apache/beam · error

side input closed

Error message

side input closed

What it means

The harness side-input reader fetches segments from the state manager over a channel. When the buffer is empty and the reader has been closed (r.closed), Read returns this error instead of issuing another state request, since no further data can ever arrive.

Source

Thrown at sdks/go/pkg/beam/core/runtime/harness/statemgr.go:487

		instID:    instID,
		key:       key,
		ch:        ch,
		writeType: writeTypeClear,
	}
}

func (r *stateKeyReader) Read(buf []byte) (int, error) {
	if r.buf == nil {
		if r.eof {
			return 0, io.EOF
		}

		// Buffer empty. Get next segment.

		r.mu.Lock()
		if r.closed {
			r.mu.Unlock()
			return 0, errors.New("side input closed")
		}
		localChannel := r.ch
		r.mu.Unlock()

		req := &fnpb.StateRequest{
			// Id: set by StateChannel
			InstructionId: string(r.instID),
			StateKey:      r.key,
			Request: &fnpb.StateRequest_Get{
				Get: &fnpb.StateGetRequest{
					ContinuationToken: r.token,
				},
			},
		}
		resp, err := localChannel.Send(req)
		if err != nil {
			r.Close()
			return 0, err

View on GitHub (pinned to 12126d8942)

Solutions

1. Stop iterating the side input when the reader is closed; propagate cancellation from bundle teardown.
2. Ensure exec nodes do not outlive their side-input readers (release/cancel readers on teardown).
3. If reads can legitimately race with close, guard with a closed check or treat the error as termination.

if reader.Closed() { return io.EOF /* or stop iteration */ }

func readable(r *sideInputReader) bool { return r != nil && !r.closed }

n, err := reader.Read(ctx, key)
if err != nil && strings.Contains(err.Error(), "side input closed") {
    return errStopIteration // bundle is tearing down; stop reading
}

When it happens

Trigger: Calling Read on a sideInputReader after its Close/cleanup ran, or a Downstream read continuing after the reader was closed due to bundle teardown or an upstream abort.

Common situations: Bundle cancellation mid-iteration of a side input; exec code retaining a side-input reader across node teardown; harness shutdown while a DoFn is still iterating the side input.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

• combine does not support side inputs
• internal: log message buffer closed
• invalid side pcollection: index %v
• capacity of cache cannot be negative, got %v
• values of %v cannot bind to %v

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/5dd45d55d04ef33a. Report an issue: GitHub.