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diff --git a/_posts/2019-08-02-program-structure-and-composability.md b/_posts/2019-08-02-program-structure-and-composability.md deleted file mode 100644 index b44c534..0000000 --- a/_posts/2019-08-02-program-structure-and-composability.md +++ /dev/null @@ -1,587 +0,0 @@ ---- -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. |