mediocre-blog/src/_posts/2019-08-02-program-structure-and-composability.md

---
title: >-
    Program Structure and Composability
description: >-
    Discussing the nature of program structure, the problems presented by
    complex structures, and a pattern that helps in solving those problems.
---

## Part 0: Introduction

This post is focused on a concept I call “program structure,” which I will try
to shed some light on before discussing complex program structures. I will then
discuss why complex structures can be problematic to deal with, and will finally
discuss a pattern for dealing with those problems.

My background is as a backend engineer working on large projects that have had
many moving parts; most had multiple programs interacting with each other, used
many different databases in various contexts, and faced large amounts of load
from millions of users. Most of this post will be framed from my perspective,
and will present problems in the way I have experienced them. I believe,
however, that the concepts and problems I discuss here are applicable to many
other domains, and I hope those with a foot in both backend systems and a second
domain can help to translate the ideas between the two.

Also note that I will be using Go as my example language, but none of the
concepts discussed here are specific to Go. To that end, I’ve decided to favor
readable code over “correct” code, and so have elided things that most gophers
hold near-and-dear, such as error checking and proper documentation, in order to
make the code as accessible as possible to non-gophers as well. As with before,
I trust that someone with a foot in Go and another language can help me
translate between the two.

## Part 1: Program Structure

In this section I will discuss the difference between directory and program
structure, show how global state is antithetical to compartmentalization (and
therefore good program structure), and finally discuss a more effective way to
think about program structure.

### Directory Structure

For a long time, I thought about program structure in terms of the hierarchy
present in the filesystem. In my mind, a program’s structure looked like this:

```
// The directory structure of a project called gobdns.
src/
    config/
    dns/
    http/
    ips/
    persist/
    repl/
    snapshot/
    main.go
```

What I grew to learn was that this conflation of “program structure” with
“directory structure” is ultimately unhelpful. While it can’t be denied that
every program has a directory structure (and if not, it ought to), this does not
mean that the way the program looks in a filesystem in any way corresponds to
how it looks in our mind’s eye.

The most notable way to show this is to consider a library package. Here is the
structure of a simple web-app which uses redis (my favorite database) as a
backend:

```
src/
    redis/
    http/
    main.go
```

If I were to ask you, based on that directory structure, what the program does
in the most abstract terms, you might say something like: “The program
establishes an http server that listens for requests. It also establishes a
connection to the redis server. The program then interacts with redis in
different ways based on the http requests that are received on the server.”

And that would be a good guess. Here’s a diagram that depicts the program
structure, wherein the root node, `main.go`, takes in requests from `http` and
processes them using `redis`.

{% include image.html
    dir="program-structure" file="diag1.jpg" width=519
    descr="Example 1"
    %}

This is certainly a viable guess for how a program with that directory
structure operates, but consider another answer: “A component of the program
called `server` establishes an http server that listens for requests. `server`
also establishes a connection to a redis server. `server` then interacts with
that redis connection in different ways based on the http requests that are
received on the http server. Additionally, `server` tracks statistics about
these interactions and makes them available to other components. The root
component of the program establishes a connection to a second redis server, and
stores those statistics in that redis server.” Here’s another diagram to depict
_that_ program.

{% include image.html
    dir="program-structure" file="diag2.jpg" width=712
    descr="Example 2"
    %}

The directory structure could apply to either description; `redis` is just a
library which allows for interaction with a redis server, but it doesn’t
specify _which_ or _how many_ servers. However, those are extremely important
factors that are definitely reflected in our concept of the program’s
structure, and not in the directory structure. **What the directory structure
reflects are the different _kinds_ of components available to use, but it does
not reflect how a program will use those components.**


### Global State vs Compartmentalization

The directory-centric view of structure often leads to the use of global
singletons to manage access to external resources like RPC servers and
databases. In examples 1 and 2 the `redis` library might contain code which
looks something like this:

```go
// A mapping of connection names to redis connections.
var globalConns = map[string]*RedisConn{}

func Get(name string) *RedisConn {
    if globalConns[name] == nil {
        globalConns[name] = makeRedisConnection(name)
    }
    return globalConns[name]
}
```

Even though this pattern would work, it breaks with our conception of the
program structure in more complex cases like example 2. Rather than the `redis`
component being owned by the `server` component, which actually uses it, it
would be practically owned by _all_ components, since all are able to use it.
Compartmentalization has been broken, and can only be held together through
sheer human discipline.

**This is the problem with all global state. It is shareable among all
components of a program, and so is accountable to none of them.** One must look
at an entire codebase to understand how a globally held component is used,
which might not even be possible for a large codebase. Therefore, the
maintainers of these shared components rely entirely on the discipline of their
fellow coders when making changes, usually discovering where that discipline
broke down once the changes have been pushed live.

Global state also makes it easier for disparate programs/components to share
datastores for completely unrelated tasks. In example 2, rather than creating a
new redis instance for the root component’s statistics storage, the coder might
have instead said, “well, there’s already a redis instance available, I’ll just
use that.” And so, compartmentalization would have been broken further. Perhaps
the two instances _could_ be coalesced into the same instance for the sake of
resource efficiency, but that decision would be better made at runtime via the
configuration of the program, rather than being hardcoded into the code.

From the perspective of team management, global state-based patterns do nothing
except slow teams down. The person/team responsible for maintaining the central
library in which shared components live (`redis`, in the above examples)
becomes the bottleneck for creating new instances for new components, which
will further lead to re-using existing instances rather than creating new ones,
further breaking compartmentalization. Additionally the person/team responsible
for the central library, rather than the team using it, often finds themselves
as the maintainers of the shared resource.

### Component Structure

So what does proper program structure look like? In my mind the structure of a
program is a hierarchy of components, or, in other words, a tree. The leaf
nodes of the tree are almost _always_ IO related components, e.g., database
connections, RPC server frameworks or clients, message queue consumers, etc.
The non-leaf nodes will _generally_ be components that bring together the
functionalities of their children in some useful way, though they may also have
some IO functionality of their own.

Let's look at an even more complex structure, still only using the `redis` and
`http` component types:

{% include image.html
    dir="program-structure" file="diag3.jpg" width=729
    descr="Example 3"
    %}

This component structure contains the addition of the `debug` component.
Clearly the `http` and `redis` components are reusable in different contexts,
but for this example the `debug` endpoint is as well. It creates a separate
http server that can be queried to perform runtime debugging of the program,
and can be tacked onto virtually any program. The `rest-api` component is
specific to this program and is therefore not reusable. Let’s dive into it a
bit to see how it might be implemented:

```go
// RestAPI is very much not thread-safe, hopefully it doesn't have to handle
// more than one request at once.
type RestAPI struct {
    redisConn *redis.RedisConn
    httpSrv   *http.Server

    // Statistics exported for other components to see
    RequestCount int
    FooRequestCount int
    BarRequestCount int
}

func NewRestAPI() *RestAPI {
    r := new(RestAPI)
    r.redisConn := redis.NewConn("127.0.0.1:6379")

    // mux will route requests to different handlers based on their URL path.
    mux := http.NewServeMux()
    mux.HandleFunc("/foo", r.fooHandler)
    mux.HandleFunc("/bar", r.barHandler)
    r.httpSrv := http.NewServer(mux)

    // Listen for requests and serve them in the background.
    go r.httpSrv.Listen(":8000")

    return r
}

func (r *RestAPI) fooHandler(rw http.ResponseWriter, r *http.Request) {
    r.redisConn.Command("INCR", "fooKey")
    r.RequestCount++
    r.FooRequestCount++
}

func (r *RestAPI) barHandler(rw http.ResponseWriter, r *http.Request) {
    r.redisConn.Command("INCR", "barKey")
    r.RequestCount++
    r.BarRequestCount++
}
```


In that snippet `rest-api` coalesced `http` and `redis` into a simple REST-like
api using pre-made library components. `main.go`, the root component, does much
the same:

```go
func main() {
    // Create debug server and start listening in the background
    debugSrv := debug.NewServer()

    // Set up the RestAPI, this will automatically start listening
    restAPI := NewRestAPI()

    // Create another redis connection and use it to store statistics
    statsRedisConn := redis.NewConn("127.0.0.1:6380")
    for {
        time.Sleep(1 * time.Second)
        statsRedisConn.Command("SET", "numReqs", restAPI.RequestCount)
        statsRedisConn.Command("SET", "numFooReqs", restAPI.FooRequestCount)
        statsRedisConn.Command("SET", "numBarReqs", restAPI.BarRequestCount)
    }
}
```

One thing that is clearly missing in this program is proper configuration,
whether from command-line or environment variables, etc. As it stands, all
configuration parameters, such as the redis addresses and http listen
addresses, are hardcoded. Proper configuration actually ends up being somewhat
difficult, as the ideal case would be for each component to set up its own
configuration variables without its parent needing to be aware. For example,
`redis` could set up `addr` and `pool-size` parameters. The problem is that there
are two `redis` components in the program, and their parameters would therefore
conflict with each other. An elegant solution to this problem is discussed in
the next section.

## Part 2: Components, Configuration, and Runtime

The key to the configuration problem is to recognize that, even if there are
two of the same component in a program, they can’t occupy the same place in the
program’s structure. In the above example, there are two `http` components: one
under `rest-api` and the other under `debug`. Because the structure is
represented as a tree of components, the “path” of any node in the tree
uniquely represents it in the structure. For example, the two `http` components
in the previous example have these paths:

```
root -> rest-api -> http
root -> debug -> http
```

If each component were to know its place in the component tree, then it would
easily be able to ensure that its configuration and initialization didn’t
conflict with other components of the same type. If the `http` component sets
up a command-line parameter to know what address to listen on, the two `http`
components in that program would set up:

```
--rest-api-listen-addr
--debug-listen-addr
```

So how can we enable each component to know its path in the component structure?
To answer this, we’ll have to take a detour through a type, called `Component`.

### Component and Configuration

The `Component` type is a made-up type (though you’ll be able to find an
implementation of it at the end of this post). It has a single primary purpose,
and that is to convey the program’s structure to new components.

To see how this is done, let's look at a couple of `Component`'s methods:

```go
// Package mcmp

// New returns a new Component which has no parents or children. It is therefore
// the root component of a component hierarchy.
func New() *Component

// Child returns a new child of the called upon Component.
func (*Component) Child(name string) *Component

// Path returns the Component's path in the component hierarchy. It will return
// an empty slice if the Component is the root component.
func (*Component) Path() []string
```

`Child` is used to create a new `Component`, corresponding to a new child node
in the component structure, and `Path` is used retrieve the path of any
`Component` within that structure. For the sake of keeping the examples simple,
let’s pretend these functions have been implemented in a package called `mcmp`.
Here’s an example of how `Component` might be used in the `redis` component’s
code:

```go
// Package redis

func NewConn(cmp *mcmp.Component, defaultAddr string) *RedisConn {
    cmp = cmp.Child("redis")
    paramPrefix := strings.Join(cmp.Path(), "-")

    addrParam := flag.String(paramPrefix+"-addr", defaultAddr, "Address of redis instance to connect to")
    // finish setup

    return redisConn
}
```

In our above example, the two `redis` components' parameters would be:

```
// This first parameter is for the stats redis, whose parent is the root and
// therefore doesn't have a prefix. Perhaps stats should be broken into its own
// component in order to fix this.
--redis-addr
--rest-api-redis-addr
```

`Component` definitely makes it easier to instantiate multiple redis components
in our program, since it allows them to know their place in the component
structure.

Having to construct the prefix for the parameters ourselves is pretty annoying,
so let’s introduce a new package, `mcfg`, which acts like `flag` but is aware
of `Component`. Then `redis.NewConn` is reduced to:

```go
// Package redis

func NewConn(cmp *mcmp.Component, defaultAddr string) *RedisConn {
    cmp = cmp.Child("redis")
    addrParam := mcfg.String(cmp, "addr", defaultAddr, "Address of redis instance to connect to")
    // finish setup

    return redisConn
}
```

Easy-peasy.

#### But What About Parse?

Sharp-eyed gophers will notice that there is a key piece missing: When is
`flag.Parse`, or its `mcfg` counterpart, called? When does `addrParam` actually
get populated? It can’t happen inside `redis.NewConn` because there might be
other components after `redis.NewConn` that want to set up parameters. To
illustrate the problem, let’s look at a simple program that wants to set up two
`redis` components:

```go
func main() {
    // Create the root Component, an empty Component.
    cmp := mcmp.New()

    // Create the Components for two sub-components, foo and bar.
    cmpFoo := cmp.Child("foo")
    cmpBar := cmp.Child("bar")

    // Now we want to try to create a redis sub-component for each component.

    // This will set up the parameter "--foo-redis-addr", but bar hasn't had a
    // chance to set up its corresponding parameter, so the command-line can't
    // be parsed yet.
    fooRedis := redis.NewConn(cmpFoo, "127.0.0.1:6379")

    // This will set up the parameter "--bar-redis-addr", but, as mentioned
    // before, redis.NewConn can't parse command-line.
    barRedis := redis.NewConn(cmpBar, "127.0.0.1:6379")

    // It is only after all components have been instantiated that the
    // command-line arguments can be parsed
    mcfg.Parse()
}
```

While this solves our argument parsing problem, fooRedis and barRedis are not
usable yet because the actual connections have not been made. This is a classic
chicken and the egg problem. The func `redis.NewConn` needs to make a connection
which it cannot do until _after_ `mcfg.Parse` is called, but `mcfg.Parse` cannot
be called until after `redis.NewConn` has returned. We will solve this problem
in the next section.

### Instantiation vs Initialization

Let’s break down `redis.NewConn` into two phases: instantiation and
initialization. Instantiation refers to creating the component on the component
structure and having it declare what it needs in order to initialize (e.g.,
configuration parameters). During instantiation, nothing external to the
program is performed; no IO, no reading of the command-line, no logging, etc.
All that’s happened is that the empty template of a `redis` component has been
created.

Initialization is the phase during which the template is filled in.
Configuration parameters are read, startup actions like the creation of database
connections are performed, and logging is output for informational and debugging
purposes.

The key to making effective use of this dichotomy is to allow _all_ components
to instantiate themselves before they initialize themselves. By doing this we
can ensure, for example, that all components have had the chance to declare
their configuration parameters before configuration parsing is done.

So let’s modify `redis.NewConn` so that it follows this dichotomy. It makes
sense to leave instantiation-related code where it is, but we need a mechanism
by which we can declare initialization code before actually calling it. For
this, I will introduce the idea of a “hook.”

#### But First: Augment Component

In order to support hooks, however, `Component` will need to be augmented with
a few new methods. Right now, it can only carry with it information about the
component structure, but here we will add the ability to carry arbitrary
key/value information as well:

```go
// Package mcmp

// SetValue sets the given key to the given value on the Component, overwriting
// any previous value for that key.
func (*Component) SetValue(key, value interface{})

// Value returns the value which has been set for the given key, or nil if the
// key was never set.
func (*Component) Value(key interface{}) interface{}

// Children returns the Component's children in the order they were created.
func (*Component) Children() []*Component
```

The final method allows us to, starting at the root `Component`, traverse the
component structure and interact with each `Component`’s key/value store. This
will be useful for implementing hooks.

#### Hooks

A hook is simply a function that will run later. We will declare a new package,
calling it `mrun`, and say that it has two new functions:

```go
// Package mrun

// InitHook registers the given hook to the given Component.
func InitHook(cmp *mcmp.Component, hook func())

// Init runs all hooks registered using InitHook. Hooks are run in the order
// they were registered.
func Init(cmp *mcmp.Component)
```

With these two functions, we are able to defer the initialization phase of
startup by using the same `Components` we were passing around for the purpose
of denoting component structure.

Now, with these few extra pieces of functionality in place, let’s reconsider the
most recent example, and make a program that creates two redis components which
exist independently of each other:

```go
// Package redis

// NOTE that NewConn has been renamed to InstConn, to reflect that the returned
// *RedisConn is merely instantiated, not initialized.

func InstConn(cmp *mcmp.Component, defaultAddr string) *RedisConn {
    cmp = cmp.Child("redis")

    // we instantiate an empty RedisConn instance and parameters for it. Neither
    // has been initialized yet. They will remain empty until initialization has
    // occurred.
    redisConn := new(RedisConn)
    addrParam := mcfg.String(cmp, "addr", defaultAddr, "Address of redis instance to connect to")

    mrun.InitHook(cmp, func() {
        // This hook will run after parameter initialization has happened, and
        // so addrParam will be usable. Once this hook as run, redisConn will be
        // usable as well.
        *redisConn = makeRedisConnection(*addrParam)
    })

    // Now that cmp has had configuration parameters and intialization hooks
    // set into it, return the empty redisConn instance back to the parent.
    return redisConn
}
```

```go
// Package main

func main() {
    // Create the root Component, an empty Component.
    cmp := mcmp.New()

    // Create the Components for two sub-components, foo and bar.
    cmpFoo := cmp.Child("foo")
    cmpBar := cmp.Child("bar")

    // Add redis components to each of the foo and bar sub-components.
    redisFoo := redis.InstConn(cmpFoo, "127.0.0.1:6379")
    redisBar := redis.InstConn(cmpBar, "127.0.0.1:6379")

    // Parse will descend into the Component and all of its children,
    // discovering all registered configuration parameters and filling them from
    // the command-line.
    mcfg.Parse(cmp)

    // Now that configuration parameters have been initialized, run the Init
    // hooks for all Components.
    mrun.Init(cmp)

    // At this point the redis components have been fully initialized and may be
    // used. For this example we'll copy all keys from one to the other.
    keys := redisFoo.Command("KEYS", "*")
    for i := range keys {
        val := redisFoo.Command("GET", keys[i])
        redisBar.Command("SET", keys[i], val)
    }
}
```

## Conclusion

While the examples given here are fairly simplistic, the pattern itself is quite
powerful. Codebases naturally accumulate small, domain-specific behaviors and
optimizations over time, especially around the IO components of the program.
Databases are used with specific options that an organization finds useful,
logging is performed in particular places, metrics are counted around certain
pieces of code, etc.

By programming with component structure in mind, we are able to keep these
optimizations while also keeping the clarity and compartmentalization of the
code intact. We can keep our code flexible and configurable, while also
re-usable and testable. Also, the simplicity of the tools involved means they
can be extended and retrofitted for nearly any situation or use-case.

Overall, this is a powerful pattern that I’ve found myself unable to do without
once I began using it.

### Implementation

As a final note, you can find an example implementation of the packages
described in this post here:

* [mcmp](https://godoc.org/github.com/mediocregopher/mediocre-go-lib/mcmp)
* [mcfg](https://godoc.org/github.com/mediocregopher/mediocre-go-lib/mcfg)
* [mrun](https://godoc.org/github.com/mediocregopher/mediocre-go-lib/mrun)

The packages are not stable and are likely to change frequently. You’ll also
find that they have been extended quite a bit from the simple descriptions found
here, based on what I’ve found useful as I’ve implemented programs using
component structures. With these two points in mind, I would encourage you to
look and take whatever functionality you find useful for yourself, and not use
the packages directly. The core pieces are not different from what has been
described in this post.