Azure Functions

Any stock Linux plan with Always On. We recommend B2 (3.5 GB) as the minimum for production:

• B2 (3.5 GB, ~£20/mo) — recommended minimum. Gives one warm worker real headroom above the ~300–400 MB a typical render peaks at, so image-rich content pages and dashboards render without memory pressure.
• B3 / EP1+ / Dedicated ≥ 7 GB — for concurrency, very heavy pages, and lower latency under load. For faster per-render latency specifically, choose Premium v3 (Pv3): its dedicated cores are far quicker than the burstable Basic core.
• B1 (1.75 GB) — only for light, self-contained HTML you control (invoices, receipts, statements). Heavy or image-rich third-party pages can exceed its memory and get OOM-killed mid-render, and its single burstable core makes renders slow. Not recommended for rendering arbitrary URLs.

Size the pool to the plan, soften the failure modes, and pre-warm at startup so the one-time bundle download (~262 MB) happens off the request path.

using CobaltPdf.WebKit;
using Microsoft.Extensions.Hosting;

var host = new HostBuilder()
    .ConfigureFunctionsWorkerDefaults()
    .Build();

CobaltEngine.SetLicense(
    Environment.GetEnvironmentVariable("COBALTPDF_LICENSE")!);

CobaltEngine.Configure(o =>
{
    o.MinSize = 1;
    o.MaxSize = 1;                                    // 1 on B2 (3.5 GB); 2 on B3/EP2 (7 GB)
    o.MaxMemoryMb = 2048;                             // clean error instead of OOM-kill
    o.BrowserLeaseTimeout = TimeSpan.FromSeconds(180); // queued requests outlive a slow render
});

// Bundle download + extract + worker warm-up, off the request path.
_ = Task.Run(() => CobaltEngine.PreWarmAsync());

host.Run();

A plain code deploy to the stock plan — the artifact is ~3 MB:

dotnet publish -c Release -o ./publish
func azure functionapp publish <YOUR_APP_NAME>

Empty HTTP 500 / worker restarts on heavy pages

The render exceeded the instance's memory and Linux OOM-killed the worker process — because the process dies, the response has no error body, just a bare 500 after a long pause. Heavy, image-rich pages need more than B1's 1.75 GB: move to B2 (3.5 GB), or B3 (7 GB) for very large pages. Watch Metrics > Memory working set to confirm.

Images missing from the rendered PDF

Many sites lazy-load images as you scroll, so a single-viewport capture never fetches the ones below the fold. Set LazyLoadPages on the request (it scrolls the page to trigger them) and keep WaitStrategy = "networkIdle" so the image requests finish before capture. Not Azure-specific — it applies to any host.

First render extremely slow or times out

The bundle cache is on /home (a network share). Set COBALT_BUNDLE_CACHE_DIR=/tmp/cobaltbundle (step 4) and restart.

"render pool saturated" under concurrency

A queued request waited longer than BrowserLeaseTimeout while a slow render held the worker. Raise it to ≥ 2× your worst-case render time (step 3), raise MaxSize on bigger plans, or return HTTP 503 to let clients retry.

Bundle download fails

Egress to github.com / release-assets.githubusercontent.com is blocked. Allow it, or self-host the bundle and set COBALT_BUNDLE_BASE_URL.

Fails on Consumption plan

Expected — the extracted bundle (~611 MB) exceeds the Consumption plan's writable temp disk, and Consumption can't keep the pool warm. Use a stock Linux plan with Always On — B2 (3.5 GB) minimum, EP1+ for interactive latency.