Streaming Data on Vercel

Vercel Functions support streaming responses, allowing you to render parts of the UI as they become ready. This lets users interact with your app before the entire page finishes loading by populating the most important components first. Common use-cases include:

• Ecommerce: Render the most important product and account data early, letting customers shop sooner
• AI applications: Streaming responses from AIs powered by LLMs lets you display response text as it arrives rather than waiting for the full result

HTTP responses typically send the entire payload to the client all at once. This approach can sometimes result in a slow user experience if the data is large or computationally intense.

The Web Streams API enables you to stream chunks of the payload as they become available, improving your users' perception of how fast data is loading. It is supported in most major web browsers and popular runtimes, such as Node.js and Deno.

The Web Streams API helps you:

• Break large data into chunks: Chunks are portions of data sent over time
• Handle backpressure: Backpressure occurs when chunks are streamed from the server faster than they can be processed in the client, causing a backup of data
• Build more responsive apps: Rendering your UI progressively as data chunks arrive can improve your users' perception of your app's performance

Chunks in web streams are fundamental data units that can be of many different types depending on the content, such as String for text or Uint8Array for binary files. Standard Function responses contain full payloads of data, while chunks are pieces of the payload that get streamed to the client as they're available.

For example, imagine you want to create an AI chat app that uses a Large Language Model to generate replies. Due to their large data sets, replies from language models can generate slowly.

Standard Function responses require you send the full reply to the client when it's done, but streaming enables you to show each word of the reply as the model generates it, improving users' perception of your chat app's speed.

Chunk sizes can be out of your control, so it's important that your code can handle chunks of any size. Chunks sizes are influenced by the following factors:

• Data source: Sometimes the original data is already broken up. For example, OpenAI's language models produce responses in tokens, or chunks of words
• Stream implementation: The server could be configured to stream small chunks quickly or large chunks at a lower pace
• Network: Factors like a network's Maximum Transmission Unit setting, or its geographical distance from the client, can cause chunk fragmentation and limit chunk size
  • In local development, chunk sizes won't be impacted by network conditions, as no network transmission is happening

For an example Function that processes chunks, see Streaming Examples.

Once you understand how to deal with chunks of different sizes, you must understand how to deal with chunks arriving faster than you can process them in the client.

When the server streams data faster than the client can process it, excess data will queue up in the client's memory. This issue is called backpressure, and it can lead to memory overflow errors, or data loss when the client's memory reaches capacity.

For example, popular social media users can receive hundreds of notifications streamed to their web client per second. If their web client can't render the notifications fast enough, some may be lost, or the client may crash if its memory overflows.

You can handle backpressure with a technique called flow control. This technique manages data transfer rates between two nodes to avoid overwhelming a slow receiver.

For an example of how to handle backpressure, see Streaming Examples.

• Streaming Quickstart: Learn how to stream responses from Serverless and Edge Functions to provide an improved user experience.
• Streaming Examples: Explore streaming on Vercel with code samples that work out of the box.
• Streaming Functions: Learn how Vercel enables streaming Function responses over time for ecommerce, AI, and more.
• Vercel AI SDK: Use Vercel's AI SDK to power your streaming AI applications.