googleapis · blakeli0 · Jun 2, 2026 · Jun 11, 2026
@@ -0,0 +1,252 @@
+# Resumable Upload Protocol (Scotty) for Java: Design Document
+
+This document proposes the architecture and design for integrating the Scotty Resumable Upload Protocol (RUP) into `gax-java` and the GAPIC code generator.
+
+---
+
+## 1. Design Principles and Requirements
+
+1. **Veneer and GAPIC Aligned**: The solution must integrate cleanly into the existing `Callable` framework of `gax-java`.
+2. **Stream-Safe Retries**: Java `InputStream` is forward-only. The design must provide a clean abstraction (`InputStreamProvider`) to recreate or seek the stream during recovery.
+3. **Double-Loop Retry & Recovery**: Implements the precise Category 1 (transient) and Category 2 (state consistency) error classification with backoffs as described in the RUP specifications.
+4. **Progress Reporting**: Supports asynchronous progress updates via a simple callback mechanism.
+5. **No unnecessary chunking**: By default, uploads send the remaining bytes in one request (avoiding unnecessary memory buffering or chunk management).
+
+---
+
+## 2. API Design (`gax` Changes)
+
+We introduce a new callable type and request/response wrappers in `com.google.api.gax.rpc`.
+
+### 2.1. `InputStreamProvider`
+
+To support seeking/rewinding, the stream source is wrapped in a functional interface that can supply fresh streams on retry:
+
+```java
+package com.google.api.gax.rpc;
+
+import java.io.IOException;
+import java.io.InputStream;
+
+/** Provides a fresh {@link InputStream} for retriable upload operations. */
+@FunctionalInterface
+public interface InputStreamProvider {
+  /** Returns a new {@link InputStream}. */
+  InputStream get() throws IOException;
+}
+```
+
+### 2.2. Progress Listener and Status
+
+```java
+package com.google.api.gax.rpc;
+
+/** Listener for tracking progress of a resumable upload. */
+public interface ResumableUploadProgressListener {
+
+  enum State {
+    NOT_STARTED,
+    IN_PROGRESS,
+    RECOVERING,
+    COMPLETED,
+    FAILED,
+    CANCELLED
+  }
+
+  void onProgress(ResumableUploadStatus status);
+}
+
+/** Status details for progress updates. */
+public final class ResumableUploadStatus {
+  private final long bytesUploaded;
+  private final long totalBytes;
+  private final ResumableUploadProgressListener.State state;
+
+  public ResumableUploadStatus(long bytesUploaded, long totalBytes, ResumableUploadProgressListener.State state) {
+    this.bytesUploaded = bytesUploaded;
+    this.totalBytes = totalBytes;
+    this.state = state;
+  }
+
+  public long getBytesUploaded() { return bytesUploaded; }
+  public long getTotalBytes() { return totalBytes; }
+  public ResumableUploadProgressListener.State getState() { return state; }
+}
+```
+
+### 2.3. Request Wrapper: `ResumableUploadRequest`
+
+```java
+package com.google.api.gax.rpc;
+
+import com.google.common.base.Preconditions;
+
+public final class ResumableUploadRequest<RequestT> {
+  private final RequestT request;
+  private final InputStreamProvider streamProvider;
+  private final long totalBytes; // -1 if unknown
+  private final ResumableUploadProgressListener progressListener;
+
+  private ResumableUploadRequest(Builder<RequestT> builder) {
+    this.request = Preconditions.checkNotNull(builder.request);
+    this.streamProvider = Preconditions.checkNotNull(builder.streamProvider);
+    this.totalBytes = builder.totalBytes;
+    this.progressListener = builder.progressListener;
+  }
+
+  public RequestT getRequest() { return request; }
+  public InputStreamProvider getStreamProvider() { return streamProvider; }
+  public long getTotalBytes() { return totalBytes; }
+  public ResumableUploadProgressListener getProgressListener() { return progressListener; }
+
+  public static <RequestT> Builder<RequestT> newBuilder() {
+    return new Builder<>();
+  }
+
+  public static class Builder<RequestT> {
+    private RequestT request;
+    private InputStreamProvider streamProvider;
+    private long totalBytes = -1;
+    private ResumableUploadProgressListener progressListener;
+
+    public Builder<RequestT> setRequest(RequestT request) {
+      this.request = request;
+      return this;
+    }
+    public Builder<RequestT> setStreamProvider(InputStreamProvider streamProvider) {
+      this.streamProvider = streamProvider;
+      return this;
+    }
+    public Builder<RequestT> setTotalBytes(long totalBytes) {
+      this.totalBytes = totalBytes;
+      return this;
+    }
+    public Builder<RequestT> setProgressListener(ResumableUploadProgressListener progressListener) {
+      this.progressListener = progressListener;
+      return this;
+    }
+    public ResumableUploadRequest<RequestT> build() {
+      return new ResumableUploadRequest<>(this);
+    }
+  }
+}
+```
+
+### 2.4. Callable Wrapper: `ResumableUploadCallable`
+
+```java
+package com.google.api.gax.rpc;
+
+import com.google.api.core.ApiFuture;
+
+public abstract class ResumableUploadCallable<RequestT, ResponseT> {
+
+  protected ResumableUploadCallable() {}
+
+  public abstract ApiFuture<ResponseT> futureCall(
+      ResumableUploadRequest<RequestT> request, ApiCallContext context);
+
+  public ResponseT call(ResumableUploadRequest<RequestT> request, ApiCallContext context) {
+    return ApiExceptions.callAndTranslateCharSequenceException(futureCall(request, context));
+  }
+
+  public ResponseT call(ResumableUploadRequest<RequestT> request) {
+    return call(request, null);
+  }
+}
+```
+
+---
+
+## 3. Transport Implementation (`gax-httpjson`)
+
+The transport layer executes the actual HTTP protocol calls using the Google HTTP Client.
+
+We introduce `HttpJsonResumableUploadCall` to coordinate the Scotty state machine.
+
+### 3.1. Error Categorization in Java
+
+```java
+private enum ErrorCategory {
+  CATEGORY_1_TRANSIENT,  // 429, 500, 502, 503, 504, TCP/Socket Timeout
+  CATEGORY_2_MISMATCH,   // 400, 412, 416
+  CATEGORY_3_FATAL       // 401, 403, 404, etc.
+}
+
+private ErrorCategory getErrorCategory(Throwable t) {
+  if (t instanceof HttpResponseException) {
+    int statusCode = ((HttpResponseException) t).getStatusCode();
+    if (statusCode == 429 || statusCode >= 500) {
+      return ErrorCategory.CATEGORY_1_TRANSIENT;
+    }
+    if (statusCode == 400 || statusCode == 412 || statusCode == 416) {
+      return ErrorCategory.CATEGORY_2_MISMATCH;
+    }
+  }
+  if (t instanceof IOException) {
+    // Socket timeouts, connection drops
+    return ErrorCategory.CATEGORY_1_TRANSIENT;
+  }
+  return ErrorCategory.CATEGORY_3_FATAL;
+}
+```
+
+### 3.2. Detailed Execution Flow (State Machine)
+
+The `HttpJsonResumableUploadCall` runs inside the user's thread (or client executor pool for future execution) and implements the following flow:
+
+```mermaid
+stateDiagram-v2
+    [*] --> StartSession
+    StartSession --> UploadLoop : Success (200 OK + active)
+    StartSession --> StartSession : Cat 1 Transient (Backoff)
+    StartSession --> [*] : Cat 3 Fatal / Deadline Exceeded
+
+    state UploadLoop {
+        [*] --> OpenStream
+        OpenStream --> SkipToOffset
+        SkipToOffset --> TransmitChunk
+        TransmitChunk --> [*] : Success (final)
+        TransmitChunk --> QueryState : Cat 2 Mismatch / Socket Drop
+        TransmitChunk --> TransmitChunk : Cat 1 Transient (Backoff)
+    }
+
+    UploadLoop --> [*] : Success
+    UploadLoop --> [*] : Cat 3 Fatal / Deadline Exceeded
+
+    state QueryState {
+        [*] --> SendQuery
+        SendQuery --> ResumeUpload : Success (active + new offset)
+        SendQuery --> [*] : Success (final)
+        SendQuery --> SendQuery : Cat 1 Transient (Backoff)
+        SendQuery --> [*] : Cat 3 Fatal
+    }
+
+    QueryState --> UploadLoop : Resume
+```
+
+#### Step 1: Start Session
+- Build standard headers + merge user-provided metadata.
+- Pre-emptively prefix headers that affect physical bodies (`Content-Length`, `Content-Type`, etc.) with `X-Goog-Upload-Header-`.
+- Set `X-Goog-Upload-Protocol: resumable` and `X-Goog-Upload-Command: start`.
+- Execute POST with the request JSON body.
+- Extract `X-Goog-Upload-URL` header value to obtain the `uploadUrl`.
+
+#### Step 2: Upload Loop (Transmit)
+- Check absolute global deadline.
+- Call `streamProvider.get()`.
+- Skip/seek to current `offset`.
+- Set `X-Goog-Upload-Command: upload, finalize` and `X-Goog-Upload-Offset: offset`.
+- Stream payload using a chunked output stream, updating the progress listener during writes.
+- If response is `final` with `2xx`: parse response and return.
+- If exception occurs: Categorize exception. If Category 2 (Mismatch) or connection drop, transition to **Query State**.
+
+#### Step 3: Query State
+- Execute POST to `uploadUrl` with `X-Goog-Upload-Command: query`.
+- If response is `active`:
+  - Extract `X-Goog-Upload-Size-Received` -> `newOffset`.
+  - If `newOffset == offset`: apply backoff (to avoid spamming server).
+  - Update `offset = newOffset` and transition back to **Upload Loop**.
+- If response is `final`: return response.
+- If Category 1 (Transient) error: retry query with backoff.
+- If Category 3 (Fatal) error: fail immediately.
@@ -41,6 +41,7 @@
 import com.google.api.gax.rpc.PagedCallSettings;
 import com.google.api.gax.rpc.ServerStreamingCallSettings;
 import com.google.api.gax.rpc.ServerStreamingCallable;
+import com.google.api.gax.rpc.ResumableUploadCallable;
 import com.google.api.gax.rpc.UnaryCallSettings;
 import com.google.api.gax.rpc.UnaryCallable;
 import com.google.api.gax.tracing.ApiTracerContext;
@@ -220,6 +221,19 @@ ServerStreamingCallable<RequestT, ResponseT> createServerStreamingCallable(
     return callable.withDefaultCallContext(clientContext.getDefaultCallContext());
   }
 
+  /**
+   * Create a resumable upload callable object. Designed for use by generated code.
+   *
+   * @param httpJsonCallSettings the http/json call settings
+   * @param clientContext {@link ClientContext} to use to connect to the service.
+   * @return {@link ResumableUploadCallable} callable object.
+   */
+  public static <RequestT, ResponseT> ResumableUploadCallable<RequestT, ResponseT> createResumableUploadCallable(
+      HttpJsonCallSettings<RequestT, ResponseT> httpJsonCallSettings,
+      ClientContext clientContext) {
+    return new HttpJsonResumableUploadCallable<>(httpJsonCallSettings, clientContext);
+  }
+
   static ApiTracerContext getApiTracerContext(@Nonnull ApiMethodDescriptor<?, ?> methodDescriptor) {
     return ApiTracerContext.newBuilder()
         .setFullMethodName(methodDescriptor.getFullMethodName())