How to use contexts in Go

• example
  • request URL in a web app
  • connection status e.g. to stop processing when the client disconnects or times out
• context package
• context.TODO() creates an empty placeholder context
• context.Background also creates an empty context
  • deciding which to use is about signaling intent to other developers
  • use Background when you "intend to start a known context"
  • it's a good default option if you aren't sure
• each layer of an app can add its own information to the context using context.WithValue(ctx, k, v) to create a new context containing that new value
  • this new context will have the original context as the "parent context"
  • contexts are immutable
• When to use a value in a context vs explicit param
  • depends on what the function is doing with the value
  • e.g. if you have a function that explicitly operates on a username, you should pass that in rather than relying on the context
  • mostly about signaling developer intent
• Ending a context
  • using Done returns a channel that can be watched for values. it will never be written to until the context is done so you can check it to know when to stop processing
  • can only perform one channel operation per select but can put that select in a loop
  • adding default to the select will keep it from blocking
  • in the example of a loop that tries to either read from Done or from another channel, default just means it starts another loop immediately instead of waiting. This is called a "busy loop". The external behavior is the same though
  • context.WithCancel returns (new context, cancellation function)
    • canceling a context does not automatically cancel the parent context
  • context canceled is a common error in http apps when the client disconnects mid-response
• context.WithDeadline
  • will cancel context automatically
  • error will be context deadline exceeded
  • there is a convenience wrapper around this called context.WithTimeout

References

• How To Use Contexts in Go | DigitalOcean