feat: add API to control number of files uploaded concurrently

packages/upload/README.md

@@ -28,6 +28,29 @@ Once installed, import the component in your application:
 import '@vaadin/upload';
 ```
 
+## Performance Considerations
+
+When uploading large numbers of files, the component automatically throttles concurrent uploads to prevent browser performance degradation. By default, a maximum of 3 files are uploaded simultaneously, with additional files queued automatically.
+
+You can customize this limit using the `max-concurrent-uploads` attribute:
+
+```html
+<!-- Limit to 5 concurrent uploads -->
+<vaadin-upload max-concurrent-uploads="5"></vaadin-upload>
+```
+
+```js
+// Or set it programmatically
+upload.maxConcurrentUploads = 5;
+```
+
+This helps prevent:
+- Browser XHR limitations (failures when uploading 2000+ files simultaneously)
+- Performance degradation with hundreds of concurrent uploads
+- Network congestion on slower connections
+
+The default value of 3 balances upload performance with network resource conservation.
+
 ## Contributing
 
 Read the [contributing guide](https://vaadin.com/docs/latest/contributing) to learn about our development process, how to propose bugfixes and improvements, and how to test your changes to Vaadin components.

packages/upload/src/vaadin-upload-mixin.d.ts

@@ -205,6 +205,16 @@ export declare class UploadMixinClass {
    */
   uploadFormat: UploadFormat;
 
+  /**
+   * Specifies the maximum number of files that can be uploaded simultaneously.
+   * This helps prevent browser performance degradation and XHR limitations when
+   * uploading large numbers of files. Files exceeding this limit will be queued
+   * and uploaded as active uploads complete.
+   * @attr {number} max-concurrent-uploads
+   * @default 3
+   */
+  maxConcurrentUploads: number;
+
   /**
    * The object used to localize this component. To change the default
    * localization, replace this with an object that provides all properties, or

packages/upload/src/vaadin-upload-mixin.js

@@ -322,6 +322,20 @@ export const UploadMixin = (superClass) =>
           value: 'raw',
         },
 
+        /**
+         * Specifies the maximum number of files that can be uploaded simultaneously.
+         * This helps prevent browser performance degradation and XHR limitations when
+         * uploading large numbers of files. Files exceeding this limit will be queued
+         * and uploaded as active uploads complete.
+         * @attr {number} max-concurrent-uploads
+         * @type {number}
+         */
+        maxConcurrentUploads: {
+          type: Number,
+          value: 3,
+          sync: true,
+        },
+
         /**
          * Pass-through to input's capture attribute. Allows user to trigger device inputs
          * such as camera or microphone immediately.
@@ -347,6 +361,18 @@ export const UploadMixin = (superClass) =>
         _files: {
           type: Array,
         },
+
+        /** @private */
+        _uploadQueue: {
+          type: Array,
+          value: () => [],
+        },
+
+        /** @private */
+        _activeUploads: {
+          type: Number,
+          value: 0,
+        },
       };
     }
 
@@ -698,12 +724,48 @@ export const UploadMixin = (superClass) =>
       Array.prototype.forEach.call(files, this._uploadFile.bind(this));
     }
 
+    /**
+     * Process the upload queue by starting uploads for queued files
+     * if there is available capacity.
+     * @private
+     */
+    _processQueue() {
+      // Process as many queued files as we have capacity for
+      while (this._uploadQueue.length > 0 && this._activeUploads < this.maxConcurrentUploads) {
+        const nextFile = this._uploadQueue.shift();
+        if (nextFile && !nextFile.complete && !nextFile.uploading) {
+          this._uploadFile(nextFile);
+        }
+      }
+    }
+
     /** @private */
     _uploadFile(file) {
       if (file.uploading) {
         return;
       }
 
+      // Check if we've reached the concurrent upload limit
+      if (this._activeUploads >= this.maxConcurrentUploads) {
+        // Add to queue if not already queued
+        if (!this._uploadQueue.includes(file)) {
+          this._uploadQueue.push(file);
+          file.held = true;
+          file.status = this.__effectiveI18n.uploading.status.held;
+          this._renderFileList();
+        }
+        return;
+      }
+
+      // Remove from queue if it was queued
+      const queueIndex = this._uploadQueue.indexOf(file);
+      if (queueIndex >= 0) {
+        this._uploadQueue.splice(queueIndex, 1);
+      }
+
+      // Increment active uploads counter
+      this._activeUploads += 1;
+
       const ini = Date.now();
       const xhr = (file.xhr = this._createXhr());
 
@@ -745,7 +807,13 @@ export const UploadMixin = (superClass) =>
         if (xhr.readyState === 4) {
           clearTimeout(stalledId);
           file.indeterminate = file.uploading = false;
+
+          // Decrement active uploads counter
+          this._activeUploads -= 1;
+
           if (file.abort) {
+            // Process queue even on abort
+            this._processQueue();
             return;
           }
           file.status = '';
@@ -759,6 +827,8 @@ export const UploadMixin = (superClass) =>
           );
 
           if (!evt) {
+            // Process queue even if event was cancelled
+            this._processQueue();
             return;
           }
           if (xhr.status === 0) {
@@ -776,6 +846,9 @@ export const UploadMixin = (superClass) =>
             }),
           );
           this._renderFileList();
+
+          // Process the queue to start the next upload
+          this._processQueue();
         }
       };
 
@@ -877,10 +950,24 @@ export const UploadMixin = (superClass) =>
       );
       if (evt) {
         file.abort = true;
+
+        // Remove from queue if it was queued
+        const queueIndex = this._uploadQueue.indexOf(file);
+        if (queueIndex >= 0) {
+          this._uploadQueue.splice(queueIndex, 1);
+        }
+
+        // Decrement active uploads if file was uploading
+        if (file.uploading) {
+          this._activeUploads -= 1;
+        }
+
         if (file.xhr) {
           file.xhr.abort();
         }
         this._removeFile(file);
+        // Process the queue to start the next upload
+        this._processQueue();
       }
     }
 

packages/upload/test/adding-files.test.js

@@ -19,7 +19,7 @@ describe('adding files', () => {
 
   beforeEach(async () => {
     upload = fixtureSync(`<vaadin-upload></vaadin-upload>`);
-    upload.target = 'http://foo.com/bar';
+    upload.target = 'https://foo.com/bar';
     upload._createXhr = xhrCreator({ size: testFileSize, uploadTime: 200, stepTime: 50 });
     await nextRender();
     files = createFiles(2, testFileSize, 'application/x-octet-stream');
@@ -332,12 +332,17 @@ describe('adding files', () => {
 
   describe('start upload', () => {
     it('should automatically start upload', () => {
+      upload.maxConcurrentUploads = 1;
       const uploadStartSpy = sinon.spy();
       upload.addEventListener('upload-start', uploadStartSpy);
 
       files.forEach(upload._addFile.bind(upload));
-      expect(uploadStartSpy.calledTwice).to.be.true;
-      expect(upload.files[0].held).to.be.false;
+      // With queue behavior, only the first file starts uploading immediately
+      expect(uploadStartSpy.calledOnce).to.be.true;
+      // Files are prepended, so the first file added is at index 1
+      expect(upload.files[1].held).to.be.false;
+      // Second file (at index 0) should be held in queue
+      expect(upload.files[0].held).to.be.true;
     });
 
     it('should not automatically start upload when noAuto flag is set', () => {