Add request/response body size tracking to workerd trace events

steebchen · claude · steebchen · commit 15358699dcee · 2026-01-17T17:19:37.000Z
Add requestSize and bodySize fields to FetchEventInfo and FetchResponseInfo so tail workers can access body size information. Request size is captured from the Content-Length header, and response size from expectedBodySize in ResponseSentTracker. Both are exposed as optional properties in JavaScript trace events.

Co-Authored-By: Claude Haiku 4.5 &lt;noreply@anthropic.com&gt;
diff --git a/src/workerd/api/trace.c++ b/src/workerd/api/trace.c++
@@ -127,8 +127,8 @@ kj::Own<TraceItem::FetchEventInfo::Request::Detail> getFetchRequestDetail(
     };
   };
 
-  return kj::refcounted<TraceItem::FetchEventInfo::Request::Detail>(
-      getCf(), getHeaders(), kj::str(eventInfo.method), kj::str(eventInfo.url));
+  return kj::refcounted<TraceItem::FetchEventInfo::Request::Detail>(getCf(), getHeaders(),
+      kj::str(eventInfo.method), kj::str(eventInfo.url), eventInfo.requestSize);
 }
 
 kj::Maybe<TraceItem::EventInfo> getTraceEvent(jsg::Lock& js, const Trace& trace) {
@@ -304,11 +304,13 @@ TraceItem::FetchEventInfo::FetchEventInfo(jsg::Lock& js,
 TraceItem::FetchEventInfo::Request::Detail::Detail(jsg::Optional<jsg::V8Ref<v8::Object>> cf,
     kj::Array<tracing::FetchEventInfo::Header> headers,
     kj::String method,
-    kj::String url)
+    kj::String url,
+    uint64_t requestSize)
     : cf(kj::mv(cf)),
       headers(kj::mv(headers)),
       method(kj::mv(method)),
-      url(kj::mv(url)) {}
+      url(kj::mv(url)),
+      requestSize(requestSize) {}
 
 jsg::Ref<TraceItem::FetchEventInfo::Request> TraceItem::FetchEventInfo::getRequest() {
   return request.addRef();
@@ -361,19 +363,36 @@ kj::String TraceItem::FetchEventInfo::Request::getUrl() {
   return (redacted ? redactUrl(detail->url) : kj::str(detail->url));
 }
 
+jsg::Optional<double> TraceItem::FetchEventInfo::Request::getBodySize() {
+  // Return null if requestSize is 0 (unknown/no body), otherwise return the size
+  if (detail->requestSize == 0) {
+    return kj::none;
+  }
+  return static_cast<double>(detail->requestSize);
+}
+
 jsg::Ref<TraceItem::FetchEventInfo::Request> TraceItem::FetchEventInfo::Request::getUnredacted(
     jsg::Lock& js) {
   return js.alloc<Request>(*detail, false /* details are not redacted */);
 }
 
 TraceItem::FetchEventInfo::Response::Response(
     const Trace& trace, const tracing::FetchResponseInfo& responseInfo)
-    : status(responseInfo.statusCode) {}
+    : status(responseInfo.statusCode),
+      bodySize(responseInfo.bodySize) {}
 
 uint16_t TraceItem::FetchEventInfo::Response::getStatus() {
   return status;
 }
 
+jsg::Optional<double> TraceItem::FetchEventInfo::Response::getBodySize() {
+  // Return null if bodySize is 0 (unknown/no body), otherwise return the size
+  if (bodySize == 0) {
+    return kj::none;
+  }
+  return static_cast<double>(bodySize);
+}
+
 TraceItem::JsRpcEventInfo::JsRpcEventInfo(
     const Trace& trace, const tracing::JsRpcEventInfo& eventInfo)
     : rpcMethod(kj::str(eventInfo.methodName)) {}
diff --git a/src/workerd/api/trace.h b/src/workerd/api/trace.h
@@ -199,11 +199,13 @@ class TraceItem::FetchEventInfo::Request final: public jsg::Object {
     kj::Array<tracing::FetchEventInfo::Header> headers;
     kj::String method;
     kj::String url;
+    uint64_t requestSize;
 
     Detail(jsg::Optional<jsg::V8Ref<v8::Object>> cf,
         kj::Array<tracing::FetchEventInfo::Header> headers,
         kj::String method,
-        kj::String url);
+        kj::String url,
+        uint64_t requestSize);
 
     JSG_MEMORY_INFO(Detail) {
       tracker.trackField("cf", cf);
@@ -224,6 +226,7 @@ class TraceItem::FetchEventInfo::Request final: public jsg::Object {
   jsg::Dict<kj::String, kj::String> getHeaders(jsg::Lock& js);
   kj::StringPtr getMethod();
   kj::String getUrl();
+  jsg::Optional<double> getBodySize();
 
   jsg::Ref<Request> getUnredacted(jsg::Lock& js);
 
@@ -232,6 +235,7 @@ class TraceItem::FetchEventInfo::Request final: public jsg::Object {
     JSG_LAZY_READONLY_INSTANCE_PROPERTY(headers, getHeaders);
     JSG_LAZY_READONLY_INSTANCE_PROPERTY(method, getMethod);
     JSG_LAZY_READONLY_INSTANCE_PROPERTY(url, getUrl);
+    JSG_LAZY_READONLY_INSTANCE_PROPERTY(bodySize, getBodySize);
 
     JSG_METHOD(getUnredacted);
   }
@@ -250,13 +254,16 @@ class TraceItem::FetchEventInfo::Response final: public jsg::Object {
   explicit Response(const Trace& trace, const tracing::FetchResponseInfo& responseInfo);
 
   uint16_t getStatus();
+  jsg::Optional<double> getBodySize();
 
   JSG_RESOURCE_TYPE(Response) {
     JSG_LAZY_READONLY_INSTANCE_PROPERTY(status, getStatus);
+    JSG_LAZY_READONLY_INSTANCE_PROPERTY(bodySize, getBodySize);
   }
 
  private:
   uint16_t status;
+  uint64_t bodySize;
 };
 
 class TraceItem::JsRpcEventInfo final: public jsg::Object {
diff --git a/src/workerd/io/trace.c++ b/src/workerd/io/trace.c++
@@ -323,17 +323,22 @@ static kj::HttpMethod validateMethod(capnp::HttpMethod method) {
 
 }  // namespace
 
-FetchEventInfo::FetchEventInfo(
-    kj::HttpMethod method, kj::String url, kj::String cfJson, kj::Array<Header> headers)
+FetchEventInfo::FetchEventInfo(kj::HttpMethod method,
+    kj::String url,
+    kj::String cfJson,
+    kj::Array<Header> headers,
+    uint64_t requestSize)
     : method(method),
       url(kj::mv(url)),
       cfJson(kj::mv(cfJson)),
-      headers(kj::mv(headers)) {}
+      headers(kj::mv(headers)),
+      requestSize(requestSize) {}
 
 FetchEventInfo::FetchEventInfo(rpc::Trace::FetchEventInfo::Reader reader)
     : method(validateMethod(reader.getMethod())),
       url(kj::str(reader.getUrl())),
-      cfJson(kj::str(reader.getCfJson())) {
+      cfJson(kj::str(reader.getCfJson())),
+      requestSize(reader.getRequestSize()) {
   kj::Vector<Header> v;
   v.addAll(reader.getHeaders());
   headers = v.releaseAsArray();
@@ -343,6 +348,7 @@ void FetchEventInfo::copyTo(rpc::Trace::FetchEventInfo::Builder builder) const {
   builder.setMethod(static_cast<capnp::HttpMethod>(method));
   builder.setUrl(url);
   builder.setCfJson(cfJson);
+  builder.setRequestSize(requestSize);
 
   auto list = builder.initHeaders(headers.size());
   for (auto i: kj::indices(headers)) {
@@ -352,7 +358,7 @@ void FetchEventInfo::copyTo(rpc::Trace::FetchEventInfo::Builder builder) const {
 
 FetchEventInfo FetchEventInfo::clone() const {
   return FetchEventInfo(
-      method, kj::str(url), kj::str(cfJson), KJ_MAP(h, headers) { return h.clone(); });
+      method, kj::str(url), kj::str(cfJson), KJ_MAP(h, headers) { return h.clone(); }, requestSize);
 }
 
 kj::String FetchEventInfo::toString() const {
@@ -595,17 +601,21 @@ HibernatableWebSocketEventInfo::Type HibernatableWebSocketEventInfo::readFrom(
   }
 }
 
-FetchResponseInfo::FetchResponseInfo(uint16_t statusCode): statusCode(statusCode) {}
+FetchResponseInfo::FetchResponseInfo(uint16_t statusCode, uint64_t bodySize)
+    : statusCode(statusCode),
+      bodySize(bodySize) {}
 
 FetchResponseInfo::FetchResponseInfo(rpc::Trace::FetchResponseInfo::Reader reader)
-    : statusCode(reader.getStatusCode()) {}
+    : statusCode(reader.getStatusCode()),
+      bodySize(reader.getBodySize()) {}
 
 void FetchResponseInfo::copyTo(rpc::Trace::FetchResponseInfo::Builder builder) const {
   builder.setStatusCode(statusCode);
+  builder.setBodySize(bodySize);
 }
 
 FetchResponseInfo FetchResponseInfo::clone() const {
-  return FetchResponseInfo(statusCode);
+  return FetchResponseInfo(statusCode, bodySize);
 }
 
 Log::Log(kj::Date timestamp, LogLevel logLevel, kj::String message)
diff --git a/src/workerd/io/trace.h b/src/workerd/io/trace.h
@@ -309,8 +309,11 @@ kj::String KJ_STRINGIFY(const SpanContext& context);
 struct FetchEventInfo final {
   struct Header;
 
-  explicit FetchEventInfo(
-      kj::HttpMethod method, kj::String url, kj::String cfJson, kj::Array<Header> headers);
+  explicit FetchEventInfo(kj::HttpMethod method,
+      kj::String url,
+      kj::String cfJson,
+      kj::Array<Header> headers,
+      uint64_t requestSize = 0);
   FetchEventInfo(rpc::Trace::FetchEventInfo::Reader reader);
   FetchEventInfo(FetchEventInfo&&) = default;
   FetchEventInfo& operator=(FetchEventInfo&&) = default;
@@ -341,6 +344,7 @@ struct FetchEventInfo final {
   // TODO(perf): It might be more efficient to store some sort of parsed JSON result instead?
   kj::String cfJson;
   kj::Array<Header> headers;
+  uint64_t requestSize = 0;  // Request body size in bytes. 0 if unknown or no body.
 
   void copyTo(rpc::Trace::FetchEventInfo::Builder builder) const;
   FetchEventInfo clone() const;
@@ -484,13 +488,14 @@ struct CustomEventInfo final {
 
 // Describes a fetch response
 struct FetchResponseInfo final {
-  explicit FetchResponseInfo(uint16_t statusCode);
+  explicit FetchResponseInfo(uint16_t statusCode, uint64_t bodySize = 0);
   FetchResponseInfo(rpc::Trace::FetchResponseInfo::Reader reader);
   FetchResponseInfo(FetchResponseInfo&&) = default;
   FetchResponseInfo& operator=(FetchResponseInfo&&) = default;
   KJ_DISALLOW_COPY(FetchResponseInfo);
 
   uint16_t statusCode;
+  uint64_t bodySize = 0;  // Response body size in bytes. 0 if unknown or no body.
 
   void copyTo(rpc::Trace::FetchResponseInfo::Builder builder) const;
   FetchResponseInfo clone() const;
diff --git a/src/workerd/io/worker-entrypoint.c++ b/src/workerd/io/worker-entrypoint.c++
@@ -138,6 +138,9 @@ class WorkerEntrypoint::ResponseSentTracker final: public kj::HttpService::Respo
   uint getHttpResponseStatus() const {
     return httpResponseStatus;
   }
+  kj::Maybe<uint64_t> getExpectedBodySize() const {
+    return expectedBodySize;
+  }
 
   kj::Own<kj::AsyncOutputStream> send(uint statusCode,
       kj::StringPtr statusText,
@@ -147,6 +150,7 @@ class WorkerEntrypoint::ResponseSentTracker final: public kj::HttpService::Respo
         "workerd", "WorkerEntrypoint::ResponseSentTracker::send()", "statusCode", statusCode);
     sent = true;
     httpResponseStatus = statusCode;
+    this->expectedBodySize = expectedBodySize;
     return inner.send(statusCode, statusText, headers, expectedBodySize);
   }
 
@@ -158,6 +162,7 @@ class WorkerEntrypoint::ResponseSentTracker final: public kj::HttpService::Respo
 
  private:
   uint httpResponseStatus = 0;
+  kj::Maybe<uint64_t> expectedBodySize;
   kj::HttpService::Response& inner;
   bool sent = false;
 };
@@ -286,8 +291,20 @@ kj::Promise<void> WorkerEntrypoint::request(kj::HttpMethod method,
       return tracing::FetchEventInfo::Header(kj::mv(entry.key), kj::strArray(entry.value, ", "));
     };
 
+    // Extract request body size from Content-Length header if present
+    uint64_t requestSize = 0;
+    KJ_IF_SOME(contentLength, headers.get(kj::HttpHeaderId::CONTENT_LENGTH)) {
+      // Parse the Content-Length value. If parsing fails, we leave requestSize as 0.
+      char* end;
+      auto parsed = strtoull(contentLength.cStr(), &end, 10);
+      if (end != contentLength.cStr() && *end == '\0') {
+        requestSize = parsed;
+      }
+    }
+
     t.setEventInfo(*incomingRequest,
-        tracing::FetchEventInfo(method, kj::str(url), kj::mv(cfJson), kj::mv(traceHeadersArray)));
+        tracing::FetchEventInfo(
+            method, kj::str(url), kj::mv(cfJson), kj::mv(traceHeadersArray), requestSize));
     workerTracer = t;
   }
 
@@ -331,7 +348,9 @@ kj::Promise<void> WorkerEntrypoint::request(kj::HttpMethod method,
     KJ_IF_SOME(t, workerTracer) {
       auto httpResponseStatus = wrappedResponse.getHttpResponseStatus();
       if (httpResponseStatus != 0) {
-        t.setReturn(context.now(), tracing::FetchResponseInfo(httpResponseStatus));
+        uint64_t responseBodySize = wrappedResponse.getExpectedBodySize().orDefault(0);
+        t.setReturn(
+            context.now(), tracing::FetchResponseInfo(httpResponseStatus, responseBodySize));
       } else {
         t.setReturn(context.now());
       }
diff --git a/src/workerd/io/worker-interface.capnp b/src/workerd/io/worker-interface.capnp
@@ -108,6 +108,8 @@ struct Trace @0x8e8d911203762d34 {
       name @0 :Text;
       value @1 :Text;
     }
+    requestSize @4 :UInt64;
+    # Request body size in bytes. 0 if unknown or no body.
   }
 
   struct JsRpcEventInfo {
@@ -158,6 +160,8 @@ struct Trace @0x8e8d911203762d34 {
   response @8 :FetchResponseInfo;
   struct FetchResponseInfo {
     statusCode @0 :UInt16;
+    bodySize @1 :UInt64;
+    # Response body size in bytes. 0 if unknown or no body.
   }
 
   cpuTime @10 :UInt64;

Original file line number	Diff line number	Diff line change
`@@ -108,6 +108,8 @@ struct Trace @0x8e8d911203762d34 {`
`108`	`108`	`name @0 :Text;`
`109`	`109`	`value @1 :Text;`
`110`	`110`	`}`
	`111`	`+ requestSize @4 :UInt64;`
	`112`	`+ # Request body size in bytes. 0 if unknown or no body.`
`111`	`113`	`}`
`112`	`114`
`113`	`115`	`struct JsRpcEventInfo {`
`@@ -158,6 +160,8 @@ struct Trace @0x8e8d911203762d34 {`
`158`	`160`	`response @8 :FetchResponseInfo;`
`159`	`161`	`struct FetchResponseInfo {`
`160`	`162`	`statusCode @0 :UInt16;`
	`163`	`+ bodySize @1 :UInt64;`
	`164`	`+ # Response body size in bytes. 0 if unknown or no body.`
`161`	`165`	`}`
`162`	`166`
`163`	`167`	`cpuTime @10 :UInt64;`