Go Package context

External

• https://pkg.go.dev/context
• https://go.dev/blog/context

Internal

• Go Modularization
• Go Concurrency

Overview

The context package provides two, somewhat unrelated, features.

One is the capability to preempt (cancel) blocking code executing on goroutines downstream in our call graph under a variety of circumstances (programmatic cancellation, timeout, deadline). This is an idiomatic preemption pattern, equivalent, but preferred to the done channel pattern. It is preferred because comes from the standard library and it is more expressive.

The second feature is the capability to store request-scoped data.

Concepts

Context

context.Context is an interface exposed by the context package. The Context instances flow across API boundaries and through the system in the same way done channels do, and their role is to care cancellation signals, deadlines and request-scoped values.

To enroll with the pattern, each function downstream from the top level concurrent call must take a Context as parameter. Conventionally, is the first parameter of the function.

The implementations ensure that they are concurrent-safe, so their methods may be called by multiple goroutines simultaneously.

The Background Context

The background Context is a non-null, empty, no-deadline and no-values Context returned by the context.Background() function. This context is never canceled, and it is typically used by the main function, initialization and tests, and as the top-level Context for incoming requests.

Context Parent/Child Relationship

Context hierarchy.

Programmatic Preemption (Cancellation)

This section documents the idiomatic pattern to preempt (cancel) a blocking function.

It consists in a someFunc() function that gets a Context instance as its first argument. The function will select reading the "done" channel returned by the Context's Done() method. When the context is externally cancelled, the "done" channel will be closed, reading on the "done" channel will unblock and allow the function to return. The recommended return value in this case is the error returned by ctx.Err().

ctx.Err() will return a standard response, as follows:

• nil is the Done channel is not closed yet.
• the context.Canceled error, if the context was canceled.
• the context.DeadlineExceeded error, if the context's deadline has passed.

// someFunc will return a context.Canceled error if the context was externally canceled
func someFunc(ctx context.Context, c <-chan interface{}) error {
	for {
		select {
		case <-ctx.Done():
			return ctx.Err() // context.Canceled is the context was externally canceled 
		case <-c: // this will ensure the goroutine will block, as we will never write on that channel
		}
	}
}

To externally cancel the context, and implicitly all functions listening on its "done" channel, invoke the cancel function returned by the context.WithCancel() function:

ctx, cancel := context.WithCancel(context.Background())

// spin off someFunc() on its own thread and get it to block by reading on the channel we will never write on
go func() {
	err := someFunc(ctx, make(chan interface{}))
	fmt.Printf("someFunc() errored out because %v\n", err)
}()

// spin off the anonymous function that will cancel someFunc() after 5 seconds
go func() {
	time.Sleep(5 * time.Second)
	cancel()
}()

TODO

One of the key differences between Go and other language is explicit context propagation. Context propagation is a mechanism of propagating an additional call argument, called context, into function calls. There is actually a type called context.Context.

The context is used for:

• Cancellation logic. You can pass a special instance of a context that can get canceled. In that case, all functions you were to call with it would be able to detect this. Can be useful for handling application shutdown or stopping any processing.
• Timeouts. Timeouts can be set for execution, by using the corresponding context functions.
• Propagating extra metadata. Additional key/value pairs can be propagate inside the context. Useful in distributed tracing. This feature should be used sparingly, in exceptional cases.

The context is immutable, but it can be cloned with extra metadata.

Function using context should accept it as their first argument.

Manage goroutines lifecycles.

If you have a component that blocks for any reason (disk, network IO, user), then it should probably take the context as a first parameter.

type reportStore interface {
  listTimes(ctx context.Context, ...) (..., error)
  writeFile(ctx context.Context, ...) error
  serveFile(ctx context.Context, ...) error
}

Using the value propagation feature of the Context is dubious. Don't use Context.value for stuff that should be regular dependencies between components. Use Context.value for data that can be passed to your program in any other way: only data that is request-scoped, stuff that is created at the beginning of a request lifecycle, like request ID, etc. If the information is available when the program starts or at any point prior to when the request starts, do not use context.value. This is the case for database handles, loggers, etc.

• https://go.dev/blog/context
• All functions performing I/O operations must accept context. Why? https://learning.oreilly.com/library/view/microservices-with-go/9781804617007/B18865_02.xhtml#:-:text=all%20functions%20performing
• What is with the _ context.Context https://learning.oreilly.com/library/view/microservices-with-go/9781804617007/B18865_02.xhtml#:-:text=(_%20context.Context