Why Large Requests Can Fail in Python

Feb 3, 2024 ยท 2 min read

When sending HTTP requests in Python using the popular Requests library, you may occasionally run into errors when submitting very large requests. This occurs because Requests automatically splits large request bodies into multiple TCP packets under the hood. If the packets get too large, transmission can fail.

How Requests Handles Large Bodies

By default, Requests will stream request bodies when they exceed 2MB. This means the body is split into multiple packets rather than being buffered fully in memory. This prevents Requests from consuming too much RAM and potentially crashing for giant payloads.

Streaming works nicely in most cases. However, very large individual packets over roughly 16KB can cause issues in some environments due to configured TCP window sizes. If a single packet exceeds the window size, you may see transmission errors or dropped connections.

Solutions for Oversized Packets

If you run into packet size limits, there are a couple ways to work around it:

  • Manually chunk the request body into smaller 16KB chunks yourself using a generator. This gives you more control over the packet size.
  • Lower the stream threshold via the stream argument of Requests, so packetization happens at smaller payload sizes.
  • Compress the data with gzip or deflate encoding if applicable. This reduces packet size.
  • Switch protocols to WebSocket or HTTP/2 which handle large messages better.
  • In summary, Requests stream handling can occasionally cause issues for huge request bodies. But with some tweaks to payload size and streaming thresholds, you can eliminate packetization problems. Carefully managing payload chunking is key.

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