A Selenium ReadTimeoutError is a symptom, not a diagnosis: it means a read in a client/server exchange did not finish in time, but the exception name alone does not identify which exchange stalled. Find the last operation in the traceback, match it to the canary run and CloudWatch Logs, and adjust only the timeout boundary or dependency implicated. The overall CloudWatch Synthetics run deadline, Selenium browser-operation timeouts, and the remote WebDriver command timeout are separate limits.
Identify which operation timed out
Start with the full exception and traceback rather than increasing every timeout. Find the last Selenium operation that was running when the failure occurred, then compare its timestamp with the canary run details and CloudWatch Logs. The operation may be a navigation, an element wait, asynchronous JavaScript, or a command waiting for the remote WebDriver endpoint. Separately determine whether Synthetics ended the whole run at its configured deadline.
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browser.get(url)or navigation: investigate page-load completion and whether the target site responds from the canary environment.- An element lookup or explicit wait: determine whether the page loaded but the condition the canary needs never became true.
- Asynchronous JavaScript: inspect the script and Selenium’s asynchronous-script timeout.
- A WebDriver command waiting on its endpoint: investigate the remote command channel and its HTTP request timeout.
- The run ends at the canary deadline: investigate the overall Synthetics timeout and the time consumed by startup and earlier steps.
Do not assume that a ReadTimeoutError is the same as Selenium’s TimeoutException for page-load completion or an overall Synthetics timeout. The traceback is needed to place a particular failure.
Distinguish the timeout layers
| Boundary | What it limits | When to investigate it |
|---|---|---|
| CloudWatch Synthetics canary timeout | The total time allowed for the canary run. | The run stops at its configured deadline, potentially before its intended work finishes. |
| Selenium page-load timeout | How long a navigation may take to complete. | The traceback stops at navigation or the page does not meet the browser’s navigation completion condition. |
| Selenium implicit or explicit element wait | How long Selenium searches for an element or waits for a stated condition. | The page is available, but a required element or application state is not ready. |
| Selenium asynchronous-script timeout | How long Selenium waits for asynchronous JavaScript to finish. | The failing operation executes asynchronous browser script. |
| Remote WebDriver HTTP request timeout | How long the client waits for a response from the WebDriver command endpoint. | A WebDriver command is pending; the browser command channel itself may be unresponsive. |
| Target-site response and network path | Whether the canary can reach the requested endpoint and receive its response. | Navigation or a separate HTTP check stalls, especially for an internal endpoint. |
These boundaries are not interchangeable. Increasing all of them can lengthen detection while leaving an unreachable endpoint, stalled browser, or incorrect readiness condition unresolved.
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Check the overall canary budget and schedule
AWS says the canary timeout defaults based on its frequency when not configured. If you set it manually, the timeout must be no longer than the canary frequency. AWS recommends allowing at least 15 seconds for Lambda cold starts and canary instrumentation startup. That is a minimum recommendation, not a guarantee that a particular journey can finish in that time.
Increase the overall budget only when the traceback and run timing show that Synthetics is ending the run before the intended work can complete. Keep the configured timeout within the schedule’s frequency and allow time for startup as well as browser actions. A longer run deadline does not make an individual navigation or WebDriver command respond faster.
When a canary times out, success metrics and artifacts such as HAR files, logs, and screenshots may not be updated; in that timeout case, artifacts may not be available in the console. Use CloudWatch Logs to inspect the execution and correlate timestamps with the failed step.
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A successful navigation does not prove that the element or application state the canary measures is ready. Conversely, a slow get() points first to page loading or reachability, not to a missing button. Use a condition-based explicit wait for the actual user-facing state instead of adding an arbitrary sleep.
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from selenium.webdriver.common.by import By
from selenium.webdriver.support import expected_conditions as EC
from selenium.webdriver.support.ui import WebDriverWait
from selenium.common.exceptions import TimeoutException
url = "https://example.com"
try:
browser.get(url)
button = WebDriverWait(browser, 10).until(
EC.element_to_be_clickable((By.CSS_SELECTOR, "button#continue"))
)
button.click()
except TimeoutException as exc:
raise RuntimeError(
"The continue button did not become clickable within 10 seconds"
) from excThis example assumes browser is the WebDriver already created by your canary, and that the selector matches the page. Replace the URL, selector, and wait duration with the journey you monitor. AWS’s Selenium sample uses WebDriverWait with an expected condition and a specific failure message for a clickable button. Preserve a clear failure when the expected user journey is broken; do not catch and suppress it merely to make the canary green.
If navigation itself is the slow operation, review Selenium’s page-load timeout separately. If navigation completes but the expected condition does not, inspect the selector, page state, and application behavior before extending the element wait.
Verify endpoint reachability and network placement
Establish whether the canary’s Lambda environment can reach the URL and whether the endpoint responds as expected. For an internal endpoint, AWS recommends placing the canary in a VPC. A browser running locally or from another network is not proof that the canary has the same route to that endpoint.
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Check the canary runtime and bundled versions
Compare the selected Synthetics runtime with AWS’s runtime matrix, including its bundled Python, Selenium, and Chromium versions. The AWS documentation lists syn-python-selenium-12.0 with Python 3.12, Selenium 4.32.0, and Chromium 150.0.7871.24. Runtime contents are volatile, so verify the matrix and the runtime selected by your canary before relying on those versions.
AWS recommends using the latest runtime. Test a runtime change safely, such as on a clone or with a supported dry run, before changing a production canary. A runtime upgrade can change bundled browser and driver behavior, so check that the canary still completes its intended journey rather than treating an upgrade as a timeout fix by itself.
Make the next failure easier to diagnose
Name the canary’s steps and record useful context around navigation and waits. AWS’s library documents step execution, and its Selenium sample gives a custom message when an element wait fails. A step name and a precise error message make it easier to match a CloudWatch Logs timestamp to the browser action that stalled.
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- Record the step being attempted before navigation or a consequential wait.
- Use a specific failure message that identifies the expected application state.
- Keep exceptions visible when the monitored user journey fails; do not convert a failed check into success.
- Use CloudWatch Logs when an overall canary timeout leaves artifacts unavailable in the console.
Or skip the browser setup
If you need a clean, on-demand screenshot rather than an AWS canary that monitors a recurring user journey, ScreenshotNeo can return an image or PDF with one request. It is not a fix for a canary’s timeout and does not replace scheduled monitoring. Its API accepts a URL and can return PNG, JPEG, WebP, or PDF.
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| What you observe | Likely area to investigate | Next action |
|---|---|---|
| The canary stops at its configured run deadline. | Overall run budget, startup time, or work exceeding the scheduled window. | Review run timing and logs; adjust the canary timeout only if needed, and keep it no longer than the frequency. |
The traceback ends during get() or navigation. |
Page-load behavior, target-site response, endpoint reachability, or the page-load timeout. | Check the endpoint from the canary environment and inspect the navigation timeout boundary before changing an element wait. |
| The page loads but the expected control never becomes ready. | Wrong selector, changed application flow, delayed rendering, or a condition that never becomes true. | Verify the selector and journey, then use an explicit wait for the correct condition with a useful failure message. |
| A WebDriver command waits without returning. | Remote WebDriver endpoint or command-channel responsiveness. | Inspect the last command and remote connection timeout; do not assume the target site alone is at fault. |
| An internal site works elsewhere but not in the canary. | Network placement or access from the Lambda environment. | Verify reachability from the canary and use a VPC placement appropriate to the internal endpoint. |
| Failures begin after a runtime change. | Bundled Python, Selenium, or Chromium version differences. | Check the selected runtime matrix and validate the upgrade on a clone or supported dry run. |
Why the traceback matters more than the error label
The reviewed official documentation describes distinct Synthetics and Selenium timeout scopes, but the exception label by itself does not establish whether a particular run stalled on browser navigation, a WebDriver command channel, or another request. Without the full traceback and run timing, naming a root cause would be guesswork. Diagnose the operation first; then change only the implicated boundary and confirm the next run reaches the expected monitored state.
Frequently Asked Questions
Does every Selenium ReadTimeoutError mean the website is down?
No. The label alone does not identify whether the stalled exchange was with the target site, the WebDriver endpoint, or another request. The full traceback and run logs are needed to tell.
Is ScreenshotNeo a replacement for an AWS Synthetics canary?
No. It provides on-demand screenshots; it does not replace a scheduled canary that checks a recurring user journey.
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