I am trying out a new tool called SST which handles configuring AWS services and full stack web apps via code. In the API tutorial we create the following function to upload a file:

import { Resource } from "sst";
import { getSignedUrl } from "@aws-sdk/s3-request-presigner";
import {
  S3Client,
  GetObjectCommand,
  PutObjectCommand,
  ListObjectsV2Command,
} from "@aws-sdk/client-s3";

const s3 = new S3Client({});

export async function upload() {
  const command = new PutObjectCommand({
    Key: crypto.randomUUID(),
    Bucket: Resource.MyBucket.name,
  });
  const body = await getSignedUrl(s3, command);

  return {
    statusCode: 200,
    body
  };
}

That is the entire upload function. As you can see there is no explicit handling of any files – I had initially thought this would create an empty file in the specified S3 bucket.

Next, once our server is live we send a curl request with a file to upload:

curl --upload-file somefile.txt "$(curl https://sometestapi.execute-api.eu-west-1.amazonaws.com)"

However when I check the S3 bucket via the AWS console I can see somefile.txt and all its content is there. I’m really not understanding a) how this file is being picked up by the API – there is nothing explicit going on here, and b) how this file is then being sent to S3.

Logging command and body I see the following:

|  +26ms       {
|  +26ms         command: PutObjectCommand {
|  +26ms           middlewareStack: {
|  +26ms             add: [Function: add],
|  +26ms             addRelativeTo: [Function: addRelativeTo],
|  +26ms             clone: [Function: clone],
|  +26ms             use: [Function: use],
|  +26ms             remove: [Function: remove],
|  +26ms             removeByTag: [Function: removeByTag],
|  +26ms             concat: [Function: concat],
|  +26ms             applyToStack: [Function: cloneTo],
|  +26ms             identify: [Function: identify],
|  +26ms             identifyOnResolve: [Function: identifyOnResolve],
|  +26ms             resolve: [Function: resolve]
|  +26ms           },
|  +26ms           serialize: [AsyncFunction: se_PutObjectCommand],
|  +26ms           deserialize: [AsyncFunction: de_PutObjectCommand],
|  +26ms           input: {
|  +26ms             Key: '860d1393-4232-48fc-4d4b-b42c0c8a6473',
|  +26ms             Bucket: 'mytestprojectname-testbucket-tokfzzao'
|  +26ms           }
|  +26ms         }
|  +26ms       }
|  +26ms       {
|  +26ms         body: 'https://mytestprojectname-testbucket-tokfzzao.s3.eu-west-1.amazonaws.com/860d1393-4232-48fc-9d4a-b42c0c8a6473?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Content-Sha256=UNSIGNED-PAYLOAD&X-Amz-Credential=<removed-credentials>&X-Amz-Date=20240607T170451Z&X-Amz-Expires=900&X-Amz-Security-Token=<removed-security-token>&X-Amz-Signature=<removed-signature>&X-Amz-SignedHeaders=host&x-id=PutObject'
|  +26ms       }

I have tried debugging all the available variables in the function to see if any contain the file being uploaded, but neither do. I had expected since no file is being explicitly handled it would not be uploaded to S3.