(0);
});

// Send the request for the port to listen on.

console.log(JSON.stringify(["get", "node_hello", "port"]));

File local.ini (Just add these to what you have):

[log]
level = info

[os_daemons]
node_hello = /path/to/node-hello-world.js

[node_hello]
port = 8000

[httpd_global_handlers]
_hello = {couch_httpd_proxy, handle_proxy_req, <<"http://127.0.0.1:8000">>}

And then start CouchDB and try:

$ curl -v http://127.0.0.1:5984/_hello
* About to connect() to 127.0.0.1 port 5984 (#0)
*   Trying 127.0.0.1... connected
* Connected to 127.0.0.1 (127.0.0.1) port 5984 (#0)
> GET /_hello HTTP/1.1
> User-Agent: curl/7.19.7 (universal-apple-darwin10.0) libcurl/7.19.7 OpenSSL/0.9.8l zlib/1.2.3
> Host: 127.0.0.1:5984
> Accept: */*
>
< HTTP/1.1 200
< Transfer-Encoding: chunked
< Server: CouchDB (Erlang/OTP)
< Date: Mon, 27 Sep 2010 01:13:37 GMT
< Content-Type: text/plain
< Connection: keep-alive
<
Hello World
* Connection #0 to host 127.0.0.1 left intact
* Closing connection #0

The corresponding CouchDB logs look like:

Apache CouchDB 1.5.0 (LogLevel=info) is starting.
Apache CouchDB has started. Time to relax.
[info] [<0.31.0>] Apache CouchDB has started on http://127.0.0.1:5984/
[info] [<0.105.0>] 127.0.0.1 - - 'GET' /_hello 200
[info] [<0.95.0>] Daemon "node-hello" :: GET /

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