This is implementing https://issues.apache.org/jira/browse/BEAM-5428 for the Go SDK.
Side inputs are valid for given window. That means they can be and correctly cached and re-used in multiple bundles if the data has already been loaded into a given worker.
Side input data for a given bundle is specified and keyed by the window and a a side input request token. The Go SDK presently doesn't use this information for a SDK side worker cache of data.
Care needs to be taken to allow the data to be garbage collected if it hasn't been used in the last minute or so. In practice, Global Window side inputs will not be evicted while still in use, but infrequently accessed data will possibly be evicted more regularly. A limit should be set to avoid running out of memory.
As presently implemented, each ProcessElement call (or StartBundle or FinishBundle) will only query for side input data on user request. However, results of user requests are not cached between elements or bundles, which means there's additional lookup and serialization time from the runner side.
It may be possible to re-use allocated side input elements, but even a solution that re-decodes data on every call would be an improvement if the data isn't re-streamed from the runner.
If https://issues.apache.org/jira/browse/BEAM-3293 has been implemented, then per-key+window caching should additionally be considered for more granular caching of data.
This optimization has an outsized impact on streaming performance as side input data can change per bundle, and streaming bundles are typically small, and quickly processed.
This work would be implemented in the exec package:
and probably the harness package