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+# OpenTelemetry Distributed Tracing for Numaflow
+
+## 1. Objective
+
+Enable distributed tracing in Numaflow to track each message's journey from Source to Sink. This provides visibility into:
+- The path a message takes through the pipeline.
+- Latency introduced by each component (Source, Map UDF, Sink UDF, platform overhead).
+- Whether latency is attributable to the Numaflow platform or user-defined functions (UDFs).
+
+## 2. Span Hierarchy: Sequential Siblings
+
+Each message produces a single trace with the following span hierarchy:
+
+```
+upstream producer (optional — carries W3C traceparent or B3 headers)
+└── pipeline.process                         (platform, root of Numaflow journey)
+    ├── source.read                          (platform, child of pipeline.process)
+    ├── map                                  (platform, child of pipeline.process)
+    │   └── udf.map.process                  (UDF, child of map)
+    │       ├── db.lookup                    (UDF child — user-created spans)
+    │       └── cache.lookup                 (UDF child — user-created spans)
+    └── sink.write                           (platform, child of pipeline.process)
+        └── udf.sink.process                 (UDF, child of sink.write)
+            ├── serialize                    (UDF child — user-created spans)
+            └── kafka.produce                (UDF child — user-created spans)
+```
+
+**Key properties:**
+- `pipeline.process` is the root span covering the full message lifecycle (source read → map → sink → ack). Its duration = total pipeline latency for that message.
+- `source.read`, `map`, and `sink.write` are **siblings** under `pipeline.process`. Each span's duration is accurate and independent — no inflation.
+- UDF spans (`udf.map.process`, `udf.sink.process`) are children of their corresponding platform span, not cross-linked.
+- Per-stage latency isolation is clean: `map` duration - `udf.map.process` duration = platform overhead for the map stage.
+
+### 2.1 Span Naming Convention
+
+| Span Name | Service | Kind | Created By |
+|---|---|---|---|
+| `numaflow.platform.pipeline.process` | numaflow-core | INTERNAL | Platform (source.rs) |
+| `numaflow.platform.source.read` | numaflow-core | PRODUCER | Platform (source.rs) |
+| `numaflow.platform.map` | numaflow-core | INTERNAL | Platform (mapper) |
+| `numaflow.platform.sink.write` | numaflow-core | CLIENT | Platform (sinker) |
+| `udf.map.process` | numaflow-udf-{name} | CONSUMER | UDF (Java/Go/Python SDK) |
+| `udf.sink.process` | numaflow-udf-{name} | CONSUMER | UDF (Java/Go/Python SDK) |
+
+### 2.2 Span Attributes (OTel Semantic Conventions)
+
+All spans carry [OTel Messaging semantic conventions](https://opentelemetry.io/docs/specs/semconv/messaging/):
+
+| Attribute | Value | Description |
+|---|---|---|
+| `messaging.system` | `numaflow` | Identifies the messaging system |
+| `messaging.operation.type` | `process` / `publish` | Operation type |
+| `messaging.operation.name` | `pipeline.process` / `source.read` / `map` / `sink.write` | Stage name |
+| `messaging.message.id` | Message offset | Per-message identifier |
+
+## 3. Key Architectural Decisions
+
+### 3.1 Format: W3C Trace Context
+
+We use the **W3C Trace Context** standard (`traceparent`, `tracestate`) for context propagation.
+- **Rationale**: Industry standard, natively supported by OpenTelemetry, compact (1-2 headers), vendor-neutral.
+
+### 3.2 Carrier: System Metadata (`sys_metadata`)
+
+Trace context is stored in the `sys_metadata` field of the Numaflow `Message` protobuf. **Not** in gRPC headers — because gRPC BiDi streaming sets headers once per stream, not per message.
+
+- **Location**: `message.metadata.sys_metadata`
+- **Key**: `"tracing"` — carries the shared parent context (`pipeline.process`)
+- **Value**: A `KeyValueGroup` containing W3C `traceparent` and `tracestate`
+
+```protobuf
+message Metadata {
+  map<string, KeyValueGroup> sys_metadata = 2;
+}
+```
+
+```json
+{
+  "sys_metadata": {
+    "tracing": {
+      "traceparent": "00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01",
+      "tracestate": "pqrs=00f067aa0ba902b7"
+    }
+  }
+}
+```
+
+### 3.3 Dual-Key sys_metadata for UDF Context
+
+Two sys_metadata keys are used for trace propagation:
+
+| Key | Contents | Written By | Read By |
+|---|---|---|---|
+| `sys_metadata["tracing"]` | `pipeline.process` context | Platform (source, once) | Map, Sink (to become siblings) |
+| `sys_metadata["tracing_udf"]` | Stage-specific context (e.g., `map` or `sink.write`) | Platform (before UDF call) | UDF (to see platform stage as parent) |
+
+**Why two keys?** The `"tracing"` key holds `pipeline.process` context and is never overwritten — this is what makes source.read, map, and sink.write siblings. The `"tracing_udf"` key is temporarily set before each UDF call so the UDF sees the correct platform stage as its parent. It is removed from result messages after the UDF returns.
+
+### 3.4 pipeline.process Span Lifecycle
+
+The `pipeline.process` span must live for the entire message lifecycle (source read → map → sink → ack) to have accurate duration. It is stored in the message's `AckHandle` — a reference-counted handle that is dropped when the message is fully processed (acked). When the `AckHandle` drops, the span ends automatically.
+
+### 3.5 Upstream Trace Stitching
+
+If the source message carries trace headers, the platform extracts them and makes `pipeline.process` a child of the upstream span. This stitches the Numaflow trace into the upstream producer's trace. Supported formats:
+
+- **W3C Trace Context** (`traceparent`, `tracestate`) — extracted directly.
+- **B3 Multi-Header** (`X-B3-TraceId`, `X-B3-SpanId`, `X-B3-Sampled`) — converted to W3C `traceparent` format. 64-bit trace IDs are left-padded to 128-bit.
+
+If no trace headers are present, `pipeline.process` starts a new root trace.
+
+## 4. Implementation: Numaflow Core (Rust)
+
+### 4.1 Dependencies
+
+```toml
+opentelemetry = "0.28"
+opentelemetry_sdk = "0.28"
+opentelemetry-otlp = "0.28"
+tracing-opentelemetry = "0.29"
+```
+
+### 4.2 Context Propagation Helper (`shared/otel.rs`)
+
+```rust
+pub const TRACING_METADATA_KEY: &str = "tracing";
+pub const TRACING_UDF_METADATA_KEY: &str = "tracing_udf";
+
+pub struct MetadataExtractor<'a>(pub &'a KeyValueGroup);
+// Implements Extractor — reads traceparent/tracestate from KeyValueGroup
+
+pub struct MetadataInjector<'a>(pub &'a mut KeyValueGroup);
+// Implements Injector — writes traceparent/tracestate to KeyValueGroup
+
+pub fn extract_trace_context(metadata: &Metadata) -> opentelemetry::Context;
+pub fn inject_context_into_metadata(metadata: &mut Metadata, key: &str, cx: &Context);
+pub fn extract_trace_context_from_headers(headers: &HashMap<String, String>) -> Context;
+```
+
+### 4.3 Component Instrumentation
+
+#### Source
+1. Extract upstream context from message headers (W3C traceparent or B3 multi-headers).
+2. Create `pipeline.process` span (parent: upstream context, or new root).
+3. Create `source.read` span (parent: `pipeline.process`). Short-lived marker.
+4. Inject `pipeline.process` context into `sys_metadata["tracing"]`.
+5. Store `pipeline.process` span in `AckHandle` for accurate duration.
+
+#### Map Vertex
+1. Extract `pipeline.process` context from `sys_metadata["tracing"]`.
+2. Create `map` span (parent: `pipeline.process`). Duration covers the full UDF call.
+3. Before UDF call: inject `map` context into `sys_metadata["tracing_udf"]`.
+4. After UDF returns: remove `tracing_udf` from result messages. Leave `tracing` (pipeline.process) untouched.
+
+#### Sink Vertex
+1. Extract `pipeline.process` context from `sys_metadata["tracing"]`.
+2. Create per-message `sink.write` spans. Duration covers the batch UDF call.
+3. Before UDF call: inject `sink.write` context into `sys_metadata["tracing_udf"]`.
+
+### 4.4 Handling Aggregation (Reduce Vertex)
+
+Reduce vertices ingest multiple messages (Fan-In) and produce a single result. Strategy:
+
+- **Primary Parent**: Select one context (e.g., first message in the window) as the direct parent of the Reduce span.
+- **Span Links**: Add contexts of other input messages as OTel Span Links. These appear as "Related Traces" in visualization tools.
+- **Limit**: Max 50 links to prevent span bloat. If the window has more messages, sample representative links (first 10, last 10, evenly spaced).
+
+### 4.5 Handling FlatMap (Fan-Out)
+
+When a Map vertex produces multiple output messages from a single input (1 → N):
+- The platform injects the **same** parent span context into `sys_metadata` for every output message.
+- Downstream vertices create N separate spans all pointing to the same parent.
+
+## 5. Implementation: Numaflow SDKs (Go/Java/Python/Rust)
+
+### 5.1 Initialization
+
+The SDK must initialize a tracer to participate in the trace. It reads:
+- `OTEL_EXPORTER_OTLP_ENDPOINT` — OTLP gRPC endpoint (e.g., `http://otel-collector:4317`)
+- `OTEL_SERVICE_NAME` — Service name for spans (e.g., `numaflow-udf-map`)
+
+### 5.2 Instrumentation
+
+In the UDF handler:
+1. **Extract**: Read parent context from `sys_metadata["tracing_udf"]` (stage-specific context). Falls back to `sys_metadata["tracing"]` (pipeline root) if `tracing_udf` is absent.
+2. **Start Span**: Create span (e.g., `udf.map.process`, kind=CONSUMER) as child of extracted context.
+3. **Execute**: Pass context to user function. Users can create child spans using standard OTel APIs.
+4. **End Span**: Close the span when the function returns.
+
+### 5.3 Example: Java Map UDF
+
+```java
+public MessageList processMessage(String[] keys, Datum datum) {
+    // Extract parent context from sys_metadata
+    Context parentCtx = NumaflowTracing.extractUdfContext(datum.getSystemMetadata());
+    Span span = NumaflowTracing.startSpan("udf.map.process", parentCtx, "map", null);
+
+    try (Scope scope = span.makeCurrent()) {
+        // User business logic — child spans auto-attach to udf.map.process
+        doWork();
+        return MessageList.newBuilder()
+                .addMessage(new Message(datum.getValue(), keys))
+                .build();
+    } finally {
+        span.end();
+    }
+}
+```
+
+## 6. End-to-End Trace Flow
+
+### MonoVertex (Source → Map → Sink)
+
+```
+┌──────────────────────────────────────────────────────────────────────────────┐
+│ Message arrives with trace headers (W3C traceparent or B3 multi-headers)    │
+│ If no headers present, a new root trace is started.                         │
+└──────────────┬───────────────────────────────────────────────────────────────┘
+               │
+               ▼
+┌─── Source (platform) ────────────────────────────────────────────────────────┐
+│  1. Extract trace context from message headers (W3C or B3 → W3C conversion) │
+│  2. Create pipeline.process span (parent: upstream, or new root)            │
+│  3. Create source.read span (parent: pipeline.process)                      │
+│  4. Inject pipeline.process context → sys_metadata["tracing"]               │
+│  5. Store pipeline.process span in AckHandle                                │
+└──────────────┬───────────────────────────────────────────────────────────────┘
+               │  sys_metadata["tracing"] = pipeline.process
+               ▼
+┌─── Map (platform) ───────────────────────────────────────────────────────────┐
+│  1. Extract pipeline.process from sys_metadata["tracing"]                   │
+│  2. Create map span (parent: pipeline.process) → sibling of source.read     │
+│  3. Inject map context → sys_metadata["tracing_udf"]                        │
+│  4. Call Map UDF via gRPC                                                   │
+│     └─ UDF reads sys_metadata["tracing_udf"] → creates udf.map.process     │
+│  5. Remove tracing_udf from result. Leave tracing (pipeline.process) intact │
+└──────────────┬───────────────────────────────────────────────────────────────┘
+               │  sys_metadata["tracing"] = pipeline.process (unchanged)
+               ▼
+┌─── Sink (platform) ──────────────────────────────────────────────────────────┐
+│  1. Extract pipeline.process from sys_metadata["tracing"]                   │
+│  2. Create sink.write span (parent: pipeline.process) → sibling of map      │
+│  3. Inject sink.write context → sys_metadata["tracing_udf"]                 │
+│  4. Call Sink UDF via gRPC                                                  │
+│     └─ UDF reads sys_metadata["tracing_udf"] → creates udf.sink.process    │
+│  5. Ack message → AckHandle drops → pipeline.process span ends              │
+└──────────────────────────────────────────────────────────────────────────────┘
+```
+
+### Pipeline (Source → Vertex₁ → ISB → Vertex₂ → ... → Sink)
+
+For multi-vertex Pipelines, the same pattern applies at each vertex. The `pipeline.process` context in `sys_metadata["tracing"]` is carried through the ISB (JetStream/Kafka) between vertices, maintaining the trace across process boundaries.
+
+## 7. Configuration & Deployment
+
+### 7.1 Enabling Tracing
+
+Add environment variables to the pipeline/monovertex spec:
+
+**Platform container** (`containerTemplate`):
+```yaml
+spec:
+  containerTemplate:
+    env:
+      - name: OTEL_EXPORTER_OTLP_ENDPOINT
+        value: "http://otel-collector.observability:4317"
+      - name: OTEL_SERVICE_NAME
+        value: "numaflow-core"
+```
+
+**UDF containers** (source, udf, sink):
+```yaml
+spec:
+  udf:
+    container:
+      env:
+        - name: OTEL_EXPORTER_OTLP_ENDPOINT
+          value: "http://otel-collector.observability:4317"
+        - name: OTEL_SERVICE_NAME
+          value: "numaflow-udf-map"
+```
+
+### 7.2 Infrastructure
+
+The user is responsible for deploying the observability stack:
+1. **OTel Collector**: Receives traces from Numaflow pods.
+2. **Backend**: Jaeger, Grafana Tempo, Datadog, etc.
+
+## 8. Reliability and Failure Handling
+
+### 8.1 OTLP Endpoint Down
+
+The tracing infrastructure is separate from the pipeline. If the OTel Collector becomes unreachable:
+- The OpenTelemetry SDK buffers spans in memory (default ~2048 spans).
+- It retries export with exponential backoff.
+- If the buffer fills, new spans are dropped. This is intentional to prevent unbounded memory growth.
+- **Pipeline impact**: None. Message processing continues normally.
+
+### 8.2 Sampling
+
+For high-throughput pipelines, recording every trace can be expensive. Head-based sampling at the Source decides "should I record this trace?" — all downstream components respect the sampled flag.
+
+- `OTEL_TRACES_SAMPLER`: Strategy (e.g., `parentbased_traceidratio`).
+- `OTEL_TRACES_SAMPLER_ARG`: Probability (e.g., `0.1` for 10% sampling).