How to Implement Resumable File Uploads in Koa.js?

Koa.js is a lightweight Node.js web framework for building fast web applications and APIs. To implement resumable file uploads in Koa.js, we need to use additional middleware and libraries to manage the logic for chunked uploads and resumable operations. The following are the general steps to implement this feature:

1. Selecting the Right Middleware and Libraries

• Use the koa-body or koa-multer middleware to handle file uploads.
• Choose libraries that support resumable uploads, such as tus-node-server, or manually handle it using streams.

2. Setting Up File Upload Middleware

javascript
const Koa = require('koa');
const koaBody = require('koa-body');

const app = new Koa();

app.use(koaBody({
  multipart: true,
  formidable: {
    // Set the temporary directory for uploaded files
    uploadDir: './uploads',
    keepExtensions: true,
  }
}));

3. Implementing Chunked Upload Logic

Chunk processing can be handled by the frontend sending chunk metadata (such as chunk index, total chunks, and file identifier) during upload, with the backend processing it accordingly:

javascript
app.use(async ctx => {
  if (ctx.url === '/upload' && ctx.method === 'POST') {
    // Get the uploaded file
    const file = ctx.request.files.file;
    const { name, path } = file;

    // Retrieve chunk metadata and other information from request body
    const { index, total, identifier } = ctx.request.body;

    // Generate a unique storage path for the chunk based on file identifier and chunk index
    const chunkPath = `./uploads/${identifier}_${index}`;

    // Move the uploaded chunk file to the chunk storage path
    const fs = require('fs');
    const readable = fs.createReadStream(path);
    const writable = fs.createWriteStream(chunkPath);
    readable.pipe(writable);

    // After confirming successful chunk upload, delete the temporary file
    fs.unlink(path, (err) => {
      if (err) throw err;
    });

    ctx.body = 'Chunk upload successful';
  }
});

4. Resumable Uploads and Chunk Reassembly Logic

• Store chunk information using a database or file system.
• Periodically check uploaded chunks to provide resumable status to the frontend.
• The frontend checks previously uploaded chunks before initiating new uploads to only send incomplete segments.
• After all chunks are uploaded, the backend merges them.

javascript
// Assume a method to merge chunks
async function mergeChunks(chunks, dest) {
  // Merge all chunks
  // ...
}

app.use(async ctx => {
  if (ctx.url === '/merge' && ctx.method === 'POST') {
    const { identifier, total } = ctx.request.body;
    const chunks = [];
    for (let i = 0; i < total; i++) {
      chunks.push(`./uploads/${identifier}_${i}`);
    }

    // Call the merge chunks method
    await mergeChunks(chunks, `./uploads/${identifier}.final`);

    ctx.body = 'File upload and merge successful';
  }
});

5. Handling Exceptions and Errors

Throughout the upload, resumption, and merge processes, handle potential exceptions and errors to ensure system stability.

6. Frontend Implementation

The frontend should use libraries supporting resumable uploads (such as tus-js-client) or implement custom logic, including chunking strategies, resumption handling, and user notifications upon completion.

The above are the general steps to implement resumable file uploads in Koa.js. The actual implementation may be more complex, including ensuring chunk integrity during concurrent uploads and handling network exceptions.