Discussion: Caching behaviour to avoid setting changes mid-plan

[olliesilvester]

The base client currently has a default expiry time for each entry, as well as a number of maximum items it can hold at once (I think this is first in first out).

If a Bluesky plan makes multiple requests using this client, and the second of these requests happens to be after the cache expiry time, or the cache fill up and drops the first entry, then the settings could change mid-plan without the user meaning to do this.

This seems like it could trip people up, but on the other hand, the default timeout if 1 hr - if we encourage people to make requests with reset_cache=True at the start of their plans, then this should be fine.

Acceptance Criteria

• Decide if default cache behaviour is good
• Document that users should use reset_cache=True at the start of their plans, and guidance on their cache_size cache_lifetime_s values

[DiamondJoseph]

I think it's a plan-dependent behaviour- maybe I always want the latest value, maybe I always want the first value, maybe I want the 1hr caching. If possible I should write my plans in a way that doesn't call the config server a second time within a plan, but if that's not possible then can I pass the caching behaviour I want in?

pseudo code

class Client:
    def with_cache_context(self, cache: CacheType) -> ContextManager:
         original_cache, self._cache = self._cache, cache
         yield
         self._cache = original_cache

with client.with_cache_context(cache=non_clearing_cache):
    my_value = client.get("/dls_sw/foo.bar")
    sleep(60 * 60 + 1)
    my_value == client.get("/dls_sw/foo.bar")

with client.with_cache_context(cache=without_caching):
    my_value = client.get("/dls_sw/foo.bar")
    my_value != client.get("/dls_sw/foo.bar")

[olliesilvester]

I think I agree. In that case, we can leave the behaviour as-is and document suggested usage? In MX though we were planning to call it multiple times throughout a plan, but add our own layer of caching to avoid accidental updates