Problem with Setting highWaterMark in AWS S3 GetObject Stream

[ujjwol05]

Checkboxes for prior research

• I've gone through Developer Guide and API reference
• I've checked AWS Forums and StackOverflow.
• I've searched for previous similar issues and didn't find any solution.

Describe the bug

When using the AWS SDK for JavaScript v3 to stream data from S3 (via GetObjectCommand), I cannot set highWaterMark , I've also set a custom highWaterMark value for stream buffers to see if that works but, the buffer size remains at the 16 KB

Regression Issue

• Select this option if this issue appears to be a regression.

SDK version number

@aws-sdk/package-name@version, ...

Which JavaScript Runtime is this issue in?

Node.js

Details of the browser/Node.js/ReactNative version

v20.17.0

Reproduction Steps

const command = new GetObjectCommand(params);
const response = await s3Client.send(command);

const stream = response.Body as NodeJS.ReadableStream;
const customStream = stream.pipe(new Stream.PassThrough({
    highWaterMark: 32 * 1024 
}));

customStream.on("data", (chunk) => {
    console.log(`Chunk size: ${chunk.length}`);
});

Observed Behavior

INFO Chunk size: 16384
INFO Chunk size: 389

Expected Behavior

INFO Chunk size: 30243
INFO Chunk size: 30243

Possible Solution

No response

Additional Information/Context

No response

[zshzbh]

Hey @ujjwol05 ,

The highWaterMark setting defines the maximum buffer size, but it doesn't guarantee that chunks will be exactly that size. The actual chunk sizes you're seeing (16384 and 389 bytes) are determined by the underlying stream implementation and the source

[ujjwol05]

@zshzbh Does that mean we have no control over the S3 stream, and if we need a fixed-size buffer, we must rebuffer it ourselves?

[zshzbh]

Depending on what you are trying to achieve.

S3 honors HTTP byte range request, so any file can be downloaded in fixed-size pieces using multiple GETs. You can use PartNumber

If this is referring to individual TCP packet sizes, then no you cannot control this.

[zshzbh]

I can share the code if you want a consistant chunk size - To get consistent 32KB chunks, you'll need to use a Transform stream instead of PassThrough. -

import { S3Client, GetObjectCommand } from "@aws-sdk/client-s3";
import { Transform } from 'stream';

const s3Client = new S3Client({
  region: "us-east-1",
});

const params = {
  Bucket: "test-s3-XXXX-mm",
  Key: "large-file.txt",
};

const command = new GetObjectCommand(params);
const response = await s3Client.send(command);
const stream = response.Body;

// Create a custom transform stream that buffers data into 32KB chunks
const chunkSize = 32 * 1024; // 32KB
let buffer = Buffer.alloc(0);

const chunker = new Transform({
    transform(chunk, encoding, callback) {
        // Add new chunk to our buffer
        buffer = Buffer.concat([buffer, chunk]);

        // While we have enough data for a full chunk
        while (buffer.length >= chunkSize) {
            // Push a chunk of exactly 32KB
            this.push(buffer.slice(0, chunkSize));
            buffer = buffer.slice(chunkSize);
        }
        callback();
    },
        // Push any remaining data when the stream ends
    flush(callback) {

        if (buffer.length > 0) {
            this.push(buffer);
        }
        callback();
    }
});

const customStream = stream.pipe(chunker);

customStream.on("data", (chunk) => {
    console.log(`Chunk size: ${chunk.length}`);
});

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