How to update a Registry value?

Questions / Help
1

Background

I have a process registered in Registry. I know this because I get the following results upon using lookup:


iex(33)> Registry.lookup(MyRegistry, {Worker, 1})                       
[{#PID<0.237.0>, [available: true]}]

Problem

The problem here is that I can’t update it no matter what:

iex(34)> Registry.update_value(MyRegistry, {Worker, 1}, fn _x -> [] end)
:error

I always get an error. Now, I know the process is registered and I know the key exists because of the lookup. But somehow I can’t update the value.

Questions

What am I doing wrong?

2

You can only update it from the registered process itself.

3

It would be rather nice to have this documented (it isn’t). How can I contribute ?

4

It’s kinda implicit in “Updates the value for key for the current process”. You could amend it on the elixir github repo.

5

So what this means is that there is no way for an external process to change the Registry value of another process, correct?

6

Yep. Just like you cannot register another process to a registry. It would be prone to race-conditions afaik.

7

I guess the only solution here would be to ask the process to update it’s own registry via a message. I find (personally) this information would be useful in the documentation, although I will only considering adding it if you and José agree.

The way I see it, for a seasoned Elixir developer (which I am not [yet]) this may be obvious.

8

For my understanding, it is documented in Registry.update_value/3 very well, the first sentence says (emphasis mine):

Updates the value for key for the current process in the unique registry .

9

You can make a PR. If there’s something wrong with it I’m sure the team will let you know how to improve it.

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10

I think we can add a note that says: “If you attempt to update a key that does not belong to the current process, it will raise an ArgumentError.”

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