Feature Request: Add TTL support to file cache backend

[Edefritz]

Hi,

While working with Tegola’s caching, I noticed that TTL configuration is currently only supported for the Redis cache backend, but not for the file cache.

When using the file cache, cached tiles persist indefinitely unless:

• the Tegola pod is restarted, or
• tiles are manually invalidated

This becomes problematic when underlying data changes infrequently but does change occasionally, those updates effectively never bust the cache in a running system.

I initially assumed I had missed an undocumented ttl parameter for the file cache, but after checking the docs and code, this doesn’t seem to exist.

Current behavior

• Redis cache supports TTL via backend configuration.
• File cache has no TTL or automatic expiration.
• Cache invalidation for file cache requires manual intervention.

Expected / Desired behavior

Support an optional TTL for the file cache backend, similar in spirit to Redis, but implemented internally by Tegola.

[iwpnd]

Hey there. First off, I would avoid using the ephemeral storage of the pod like this as this can cause node pressure eviction events that can terminate your pods abruptly. On a second note, I agree that tegola should support a ttl for the file cache. If @gdey and @ARolek agree, I could add a simple lazy invalidation that would check expiration against the mtime of a file on request at least.

[ARolek]

@iwpnd I support the idea of a TTL on the file cache and the use of mod time makes sense to me!

For the implementation, if the mod time is greater than the TTL do you plan on deleting the tile and return inga cache MISS, or just return a cache MISS and wait for the expected subsequent write?

[iwpnd]

I would wait for the next write, similar to how redis would do it in that case.

[ARolek]

@iwpnd yeah I think that's the right move.

[Edefritz]

Just tested this with the edge image and the new TTL setting for file cache works brilliantly, thank you very much for the improvement. @ARolek do you know whether this feature is planned to be included in an upcoming release?

[ARolek]

@Edefritz yep, this will be part of the upcoming release. I just need to take care of some of the dependabot issues and then I can cut a release. I might also hold out for the #1071 PR to land. Let me see how that PR progresses.

Glad this is going to work for you, and thank you @iwpnd for the quick turn around