I propose the addition of a function for getting all the keys (and values
associated with them) out of a registry. Currently the only way of doing that
is knowing all the pids or keys, and looping over them to request the rest of
the information from the registry, which is awkward and under-performant.
I came across the need for functionality like this when I was setting up an
architecture where an outside source, a list of keys, should be used to
determine what processes to run. For each key in that list a process should be
started. Over time, the list will change, and the processes running should be
adjusted to represent this. Assuming there's a DynamicSupervisor owning the
workers and a coordinating process starting and stopping them, I need a list of
the current running processes and their keys to compare with the latest version
of the external state.
A simple solution to the problem would be to rely on the changes between the
latest version of the list and the last version, and trust the supervisor to
restart dying processes. This, however, means the workers can't be
`:temporary`, because we would risk reality getting out of sync of the list of
workers on the coordinator. But even if I go for `:transient` workers, I still
risk getting out of sync if the DynamicSupervisor crashes. Even if I keep that
process clean to avoid crashes, some worker crashing repeatedly can still bring
it down, and all the workers with it. At that point, the only way to
synchronize back to the correct state would be restarting the server or
implement some complex code where the coordinator attempts to verify the status
of each key by looking it up in the registry, but then we're back to
inefficiently getting all keys.
I propose a function that looks and works something like this (naming and
ordering of return values is obviously up for discussion)
```
Registry.all(MyRegistry)
=> [{key, pid, value}, {key, pid, value}, ...]
```
Just like `Registry.count/1` has a corresponding `Registry.count_match/{3,4}`,
it could be designed to take a match specification.
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