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[Feature Request]: Support random access in the Go Filesystem API (io.ReaderAt / io.ReadSeeker) #39176
Description
What would you like to happen?
The Go Filesystem API should support random access when the underlying filesystem provides it.
Currently, filesystem.Interface.OpenRead returns an io.ReadCloser, which only supports sequential reads. This makes it difficult to implement readers for file formats such as Parquet, ORC, Arrow IPC, and similar formats that rely on seeking to arbitrary offsets within a file.
Although storage backends such as GCS, S3, and local files support range reads or random access, that capability is not exposed through the Go Filesystem abstraction. As a result, implementations either need to buffer the entire file into memory or bypass the Beam Filesystem API and use storage-specific clients directly.
It would be useful if the Go Filesystem API exposed a random access interface, for example by returning an io.ReaderAt, io.ReadSeeker, or another abstraction that supports efficient seeking when the underlying filesystem is capable of it.
This would make it easier to build portable, efficient Beam I/O connectors and readers for columnar and indexed file formats without sacrificing performance or bypassing the Beam filesystem abstraction.
Issue Priority
Priority: 2 (default / most feature requests should be filed as P2)
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Activity
Gaurav598 commentedon Sep 1, 2026
I did a repository-level investigation of this issue to understand the current Go filesystem architecture and possible API designs.
A few findings that may be useful for whoever works on this:
filesystem.Interface.OpenReadcurrently returnsio.ReadCloser, so the Beam abstraction only exposes sequential reads.Copier,Renamer, andRemover, which makes an additive optional random-access capability a natural fit.*os.Fileonly as anio.ReadCloser, even though the underlying file supports positional reads/seeking.memfssimilarly uses abytes.Reader, whose random-access capabilities are currently hidden by theio.ReadCloserabstraction.OpenReaddirectly toio.ReaderAtorio.ReadSeekerwould be a breaking API change for existing filesystem implementations.io.ReadSeeker.The main design question seems to be whether the optional capability should expose an
io.ReaderAt-style interface with explicit lifecycle semantics, or a Beam-specific range-read abstraction that maps directly to remote range requests.I also noticed that
parquetiocurrently reads the entire file into memory before passing it to the Parquet reader, which is a concrete downstream example of the limitation described in this issue.I hope these findings are useful to whoever takes this issue forward.
goutamadwant commentedon Sep 27, 2026
@mdreizin is an optional
io.ReaderAtinterface with a buffered fallback the right approach here? I can put together the API and filesystem tests. Let me know. thanks!