Intern hostnames in caching directory lister

[sopel39]

# Section
* Reduce coordinator memory usage when caching of file metadata is enabled ({issue}`issuenumber`)

[sopel39]

fyi @romanvainb

[findepi]

I think we generally avoid using weak-valued caches because this brings less predictibility for the memory management. So this small single line is a big change to me.

however, what about scoping the interner in the caller, eg in InternalHiveSplitFactory?

[findepi]

cc @electrum @dain @losipiuk @arhimondr

[sopel39]

I think we generally avoid using weak-valued caches because this brings less predictibility for the memory management. So this small single line is a big change to me.

The assumption is there won't be many hosts in the cluster so it should be negligible in practice.

however, what about scoping the interner in the caller, eg in InternalHiveSplitFactory?

I'm not sure what would that change. For example CachingDirectoryLister has global scope and it stores BlockLocations indirectly.
I wanted to avoid extra complexity here since the number of hosts should be small anyway

[findepi]

The assumption is there won't be many hosts in the cluster so it should be negligible in practice.

good point. can you add this as a code comment?

[hashhar]

stupid question: what about deployments which scale up and down over time? The number of interned hosts will keep increasing. What causes it to release hostnames which are no longer in use? GC?

[raunaqmorarka]

not a stupid question, since I was about to ask the same one :P

[sopel39]

stupid question: what about deployments which scale up and down over time? The number of interned hosts will keep increasing.

Yes. Even if extreme cases I assume like 1000 hosts. They cannot go up/down very frequently, so the size of this structure will remain relatively stable over time (and small too).

GC will collect weak references eagerly (as soon as no-one references them)

[findepi]

% add a comment we assume the HOST_INTERNER will intern only very small limited number of names over time.