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Load balance headless browser sessions by limiting how many browser connections run at once, placing excess jobs in a queue, and releasing each session in unconditional cleanup. A provider’s queue can absorb bursts, but an application-side limit still helps protect target sites and makes waiting time visible. The right cap depends on your provider quota or measured self-hosted capacity—not a universal sessions-per-machine formula.
What session concurrency means
A browser session is an active browser connection doing work for an automation job. Concurrency is the number of sessions active simultaneously. Browserless defines it as “the maximum number of browser sessions that can run simultaneously on a Browserless instance.” Browserless terminology also explains capacity pressure and queuing.
More workers do not necessarily mean more useful throughput. If the provider or your own fleet cannot serve all active sessions, additional jobs wait, fail, or consume resources without improving completion time. Set an explicit cap and treat queued work separately from active browser sessions.
Use a bounded control loop
A bounded worker pool or semaphore is a practical way to keep application demand within an intended capacity. The sequence is: enqueue the job, acquire a slot, connect or launch a browser, run the work, then release the slot whether the work succeeds or fails.
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- Choose an initial cap. For a managed service, check the current plan concurrency and session-duration limits in its live documentation. For self-hosting, begin conservatively and raise the cap only after representative load tests.
- Queue work before opening sessions. Keep waiting jobs in an application queue rather than creating a large number of browser connections that all wait on the provider.
- Acquire a slot before connecting. A semaphore or fixed-size worker pool should prevent active connections from exceeding the cap.
- Release capacity in a
finallyblock. Close the page and remote browser connection even if navigation, assertions, or extraction throws. Browserless specifically warns that failing to close sessions can exhaust concurrency: see Best Practices. - Observe pressure. Track active sessions, queued jobs, queue wait, session duration, failures, and provider capacity or pressure signals when available. Thresholds are deployment-specific; do not treat any single metric policy as universal.
Playwright example with remote Chromium
This Node.js example uses a local semaphore to cap active sessions at two. Replace the endpoint with the current WebSocket endpoint and authentication format supplied for your Browserless region or deployment. The cap is an example, not a recommendation for every plan or workload.
import { chromium } from 'playwright';
const endpoint = process.env.BROWSER_WS_ENDPOINT;
const maxSessions = 2;
let active = 0;
const waiters = [];
async function acquire() {
if (active < maxSessions) {
active++;
return;
}
await new Promise(resolve => waiters.push(resolve));
active++;
}
function release() {
active--;
const next = waiters.shift();
if (next) next();
}
async function capture(url) {
await acquire();
let browser;
try {
browser = await chromium.connectOverCDP(endpoint);
// Use the default context when launch-level proxy or profile settings
// need to carry through; verify behavior for your endpoint and versions.
const context = browser.contexts()[0];
const page = await context.newPage();
try {
await page.goto(url, { waitUntil: 'domcontentloaded', timeout: 30000 });
return await page.title();
} finally {
await page.close();
}
} finally {
try {
if (browser) await browser.close();
} finally {
release();
}
}
}
const urls = ['https://example.com', 'https://playwright.dev'];
const results = await Promise.all(urls.map(capture));
console.log(results);In this pattern, each task owns a remote connection and closes it before returning its slot. If your provider’s connection model treats multiple pages or contexts differently, confirm the session semantics before adapting the ownership and cleanup boundaries. Browserless’s concurrent-session examples describe independent sessions and note that a newly created Playwright context may not inherit launch-level proxy or profile settings; verify that detail against the deployed endpoint and library versions. See Run concurrent browser sessions.
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How provider queuing fits
Some managed browser services queue connections when capacity is occupied. Browserless documents automatic queuing and a client-side concurrency cap, including the rationale of avoiding excessive pressure on target sites: concurrent sessions and terminology.
Provider queuing is burst handling, not a replacement for workload control. A local cap lets you regulate request rate toward a site, preserve a visible application queue, and avoid sending more connections than your workflow can use. Do not assume queued requests avoid timeouts or have no effect on throughput; confirm queue behavior and timeout semantics for the service and plan you use.
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Managed service or self-hosted fleet?
Both approaches can support remote browser automation; the trade-off is who operates the browser capacity. Browserless describes managed Browsers as a Service and self-hosted choices, but its documentation does not establish a general cost or performance break-even point. See Browsers as a Service.
| Decision | Managed browser service | Self-hosted fleet |
|---|---|---|
| Operations | Provider manages the browser pool and runtime operations. | Your team operates deployment, capacity, and updates. |
| Control | Use provider endpoints and supported controls. | More direct control over deployment and configuration. |
| Capacity | Plan limits and provider queueing may apply; verify current terms. | Configure and operate concurrency in your own deployment. |
| Geography | Choose among supported provider regions and endpoint locations. | Choose infrastructure regions under your control. |
| Validation | Check current quotas, timeouts, endpoint, and session semantics. | Validate worker sizing, scaling, health, updates, and cleanup. |
For a managed service
Confirm its current concurrency cap, maximum session duration, queue behavior, timeout policy, and endpoint regions before choosing an application cap. Those are provider and plan details that can change. Browserless’s best-practices page lists plan-specific limits, but they should be checked live rather than treated as stable figures: Best Practices.
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For self-hosting
Size and scale workers using measurements from the pages and automation you actually run. Browserless documentation describes increasing worker size or adding worker instances, but does not offer a portable sessions-per-CPU or sessions-per-GB rule. Load-test representative pages, browser builds, contexts, and resource profiles in the target deployment. Keep headless browser builds aligned with supported browser versions; Playwright documents its supported browser builds and headless-mode distinctions at Browsers.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choose regions and verify connection behavior
When latency matters, use a supported region close to the workload or users and verify the current endpoint map. Browserless recommends a nearby region to reduce latency, and its endpoint documentation lists connection URLs and regions: Connection URLs and Endpoints. Endpoint hostnames and availability can change, so copy the current endpoint for the service and region rather than relying on an old example.
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With Playwright CDP connections, Browserless examples advise using the default context where launch-level proxy or profile configuration needs to carry through; creating a separate context may not inherit those settings. Treat this as an integration detail to validate with your deployed endpoint and Playwright version, not a guarantee for every remote-browser setup. See Run concurrent browser sessions.
Troubleshoot concurrency and session failures
Jobs wait longer than expected
- Likely cause: The application cap or provider capacity is already occupied, or provider queuing is active.
- What to check: Compare active sessions, queue depth, queue wait, and the current plan or fleet capacity. Reduce incoming work or add measured capacity; do not simply increase the local cap without checking the bottleneck.
New jobs fail after earlier jobs finish
- Likely cause: A browser connection or page was not closed on an exception path, leaving capacity occupied.
- What to check: Put cleanup in unconditional error handling such as
finally; ensure the semaphore slot is released even if browser closure itself fails.
Sessions time out in the queue or during work
- Likely cause: Queue delay, navigation duration, or provider maximum-session duration exceeds a configured timeout.
- What to check: Verify the selected provider’s current timeout and queue behavior. Set application deadlines that account for queue wait as well as execution, and avoid retry loops that add pressure during an outage.
Proxy or profile settings do not apply
- Likely cause: The automation created a new context that does not inherit connection-level settings in the deployed integration.
- What to check: Test the default context against a newly created context and confirm behavior for your endpoint and Playwright versions.
Self-hosted throughput drops under load
- Likely cause: The workload’s pages, contexts, or resource usage exceed the capacity assumptions used to set the worker cap.
- What to check: Repeat load tests with representative pages and browser builds, then adjust worker size, instance count, or concurrency based on observed behavior rather than a generic ratio.
Or skip the browser setup
If your task is to capture website screenshots rather than operate a general browser automation fleet, ScreenshotNeo offers a one-request screenshot API. Cookie banners, popups, and chat widgets are removed before the shot; bot checks, blank pages, and failed loads are never billed. It also provides an MCP server so AI agents can take screenshots. The free plan includes 1,000 screenshots a month with no card; paid plans start at $5 for 3,000.
cURL example (see the ScreenshotNeo documentation for options):
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Quick Recap
Deployment checklist
- Confirm the current provider quota, maximum session duration, queue behavior, and timeout policy—or establish measured self-hosted capacity.
- Set a bounded application worker pool or semaphore below the capacity you intend to consume.
- Queue waiting work instead of opening unbounded browser connections.
- Close pages, contexts, and remote sessions in unconditional cleanup, including on errors.
- Track active sessions, queue depth and wait, session duration, failures, and capacity pressure where available.
- Verify region-specific endpoints and choose a nearby supported region when latency matters.
- Test Playwright context, proxy, and profile behavior with the exact endpoint and library versions deployed.
- Load-test representative sites and automation flows before increasing self-hosted concurrency.
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