Fab.js: Filter

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Ooh! Shiny new apps are available in fab.js, so I must give at least one a whirl. I opt for fab.filter because it seems like a prime candidate to interact with the CouchDB changes API (and this after I swore off CouchDB / fab.js interop last night).

Again, I start with my skeleton fab.js script:

with ( require( "../" ) )

( fab )

  ( listen, 0xFAB )

  // code here

  ( 404 );

Between listening and 404 land, I define a "change" handler that will proxy requests back and forth between my CouchDB seed database:

  ( /^\/changes/ )

    ( fab.nodejs.http, "http://localhost:5984/seed/_changes?feed=continuous" )

Nothing new there, I am just making use of fab.nodejs.http to request the changes API to send changed documents continuously. But what happens if I am only interested in changes to "bar" documents? Sure I could load a filter design document into the CouchDB database, but who wants that hassle? I'd like to filter them in my fab.js server.

My first attempt:

  ( /^\/changes/ )

    ( fab.filter ( function (obj) {
   return /bar/.test(obj.body);  } ) )

    ( fab.nodejs.http, "http://localhost:5984/seed/_changes?feed=continuous" )

Running the fab server and then accessing the changes resource, I find... nothing.

Ugh.

At this point, I take some time to introduce my favorite debugging tool into my fab.js arsenal: print-stderr. Print-stderr is a timeless tool, that can be used in fab.js by defining "puts" at the top of the file thusly:

var puts = require( "sys" ).puts;

...and then using it liberally:

( fab.filter ( function (obj) {
   for (var prop in obj) {puts(prop)};
   return /bar/.test(obj.body);  } ) )

The output from my print-stderr in this case is:

cstrom@whitefall:~/repos/fab$ node ./play/changes.js
status
headers

Ah! There is a reason that the argument is obj and not app—it might only contain the header response. So I adjust my filter to ignore the header and only return true on a "bar" type change:

( fab.filter ( function (obj) {
   if (!obj.body) return false;

   return /bar/.test(obj.body);  } ) )

Now, when I update a bar document, I see:

cstrom@whitefall:~/repos/fab$ curl -n localhost:4011/changes/foo
{"seq":124,"id":"bar","changes":[{"rev":"1-967a00dff5e02add41819138abb3284d"}]}
{"seq":125,"id":"bar","changes":[{"rev":"2-a4b323f79cdc4ed3ce57b0f9145ea154"}]}

Changes to the "foo" document (or any other change) are ignored.

Day #84