Limit concurrency for caching

[cisaacstern]

On the first production run of https://github.com/pangeo-forge/terraclimate-feedstock, Dataflow autoscaled the cluster to 1000 workers, in response to the slow throughput of caching ~882 inputs (totaling ~1.9 TB).

We should be able to limit concurrency for caching, given that the source file servers will generally be bandwidth-constrained. Dataflow provides a max_num_workers option to cap the size of the worker pool, but this issue is separate from that concern: concurrency should be limited only for the caching step, and then we should support larger scale-out after data is cached.

There must be a more formal discussion of this somewhere in the Beam docs, but for now the most direct discussion I've found is in the replies to https://stackoverflow.com/a/65634538, which suggest GroupByKey might be used to achieve this.

I believe this will require pulling caching out from OpenURLWithFSSpec. Currently, if a cache argument is provided to OpenURLWithFSSpec, the input is cached and then immediately opened from the cache

pangeo-forge-recipes/pangeo_forge_recipes/openers.py

Lines 31 to 32 in bdb32f2

	cache.cache_file(url, secrets, **kw)
	open_file = cache.open_file(url, mode="rb")

In order to limit concurrency for the caching, but not for the opening, I believe caching will need to be its own transform, the output of which is then passed to OpenURLWithFSSpec, which does not do any caching.

cc @rabernat @alxmrs, xref #376

[alxmrs]

Something that I just learned about in Beam is resource hints (https://beam.apache.org/documentation/runtime/resource-hints/). It sounds like this could pair really well with breaking out file caching.

[alxmrs]

After TAL at OpenURLWifFSSpec, I agree that Caching should be a separate PTransform.

[cisaacstern]

Noting that pangeo-forge/paleo-pism-feedstock#2 is blocked by this. Looks like we will not be able to deploy a production run of that feedstock until we have some way to limit concurrency during the caching stage. cc @jkingslake

[alxmrs]

Here's an example that could help: for this kind of problem we use this RateLimit transform. https://github.com/google/weather-tools/blob/main/weather_mv/loader_pipeline/util.py#L282

[cisaacstern]

Belated thanks for sharing this example, @alxmrs. 🙏

[jkingslake]

Hi all,
As always, thanks for all the work you do with these tools!

Any updates on this issue? As noted above, it is stopping progress on pangeo-forge/paleo-pism-feedstock#2

[cisaacstern]

For those still following this, I am now working on a fix in #557

[cisaacstern]

This is fixed by #557, @jkingslake please feel free to ping me on your recipe thread if you'd like to work together to revive it in light of this fix. And thanks for your patience!