There are two primary ways to write web applications these days:

• By building a single, monolithic web application (where all your code is in a single project and runs on a single domain).
• By building lots of small independent web services, each one with their own codebase and URL endpoint. (This methodology is known as Service Oriented Architecture.)

There are benefits and drawbacks to each approach.

What this article is about, however, are the side effects of writing your applications as web services as opposed to monolithic web applications. This article assumes you have some basic knowledge of writing web applications.

To get started, lets discuss the basics: the monolithic pattern and the service pattern.

The Monolithic Pattern

If I had to guess, I’d say that close to 99% of all active web applications are written as monolithic web apps. Why? I think that with so little practical information around on building service oriented web applications, it is only natural to build monolithic applications instead.

For starters, it’s a lot easier (at least in the beginning) to put all your web app code into a single project. Over time, as your project grows and more and more complexity is added, it’s a lot easier to simply refactor the old stuff than it is to rip your application apart into multiple independent services.

With so many monolithic applications out there, certain patterns have emerged over the years that make building and scaling monolithic web applications simpler–primarily, message queues.

When you’re working on a large monolithic web application, and need to:

• Speed up your response time,
• Render things asynchronously, or
• Perform time intensive operations on separate hardware…

The de-facto way to do so is by relying on the message queueing pattern. What this allows you to do is dump tasks into a message queue (something like RabbitMQ, Amazon SQS, or Redis), which will then be read (consumed) by one or more worker servers which read the task, process it, and dump the resulting values into some place that your web application can retrieve it.

Using message queues allows you to fire-and-forget slow operations, letting them happen naturally behind the scenes without delaying your web response time.

The Service Pattern

While service oriented architecture is certainly a hot topic in tech circles, it has still not gained widespread adoption, primarily due to the fact that it can be complex to implement, and has a steeper learning curve for new developers.

Service oriented web applications differ from monolithic web applications in several main ways:

• They are composed of multiple, small, independent services (think: www, api, accounts, portal).
• Each of these small services talks via HTTP to the other services.
• Each of these small services has less code in it (since they do less things), making them generally easier to maintain over long periods of time.
• Each of these small services has their own resources: web servers, databases, etc.

As you can probably see, building service oriented web applications essentially gives you many small moving parts as opposed to a single large part.

Effect 1: Maintainability

The first side effect you get by writing service oriented web applications is simple: better maintainability for your web application.

By having multiple small code bases that are each responsible for a very small amount of logic, you’ll typically have an easier time maintaining your code in the long term–since complexity is always kept to a minimum and the code is easy to navigate.

Let’s say you’ve written an accounts web service, which is solely responsible for creating, updating, and removing user accounts. This means that if you need to update some account logic, you know exactly where it is. There is no searching through thousands of lines of view code, database wrappers, or anything else–as this logic exists in only a single small place in your application.

Compare this with maintaining monolithic applications with lots of complexity in a single place, it becomes harder to make code changes without side effects, more difficult for new developers to dive in (where do I get started?), and tricky to refactor without breaking things.

Effect 2: Scalability

One of my favorite effects of writing service oriented web applications is the passive effect it has on scalability.

By having multiple small web services, each with their own resources (databases, message queues, web servers), this gives you the ultimate flexibility in scaling your web application. Let’s say your application consists of the following services:

• myapp-api, your public facing web API for developers.
• myapp-www, your public facing website.
• myapp-accounts, your internal API for managing user accounts.

Let’s say that each time a developer makes a request to your myapp-api project, you first have to authenticate the user against your myapp-accounts project (real-world scenario). Let’s also say that each time users log into their account on myapp-www, they ALSO must authenticate against your myapp-accounts project.

Since both your myapp-api and myapp-www projects rely on myapp-accounts for authentication data, it is likely that your myapp-accounts project will be getting a lot more usage than either your myapp-api or myapp-www projects.

Luckily, since you’ve built your web application as a composition of independent services, you now have several options to scale your myapp-accounts project:

• You can add more web servers.
• You can add a larger database.
• You can scale your database queries out (through read slaves).
• You can add or refactor your logic to make better use of message queues for time intensive tasks.

And you can do this all without affecting either of your other two services: myapp-api or myapp-www!

Compare this with scaling monolithic web applications: if you need to increase capacity for users authenticating against your database, you must scale your entire application–you can’t selectively choose which parts to scale, and which parts to leave alone (note: this isn’t *entirely* true, but I’m saying it anyway).

Effect 3: Simplicity

Another effect of writing service oriented web applications is that you generally have a much simpler back end system.

As I mentioned at the start of this article, one of the primary ways to scale monolithic web applications is by heavily relying on message queues to process time intensive tasks. When you begin writing services as opposed to monolithic apps, you can actually rely much less on messages queues than you’d think, eliminating technical complexity and architectural burden.

Let’s say you have a monolithic application that fires off a time intensive task (via a message queue) which subtracts money from a user’s bank account. The reason you fire this off as a task is because this function must perform a lot of sanity checks before removing the user’s money, and you want to ensure this happens safely outside of your user’s request-response cycle.

What if one of your internal services handled all banking transactions for users? Instead of:

• Running a message queue,
• Running a backup message queue (in case of failure), and
• Running multiple worker servers…

You could simply fire off an HTTP POST request to your internal myapp-transactions project and let it handle things in real time. The reason you could do this is because, knowing your transactions application requires fast servers, you could:

• Ensure your myapp-transactions service runs on very fast web servers,
• Can quickly process and handle banking transactions,
• Keep your code simple and independent of external dependencies.

Instead of relying on a message queue, you could: rip it out of your architecture all together, have a small (easily maintainable) code base for your transactions, expose a simple HTTP service that your other applications can talk with, and scale your transactions service independent of all your other services (this way, if you suddenly start doing a lot of transactions, you only need to focus on scaling a single small code base).

Wrap Up

In my experience, building service oriented web applications generally leads to simpler, faster, and more scalable web applications. Without realizing it, you can:

• Make maintaining and expanding your code base easier.
• Lower the bar for new developers.
• Make scaling your application a simple process.
• Reduce architectural overhead and complexity.

Also: if you’re at all interested in building service oriented web applications, you may want to check out my book: The Heroku Hacker’s Guide, it teaches you how to use Heroku’s platform to build fast, small, independent web services.