Seamless Large File Uploads with Chunking Strategy

Uploading large files has always been a challenge in web applications. Whether it’s videos, high-resolution images, or large datasets, traditional upload methods often fail due to server timeouts, unstable connections, or browser limitations. This can lead to frustration for users and extra load for developers trying to ensure reliability.

One effective solution to this problem is chunk upload. Instead of sending the entire file in a single request, the file is split into smaller, manageable pieces called chunks. These chunks are uploaded sequentially (or in parallel), and once all parts are successfully received, the server reassembles them into the original file.

This approach not only makes large file uploads more reliable but also allows features like upload resumption, better error handling, and improved performance. In this article, we’ll explore the problems with large file uploads, how chunk upload works, and why it’s becoming the preferred method for modern applications.

The Main Problem: Uploading Large Files

Uploading large files is one of the most common pain points in modern web applications. Imagine a user trying to upload a 2GB video or a high-resolution dataset:

• If the internet connection is unstable, a small disruption can cause the entire upload to fail.
• Many servers have strict timeout limits, so long-running uploads may get cut off.
• A single large request can put heavy strain on the backend, leading to performance issues.
• Users often have to restart from zero if something goes wrong — which is frustrating and inefficient.

Traditional single-request uploads are simply not reliable for big files. This is where chunk upload comes in as a practical solution.

What is Chunk Upload?

• Definition: splitting a file into smaller, manageable pieces (chunks).
• Workflow overview:
  1. The file is split into chunks on the client side.
  2. Each chunk is uploaded sequentially (or in parallel) to the server.
  3. The server temporarily stores the chunks.
  4. Once all chunks are received, the server merges them back into the original file.
• Advantages: more reliable uploads, resumable transfers, better error handling, improved performance.

Flow of Chunk Upload

Client-Side (Browser / Mobile App)

• Select File: User chooses a file (e.g., video, dataset, or large image).
• Split File: Client splits the file into smaller pieces (chunks), e.g. 5MB each.
• Assign Metadata (sent with each chunk):
  • fileId → unique identifier for this file (UUID or hash).
  • chunkIndex → the current chunk number (0, 1, 2, …).
  • totalChunks → how many chunks in total.
  • fileName → original file name.
• Upload Chunks: Send each chunk to the server via HTTP (sequentially or in parallel).
• Notify Completion: After all chunks are sent, the client calls a final API to tell the server: “I’ve uploaded all chunks, please merge them.”

Server-Side

• Receive Chunks: Each chunk is saved temporarily, usually in a folder like chunks/{fileId}/.
• Track Chunks: The server uses chunkIndex and totalChunks to know which pieces belong to which file.
• Merge Chunks: Once the client notifies completion (or server detects all chunks uploaded), the server concatenates all chunks in order to recreate the original file.
• Clean Up: Temporary chunk files are deleted after merging.

One of the biggest advantages of chunk upload is that it can be extended with powerful features to make large file transfers more reliable and efficient.

Resumable Uploads allow users to continue uploading from where they left off if their connection is interrupted. Instead of starting over, the client first checks with the server to see which chunks have already been stored. It then only sends the missing pieces, saving both time and bandwidth.

Parallel Uploads make the process faster by sending multiple chunks at the same time. Since each chunk is independent, the client can open several connections to the server and upload chunks concurrently. This approach can significantly reduce the overall upload time, especially for very large files.

Validation ensures that the uploaded file is complete and uncorrupted. The server can verify the size, type, and even the hash of the file after merging all chunks. This helps guarantee that the final file matches the original and that no data has been lost or altered during transmission.

By combining these features, chunk upload becomes not only a solution for large file handling but also a robust, user-friendly, and reliable method for modern applications.

Example code

Upload Session / File Token

The secure way is to create an upload session or a fileId + unique token every time a user wants to upload a file.

1. Client requests upload permission → sends the file name, size, and user login token.
2. Server responds → provides a fileId and an uploadToken (unique, single-use, often with an expiration).
3. Client uploads chunks → each request includes the fileId and the uploadToken.
4. Server validates:
   • Is the uploadToken valid?
   • Does the fileId match this token?
   • Is the upload status still “active”?
5. Once all chunks are uploaded and merged, the server can mark the session as complete → the uploadToken immediately becomes invalid.