Ideally, every function that writes to the stdout probably should ask for a io.Writer and write to it instead. However, it’s common to encounter functions like this: func frobnicate() { fmt.Println("do something") } This would be easier to test if frobnicate would ask for a writer to write to. For instance: func frobnicate(w io.Writer) { fmt.Fprintln(w, "do something") } You could pass os.Stdout to frobnicate explicitly to write to the console: ...
While watching Mitchell Hashimoto’s Advanced Testing with Go talk, I came across this neat technique for deferring teardown to the caller. Let’s say you have a helper function in a test that needs to perform some cleanup afterward. You can’t run the teardown inside the helper itself because the test still needs the setup. For example, in the following case, the helper runs its teardown immediately: func TestFoo(t *testing.T) { helper(t) // Test logic here: resources may already be cleaned up! } func helper(t *testing.T) { t.Helper() // Setup code here. // Teardown code here. defer func() { // Clean up something. }() } When helper is called, it defers its teardown - which executes at the end of the helper function, not the test. But the test logic still depends on whatever the helper set up. So this approach doesn’t work. ...
There are primarily three ways of sorting slices in Go. Early on, we had the verbose but flexible method of implementing sort.Interface to sort the elements in a slice. Later, Go 1.8 introduced sort.Slice to reduce boilerplate with inline comparison functions. Most recently, Go 1.21 brought generic sorting via the slices package, which offers a concise syntax and compile-time type safety. These days, I mostly use the generic sorting syntax, but I wanted to document all three approaches for posterity. ...
Comparing interface values in Go has caught me off guard a few times, especially with nils. Often, I’d expect a comparison to evaluate to true but got false instead. Many moons ago, Russ Cox wrote a fantastic post on Go interface internals that clarified my confusion. This post is a distillation of my exploration of interfaces and nil comparisons. Interface internals Roughly speaking, an interface in Go has three components: ...
Middleware is usually the go-to pattern in Go HTTP servers for tweaking request behavior. Typically, you wrap your base handler with layers of middleware - one might log every request, while another intercepts specific routes like /special to serve a custom response. However, I often find the indirections introduced by this pattern a bit hard to read and debug. I recently came across the embedded delegation pattern while browsing Gin’s HTTP router source code. Here, I explore both patterns and explain why I usually start with delegation whenever I need to modify HTTP requests in my Go services. ...
I’ve always found the signature of io.Reader a bit odd: type Reader interface { Read(p []byte) (n int, err error) } Why take a byte slice and write data into it? Wouldn’t it be simpler to create the slice inside Read, load the data, and return it instead? // Hypothetical; what I *thought* it should be Read() (p []byte, err error) This felt more intuitive to me - you call Read, and it gives you a slice filled with data, no need to pass anything. ...
Just like any other dynamically growable container structure, Go slices come with a few gotchas. I don’t always remember all the rules I need to be aware of. So this is an attempt to list some of the most common mistakes I’ve made at least once. Slices are views over arrays In Go, a slice is a lightweight wrapper around an array. Instead of storing data itself, it keeps track of three things: a pointer to an underlying array where the data is stored, the number of elements it currently holds, and the total capacity before it needs more space. The Go runtime defines it like this: ...
I’ve been having a ton of fun fiddling with Tailscale over the past few days. While setting it up on a server, I came across this ufw firewall script that configures the firewall on Linux to ensure direct communication across different nodes in my tailnet. It has the following block of code that I found interesting (added comments for clarity): #!/usr/bin/env bash # Define PID file path using script's name for uniqueness PIDFILE="/tmp/$(basename "${BASH_SOURCE[0]%.*}.pid")" # Open file descriptor 200 for the PID file exec 200>"${PIDFILE}" # Try to acquire a non-blocking lock; exit if the script is already running flock -n 200 \ || { echo "${BASH_SOURCE[0]} already running. Aborting..."; exit 1; } # Store the current process ID (PID) in the lock file for reference PID=$$ echo "${PID}" 1>&200 # Do work (in the original script, real work happens here) sleep 999 Here, flock is a Linux command that ensures only one instance of the script runs at a time by locking a specified file (e.g., PIDFILE) through a file descriptor (e.g., 200). If another process already holds the lock, the script either waits or exits immediately. Above, it bails with an error message and exit code 1. ...
People love single-method interfaces (SMIs) in Go. They’re simple to implement and easy to reason about. The standard library is packed with SMIs like io.Reader, io.Writer, io.Closer, io.Seeker, and more. One cool thing about SMIs is that you don’t always need to create a full-blown struct with a method to satisfy the interface. You can define a function type, attach the interface method to it, and use it right away. This approach works well when there’s no state to maintain, so the extra struct becomes unnecessary. However, I find the syntax for this a bit abstruse. So, I’m jotting down a few examples here to reference later. ...
I was fiddling with graphlib in the Python stdlib and found it quite nifty. It processes a Directed Acyclic Graph (DAG), where tasks (nodes) are connected by directed edges (dependencies), and returns the correct execution order. The “acyclic” part ensures no circular dependencies. Topological sorting is useful for arranging tasks so that each one follows its dependencies. It’s widely used in scheduling, build systems, dependency resolution, and database migrations. For example, consider these tasks: ...
Besides retries, circuit breakers are probably one of the most commonly employed resilience patterns in distributed systems. While writing a retry routine is pretty simple, implementing a circuit breaker needs a little bit of work. I realized that I usually just go for off-the-shelf libraries for circuit breaking and haven’t written one from scratch before. So, this is an attempt to create a sloppy one in Go. I picked Go instead of Python because I didn’t want to deal with sync-async idiosyncrasies or abstract things away under a soup of decorators. ...
One neat use case for the HTTP ETag header is client-side HTTP caching for GET requests. Along with the ETag header, the caching workflow requires you to fiddle with other conditional HTTP headers like If-Match or If-None-Match. However, their interaction can feel a bit confusing at times. Every time I need to tackle this, I end up spending some time browsing through the relevant MDN docs on ETag, If-Match, and If-None-Match to jog my memory. At this point, I’ve done it enough times to justify spending the time to write this. ...
Every once in a while, I find myself skimming through the MDN docs to jog my memory on how CORS works and which HTTP headers are associated with it. This is particularly true when a frontend app can’t talk to a backend service I manage due to a CORS error. MDN’s CORS documentation is excellent but can be a bit verbose for someone just looking for a way to quickly troubleshoot and fix the issue at hand. ...
Ever since Rob Pike published the text on the functional options pattern, there’s been no shortage of blogs, talks, or comments on how it improves or obfuscates configuration ergonomics. While the necessity of such a pattern is quite evident in a language that lacks default arguments in functions, more often than not, it needlessly complicates things. The situation gets worse if you have to maintain a public API where multiple configurations are controlled in this manner. ...
These days, I don’t build hierarchical types through inheritance even when writing languages that support it. Type composition has replaced almost all of my use cases where I would’ve reached for inheritance before. I’ve written about how to escape the template pattern hellscape and replace that with strategy pattern in Python before. While by default, Go saves you from shooting yourself in the foot by disallowing inheritance, it wasn’t obvious to me how I could apply the strategy pattern to make things more composable and testable. ...
While I like Go’s approach of treating errors as values as much as the next person, it inevitably leads to a situation where there isn’t a one-size-fits-all strategy for error handling like in Python or JavaScript. The usual way of dealing with errors entails returning error values from the bottom of the call chain and then handling them at the top. But it’s not universal since there are cases where you might want to handle errors as early as possible and fail catastrophically. Yet, it’s common enough that we can use it as the base of our conversation. ...
I used to reach for reflection whenever I needed a Retry function in Go. It’s fun to write, but gets messy quite quickly. Here’s a rudimentary Retry function that does the following: It takes in another function that accepts arbitrary arguments. Then tries to execute the wrapped function. If the wrapped function returns an error after execution, Retry attempts to run the underlying function n times with some backoff. The following implementation leverages the reflect module to achieve the above goals. We’re intentionally avoiding complex retry logic for brevity: ...
Despite moonlighting as a gopher for a while, the syntax for type assertion and type switches still trips me up every time I need to go for one of them. So, to avoid digging through the docs or crafting stodgy LLM prompts multiple times, I decided to jot this down in a Go by Example style for the next run. Type assertion Type assertion in Go allows you to access an interface variable’s underlying concrete type. After a successful assertion, the variable of interface type is converted to the concrete type to which it’s asserted. ...
As of now, unlike Python or NodeJS, Go doesn’t allow you to specify your development dependencies separately from those of the application. However, I like to specify the dev dependencies explicitly for better reproducibility. While working on link-patrol, a CLI for checking dead URLs in Markdown files, I came across this neat convention: you can specify dev dependencies in a tools.go file and then exclude them while building the binary using a build tag. ...
I love dynamically typed languages as much as the next person. They let us make ergonomic API calls like this: import httpx # Sync call for simplicity r = httpx.get("https://dummyjson.com/products/1").json() print(r["id"], r["title"], r["description"]) or this: fetch("https://dummyjson.com/products/1") .then((res) => res.json()) .then((json) => console.log(json.id, json.type, json.description)); In both cases, running the snippets will return: 1 'iPhone 9' 'An apple mobile which is nothing like apple' Unless you’ve worked with a statically typed language that enforces more constraints, it’s hard to appreciate how incredibly convenient it is to be able to call and use an API endpoint without having to deal with types or knowing anything about its payload structure. You can treat the API response as a black box and deal with everything in runtime. ...