Monthly Archives: July 2007

How not to bridge XMPP and HTTP

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BOSH is a specification that defines how XMPP can be used over HTTP. It’s obviously written by people who know what they’re talking about, because they’ve got good requirements, and get into great detail about the design choices they’ve made. Unfortunately, BOSH makes the one big mistake that so many others make; treating HTTP as a transport protocol. To wit;

POST /webclient HTTP/1.1
Host: httpcm.jabber.org
Accept-Encoding: gzip, deflate
Content-Type: text/xml; charset=utf-8
Content-Length: 188

<body rid='1249243562'
      sid='SomeSID'
      xmlns='http://jabber.org/protocol/httpbind'>
  <message to='contact@example.com'
           xmlns='jabber:client'>
    <body>I said "Hi!"</body>
  </message>

  <message to='friend@example.com'
           xmlns='jabber:client'>
    <body>I said "Hi!"</body>
  </message>
</body>

(you might also note that all of their example requests are POSTs to /webclient – a warning sign if ever there was one)

The intent of that message is to send two messages, one to each of the recipients at example.com. If we were treating HTTP as an application protocol, that would be done like this;

POST mailto:contact@example.com HTTP/1.1
Host: httpcm.jabber.org
Accept-Encoding: gzip, deflate
Content-Type: text/xml; charset=utf-8
Content-Length: nnn

<body rid='1249243562'
      sid='SomeSID'
      xmlns='http://jabber.org/protocol/httpbind'>
  <message xmlns='jabber:client'>
    <body>I said "Hi!"</body>
  </message>
<body>

POST mailto:friend@example.com HTTP/1.1
Host: httpcm.jabber.org
Accept-Encoding: gzip, deflate
Content-Type: text/xml; charset=utf-8
Content-Length: mmm

<body rid='1249243562'
      sid='SomeSID'
      xmlns='http://jabber.org/protocol/httpbind'>
  <message xmlns='jabber:client'>
    <body>I said "Hi!"</body>
  </message>
</body>

Alternately, if you don’t like proxies, the mailto URIs could be swapped out for an http URI specific to each mail address. But the point is that HTTP semantics be reused by recasting XMPP to them, rather than the current approach of grafting XMPP on top (read: obliterating). Don’t like two messages? Try pipelining them. Can’t pipeline? Does some other feature not map well onto HTTP in this way? Then it wasn’t meant to be.

We use HTTP (and the Web) because we want to be part of the Web; participate in the network effects, make information freely available (like, say, my presence status), etc.. We don’t do it because we need a way to get past firewalls. Good admins will avoid deploying software behind their firewall which subverts the intent of the firewall.

Crud, more CRUD crud

2 Replies

I’ve said it before, but apparently not enough as David Chappel chimes in with this;

A RESTful approach is a natural for data-oriented applications that focus on create/read/update/delete scenarios.

He’ll get no argument from me that REST is good for data-oriented applications, but saying that it’s only good for the subset that fit the CRUD model is wrong. It’s wrong because CRUD doesn’t have an equivalent for HTTP POST. Once you incorporate POST into your repertoire, then you can do all kinds of interesting things like, say, ordering stuff.

Perhaps David – or anybody else – could point me towards a data oriented application which can’t fit (well) into such a model (not REST, just the uniform interface part).