Component-Oriented Programming

by Brian Picciano - (16 min read)

A previous post in this blog focused on a framework developed to make designing component-based programs easier. In retrospect, the proposed pattern/framework was over-engineered. This post attempts to present the same ideas in a more distilled form, as a simple programming pattern and without the unnecessary framework.


Many languages, libraries, and patterns make use of a concept called a “component,” but in each case the meaning of “component” might be slightly different. Therefore, to begin talking about components, it is necessary to first describe what is meant by “component” in this post.

For the purposes of this post, the properties of components include the following.

 1… Abstract: A component is an interface consisting of one or more methods.

   1a… A function might be considered a single-method component if the language supports first-class functions.

   1b… A component, being an interface, may have one or more implementations. Generally, there will be a primary implementation, which is used during a program’s runtime, and secondary “mock” implementations, which are only used when testing other components.

 2… Instantiatable: An instance of a component, given some set of parameters, can be instantiated as a standalone entity. More than one of the same component can be instantiated, as needed.

 3… Composable: A component may be used as a parameter of another component’s instantiation. This would make it a child component of the one being instantiated (the parent).

 4… Pure: A component may not use mutable global variables (i.e., singletons) or impure global functions (e.g., system calls). It may only use constants and variables/components given to it during instantiation.

 5… Ephemeral: A component may have a specific method used to clean up all resources that it’s holding (e.g., network connections, file handles, language-specific lightweight threads, etc.).

   5a… This cleanup method should not clean up any child components given as instantiation parameters.

   5b… This cleanup method should not return until the component’s cleanup is complete.

   5c… A component should not be cleaned up until all its parent components are cleaned up.

Components are composed together to create component-oriented programs. This is done by passing components as parameters to other components during instantiation. The main procedure of the program is responsible for instantiating and composing the components of the program.


It’s easier to show than to tell. This section posits a simple program and then describes how it would be implemented in a component-oriented way. The program chooses a random number and exposes an HTTP interface that allows users to try and guess that number. The following are requirements of the program:

  • A guess consists of a name that identifies the user performing the guess and the number that is being guessed;

  • A score is kept for each user who has performed a guess;

  • Upon an incorrect guess, the user should be informed of whether they guessed too high or too low, and 1 point should be deducted from their score;

  • Upon a correct guess, the program should pick a new random number against which to check subsequent guesses, and 1000 points should be added to the user’s score;

  • The HTTP interface should have two endpoints: one for users to submit guesses, and another that lists out user scores from highest to lowest;

  • Scores should be saved to disk so they survive program restarts.

It seems clear that there will be two major areas of functionality for our program: score-keeping and user interaction via HTTP. Each of these can be encapsulated into components called scoreboard and httpHandlers, respectively.

scoreboard will need to interact with a filesystem component to save/restore scores (because it can’t use system calls directly; see property 4). It would be wasteful for scoreboard to save the scores to disk on every score update, so instead it will do so every 5 seconds. A time component will be required to support this.

httpHandlers will be choosing the random number which is being guessed, and will therefore need a component that produces random numbers. httpHandlers will also be recording score changes to scoreboard, so it will need access to scoreboard.

The example implementation will be written in go, which makes differentiating HTTP handler functionality from the actual HTTP server quite easy; thus, there will be an httpServer component that uses httpHandlers.

Finally, a logger component will be used in various places to log useful information during runtime.

The example implementation can be found here. While most of it can be skimmed, it is recommended to at least read through the main function to see how components are composed together. Note that main is where all components are instantiated, and that all components’ take in their child components as part of their instantiation.


One way to look at a component-oriented program is as a directed acyclic graph (DAG), where each node in the graph represents a component, and each edge indicates that one component depends upon another component for instantiation. For the previous program, it’s quite easy to construct such a DAG just by looking at main, as in the following:

net.Listener     rand.Rand        os.File
     ^               ^               ^
     |               |               |
 httpServer --> httpHandlers --> scoreboard --> time.Ticker
     |               |               |
     +---------------+---------------+--> log.Logger

Note that all the leaves of the DAG (i.e., nodes with no children) describe the points where the program meets the operating system via system calls. The leaves are, in essence, the program’s interface with the outside world.

While it’s not necessary to actually draw out the DAG for every program one writes, it can be helpful to at least think about the program’s structure in these terms.


Looking at the previous example implementation, one would be forgiven for having the immediate reaction of “This seems like a lot of extra work for little gain. Why can’t I just make the system calls where I need to, and not bother with wrapping them in interfaces and all these other rules?”

The following sections will answer that concern by showing the benefits gained by following a component-oriented pattern.


Testing is important, that much is being assumed.

A distinction to be made with testing is between unit and non-unit tests. Unit tests are those for which there are no requirements for the environment outside the test, such as the existence of global variables, running databases, filesystems, or network services. Unit tests do not interact with the world outside the testing procedure, but instead use mocks in place of the functionality that would be expected by that world.

Unit tests are important because they are faster to run and more consistent than non-unit tests. Unit tests also force the programmer to consider different possible states of a component’s dependencies during the mocking process.

Unit tests are often not employed by programmers, because they are difficult to implement for code that does not expose any way to swap out dependencies for mocks of those dependencies. The primary culprit of this difficulty is the direct usage of singletons and impure global functions. For component-oriented programs, all components inherently allow for the swapping out of any dependencies via their instantiation parameters, so there’s no extra effort needed to support unit tests.

Tests for the example implementation can be found here. Note that all dependencies of each component being tested are mocked/stubbed next to them.


Practically all programs require some level of runtime configuration. This may take the form of command-line arguments, environment variables, configuration files, etc.

For a component-oriented program, all components are instantiated in the same place, main, so it’s very easy to expose any arbitrary parameter to the user via configuration. For any component that is affected by a configurable parameter, that component merely needs to take an instantiation parameter for that configurable parameter; main can connect the two together. This accounts for the unit testing of a component with different configurations, while still allowing for the configuration of any arbitrary internal functionality.

For more complex configuration systems, it is also possible to implement a configuration component that wraps whatever configuration-related functionality is needed, which other components use as a sub-component. The effect is the same.

To demonstrate how configuration works in a component-oriented program, the example program’s requirements will be augmented to include the following:

  • The point change values for both correct and incorrect guesses (currently hardcoded at 1000 and 1, respectively) should be configurable on the command-line;

  • The save file’s path, HTTP listen address, and save interval should all be configurable on the command-line.

The new implementation, with newly configurable parameters, can be found here. Most of the program has remained the same, and all unit tests from before remain valid. The primary difference is that scoreboard takes in two new parameters for the point change values, and configuration is set up inside main using the flags package.


A program can be split into three stages: setup, runtime, and cleanup. Setup is the stage during which the internal state is assembled to make runtime possible. Runtime is the stage during which a program’s actual function is being performed. Cleanup is the stage during which the runtime stops and internal state is disassembled.

A graceful (i.e., reliably correct) setup is quite natural to accomplish for most. On the other hand, a graceful cleanup is, unfortunately, not a programmer’s first concern (if it is a concern at all).

When building reliable and correct programs, a graceful cleanup is as important as a graceful setup and runtime. A program is still running while it is being cleaned up, and it’s possibly still acting on the outside world. Shouldn’t it behave correctly during that time?

Achieving a graceful setup and cleanup with components is quite simple.

During setup, a single-threaded procedure (main) first constructs the leaf components, then the components that take those leaves as parameters, then the components that take those as parameters, and so on, until the component DAG is fully constructed.

At this point, the program’s runtime has begun.

Once the runtime is over, signified by a process signal or some other mechanism, it’s only necessary to call each component’s cleanup method (if any; see property 5) in the reverse of the order in which the components were instantiated. This order is inherently deterministic, as the components were instantiated by a single-threaded procedure.

Inherent to this pattern is the fact that each component will certainly be cleaned up before any of its child components, as its child components must have been instantiated first, and a component will not clean up child components given as parameters (properties 5a and 5c). Therefore, the pattern avoids use-after-cleanup situations.

To demonstrate a graceful cleanup in a component-oriented program, the example program’s requirements will be augmented to include the following:

  • The program will terminate itself upon an interrupt signal;

  • During termination (cleanup), the program will save the latest set of scores to disk one final time.

The new implementation that accounts for these new requirements can be found here. For this example, go’s defer feature could have been used instead, which would have been even cleaner, but was omitted for the sake of those using other languages.


The component pattern helps make programs more reliable with only a small amount of extra effort incurred. In fact, most of the pattern has to do with establishing sensible abstractions around global functionality and remembering certain idioms for how those abstractions should be composed together, something most of us already do to some extent anyway.

While beneficial in many ways, component-oriented programming is merely a tool that can be applied in many cases. It is certain that there are cases where it is not the right tool for the job, so apply it deliberately and intelligently.


In lieu of a FAQ, I will attempt to premeditate questions and criticisms of the component-oriented programming pattern laid out in this post.

This seems like a lot of extra work.

Building reliable programs is a lot of work, just as building a reliable anything is a lot of work. Many of us work in an industry that likes to balance reliability (sometimes referred to by the more specious “quality”) with malleability and deliverability, which naturally leads to skepticism of any suggestions requiring more time spent on reliability. This is not necessarily a bad thing, it’s just how the industry functions.

All that said, a pattern need not be followed perfectly to be worthwhile, and the amount of extra work incurred by it can be decided based on practical considerations. I merely maintain that code which is (mostly) component-oriented is easier to maintain in the long run, even if it might be harder to get off the ground initially.

My language makes this difficult.

I don’t know of any language which makes this pattern particularly easier than others, so, unfortunately, we’re all in the same boat to some extent (though I recognize that some languages, or their ecosystems, make it more difficult than others). It seems to me that this pattern shouldn’t be unbearably difficult for anyone to implement in any language either, however, as the only language feature required is abstract typing.

It would be nice to one day see a language that explicitly supports this pattern by baking the component properties in as compiler-checked rules.

My main is too big

There’s no law saying all component construction needs to happen in main, that’s just the most sensible place for it. If there are large sections of your program that are independent of each other, then they could each have their own construction functions that main then calls.

Other questions that are worth asking include: Can my program be split up into multiple programs? Can the responsibilities of any of my components be refactored to reduce the overall complexity of the component DAG? Can the instantiation of any components be moved within their parent’s instantiation function?

(This last suggestion may seem to be disallowed, but is fine as long as the parent’s instantiation function remains pure.)

Won’t this will result in over-abstraction?

Abstraction is a necessary tool in a programmer’s toolkit, there is simply no way around it. The only questions are “how much?” and “where?”

The use of this pattern does not affect how those questions are answered, in my opinion, but instead aims to more clearly delineate the relationships and interactions between the different abstracted types once they’ve been established using other methods. Over-abstraction is possible and avoidable regardless of which language, pattern, or framework is being used.

Does CoP conflict with object-oriented or functional programming?

I don’t think so. OoP languages will have abstract types as part of their core feature-set; most difficulties are going to be with deliberately not using other features of an OoP language, and with imported libraries in the language perhaps making life inconvenient by not following CoP (specifically regarding cleanup and the use of singletons).

For functional programming, it may well be that, depending on the language, CoP is technically being used, as functional languages are already generally antagonistic toward globals and impure functions, which is most of the battle. If anything, the transition from functional to component-oriented programming will generally be an organizational task.