select-slice-of-context

Select on a Slice of Closing Channels

i.e. select on []ctx.Done()

Problem

This example is born out of a need to monitor "from the side" a variable set of channels and associated contexts. The specific use-case is a layer in the middle of a Watch functionality where components can register, and de-register, watching for changes in a subset of a repository (i.e. etcd like).

The "watch" is constructed from a context and an query (key) against the repository and the resultant watch object is simply a Golang channel. In order to perform an orderly shutdown of the repository, and consequently shutdown each watch, there must be a means of canceling each context that was passed in on construction. As the repository, and it's watch() function, sit in a middle layer between client and underlying store, the repository can wrap the caller's context WithCancel(), holding on to the cancellation function and passing the wrapped context down to the underlying store. The cancellation function can then be held, and used, by the repository layer to cancel and tear-down each constructed watch at the time the repository initiates destruction.

The catch, which will be demonstrated in this example is that each watcher can be torn down (de-regitered) at an arbitrary time during the life of the repository. In order to prevent unbounded growth of a list of watchers', context cancellation functions, the closure of the watch channel will be indirectly detected by selecting against the cancellation of the watcher's context, passed at construction. In order to detect cancellation, each active watcher's context must have a select applied against it's Done() function. In short, a select against a slice of closing channels.

Solution(s)

The traditional approach of constructing a slice of channels and then performing a select against the slice is afforded by Golang through the reflect package and reflect.Select(). Additionally, the Golang community has posed an alternative approach which involves an "aggregating channel" and individual goroutines per select. Both approaches are detailed in stack-overflow-1.

Update and Simplification

After attempting to apply the pattern developed in this example to the real world problem I identified some simplifications that removed a reasonable amount of the code in Middleware. The original solution is visible from the tag go-select-slice-v1:

• https://github.com/egustafson/sandbox/tree/go-select-slice-v1/go/select-slice-of-context

The simplification is the latest commit.