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On the Missed Opportunities of Static Types

Recently, a colleague spent a bunch of time working on a bug. When attempting to perform some action, the system would provide feedback that everything was going as planned, and then “go dark,” seemingly out of no where. It’s important to realize that this is a complex distributed system, and that there is a network connection between the “part that worked” and “the part that went dark.” But, there wasn’t a change to the connection logic, and it worked before, so that “couldn’t possibly be it.” (Yes. I already told you where this is going—Ed)

Suppose you’re writing a Go program and you want to perform a different action based on the value of another value. You might do this:

func Greeter(lang, name string) {
    switch lang {
    case "english":
        fmt.Printf("Hello, %s!\n", name)
    case "spanish":
        fmt.Printf("Hola, %s!\n", name)
        fmt.Printf("excuse me?\n")

Based on lang, the function will (naively) print to the screen a greeting, for name, in the specified language.

Greeter("spanish", "Andrew")  // outputs: Hola, Andrew!
Greeter("english", "Andrew") // outputs: Hello, Andrew!

(Now, you can argue with me about whether strings are appropriate here, when an iota would do, but that would be silly, as that’s not the point. In the bug, strings were used, so that’s what we’re going to use here.—Ed)

In the case of Greeter, there’s no dependency between lang and name. name is perfectly valid for each lang, unless you’re fancy and do a translation of “Andrew” to “Andres”. Again, not the point.

In a more complicated setting there is a dependency between the two values. Let’s take a look at a Go API many of us are familiar with because it’s front and center on the package documentation for net.

conn, err := net.Dial("tcp", "")
if err != nil {
    // handle error

This demonstrates something beautiful; flexibility! If I want to Dial a unix socket, the right parameters are "unix", "/path/to/socket", and hostname:port is meaningless. This works because Dial implements validation logic to ensure that the address provided makes sense for the protocol, and returns an error if not. This is helpful in reducing the amount of code required to dynamically Dial a socket specified from external configuration, but it pushes error checking to runtime which is an aphorism for “you’re eventually going to be woken up due to this bug.”

If we redesign the API, we can get the compiler to ensure we’re not doing something silly; but we need to make a few concessions.

DialUnix(addr UnixSocket) could ensure that addr is a path to a unix socket. It may not delegate everything to the compiler, but obvious errors could be detected. You wouldn’t be able to, say, DialUnix(HostPort("", 80)) since HostPort is not of type UnixSocket. This particular example works better if file paths aren’t represented as strings, a sore point of sorts. Then, UnixSocket can be an effective alias to Path, for instance.

But what is the cost here? Well, in the case of Dial, the interface that the net package exposes becomes much larger, creating more cognitive overhead for programmers, and net developers. Go net developers have to create a new function, and type for every new type of supported socket. Programmerss must adopt those new functions, increase the things they know about, and no one is truly happy with the experience.

Go doesn’t provide an obvious mechanism to make this more generic, and while the generics proposal may make solving this possible that’s not going to help us for quite some time.

So what is a programmer to do?

To recap the problem here: when using strings as arguments to simplify an interface, the compiler cannot create assurances that the two strings relate to each other in any meaningful way, which means that all validation must be done at runtime, which almost certainly means bugs are gonna slip into production, despite the fact that we already pay for static types in the language, and the compiler.

The other possibility, which keeps a singular function, is to have Dial and an interface instead, say, ProtoAddr:

// SIDENOTE: This, effectively, already exists as net.Addr ... :tableflip:
type ProtoAddr interface {
    Protocol() string // wait, wuh????
    Addr() string

func Dial(pa ProtoAddr) (net.Conn, err) {
    return originalDial(pa.Protocol(), pa.Addr())

We can then create helper functions and types that constrain our inputs and allow for more assurances that the caller isn’t doing something silly, like providing a filesystem path instead of an IP address. Want to Dial an IP address? Dial(TCPIPv4Addr{net.IPv4{127,0,0,1}, 8000}).

Similar types can just as easily be constructed for hostname:port addresses, Unix Sockets, etc. The important aspect here is that the caller’s intentions are stated, in a way that the compiler can catch, and that the library author can quickly assert validity of.

type TCPIPv4Addr struct {
    IP IPv4
    Port uint16

func (t TCPIPv4Addr) Protocol() string { return "tcp" }
func (t TCPIPV4Addr Addr() string { return fmt.Sprint("%s:%d", t.IP.String(), t.Port) }

func NewTCPIPv4Addr(ip IPv4, port uint16) TCPIPv4Addr ...

type UnixAddr struct {
    Socket filesystem.Path // made up, because, in Go, strings are totally fine for everything, including file paths.

func (u UnixAddr) Protocol() string { return "unix" }
func (u UnixAddr) Addr() string { return u.Socket.String() }

In the above code snippets, you’ll notice something odd–that string’s are ultimately passed into originalDial as before. This is meant to address and illustrate how you might take Dial and evolve it, just as you might evolve instances in your own codebase. This whole idea, of course, fundamentally is not backward compatible.

“But wait a minute, Andrew! What the hell? All you’ve done is add more boilerplate for me to type out, and I already have enough of that with Go’s error checking. What’s this got to do with me? Why is this any better?”, you say.

The primary job of a program is to take input and produce some output. This is vague; yes. The primary job of a programmer is to ensure that the input given to a program makes sense in order to produce the desired output.

In other words, the fact that we build APIs that take arbitrary strings, like Dial, doesn’t give us a pass from validating that the input makes sense, and indeed Dial heavily validates input. But, why not leverage the type system to check as much of your work as possible at compile time?

Go’s type system doesn’t provide us with enough expressive power to solve this simply, unfortunately. And so, as written, it’s not possible to prevent a silly programmer from calling the new Dial with a LOLAddr defined like so:

type LOLAddr struct { 
   What string
   Lol string

func (l LOLAddr) Protocol() string { return l.What }
func (l LOLAddr) Addr() string { return l.Lol }

Dial(LOLAddr{"unix", "localhost:8383"})

Because of this, you might think that the implementation I’ve described is totally useless; I don’t think so. Go does provide a mechanism that you can use which gives you all of the good properties of what I described without the bad.

type ClosedProtoAddr interface {
    Protocol() string
    Addr() string
    closed() struct{}

This interface is impossible to implement outside of the package it’s defined in, because closed() struct{} is not exported. Using this, we can constrain the input to known good values, and exhaustively create all of the supported Dial compatible ProtoAddrs in the package. Dial just has to handle the possibility that nil was passed in, and validate that nothing sneaky is hiding in one of those strings, something that is done right now in the old interface, too.

So about that bug? Well, it was especially sinister as it was contained in a subsystem that implements a proxy with a virtual network address overlay. The proxy’s connection is always supposed to be alive and so it reconnects in a loop, without providing much in the way of visibility in regards to its status. There’s no timeout errors. There’s no error logs. There’s an assumption made that if the link goes down, a retry will eventually succeed.

There are, also, two virtual address types. The two types of addresses are there to allow you to say “connect to this specific node” or “connect to any of the nodes in this cluster”. These are reasonable things. The addresses look different, but are each represented by strings. The address type, itself, is represented by strings. There is no DialRandomNodeInCluster(clusterId). There is no DialHost(hostId). There is an analog to Dial(type, addr), and then a lot of screaming when you realize that the feature you are working on required you to dial a specific host—not a random one—and you didn’t change the first argument that provides the context for how to interpret the address.

As developers, it is important to create clear guidance on how not to be frustrated when attempting to use our libraries. This, too often, is delegated to poorly written documentation, even when the best documentation, the code, can be used to point out our misuse, by our friend the type checker.

- published: 2021/02/05. written: 2020/11/15