Discussion on FastAPI performance

[wu-clan]

After reviewing the fastapi-0.108.0 source code

I have the following points:

1. response_model:
   • When using pydantic-v1, don't use response_model in the interface parameters and don't specify that the interface returns a model; before the interface responds, fastapi will first do a lot of adaptation and validation of the v1 model, then serialize it according to the model, and finally parse the response through jsonable_encoder. and return.
   • When using pydantic-v2, it is recommended to use it, fastapi will give the response content directly to pydantic to process (including validation) and return
2. jsonable_encoder:
   • Additional performance overhead (data parsing)
3. response_class:
   • fastapi will use the official json to encode the response content by default.

How to improve performance?

1. when using pydantic-v2, prioritize the use of response_model
2. don't use jsonable_encoder unless necessary
3. high performance response_class

What we've done so far:

Use jsonable_encoder to parse the response in advance, and then have fastapi to handle the content.

TODO:

Obviously, we need to change this. The reason we use a custom ResponseBase is to unify the return structure and simplify the serialization configuration;

• using the response_model globally
• Do not use jsonable_encoder within ResponseModel
• for serialization configuration, use interface parameter settings, e.g.: response_model_exclude...
• high-performance serializer（Pydantic serialization enforces binding internally, which only works on the final serialization）

Prompt but not implemented

1. Do not use response_model
   • Because Pydantic serialization enforces binding internally, if the interface returns Schema, the serialization speed will be affected by pydantic. The solution is to use MsgSpecJSONResponse serialization and return directly.

[wu-clan]

Fastapi makes heavy use of anyio to handle synchronization functions, but there is a performance overhead with anyio, check out the：agronholm/anyio#543

[wu-clan]

Json serialization comparison：https://www.libhunt.com/compare-orjson-vs-msgspec

msgspec docs: https://jcristharif.com/msgspec/benchmarks.html

[downdawn]

It seems that someone has already submitted a pull request (PR).
tiangolo/fastapi#10631

[wu-clan]

If you want to avoid pydantic as much as possible and have full performance, you need to completely rewrite the Route class, which is destructive (not recommended).