changes from proofread of CoP post
This commit is contained in:
Brian Picciano
parent
d40fe10213
commit
daae1b44bf
@ -8,29 +8,30 @@ description: >-
|
||||
[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 pattern/framework proposed was over-engineered. This post
|
||||
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
|
||||
"component", but in each case the meaning of "component" might be slightly
|
||||
different. Therefore to begin talking about components it is necessary to first
|
||||
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, properties of components include:
|
||||
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 to be a single-method
|
||||
component _if_ the language supports first-class functions.
|
||||
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.
|
||||
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
|
||||
@ -40,13 +41,13 @@ same component can be instantiated, as needed.
|
||||
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
|
||||
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).
|
||||
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.
|
||||
@ -54,76 +55,75 @@ 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 of its
|
||||
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
|
||||
Components are composed together to create component-oriented programs. This is
|
||||
done by passing components as parameters to other components during
|
||||
instantiation. The `main` process of the program is responsible for
|
||||
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 will posit a simple program and
|
||||
then describe how it would be implemented in a component-oriented way. The
|
||||
program chooses a random number and exposes an HTTP interface which allows
|
||||
users to try and guess that number. The following are requirements 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 identifying the user performing the guess and the
|
||||
number which is being guessed.
|
||||
* 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.
|
||||
* 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 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 to check
|
||||
subsequent guesses against, and 1000 points should be added to the user's
|
||||
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 which lists out user scores from highest to lowest.
|
||||
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 to our
|
||||
program: keeping scores and user interaction via HTTP. Each of these can be
|
||||
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 in order to
|
||||
save/restore scores (since 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.
|
||||
`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 so
|
||||
will need a component which produces random numbers. `httpHandlers` will also be
|
||||
recording score changes to the `scoreboard`, so will need access to the
|
||||
`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, so there will
|
||||
be an `httpServer` component which uses the `httpHandlers`.
|
||||
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
|
||||
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 how `main` is where all components
|
||||
are instantiated, and how all components' take in their child components as part
|
||||
of their instantiation.
|
||||
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 the 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`:
|
||||
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
|
||||
@ -134,7 +134,7 @@ net.Listener rand.Rand os.File
|
||||
+---------------+---------------+--> log.Logger
|
||||
```
|
||||
|
||||
Note that all the leaves of the DAG (i.e. nodes with no children) describe the
|
||||
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.
|
||||
|
||||
@ -156,24 +156,24 @@ 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 (sometimes
|
||||
called "integration") tests. Unit tests are those which do not make any
|
||||
requirements of the environment outside the test, such as the existence of a
|
||||
running database, filesystem, or network service. Unit tests do not interact
|
||||
with the world outside the testing process, but instead use mocks in place of
|
||||
functionality which would be expected by that world.
|
||||
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 which does not expose any way of swapping out dependencies
|
||||
for mocks of those dependencies. The primary culprit of this difficulty is
|
||||
direct usage of singletons and impure global functions. With component-oriented
|
||||
programs all components inherently allow for swapping out any dependencies via
|
||||
their instantiation parameters, so there's no extra effort needed to support
|
||||
unit tests.
|
||||
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
|
||||
@ -185,25 +185,25 @@ Practically all programs require some level of runtime configuration. This may
|
||||
take the form of command-line arguments, environment variables, configuration
|
||||
files, etc.
|
||||
|
||||
With 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.
|
||||
For any component which a configurable parameter effects, that component merely
|
||||
needs to take an instantiation parameter for that configurable parameter;
|
||||
`main` can connect the two together. This accounts for unit testing a
|
||||
component with different configurations, while still allowing for configuring
|
||||
any arbitrary internal functionality.
|
||||
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, wrapping whatever configuration-related functionality
|
||||
is needed, which other components use as a sub-component. The effect is the
|
||||
same.
|
||||
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
|
||||
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 amounts for both correct and incorrect guesses (currently
|
||||
* The point change values for both correct and incorrect guesses (currently
|
||||
hardcoded at 1000 and 1, respectively) should be configurable on the
|
||||
command-line.
|
||||
command-line;
|
||||
|
||||
* The save file's path, HTTP listen address, and save interval should all be
|
||||
configurable on the command-line.
|
||||
@ -212,56 +212,56 @@ example program's requirements will be augmented to include the following:
|
||||
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
|
||||
amounts, and configuration is set up inside `main`.
|
||||
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 internal state is assembled in order to make runtime
|
||||
possible. Runtime is the stage during which a program's actual function is being
|
||||
performed. Cleanup is the stage during which runtime stops and internal state is
|
||||
disassembled.
|
||||
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 (frequently it is not a concern at all).
|
||||
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
|
||||
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 even acting on the outside world still. Shouldn't
|
||||
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:
|
||||
Achieving a graceful setup and cleanup with components is quite simple.
|
||||
|
||||
During setup a single-threaded procedure (`main`) constructs the leaf components
|
||||
first, then the components which take those leaves as parameters, then the
|
||||
components which take _those_ as parameters, and so on, until the component DAG
|
||||
is constructed.
|
||||
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.
|
||||
At this point, the program's runtime has begun.
|
||||
|
||||
Once 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 the components were instantiated in.
|
||||
This order is inherently deterministic, since the components were instantiated
|
||||
by a single-threaded procedure.
|
||||
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, since 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
|
||||
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
|
||||
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.
|
||||
* 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.
|
||||
* During termination (cleanup), the program will save the latest set of scores
|
||||
to disk one final time.
|
||||
|
||||
[The new implementation which accounts for these new requirements can be found
|
||||
here.](/assets/component-oriented-design/v3/main.html) For this example go's
|
||||
[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.
|
||||
|
||||
@ -269,26 +269,26 @@ 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
|
||||
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 do to some extent already anyway.
|
||||
most of us already do to some extent anyway.
|
||||
|
||||
While beneficial in many ways, component-oriented programming is merely a tool
|
||||
which can be applied in many cases. It is certain that there are cases where it
|
||||
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:
|
||||
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 which likes
|
||||
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 maleability and deliverability, which naturally leads to skepticism of any
|
||||
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.
|
||||
|
||||
@ -301,52 +301,52 @@ 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
|
||||
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 needed is abstract typing.
|
||||
feature required is abstract typing.
|
||||
|
||||
It would be nice to one day see a language which explicitly supported this
|
||||
pattern by baking the component properties in as compiler checked rules.
|
||||
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's large sections of your
|
||||
program which are independent of each other then they could each have their own
|
||||
construction functions which `main` then calls.
|
||||
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 which are worth asking: Can my program be split up
|
||||
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 in fact fine as long as
|
||||
the parent's instantiation function remains pure.)
|
||||
(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?".
|
||||
way around it. The only questions are "how much?" and "where?"
|
||||
|
||||
The use of this pattern does not effect how those questions are answered, in my
|
||||
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 no
|
||||
matter what language, pattern, or framework is being used.
|
||||
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 when it
|
||||
comes to cleanup and use of singletons).
|
||||
perhaps making life inconvenient by not following CoP (specifically regarding
|
||||
cleanup and the use of singletons).
|
||||
|
||||
With functional programming it may well be, depending on the language, that CoP
|
||||
is technically being used, as functional languages are generally antagonistic
|
||||
towards to globals and impure functions already, which is most of the battle.
|
||||
Going from functional to component-oriented programming will generally be a
|
||||
problem of organization.
|
||||
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.
|
||||
|
||||
Loading…
Reference in New Issue
Block a user