353 lines
16 KiB
Markdown
353 lines
16 KiB
Markdown
---
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title: >-
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Component-Oriented Programming
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description: >-
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A concise description of.
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---
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[A previous post in this
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blog](/2019/08/02/program-structure-and-composability.html) focused on a
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framework developed to make designing component-based programs easier. In
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retrospect, the proposed pattern/framework was over-engineered. This post
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attempts to present the same ideas in a more distilled form, as a simple
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programming pattern and without the unnecessary framework.
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## Components
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Many languages, libraries, and patterns make use of a concept called a
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"component," but in each case the meaning of "component" might be slightly
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different. Therefore, to begin talking about components, it is necessary to first
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describe what is meant by "component" in this post.
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For the purposes of this post, the properties of components include the
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following.
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1... **Abstract**: A component is an interface consisting of one or more
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methods.
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1a... A function might be considered a single-method component
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_if_ the language supports first-class functions.
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1b... A component, being an interface, may have one or more
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implementations. Generally, there will be a primary implementation, which is
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used during a program's runtime, and secondary "mock" implementations, which are
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only used when testing other components.
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2... **Instantiatable**: An instance of a component, given some set of
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parameters, can be instantiated as a standalone entity. More than one of the
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same component can be instantiated, as needed.
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3... **Composable**: A component may be used as a parameter of another
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component's instantiation. This would make it a child component of the one being
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instantiated (the parent).
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4... **Pure**: A component may not use mutable global variables (i.e.,
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singletons) or impure global functions (e.g., system calls). It may only use
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constants and variables/components given to it during instantiation.
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5... **Ephemeral**: A component may have a specific method used to clean
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up all resources that it's holding (e.g., network connections, file handles,
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language-specific lightweight threads, etc.).
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5a... This cleanup method should _not_ clean up any child
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components given as instantiation parameters.
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5b... This cleanup method should not return until the
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component's cleanup is complete.
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5c... A component should not be cleaned up until all its
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parent components are cleaned up.
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Components are composed together to create component-oriented programs. This is
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done by passing components as parameters to other components during
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instantiation. The `main` procedure of the program is responsible for
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instantiating and composing the components of the program.
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## Example
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It's easier to show than to tell. This section posits a simple program and then
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describes how it would be implemented in a component-oriented way. The program
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chooses a random number and exposes an HTTP interface that allows users to try
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and guess that number. The following are requirements of the program:
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* A guess consists of a name that identifies the user performing the guess and
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the number that is being guessed;
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* A score is kept for each user who has performed a guess;
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* Upon an incorrect guess, the user should be informed of whether they guessed
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too high or too low, and 1 point should be deducted from their score;
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* Upon a correct guess, the program should pick a new random number against
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which to check subsequent guesses, and 1000 points should be added to the
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user's score;
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* The HTTP interface should have two endpoints: one for users to submit guesses,
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and another that lists out user scores from highest to lowest;
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* Scores should be saved to disk so they survive program restarts.
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It seems clear that there will be two major areas of functionality for our
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program: score-keeping and user interaction via HTTP. Each of these can be
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encapsulated into components called `scoreboard` and `httpHandlers`,
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respectively.
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`scoreboard` will need to interact with a filesystem component to save/restore
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scores (because it can't use system calls directly; see property 4). It would be
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wasteful for `scoreboard` to save the scores to disk on every score update, so
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instead it will do so every 5 seconds. A time component will be required to
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support this.
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`httpHandlers` will be choosing the random number which is being guessed, and
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will therefore need a component that produces random numbers. `httpHandlers`
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will also be recording score changes to `scoreboard`, so it will need access to
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`scoreboard`.
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The example implementation will be written in go, which makes differentiating
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HTTP handler functionality from the actual HTTP server quite easy; thus, there
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will be an `httpServer` component that uses `httpHandlers`.
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Finally, a `logger` component will be used in various places to log useful
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information during runtime.
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[The example implementation can be found
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here.](/assets/component-oriented-design/v1/main.html) While most of it can be
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skimmed, it is recommended to at least read through the `main` function to see
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how components are composed together. Note that `main` is where all components
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are instantiated, and that all components' take in their child components as
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part of their instantiation.
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## DAG
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One way to look at a component-oriented program is as a directed acyclic graph
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(DAG), where each node in the graph represents a component, and each edge
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indicates that one component depends upon another component for instantiation.
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For the previous program, it's quite easy to construct such a DAG just by
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looking at `main`, as in the following:
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```
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net.Listener rand.Rand os.File
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^ ^ ^
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httpServer --> httpHandlers --> scoreboard --> time.Ticker
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+---------------+---------------+--> log.Logger
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```
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Note that all the leaves of the DAG (i.e., nodes with no children) describe the
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points where the program meets the operating system via system calls. The leaves
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are, in essence, the program's interface with the outside world.
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While it's not necessary to actually draw out the DAG for every program one
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writes, it can be helpful to at least think about the program's structure in
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these terms.
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## Benefits
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Looking at the previous example implementation, one would be forgiven for having
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the immediate reaction of "This seems like a lot of extra work for little gain.
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Why can't I just make the system calls where I need to, and not bother with
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wrapping them in interfaces and all these other rules?"
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The following sections will answer that concern by showing the benefits gained
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by following a component-oriented pattern.
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### Testing
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Testing is important, that much is being assumed.
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A distinction to be made with testing is between unit and non-unit tests. Unit
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tests are those for which there are no requirements for the environment outside
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the test, such as the existence of global variables, running databases,
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filesystems, or network services. Unit tests do not interact with the world
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outside the testing procedure, but instead use mocks in place of the
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functionality that would be expected by that world.
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Unit tests are important because they are faster to run and more consistent than
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non-unit tests. Unit tests also force the programmer to consider different
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possible states of a component's dependencies during the mocking process.
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Unit tests are often not employed by programmers, because they are difficult to
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implement for code that does not expose any way to swap out dependencies for
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mocks of those dependencies. The primary culprit of this difficulty is the
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direct usage of singletons and impure global functions. For component-oriented
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programs, all components inherently allow for the swapping out of any
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dependencies via their instantiation parameters, so there's no extra effort
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needed to support unit tests.
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[Tests for the example implementation can be found
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here.](/assets/component-oriented-design/v1/main_test.html) Note that all
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dependencies of each component being tested are mocked/stubbed next to them.
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### Configuration
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Practically all programs require some level of runtime configuration. This may
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take the form of command-line arguments, environment variables, configuration
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files, etc.
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For a component-oriented program, all components are instantiated in the same
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place, `main`, so it's very easy to expose any arbitrary parameter to the user
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via configuration. For any component that is affected by a configurable
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parameter, that component merely needs to take an instantiation parameter for
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that configurable parameter; `main` can connect the two together. This accounts
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for the unit testing of a component with different configurations, while still
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allowing for the configuration of any arbitrary internal functionality.
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For more complex configuration systems, it is also possible to implement a
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`configuration` component that wraps whatever configuration-related
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functionality is needed, which other components use as a sub-component. The
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effect is the same.
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To demonstrate how configuration works in a component-oriented program, the
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example program's requirements will be augmented to include the following:
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* The point change values for both correct and incorrect guesses (currently
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hardcoded at 1000 and 1, respectively) should be configurable on the
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command-line;
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* The save file's path, HTTP listen address, and save interval should all be
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configurable on the command-line.
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[The new implementation, with newly configurable parameters, can be found
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here.](/assets/component-oriented-design/v2/main.html) Most of the program has
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remained the same, and all unit tests from before remain valid. The primary
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difference is that `scoreboard` takes in two new parameters for the point change
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values, and configuration is set up inside `main` using the `flags` package.
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### Setup/Runtime/Cleanup
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A program can be split into three stages: setup, runtime, and cleanup. Setup is
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the stage during which the internal state is assembled to make runtime possible.
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Runtime is the stage during which a program's actual function is being
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performed. Cleanup is the stage during which the runtime stops and internal
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state is disassembled.
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A graceful (i.e., reliably correct) setup is quite natural to accomplish for
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most. On the other hand, a graceful cleanup is, unfortunately, not a programmer's
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first concern (if it is a concern at all).
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When building reliable and correct programs, a graceful cleanup is as important
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as a graceful setup and runtime. A program is still running while it is being
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cleaned up, and it's possibly still acting on the outside world. Shouldn't
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it behave correctly during that time?
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Achieving a graceful setup and cleanup with components is quite simple.
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During setup, a single-threaded procedure (`main`) first constructs the leaf
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components, then the components that take those leaves as parameters, then the
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components that take _those_ as parameters, and so on, until the component DAG
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is fully constructed.
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At this point, the program's runtime has begun.
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Once the runtime is over, signified by a process signal or some other mechanism,
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it's only necessary to call each component's cleanup method (if any; see
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property 5) in the reverse of the order in which the components were
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instantiated. This order is inherently deterministic, as the components were
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instantiated by a single-threaded procedure.
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Inherent to this pattern is the fact that each component will certainly be
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cleaned up before any of its child components, as its child components must have
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been instantiated first, and a component will not clean up child components
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given as parameters (properties 5a and 5c). Therefore, the pattern avoids
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use-after-cleanup situations.
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To demonstrate a graceful cleanup in a component-oriented program, the example
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program's requirements will be augmented to include the following:
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* The program will terminate itself upon an interrupt signal;
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* During termination (cleanup), the program will save the latest set of scores
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to disk one final time.
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[The new implementation that accounts for these new requirements can be found
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here.](/assets/component-oriented-design/v3/main.html) For this example, go's
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`defer` feature could have been used instead, which would have been even
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cleaner, but was omitted for the sake of those using other languages.
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## Conclusion
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The component pattern helps make programs more reliable with only a small amount
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of extra effort incurred. In fact, most of the pattern has to do with
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establishing sensible abstractions around global functionality and remembering
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certain idioms for how those abstractions should be composed together, something
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most of us already do to some extent anyway.
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While beneficial in many ways, component-oriented programming is merely a tool
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that can be applied in many cases. It is certain that there are cases where it
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is not the right tool for the job, so apply it deliberately and intelligently.
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## Criticisms/Questions
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In lieu of a FAQ, I will attempt to premeditate questions and criticisms of the
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component-oriented programming pattern laid out in this post.
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**This seems like a lot of extra work.**
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Building reliable programs is a lot of work, just as building a
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reliable _anything_ is a lot of work. Many of us work in an industry that likes
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to balance reliability (sometimes referred to by the more specious "quality")
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with malleability and deliverability, which naturally leads to skepticism of any
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suggestions requiring more time spent on reliability. This is not necessarily a
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bad thing, it's just how the industry functions.
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All that said, a pattern need not be followed perfectly to be worthwhile, and
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the amount of extra work incurred by it can be decided based on practical
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considerations. I merely maintain that code which is (mostly) component-oriented
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is easier to maintain in the long run, even if it might be harder to get off the
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ground initially.
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**My language makes this difficult.**
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I don't know of any language which makes this pattern particularly easier than
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others, so, unfortunately, we're all in the same boat to some extent (though I
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recognize that some languages, or their ecosystems, make it more difficult than
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others). It seems to me that this pattern shouldn't be unbearably difficult for
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anyone to implement in any language either, however, as the only language
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feature required is abstract typing.
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It would be nice to one day see a language that explicitly supports this
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pattern by baking the component properties in as compiler-checked rules.
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**My `main` is too big**
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There's no law saying all component construction needs to happen in `main`,
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that's just the most sensible place for it. If there are large sections of your
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program that are independent of each other, then they could each have their own
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construction functions that `main` then calls.
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Other questions that are worth asking include: Can my program be split up
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into multiple programs? Can the responsibilities of any of my components be
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refactored to reduce the overall complexity of the component DAG? Can the
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instantiation of any components be moved within their parent's
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instantiation function?
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(This last suggestion may seem to be disallowed, but is fine as long as the
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parent's instantiation function remains pure.)
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**Won't this will result in over-abstraction?**
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Abstraction is a necessary tool in a programmer's toolkit, there is simply no
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way around it. The only questions are "how much?" and "where?"
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The use of this pattern does not affect how those questions are answered, in my
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opinion, but instead aims to more clearly delineate the relationships and
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interactions between the different abstracted types once they've been
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established using other methods. Over-abstraction is possible and avoidable
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regardless of which language, pattern, or framework is being used.
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**Does CoP conflict with object-oriented or functional programming?**
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I don't think so. OoP languages will have abstract types as part of their core
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feature-set; most difficulties are going to be with deliberately _not_ using
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other features of an OoP language, and with imported libraries in the language
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perhaps making life inconvenient by not following CoP (specifically regarding
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cleanup and the use of singletons).
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For functional programming, it may well be that, depending on the language, CoP
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is technically being used, as functional languages are already generally
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antagonistic toward globals and impure functions, which is most of the battle.
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If anything, the transition from functional to component-oriented programming
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will generally be an organizational task.
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