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----
-title: >-
-    Component-Oriented Programming
-description: >-
-    A concise description of.
-tags: tech
----
-
-[A previous post in this
-blog](/2019/08/02/program-structure-and-composability.html) 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.
-
-## Components
-
-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.
-
-## Example
-
-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.](/assets/component-oriented-design/v1/main.html) 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.
-
-## DAG
-
-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.
-
-## Benefits
-
-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
-
-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.](/assets/component-oriented-design/v1/main_test.html) Note that all
-dependencies of each component being tested are mocked/stubbed next to them.
-
-### Configuration
-
-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.](/assets/component-oriented-design/v2/main.html) 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.
-
-### Setup/Runtime/Cleanup
-
-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.](/assets/component-oriented-design/v3/main.html) 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.
-
-
-## Conclusion
-
-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.
-
-## Criticisms/Questions
-
-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.