Ensuring cache feature is thread safe?

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Summary

• Crystal has supported Thread for some time. (I.e. -Dpreview_mt)
• Hash is not thread-safe, Hash is volatile.
• Once multiple threads try to write the same key value at the same time, the program crashes.
• This issue has appeared in my other project, so I raised an issue.

Solution

• Using lucaong / immutable can solve this problem.
• This is currently the only thread-safe Hash of Crystal.
• I used it in my branch organization, You don't need to change too much.

Improve

• maxminddb.cr/src/maxminddb/cache.cr

  
  require "./any.cr"

module MaxMindDB struct Cache(K, V) property capacity : Int32 property storage : Immutable::Map(K, V)

def initialize(@capacity : Int32)
  @storage = Immutable::Map(K, V).new
end

def fetch(key : K, &block : K -> V) : V
  value = storage[key]?
  return value if value

  value = yield key

  unless full?
    _storage = storage.set key, value
    self.storage = _storage
  end

  value
end

def full?
  storage.size >= capacity
end

end end

* [shard.yml](https://github.com/delef/maxminddb.cr/blob/master/shard.yml)

```yaml
dependencies:
  immutable:
    github: lucaong/immutable

References

• Thread-safe, persistent, immutable collections for the Crystal language

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Finally

I just provide it as a reference, if you are interested, you can always improve it yourself.