Note: the following article is in the context of the Ruby community and makes many Ruby-specific references, but the principles discussed are generally language-agnostic.

Intro

By now the benefits provided by macro-architecture in the context of web applications and web services via frameworks like Rails are well-understood and do not need to be re-explained. Changes in Rails and the introduction of ActiveResource have also contributed to the current acceptance of REST’ful principles as being beneficial in web apps.

However, one area that has been gaining more support in the community but has not been directly examined is that of micro-architectures. For the sake of this article I am defining a micro-architecture as a REST’ful web applications or service that is composed of several self-contained services communicated with over HTTP. Herein the general benefits of micro-architectures, as well as when and how to effectively move to them, well be discussed.

Test-driving the public interface

As with most software, begin TDD-style with tests first. I recommend outside-in style development, with heavy emphasis on the “outside” and little on the “in”. In other words, favor integration tests over unit tests. For example, in typical software development your goal might be the algorithmic transformation of some data set. In this case you would start out with some example base cases of how your library works, and implement them in the red-green-refactor cycle. If the implementation starts becoming too complicated then introduce some unit tests to help you manage complexity. However, remember that the purpose of the api is just to get the integration tests to pass, and any extra unit tests just make refactoring more difficult. Thus, unit tests (and even more extremely, mocking) are only a result of necessity due to complexity or test-suite run time savings.

The analogy in the web app/service world to our algorithmic transform library then would be to use integration tests at the highest level possible, namely through the http api. For a web application (something consumed by a human in a browser) use webrat so that you can exercise the same navigation and data-submission structure that users do in the browser in html. For testing web services (consumed programmatically as apis, mostly simple xml or json serialization formats), I recommend testing with rack-test.

One common and valid argument against micro-architectures is that they are a bit complicated to setup automated tests for, although I think this is beginning to change. Either way both webrat and rack-test can be used to test rack-based applications in-memory. It is advised to start this way to avoid setup pains.

Once you have your tests passing you can easily convert them to shoot real http requests at servers rather than in-memory rack-object interface testing. You have many options here for webrat, including mechanize if you want to go headless and selenium if you want to spawn up a real browser. I recommend going headless, at least during development so you don’t slow down your feedback loop. If you are using rack-test then something to look out for is rack-client. Although the library isn’t mature yet, it offers a single line of code change to go from in-memory rack-test to http-based rack-test. Since webrat has a rack-test adapter, it could also be used with rack-client instead of mechanize. First get your in-memory tests passing, then write up some rackup .ru files to manually spawn servers with, and then convert the tests to http with the methods described above. After that is done, you can automate the spawning servers with your testing framework’s equivalent of RSpec before(:all) and after(:all).

Development guidelines

I will assume that we are building some single web app or service to be hosted at some top-level domain (e.g. facebook, google, etc). As such, the customer will be facing an api that from the outside looks like a macro-architecture, where many things are possible in a somewhat complicated service. The opposite would be many Amazon AWS-style mini services like S3, SimpleDB, EC2, etc, but these kind of public micro-services are not too common.

So, once again we start building our public macro-service. Because such services have growth and complexity in mind, it’s a good idea to use a larger framework like Rails or Merb with enough conventions and organization to handle that complexity. It is easier to develop an entire macro-service with a macro-architecture, putting all of your controllers, models, views, etc in the same codebase to makeup your resources. In contrast separating code out into isolated internal micro-services takes some work, in the same way that organizing a pile of legos into piles of common color is harder than throwing everything into a single pile.

Additionally, while setting up http-level integration testing by forking a process in before(:all) and killing it in after(:all) is manageable, doing the same with several internal services consumed by one main one is a bit more complicated to setup. Nonetheless, it is still advisable to manually do this until some nice library pops up that can handle expressing these connections and spawning/tearing down servers from them elegantly.

You should design your macro-architecture in a way that makes separating out micro-services easy. In other words, think about how whatever part of your app that you are building could be backed by some service that stands on its own (e.g. a blog post comments controller could be backed by a generic annotation service that accepts a uri to some resource, as well as annotation text and author information). Get your tests passing this way against your integration-level macro-service api. Note that you don’t have to test the micro-service api, that is the fun part of refactoring (but if necessary due to complexity, be sure to add tests… use the same discretion between integration, unit, and mock tests as used in the simple algorithmic transform example above).

The reason why you should think about the internal-service that could comprise your macro-service is to avoid sharing code between this part and the rest of the application. If you haven’t noticed already this article is really about taking classic established software principles and applying to the world of web apps. In this case we want high cohesion and loose coupling between code in various parts of our public macro-service api. This is a good practice in general, and as long as you maintain it in a slightly stricter fashion than normal then it will make extracting a micro-service possible. Also when I avoid code sharing, that doesn’t prevent you from extracting common utility code into shared gems, just stay away from sharing business logic code that will eventually make refactoring a service out unbearable.

Being pragmatic

Note that I am not saying that you should always follow this practice of avoiding code reuse and thinking about how to make public-api resources independent for each part of your application. Software development is a constant rearrangement of where complexity resides, whether it be in a specific, isolated, and visible portion of code, or whether it lies in the complex relationships between too many abstractions. Therefore, as always use discretion as to what makes sense to keep in the internal macro-architecture of the public macro-service, and what should be split off into internal micro-services. What I am saying is that you should be aware of when you reuse code across resources and consciously make the decision to do so. Just try to think about whether or not a resource could easily explode in complexity (making the encapsulation of the complex service worth it), or whether it could easily become a performance bottleneck (making isolated performance monitoring worth it). Those two reasons, separating out complexity via encapsulation, and performance monitoring, are the big motivators for creating a micro-architecture.

In any case, if you’ve identified a resource that is a good candidate for becoming a micro-service, and has already been tested implicitly via the public macro-service api, then there is a convenient middle ground to take. Rack and its notion of middleware, in addition to bigger frameworks supporting it with things like Rails Metal, make this middle ground possible. I generally like using a class that inherits from Sinatra::Base as a micro-service, as the api does not contrain your resource design like bigger frameworks with more conventions do, and keeping the amount of files and directories to a minimum is possible. Sinatra provides just enough sugar to make building a Rack-middleware a pleasant experience (although the Rails 3.0 internals are shaping up to be flexible enough to be used in a very similar way, so watch for that as well). By creating your resource as a middleware service you more explicitly mark it as code to be isolated from the rest of the internal macro-architecture, but because of Rack you get the benefit of being able to test the entire public macro-service in one process. Doing this during the development phase of your application prevents you from being barred down by setting up the integration-testing infrastructure for running several rack-apps pulled out of the middleware stack but still consumed by the main application. In fact, if the separation is just for organization encapsulation purposes then feel free to deploy your monolithic app with the micro-services as middleware. However, if you have micro-services that were separated due to performance concerns that need to be monitored then be sure to pull them out and integration test them before your public deployment.

Common criticisms

The most common criticism that I have not addressed when it comes to using all these micro services is complexity introduced by latency issues when dealing with many http apis. To deal with this transparently and most elegantly, I recommend Ruby 1.9 + Neverblock. Other options include manual event handling with em-http-request, or explicit thread-based latency hiding with something like “need_later” futures in Dataflow.

Another common criticism is the complexity increased in an application because of all the new moving parts, which is indeed a problem and should be weighed against the two major motivations. Notably though, with the clear boundaries introduced via the separate applications, you can catch exceptions if one micro-service is not responding and handle error messages on the specific page that uses it. Compare this to a monolithic architecture, where partial system availability is much more difficult to design.

Final remarks

Please note that for simplicity I have been implying a ratio of one public api resource to one internal micro-service, but in reality a single micro service could be used in part by many public api resources, and a single public api resource could make use of several internal micro-services.

This concludes my summary of how to effectively manage and take advantage of an internal micro-service architecture. Micro-services are still in their young and “hip” stages compared to their macro counterparts, so expect best practices regarding them to evolve as we collectively gain experience. I think the general advice in this post is sane though, so hopefully it can serve as a reference to people not yet familiar with the topic. As always, comments/criticism welcome.

Benefits

For brevity I tried to not cover specific benefits of micro-architecture within the two major categories of encapsulation and performance monitoring isolation. Here is a list that I will update if anyone has more suggestions in the comments.

• library/gem dependency isolation
• choosing appropriately different servers for services (e.g. passenger vs thin)
• partial-site functioning when isolated services are down and error handling is in place
• full http reverse-proxy caching via ESI pulling in bits from disparate services

As some of you know, when I’m not working I’m taking classes in Information Systems at UCF. One of the classes I took this semester was Web Systems II, which is essentially a server-side course that focuses on ASP.NET development.

One particular assignment was creating a “Category Code Manager”, which I had never heard of previously. I think the professor made the term up, but the concept is simple nonetheless. There are categories (ie: fruits) that each can have many codes (ie: apples, oranges). I think the purpose of the assignment was to get used to working with foreign keys, and building dynamic drop downs as a tool to display this sort of relationship. Another gotcha is that the categories can have parent categories.

The class had a participation grade that I must have overlooked… Needless to say that didn’t bode well for me, so I created this screencast as an introduction to Ruby on Rails (beginner level) using the new Rails 2.0 (final source code included.) I tried to follow best practices where possible (ie: TDD), and covered the topics of:

• Database agnosticism & environments
• Using rake to create databases, and to run
• Scaffold generator
• has_many & belongs_to relationships within ActiveRecord
• The interactive Rails console
• The new integration of the ruby debugger, and a drop of live metaprogramming
• Test Driven Development with Test::Unit
• Intro to REST’ful architecture within Rails

But, I left some things out to avoid making the screencast even longer, and avoid it being confusing for beginners. Just so you all know, some changes I would have made include:

• Create some helper methods for things like populating the select tags
• Use nested routing (I did this first, but renaming all the named routes, having to explain the routing, etc made this too complex for a beginner video)
• Use RSpec for testing (RSpec’s scaffold uses mocks and stubs, which are just a little too much to explain when already introducing all of Rails and testing)
• Use a REST’ful abstraction plugin (we developed an internal one at work, but unfortunately we haven’t been able to open source it yet… until then check out make_resourceful)
• Use the ObjectMother pattern for creating objects during tests

Enjoy the screencast!

I’ve been doing a lot with REST and ROA on and off the job lately (mainly with Eric and Dray, who still does not have a blog.)

The way Rails implements resources is through Controller classes. One thing that came up yesterday, was the argument that the implementation is improper due to the inclusion of both an index and a show action in one Controller. In REST, a resource can be its own entity (ie: an apple), or a collection of of entities (ie: a fruits resource composed of apples, grapes, oranges, etc.) At first glance, the challenge to Rails Controllers seems to stand because a GET to the index action returns a collection of resources, whereas a GET to the show action just returns one resource.

Upon closer inspection though, you’ll see that the confusion comes from focusing on Controller actions. What should really be looked at is the resource (the controller), and how those two requests are addressed (the URI.) Firstly, notice what kind of resource we have… it’s a fruits resource. Next, look at the URI, it is /fruits.

Now that we’ve established that our FruitsController represents a plural resource (a collection of fruit resources), we can move on to explaining the confusion that the index and show actions introduce. A request to /fruits will return all of our fruits. But, the world has many fruits and we don’t want to request them all. Instead, we will fire a GET at /fruits?page=2?limit=10. Although most of you will recognize this as simple pagination, in ROA it is known as the concept of addressibiliy to indicate state in the URI.

Here’s where the confusion is cleared: a request to /fruits/1337 is not requesting a singular resource (in this case a cherry, the most elite of all fruits.) Instead, it is requesting a collection resource (fruits) but using addressibility to indicate state. It is okay to use the URI in REST different ways for addressiblity, because REST is a style and does not have a specification for addressing via URI’s. For example, another way of getting to /fruits/1337 might be /fruits/?page=1337?limit=1.

As a closing note, even though we used the fruits resource collection here and addressed cherry, we could have done it differently still. If you really wanted a singular cherry resource, you can do so in routes.rb by replacing map.resources :fruits with map.resource :cherry (notice that “resource” is singular.) This would map to a singular CherryController to implement the resource. You could access it with a get request to /cherry, which would map to the show action (in this case the index, or list, action does not make sense because cherry is singular.)

Wow… RailsConf07 was absolutely amazing. The interesting thing is that I am not talking about the presentations, although they were top-notch. Instead, the Rails community is what made this conference so… heart-touching, even.

Chad Fowler directly announced that he picked the keynotes and sessions as his personal mold for what we should be thinking about. To this end, I thank him deeply for his choices.

While there, I came to recognize the Rails community in a new light. I don’t think I could have understood this unless actually being there either (as cliche as it sounds.) Our community stereotype as elitist, arrogant developers did not show its face.

I hope that the community stays like this, and that everyone has a chance to go to at least one conference like this.

All of the attendees were teeming with positive energy and friendly. The diversity of people was great, including people from different states and countries, and people in suits (maybe one or two) to people dressed like hippies and supermodels (you can guess here…). Breakfast and lunch was served in an enormous open room full of tables and seats. But, everyone there was inviting, and immediately started conversations naturally. This was true of people in the hallways too, and included speakers and even DHH himself. Chad even told us to introduce ourselves to somebody new during one of the breaks, although this was happening naturally anyway.

The ad-hoc organization of lightning talks and RejectConf led to some of the more interesting demonstrations. Similarly, post-session conversations with speakers that led to group discussions, which then led to BoFs ,fully capture the spirit of the conference. The BoFs in general stood on their own as hallmarks of open, informative discussion… not just presentation.

Each day opened with one of the hilarious Mac VS PC spoofs by Gregg and Jason. Keynotes were accompanied by accordion and ukulele (courtesy Joey deVilla and Chad Fowler, respectively), and even a spoof serenade (to the tune of ) to our beloved creator DHH.

DHH stressed the importance of plugins both as a community way to share common functionality and testing-grounds for Rails core-features.Avi Bryant (creator of Seaside) gave a jaw dropping and eye opening talk exposing similarities between Smalltalk and Ruby. He strongly recommending stealing from Smalltalk’s maturity in IDE and especially VM implementations. Specifically, a highly distributed virtually machine network by a local Portland company called Gem
Stone. Supposedly, converting this to Ruby would not be overly difficult. But, it would make Ruby run as fast as Smalltalk, which is approximately 10 times faster than Python (and 3 to 4 times slower than Java).
Besides that, Avi stressed his belief that state is good, and that like wine, “objects get better with time”. In his opinion, that is the path that Ruby should follow, effectively becoming Smalltalk =p. This was not a unique opinion though, as Chad seemed to support Smalltalk. As a bit of comic relief, Avi introduced himself as “from the future” and had a hilarious metaphor from 2001: A Space Odyssey (we are the apes…)

Dave Thomas even attacked the browser with its transition from form based requests to AJAX as a medium for the web. He compared it to a similar evolution of ideas that happened in early computers, and warned to not get stuck in a loop of repeating history. Dave went on to get the audience to come up with more examples of this “cargo cult”, evoking responses as extreme as REST, Mongrel, and Rails itself. Although, he did boldly reject the idea that conferences could ever be a cargo cult =p.

Dave Thomas had a donation-based all day tutorial that raised over $12,000 by itself. After witnessing this accomplishment, Chad Fowler (and many other speakers that followed) encouraged the audience to continue donating throughout the conference. Eventually, a challenge to make this the standard for tech conferences was put in place. It is still going on, so anyone is still encouraged to donate.

As a whole, RailsConf07 was one of the most spectacular things I’ve ever experienced. There were many other interesting things happening, such as the effort to understand how “the enterprise” will fit into Rails, and various thought provoking sessions. But, by far the Rails community showed that it is concerned, responsible, and innovative, and I am extremely proud to be a part of it. If you were there, please take the time to blog your version of the event (and encourage others to do so as well) so the rest of the community can understand what is happening and share the excitement.

I downloaded and browsed the source code for the Beast forum today. Because it’s REST’ful, open source, and developed by Rick Olson, I recommend any Rails developer to look at it. What makes it even better is that a goal of Beast is to “stay around 500 lines”, meaning the app doesn’t take much time to figure out and can be studied leisurely.

Anything you define in YourController#rescue_action will be run when an error is raised, so check out what I found in Beast:

def rescue_action(exception)
exception.is_a?(ActiveRecord::RecordInvalid) ? render_invalid_record(exception.record) : super
end

def render_invalid_record(record)
render :action => (record.new_record? ? ‘new’ : ‘edit’)
end

As you can see, now any errors that are invalid AR objects will call a special method. This method render’s new or edit depending on whether the record exists yet or not.

So…rather than doing this in our controllers:

def create
@record = Record.new(params[:record])
if @record.save
flash[:notice] = ‘Saved successfully’
redirect_to :action => ‘index’
else
render :action => ‘new’
end
end

We simply do this:

def create
@record = Record.new(params[:record])
@record.save! and flash[:notice]=(’Saved successfully’) and redirect_to(:action => ‘index’)
end

The magic is in the save! method, as the “bang” raises errors instead of returning false when validation fails, not bad!

On another note, I’ve been spending a lot of my time recently on reading books about and developing a social site… As you know, my ambition to blog a lot since the creation of larrytheliquid.com has quickly diminished. I’m making this app to change that for myself and all bloggers… but that’s enough for now =)

Ruby has a nifty feature to be able to use a special constant that represents infinity, accessible as follows:

irb(main):001:0> 1.0/0
=> Infinity

Recently, I needed to validate that a field in my Rails model would be positive. While there are countless ways to do this, I was very happy to find a fun and practical use for infinity.

validates_inclusion_of :some_attribute, :within => 1..1.0/0, :allow_nil => true, :message => 'should be positive'

The attribute is validated to be within positive one (inclusive) and positive infinity (technically exclusive.) This is done with Rails’ validates_inclusion_of for numeric data, Ruby’s range operator .., and the infinity constant 1.0/0.

Huzzah!