Use erlang:whereis/1 for checking if a store is running

[the-mikedavis]

A store's process uses its StoreId as its registered name. This is a public interface of Ra so we can depend on it. Reading from key metrics counters is already very fast but switching to whereis/1 eliminates basically all overhead of this function. When used heavily (for example in RabbitMQ while publishing and consuming rapidly) the CPU time spent on is_store_running/1 disappears from the output of a perf recording and a flamegraph.

This is a follow-up to #292 based on the post-merge discussion.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.65%. Comparing base (2d04459) to head (ef07168).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #296      +/-   ##
==========================================
+ Coverage   89.54%   89.65%   +0.11%     
==========================================
  Files          21       21              
  Lines        3195     3191       -4     
==========================================
  Hits         2861     2861              
+ Misses        334      330       -4     

Flag	Coverage Δ
erlang-25	88.90% <100.00%> (+0.20%)	⬆️
erlang-26	89.28% <100.00%> (+0.04%)	⬆️
erlang-27	89.28% <100.00%> (-0.02%)	⬇️
os-ubuntu-latest	89.62% <100.00%> (+0.26%)	⬆️
os-windows-latest	89.56% <100.00%> (+0.11%)	⬆️

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[dumbbell]

I still consider this is knowledge of an implementation detail of Ra.

I know that the ra_server_id() type is described as a registered process name in src/ra.hrl. However, it is explicitly documented as an internal thing:

• The Ra server ID concept is described in docs/internals/INTERNALS.md.
• When the server_id() type is defined and exported, it is also marked as internal:

  %% export some internal types
  ...
  -type server_id() :: ra_server_id().
  ...

Therefore, I’m ok to accept the change, but I would like that a comment is added to clarify this. For instance:

    %% FIXME: Ra has no API to know if a Ra server is running or not. We could
    %% use a public API such as `ra:key_metrics/2', but unfortunalety, it is
    %% not as efficient as querying the process directly. Therefore, we bypass
    %% Ra and rely on the fact that the server ID is internally a registered
    %% process name and resolve it to determine if it is running.
    Runs = erlang:whereis(StoreId) =/= undefined,

[dumbbell]

Thanks!

[michaelklishin]

We don't have a 0.15.1 milestone (maybe we should) so I have set it to 0.16.0 because it's better than not having any details.

[dumbbell]

Thank you, I forgot to set the milestone…