[zzylol]

Why

The backend cannot ingest the Collector's output yet. Until it can, the edge Collector's two outputs (sketch metrics over OTLP/gRPC and raw Gorilla fragments over HTTP) need somewhere to go that accepts them, reports what arrived, and can save the bodies for an offline accuracy check.

What

otel-app/sink becomes a fake backend that only receives:

• OTLP/gRPC Export (-grpc, default :4317). Sketch data points are counted per sketch kind and encoding: DDSketch, KLLSketch, CountSketch, CountMinSketch, HLLSketch and SumAgg, each with Proto/ProtoDelta/Delta/Msgpack/MsgpackDelta. Any *Delta encoding is a delta payload, the rest are full state. Plain gauge/sum/histogram/summary points keep the existing SINK_STATS counts. The sink now accepts gzip-compressed exports.
• HTTP POST /ingest/gorilla (-http, default :10908, the gorilla-merger port). This takes the cold fragment shipper's gzipped ASAPFRG1 frame. The sink counts requests, gzip and decompressed bytes, fragments, distinct series and samples.
• -dump-dir (optional). Each request body goes to its own file, <receive-unix-ns>-<otlp|gorilla>-<seq>.bin. Without the flag, nothing is written.
• -stats-every (optional). Prints the totals periodically. The totals are always printed on SIGINT/SIGTERM.

The sink does no merging, querying or accuracy computation.

How

• OTLP is decoded with the patched collector pdata (pmetricotlp), the module the asapedgeprocessor encodes with (asap-precompute-go/otel.Encode). The patched proto bindings that the sink used before have no SumAgg variant, so Sum aggregations were invisible to it. otel-app/go.mod gains a replace go.opentelemetry.io/collector/pdata => ../opentelemetry-collector/pdata, set up the same way as the existing opentelemetry-go and proto replaces.
• Full vs delta is read from each data point's encoding enum. That enum is the only place the edge marks it: otel.Encode maps precompute.EncodingProtoDelta/EncodingMsgpackDelta to the *Delta values.
• Fragments are decoded with the existing gorilla.DecodeFragmentBatch and gorilla.DecodeFragmentSamples. Series are keyed by gorilla.FragmentLabels.
• Gorilla dump files hold the body exactly as received (gzipped). OTLP dump files hold the ExportMetricsServiceRequest protobuf re-marshalled from the decoded request, because gRPC decodes the message before the handler runs.
• The gRPC listen address moves from a positional argument to -grpc. No script in the repo passed it.

Before

• The sink accepted only OTLP/gRPC and counted plain datapoints.
• Sketch metrics were not broken down.
• SumAgg was not seen.
• gzip-compressed exports were rejected because no gzip decompressor was registered.
• There was no endpoint for cold fragments and no way to keep the bodies.

After

SINK_STATS reqs=… datapoints=… bytes=…
SINK_SKETCH kind=DDSketch encoding=ProtoDelta mode=delta msgs=… bytes=…
SINK_RAW reqs=… gzip_bytes=… bytes=… fragments=… series=… samples=…

To use it, point the edge Collector's OTLP exporter at -grpc and cold.ship_endpoint at http://<sink>:10908/ingest/gorilla.

Verification

TestSinkCountsAndDumps sends:

• one DDSketch delta envelope, built with otel.Encode and exported over gzip gRPC;
• one gzipped ASAPFRG1 frame with 2 series and 5 samples, built with StreamingFragmentEncoder and EncodeFragmentBatch.

It checks every counter. It also checks that the dump dir holds exactly two files whose bytes equal the sent bodies.

go test -count=1 ./sink   # ok
go vet ./sink             # ok
go build .                # otel-app main still builds with the go.mod change

These were run against sketchlib-go main (cf7dac6) and again against the earlier CI pin (148643f). Both passed.