Positive Incline Mike Burrows (@asplake) moving on up, positively

August 3, 2009

One link to rule them all

Filed under: Uncategorized,Web Integration — Tags: , , — Mike @ 11:22 am

A quick demonstration of the described_routes discovery protocol is in order. It’s all done with link headers, one per resource.

Starting with some arbitrary resource in the application:

$ curl --silent --head http://localhost:3000/users/dojo | grep Link
Link: ; rel="describedby"; meta="ResourceTemplate"
$ curl --silent --head http://localhost:3000/described_routes/user?user_id=dojo | grep Link
Link: ; rel="index"; meta="ResourceTemplates"

So after the first HEAD request we have the location of the resource’s ResourceTemplate metadata; after the second we have the location of the site ResourceTemplates (plural) metadata.

A HEAD request to the site metadata location merely confirms (via the meta tag) what we already knew:

$ curl --silent --head http://localhost:3000/described_routes | grep Link
Link: ; rel="self"; meta="ResourceTemplates"

On the root resource, a single HEAD request links us to the site metadata location directly:

$ curl --silent --head http://localhost:3000 | grep Link
Link: ; rel="describedby"; meta="ResourceTemplates"

After that, it’s up to the client.

When you create a path-to client thus:

app = PathTo::DescribedRoutes::Application.discover("http://localhost:3000/users/dojo")

path-to discovers the site metadata and GETs it, specifying JSON as the acceptable format. The JSON response is converted to basic Ruby objects and used to initialize the Application object app. The client now knows the application’s resource structure:

app.users['dojo'].uri
=> "http://localhost:3000/users/dojo"

and we’re back where we started!

July 27, 2009

Putting it all together – ResourceTemplates, described_routes and path-to

Filed under: Web Integration — Tags: , , , — Mike @ 10:18 pm

We’ve watched described_routes and path-to grow here over the past few weeks. And fun though it has been for me, it must be hard to get a good overview from blog posts triggered by design challenges! So here goes: an attempt at a one-post overview.

In a sentence?

Add described_routes to your Rails application to give it header-based site discovery with ResourceTemplate metadata that enables instant Ruby APIs on the client side with path-to.

And if I’m not using Rails or even Ruby?

There’s library support for ResourceTemplate metadata in Ruby (for the moment it’s included as part of described_routes) and Python (see described_routes-py). There’s a simpler version of path-to available in Python also, namely path-to-py. And there’s nothing Rails-specific or complicated about ResourceTemplates either – strip away the JSON, YAML or XML formatting and they’re not much more than named resources arranged hierarchically with their URI templates and supported HTTP methods as properties – and (so I’m told) can be useful even without path-to.

OK – I’m sold! What do I have to do?

Just follow these simple steps:

  1. Install described_routes for the server
  2. Add build-time integration to the server
  3. Add basic run-time integration to the server
  4. Add site discovery to the server
  5. Install and run path-to
  6. Profit!

The README files of the two gems tell you all you need to know in detail, but here in one place:

1. Install described_routes for the server

$ sudo gem install described_routes

2. Add build-time integration to the server

This is just a set of Rake tasks that lets you see immediately what the metadata looks like. In your Rakefile:

require 'tasks/described_routes'

Then try it out:

$ rake --tasks described_routes
rake described_routes:json        # Describe resource structure in JSON format
rake described_routes:xml         # Describe resource structure in XML format
rake described_routes:yaml        # Describe resource structure in YAML format
rake described_routes:text        # Describe resource structure in text (comparable to "rake routes")

Specify the base URI of your app with "BASE=http://..." to see full URIs in the output.

3. Add basic run-time integration to the server

Somewhere in your application include the controller, perhaps in an initializer:

require 'described_routes/rails_controller'

Add the following route in config/routes.rb:

map.resources :described_routes, :controller => "described_routes/rails"

You can now browse to /described_routes(.format) and /described_routes/{controller_name}(.format) and see the data generated at run time.

4. Add site discovery to the server

Site discovery (linking resources to their resource-specific and site-wide metadata) works via link headers (“Link:“) added to the responses served by one or more controllers. This has a double benefit:

i) Resources gain some type information derived from the Rails route name of the resource that may be of value to clients
ii) A path-to client (or any other client interested in the ResourceTemplate metadata) can be initialised from a regular resource URI; prior knowledge of metadata location is not needed.

According to your requirements, add the set_link_header filter to either the controller of your root resource (&/or or other specific controllers) or to ApplicationController in order to benefit all controllers:

require 'described_routes/helpers/described_routes_helper'

class MyController < ApplicationController
  include DescribedRoutes::DescribedRoutesHelper
  after_filter :set_link_header
end

Install and run path-to, …profit!

$ sudo gem install path-to

It is now a one-liner to bootstrap a client application, in this example a test blog with user and article resources:

require "path-to/described_routes"

# bootstrap a path-to client from the test_rails_app provided in described_routes
app = PathTo::DescribedRoutes::Application.discover("http://localhost:3000/")
#=> <PathTo::DescribedRoutes::Application>

# get user 'dojo'
puts app.users['dojo'].get
#=> '<html>...</html>

#get a JSON representation of the recent articles of user 'dojo'
puts app.users['dojo'].articles.recent['format' => 'json'].get
#=> [...]

Profit!

This bit is up to you, but metadata-enhanced web apps and instant client APIs achieved for so little work has to be worth something!

July 23, 2009

New link_header gem

Filed under: Web Integration — Tags: , , , , , — Mike @ 9:26 pm

My latest project on github and Rubyforge is link_header, a small rubygem for parsing and generating HTTP link headers as per the latest spec draft-nottingham-http-link-header-06.txt.

Usage

The usual install:

sudo gem install link_header

The library’s LinkHeader and LinkHeader::Link classes follow a pattern established in the ResourceTemplate and ResourceTemplate classes in that they offer easy conversions both to & from Ruby primitives, i.e. Arrays, Strings etc. This in turn makes them easy to prettyprint, convert to & from JSON and YAML, create from test fixtures, and so on. [Aside: @kevinrutherford and I discussed this idea on Twitter a few days ago in response to his blog post “factory method in ruby“. It’s worth a read.]

Link attribute names can appear more than once, so I have chosen a list of attribute/value pairs rather than a hash to represent link attributes. Link objects do however have an #attrs method that will lazily generate a hash if that’s convenient (it’s left to you to decide whether it’s safe!). There’s an example of this below.

So:

require "link_header"
require "pp"

#
# Create a LinkHeader with Link objects
#
link_header = LinkHeader.new([
  LinkHeader::Link.new("http://example.com/foo", [["rel", "self"]]),
  LinkHeader::Link.new("http://example.com/",    [["rel", "up"]])])

puts link_header.to_s
#=> <http://example.com/foo>; rel="self", <http://example.com/>; rel="up"

link_header.links.map do |link|
  puts "href #{link.href.inspect}, attr_pairs #{link.attr_pairs.inspect}, attrs #{link.attrs.inspect}"
end
#=> href "http://example.com/foo", attr_pairs [["rel", "self"]], attrs {"rel"=>"self"}
#   href "http://example.com/", attr_pairs [["rel", "up"]], attrs {"rel"=>"up"}

#
# Create a LinkHeader from raw (JSON-friendly) data
#
puts LinkHeader.new([
  ["http://example.com/foo", [["rel", "self"]]],
  ["http://example.com/",    [["rel", "up"]]]]).to_s
#=> <http://example.com/foo>; rel="self", <http://example.com/>; rel="up"

#
# Parse a link header into a LinkHeader object then produce its raw data representation
#
pp LinkHeader.parse('<http://example.com/foo>; rel="self", <http://example.com/>; rel = "up"').to_a
#=> [["http://example.com/foo", [["rel", "self"]]],
#    ["http://example.com/", [["rel", "up"]]]]

Later…

My next programming task will be some minor refactoring on described_routes and path-to take advantage of this new gem. The driver behind this all is an efficient discovery protocol and a significant reduction in the number of links reported by default – I realised that it was wasteful to produce multiple links on every request that are (let’s be honest) be of no interest at all to clients, when just one of those links points to metadata that carries that same information and more! Then for those server apps that generate the correct headers, a short one-liner will initialize a path-to client given the address of any resource served by the application, i.e. without special knowledge of the location of the app’s metadata resources.

Refactoring aside, the described_routes part of this is done (so servers support the protocol already); I just need to finish path-to part to take advantage of it on the client side.

I can’t make any promises about timelines at the moment (new job starts soon) but a Python version should be forthcoming soon(ish) also. Meanwhile, enjoy the Ruby version if you can!

July 13, 2009

Distracted

Filed under: Web Integration,Work — Tags: , , , , , — Mike @ 11:12 am

Yes, things are a bit slow on the blog front at the moment. Excuses: we move to a Georgian cottage in the Derbyshire Dales on Wednesday, I start an exciting new job on August 3rd (more on that nearer the time), and we have a new miniature schnauzer puppy named Klaus!

I haven’t forgotten described_routes and path-to though. There’s a serious link header tidy-up underway that delivers an efficient discovery protocol. Details to follow…

All in all, I’ve had a very enjoyable and quite productive 3 month break from full-time work. As well as the Ruby and Python programming I’ve been hanging out (as @asplake) with the #kanban guys on Twitter – a great bunch of people – and enjoyed @dpjoyce’s SkillsMatter presentation on kanban at the BBC in person a few weeks ago. It has also been good to clear the house of clutter (how Lean is that!), though to be honest, most of the credit for that goes to my very hardworking wife Sharon, who has been wonderful throughout this period of great change. Credit too to my teenage boys for agreeing to take this leap into the unknown!

June 30, 2009

Bootstrapping REST

Filed under: Web Integration — Tags: , , , , , , — Mike @ 6:55 pm

In this article I’m approaching REST from the perspective of client/server interaction, most especially (and slightly unusually I think) on the needs of the client and you, the client’s developer. I hope you find it helpful.

A caveat (no apology): my interest is in structure and navigation. HTTP has mechanisms already for clients and servers to negotiate content types and I’ve made a conscious design decision to rely on their existence. In other words, I ignore it completely here. Orthogonality is a wonderful thing!

We start at the beginning:

The completely hand-crafted client

At the most extreme, you’re working against an undocumented, unsupported server API that lacks regular structure. Any complexity implies significant reverse-engineering effort, made worse if you’re tracking a moving target. If you’re lucky though, you’ll be working against a supported and stable API.

Relying on regularity

Servers built using frameworks such as Rails (and even many that aren’t) will tend to exhibit some predictable patterns. The details may be framework-specific, but a collection resource, say “users”, can be expected to support these (or similar) operations and subresources:

GET, POST         http://www.example.com/users
GET               http://www.example.com/users/new
GET, PUT, DELETE  http://www.example.com/users/{user_id}
GET               http://www.example.com/users/{user_id}/edit

and a nested collection resource (each user’s articles, say) can be expected to follow a similar structure, rooted on a specific user resource.

GET, POST         http://www.example.com/users/{user_id}/articles
GET               http://www.example.com/users/{user_id}/articles/new
GET, PUT, DELETE  http://www.example.com/users/{user_id}/articles/{article_id}
GET               http://www.example.com/users/{user_id}/articles/{article_id}/edit

Armed therefore with just the knowledge of the user and articles collections, your client’s internal object model might easily support expressions such as

app.users
app.users.new
app.users[user_id]
app.users[user_id].edit
app.users[user_id].articles
app.users[user_id].articles.new
app.users[user_id].articles[article_id]
app.users[user_id].articles[article_id].edit

You’ll pay a price when the application departs from its usual pattern, but most of the time you’ll be fine. In fact for many client applications the edit and new are irrelevant, so all you need is the basic hierarchical structure and knowledge of the representations you can GET, PUT, POST there, also how to DELETE things.

Model-driven development

One approach to speeding application development is to abstract out that object model and generate skeleton code for both client and server from some common, logical representation. Not only are you spared the task of building the object model, but (given the right toolset) you can expect to have the web client functionality baked in for you. Most importantly, it’s much cheaper to track any changes made on the server.

The downside? You are now tied to a toolset. That might be a price worth paying if you’re in control of both client and server (for an internal application, say), but for many this will be unpalatable.

The no-model option

[Stick with me – this apparent digression makes metadata-driven clients easier to introduce!]

Dynamic languages make it possible to support expression like

app.users[user_id].articles.new

even if your app object doesn’t actually have a users member (method, attribute or property) and articles and new exist nowhere either. See [1] and [2] to read how (generally) this can be done in Ruby and Python, and [3] for a description of path-to [4], an actual client that can work this way.

Even without these dynamic features, a similar effect can be achieved with operator overloading. This, for example, is perfectly valid Java:

app / users / member(user_id) / articles / _new

The objects returned by such expressions can easily have a URI representation, request methods and so on.

But beware! Suppose a user resource defines an “up” link. Take for example

app.users["dojo"].up

To which of these URIs (below) does this expression refer?

http://www.example.com/users/dojo/up
http://www.example.com/users

We’ll come back to this issue later.

Metadata-driven clients

You can take the no-model approach and validate each navigation against it, so that for a non-existent path foo,

app.foo

raises an error just as the model-driven version would have done. To your client application code, this implied object model looks no different to the generated one, even though (in a way) it doesn’t really exist! I like to think of this as the model-driven approach turned inside-out – instead of running code generated from a model, we run code that interprets a model.

If that metadata comes from the server (so much better than a build-time configuration step), you’re getting closer to that RESTful goal of application interaction driven by links, or

Hypertext as the engine of application state (HATEOAS) [5]

But before claiming this goal genuinely you will need to address a couple of issues that we’ve glossed over so far:

  1. how to generate URIs for collection members, for example turning users["dojo"] into http://example.com/users/dojo
  2. how to generate URIs for navigations like up and first

The cleanest answer to the first issue is the URI Template (a parameterised URI with a standardised syntax), sourced (naturally) from the server. With Fielding’s quote in mind it’s clear that the resolution to the second issue must lie also in server-provided information. In fact we’ll see both issues come together as we understand that the problem really isn’t one of describing URI structure but instead one of describing resource relationships and their manifestations in links.

URI Templates and Resource Templates

A URI Template that maps users["dojo"] into http://example.com/users/dojo might look this this:

http://example.com/users/{user_id}

The syntax isn’t that important (the draft standard looks set to change significantly), and yes, we’ve seen them here already without introduction. They’re pretty self-documenting and (with a good library implementation) easy to use. If you stick to a standard there’s nothing unRESTful about them – having your application create a URI from template is really no different from a browser creating a URI as the result of GET-based HTML form.

In more complex cases – in particular where there is a mixture of mandatory and optional parameters – it’s advantageous to wrap the URI Template in some basic metadata. WADL [6] is a well-supported example of this idea (well almost – see the comments [7]) ; another is my own Resource Template format [8] (actually more a logical schema than a format since it’s available in JSON, YAML and XML).

WADL and Resource Templates typically describe an entire application, each element describing a class of resource, its relationships, its URI template (so that relationships can be turned into links), parameters, supported HTTP methods and so on.

Integrated with Rails, Resource Templates are generated by the server at run-time from the application’s routes, information already available. Although (most typically) they can be generated and consumed for the whole application in one go, they can be generated for a single resource already pre-populated with that specific resource’s known parameters.

At last: links!

From those pre-populated, resource-specific Resource Templates we can generate links – as HTML or XML elements or as HTTP headers – that our client can consume and navigate, resource by self-described resource. Every resource becomes a potential entry point to a virtual application that can easily span multiple servers. Finally, we got there: true hyperlink-based client/server interaction, and where the framework support exists, for very little effort too.

Discussion

In this article I have described the journey I travelled myself over a period of weeks following my decision to expose a complex existing application to scripting via HTTP. For me, technology-neutrality was a determining principle, and selecting HTTP and REST was an easy decision, first to make and then to sell. A proof of concept and a good demo based on a hand-crafted object model came quickly; metadata and templates were the result of some serious refactoring. Resource Templates came after I left the project, as did Rails integration (not a project requirement) and the path-to client in both Ruby and Python.

At the time of writing, link generation is supported on the server side but not yet on the client. The purist in me likes the fact that it is theoretically possible and will strive to keep it that way; the engineer in me is for the moment content with slurping up a whole application’s worth of metadata in one go and thereafter restricting interactions and payloads to the application level. Even as a purist, I see elegance in a simple metadata format, avoidance of hand-coded generation of URIs, respect for the power of HTTP and the orthogonality of formats; and if you believe as I do that RESTfulness lies in how clients and servers interact and not just in how server resources support the HTTP methods, I’ve probably succeeded in taking it further than most.

References

  1. Dynamically extending object behaviour in Ruby and Python [positiveincline.com]
  2. Programmatically adding methods to classes and objects: more Ruby/Python comparisons [positiveincline.com]
  3. path-to is born: nice client-side APIs to web applications in Ruby, also articles tagged path-to [positiveincline.com]
  4. path-to (Ruby) and path-to-py (Python) [github.com]
  5. Representational State Transfer (REST) (Chapter 5 of Architectural Styles and the Design of Network-based Software Architectures) [www.ics.uci.edu]
  6. Web Application Description Language [wikipedia.com]
  7. Partial template expansion in described_routes (comments)” [positiveincline.com]
  8. described_routes: hierarchical, framework-neutral and machine-readable descriptions of Rails routes [positiveincline.com], described_routes (Ruby) and described_routes-py (Python) [github.com]

June 28, 2009

Two new described_routes versions

Filed under: Web Integration — Tags: — Mike @ 4:12 pm

0.6.0 adds the automatic link element and link header generation described in the last few blog posts.

0.6.1 adds #all_preorder and #all_postorder methods to the ResourceTemplate and ResourceTemplates classes, so that the hierarchy can be navigated deterministically.

Installation is the usual

  gem install described_routes

Enjoy!

June 25, 2009

Resolutions

Filed under: Web Integration — Tags: , , , , — Mike @ 1:49 pm

In my previous post I was casting around for an alternative to type as the name of a link header attribute, since in the standard this refers to a content type, not the “logical” type of the resource pointed to by the link.

I have decided to go with role, borrowed from XLink. I know that we’re not dealing with XML here and XLink isn’t wildly popular, but it’s documented, accepted and understood, so why reinvent the wheel?

Consistent with both XLink and the existing rel attributes, I’ve made these role attributes contain URIs, as in this example:

Link: <http://example.com/users/dojo>; rel="self"; role="http://example.com/described_routes#user",
           <http://example.com/described_routes/user>; rel="describedby"; role="http://example.com/described_routes#described_route",
           <http://example.com/described_routes/user?user_id=dojo>; rel="describedby"; role="http://example.com/described_routes#ResourceTemplate",
           <http://example.com/users>; rel="up"; role="http://example.com/described_routes#users",
           <http://example.com/users/dojo/edit>; rel="edit"; rel="http://example.com/described_routes/user#edit"; role="http://example.com/described_routes#edit_user",
           <http://example.com/users/dojo/articles>; rel="http://example.com/described_routes/user#articles"; role="http://example.com/described_routes#user_articles"

You can think of this in terms of entities and relationships in the ER model:

  role="http://example.com/described_routes#user_articles"

identifies a target entity (“user_articles” in this example, named after its Rails route) within the overall schema, and

  rel="http://example.com/described_routes/user#articles"

identifies a relationship (“articles”) within the description of the source entity (“user”).

These URIs are globally unique, and even if they are never dereferenced by the client (i.e. the client isn’t interested in the ResourceTemplate metadata there – shame on them!), these values can be used by clients to select links reliably.

Postscript

One of my sources of both good practice and inspiration has been Tim Berners-Lee’s Design Issues. I hadn’t got to the Metadata Architecture article until after drafting the above, and it’s quite gratifying to read it now.  Tim makes the ER comparison too, but rather than rehash that I will quote some of his key principles:

Metadata about one document can occur within the document, or within a separate document, or it may be transferred accompanying the document.

Link headers and link elements pointing to ResourceTemplate documents exemplify all three possibilities.

The URL space is an appropriate space for the definition of attribute names

Not only appropriate, but a lot more powerful than arbitrary tokens.  My initial plan to use simple names for rel and type attributes were definitely misguided.

As much as possible of the syntax and semantics should be able to be acquired by reference from a metadata document.

Within the scope of defining navigations around a web application I think this is achieved successfully.  A lot can be done by the client even without the metadata document, though clients will still need help in constructing URIs for members of collections.  I think there would a be place for the ResourceTemplate format (or something very much like it) even if there was a standard for URI Templates in link headers, but that’s a whole new article, perhaps my next one.

June 23, 2009

Link header dilemmas

Filed under: Web Integration — Tags: , , , , — Mike @ 12:07 pm

I received a very reasonable answer to my question (thank you Phil), but as a result of trawling through 12 months of archive came to realise that I’d misunderstood the guidance (or rather the relative lack thereof) regarding the use of the type link attribute.  The spec cites RFC 4287 The Atom Syndication Format and it appears that I have abused it by populating it with resource template names (sourced from Rails route names).  My fault entirely, lesson learned.

I’ll need to choose a more appropriate name for it therefore – either that or remove it altogether!  Possibilities:

  • name?  I quite like this, but is it too generic?
  • resourcename? Is this any better than name?
  • routename? Accurate when driven from Rails, but otherwise too implementation-specific I think
  • linkname? I quite link this too, but why not call the linkname on a link just name?
  • target?  Already taken, or at least too reminiscent of the target frame/window attribute.  Perhaps I’m on a better track here though? targettype maybe?

Aarrgghh!  We know that naming is hard; naming a name is doubly so.  I’ll sit on this for a while and hope that either inspiration or feedback comes…

June 19, 2009

Link headers, link elements, and REST

Filed under: Web Integration — Tags: , , , , , — Mike @ 3:14 pm

The house move looms (still no date yet, frustratingly) so I’ve had less time for programming (or blogging for that matter!).  I do however have an experimental and unreleased enhancement to the Ruby version of described_routes that generates link elements (for the <head> section of your html) or the yet-to-be-standardized link headers completely automatically.  Integrating them into your Rails application requires just a one-liner per layout or controller (or you can do all controllers in one go).  Drop me a line if you would like to play with it.

I have one question outstanding on the new spec (see it here on the ietf-http-wg list archives) but I have to say that I’m pretty happy with it all.  It takes the ideas of Partial template expansion in described_routes and adds a standardised way to relate content and metadata, making it even easier clients to navigate web applications without necessarily slurping up the entire site’s schema in one go.  It allows interaction to be 100% RESTful (HATEOAS and everything), and even if you prefer not to go that way as a client you can at least be confident that the application will be transparent, self-documenting and well-behaved.

June 4, 2009

Alternative URI Templates implementation in Python; fun with decorators

Filed under: Programming,Web Integration — Tags: , , — Mike @ 8:20 am

I’ve added uri_templates.py to the described_routes-py repository.  Feature-wise, it differs from Joe’s in that it supports a partial expansion mode (see this post for motivation).  Implementation-wise, it’s all done with regexps – no special parsers required.

There are a couple of missing features (I own up to them in the docstring) but they probably won’t get fixed. Not only do I not need them, but we’re expecting the spec to undergo some some significant change soon and it would probably be wasted effort.

Fun with decorators

For a bit of Python newbie fun, I used a decorator to populate the operator dispatch table:

def operator(name):
    def save_operator(func):
        operators[name] = func
        return func
    return save_operator

@operator('opt')
def op_opt(arg, variables, params, encoding, partial):
    etc

They then get called by:

    operators[operator](arg, variables, params, encoding, partial)

Easy! Perhaps this is overkill, but it seems nicer than techniques like calling functions via constructed function names and it might allow new operators to be added from outside the module.

This must be the simplest example of parameterised decorators you’re ever likely to find as there’s no function wrapping involved. The one thing to understand is that operator('opt') returns a new function that “remembers” the name ‘opt’.

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