printPDF to return a stream

[jifeon]

Currently printToPDF returns a base64 encoded string. It works great in general, but we are generating a bit huge PDFs - min 40MBs, they include lot of pages with high resolution images. Keeping such big data in memory causes performance and memories issues with node. It would be nice to take PDF content as a stream, or as a file.

Currently we are thinking about workaround with page per page printing. But it dramatically increases complexity of the service. So my question is: are there any plans to support streams for printToPDF?

[cyrus-and]

While I agree that it would be a nice thing to have, this feature request is off topic here. Ping the Google Group, the official protocol repo or even file an upstream issue.

[jifeon]

@cyrus-and thx for quick response!

[nyroDev]

There is now a transferMode (experimental) option available in the printToPDF method, that accept "ReturnAsStream" value.
How can we use it in node to use the returned stream?

[cyrus-and]

@nyroDev something along the lines of:

const CDP = require('chrome-remote-interface');
const fs = require('fs');

async function streamPdfToFile(url, file, chunkSize = undefined /* means auto */) {
    const client = await CDP();

    try {
        const {IO, Page} = client;

        console.log(`Navigating to ${url}`);

        await Page.enable();
        await Page.navigate({url});
        await Page.loadEventFired();

        console.log(`Page loaded, requesting the PDF stream`);

        const {stream: handle} = await Page.printToPDF({
            transferMode: 'ReturnAsStream'
        });

        let fileStream;
        while (true) {
            const {base64Encoded, data, eof} = await IO.read({
                handle,
                size: chunkSize
            });

            if (!fileStream) {
                fileStream = fs.createWriteStream(file, {
                    encoding: base64Encoded ? 'base64' : 'binary'
                });
            }

            if (eof) {
                console.log(`PDF stream finished, saved to ${file}`);

                await IO.close({handle});
                fileStream.close();
                break;
            }

            console.log(`Processing the next chunk of ${data.length} (decoded) bytes`);

            fileStream.write(data);
        }
    } finally {
        client.close();
    }
}

streamPdfToFile('https://nodejs.org/api/fs.html', '/tmp/out.pdf', 1 << 20);

[nyroDev]

@cyrus-and thank you so much for your quick and detailed response, it works perfectly!

[nyroDev]

@cyrus-and Please not that in some edges cases (particullary with large images), letting the chunkSize to undefined creates a truncated (corrupted) PDF file.

After reading this comment, I updated the code below to set the default chunk size to 16384*2 and so far it's working great!