Help - how does multipart works under the hood

[Rantoledo]

💬 Question here

How does @fastify/multipart work?
I'm trying to upload a file approximately 20-50 mib size.
I'm planning on using multipart as a content type, so I used @fastify/multipart.
But, I have nginx reverse proxy, so I'm limited to 1mb to the request size.
My question is does multipart send the data in chunks? does it do that for me? my testing saying it doesn't, but just wanted to ask to make sure.

• node version: 22
• fastify version: 5.1.0
• os: Linux

[climba03003]

My question is does multipart send the data in chunks?

Yes, by the nature of stream it is always send by chunk. The only difference is the chunk size.

does it do that for me?

By default, using the Stream API should be process by chunk.
However, @fastify/multipart offers the ability to buffer in memory. So, it may gives some negative image.

my testing saying it doesn't, but just wanted to ask to make sure.

You should probably post the code you test. There is no way to ensure your implementation is doing things right.

But, I have nginx reverse proxy, so I'm limited to 1mb to the request size.

Since your setup involve nginx as extra layer.
nginx by default buffered the whole request body before sending to next server.
You should either disable proxy_request_buffering or set proxy_http_version to 1.1.

That maybe the reason why your test always received the whole payload instead of chunk.

Refs: https://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_request_buffering
Refs: https://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_http_version

[Rantoledo]

@climba03003
So, you're saying that each chunk is sent in a separate request, and the only reason my backend receives a single request with the entire file is because of nginx?

[climba03003]

Possibly, yes.

If you want to test things work properly, you should reduce the intermediate parts (e.g. nginx) between client and server.