Debugging rendering problems at scale

Debugging rendering
problems at scale
Giacomo Zecchini | Verve Search
SLIDESHARE.NET/GIACOMOZECCHINI
@GIACOMOZECCHINI

Hi, I’m Giacomo. Technical
Director at Verve Search.
Technical background and
previous experiences in
development.
@giacomozecchini
#brightonSEO

Today we are going to talk about
rendering errors, the challenges
of debugging at scale and a new
approach to solve these issues.
@giacomozecchini
#brightonSEO

The search engine's rendering
process is very similar to
Schrödinger's cat paradox.
https://en.wikipedia.org/wiki/Schrödinger's_cat
@giacomozecchini
#brightonSEO

A hypothetical cat page may be
considered simultaneously both
alive correctly rendered and
dead not correctly rendered.
@giacomozecchini
#brightonSEO

Let me explain..
@giacomozecchini
#brightonSEO

Search engines get web pages
and put them in web rendering
services.
https://developers.google.com/search/docs/guides/javascript-seo-basics
@giacomozecchini
#brightonSEO

Inside the web rendering
services, the pages are rendered
similarly to a browser.
@giacomozecchini
#brightonSEO

Then, the search engines can
extract all information they
need from those rendered
pages.
@giacomozecchini
#brightonSEO

This is an oversimpliﬁcation of a
complex process.
https://www.youtube.com/watch?v=Qxd_d9m9vzo
@giacomozecchini
#brightonSEO

If you want to know more about
this I’d suggest to watch Martin
Splitt’s TechSEO Boost 2019
talk.
https://www.youtube.com/watch?v=Qxd_d9m9vzo
@giacomozecchini
#brightonSEO

What’s the problem
with Web Rendering
Services?
@giacomozecchini
#brightonSEO

What happens inside the web
rendering services is something
hidden from our eyes, like in a
closed box.
@giacomozecchini
#brightonSEO

You don’t know if a page has
been correctly rendered until you
check it manually.
@giacomozecchini
#brightonSEO

Search engines are capable of
rendering your pages and most
of the time the process will be
ﬁne.
@giacomozecchini
#brightonSEO

Nonetheless, some pages have
rendering problems.
@giacomozecchini
#brightonSEO

When is a page not
correctly rendered?
@giacomozecchini
#brightonSEO

A page is “not correctly
rendered” when is not possible
for the WRS to get an asset or
when an error blocks the
process.
@giacomozecchini
#brightonSEO

Not only pages with Javascript
have problems!
@giacomozecchini
#brightonSEO

Let's have a look at a few
examples...
@giacomozecchini
#brightonSEO

HTTP / DNS / Network errors
@giacomozecchini
#brightonSEO
https://developers.google.com/search/docs/advanced/crawling/http-network-errors
Crawler
WRS
Cache
SEARCH ENGINE
* Icons made by Freepik from www.ﬂaticon.com

Robots.txt blocks a resources
@giacomozecchini
#brightonSEO
https://developers.google.com/search/docs/advanced/robots/intro
Crawler
WRS
Cache
SEARCH ENGINE

Fetch timeout
@giacomozecchini
#brightonSEO
Crawler
WRS
Cache
SEARCH ENGINE
* this doesn’t seem very common,
but it can happen

https/http mixed content
@giacomozecchini
#brightonSEO
If your website has an HTTPS
URL but one of the Javascript
ﬁles has an HTTP URL and the
HTTPS version is not available,
the script won't be used!

Cache mismatch, user
permission for speciﬁc
features (e.g. geolocation),
service worker registration,
Javascript syntax errors, etc.
@giacomozecchini
#brightonSEO

What if a page is not
correctly rendered?
@giacomozecchini
#brightonSEO

If WRS can’t get your CSS the
page layout won’t be correct and
you may also have Mobile
Usability issues.
@giacomozecchini
#brightonSEO

If WRS can’t get or execute your
JS ﬁles correctly, your page may
be blank or broken.
@giacomozecchini
#brightonSEO

Eventually, WRS may need to
render again your page, which
means slower indexing.
@giacomozecchini
#brightonSEO

Debugging at scale
@giacomozecchini
#brightonSEO

Manually checking page per
page might work on very small
websites.
@giacomozecchini
#brightonSEO

When you start having a lot of
pages.. That’s a problem!
@giacomozecchini
#brightonSEO

You can prioritise and group
pages with similar HTML and
resources together, but..
@giacomozecchini
#brightonSEO

..the rendering of a page can fail
regardless of what happens to
other similar pages.
@giacomozecchini
#brightonSEO

You still have to manually check
pages to be 100% sure those are
correctly rendered.
@giacomozecchini
#brightonSEO

WRS capabilities
vs Debugging
@giacomozecchini
#brightonSEO

Understanding what a Web
Rendering Service can or can’t
do is a one time task.
@giacomozecchini
#brightonSEO

You can build a page with a
speciﬁc feature and test it. If it
works once it will work again on
other pages.
@giacomozecchini
#brightonSEO

When debugging issues you are
not focusing on a single feature
but on having an overall correct
rendering.
@giacomozecchini
#brightonSEO

View crawled page is the way.
@giacomozecchini
#brightonSEO

A lot of information.
@giacomozecchini
#brightonSEO

But that’s not enough, you want
more. For instance, Javascript
console messages are
coalesced and not shown.
@giacomozecchini
#brightonSEO

Yes, you can get JavaScript
console errors from the Mobile
Friendly test or other live tests
but it’s not the same!
@giacomozecchini
#brightonSEO

Mobile-Friendly Test and the
other live tests bypass the
cache, have shorter timeouts,
and few other differences.
@giacomozecchini
#brightonSEO

A new hope
approach
@giacomozecchini
#brightonSEO

I started my research by getting
and printing the information I
needed on the page with some
Javascript, in a hidden <DIV>.
@giacomozecchini
#brightonSEO

<html>
…
<div id="info" style="display:none"></div>
…
<script>
…
function getInformation(){
// do stuff!
}
…
var div = document.getElementById("info");
var p = document.createElement("p");
p.innerText = getInformation();
div.appendChild(p);
…
</script>
…
</html>
@giacomozecchini
#brightonSEO
This prints the
information you need
in the DIV at
rendering time and
then you can get
them in Search
Console view crawled
page HTML.

But waiting for a page to be
crawled, rendered and indexed
again is time consuming and not
scalable.
@giacomozecchini
#brightonSEO

It’s a nice way of discovering
new things but you still have to
manually check all pages.
@giacomozecchini
#brightonSEO

Then, I thought of using 1x1 px
images, appending errors or
information in the URL:
https://www.example.com/image.jpg
?u=page_url&e=error
@giacomozecchini
#brightonSEO

The idea was to look in the
server access log and ﬁnd all
errors that occurred during the
rendering.
@giacomozecchini
#brightonSEO

But Google’s WRS doesn’t
download images during the
rendering of a page.
@giacomozecchini
#brightonSEO

But then..
@giacomozecchini
#brightonSEO

The answer was always in
front of my eyes:
Javascript + POST requests!
@giacomozecchini
#brightonSEO

Google’s WRS cache GET
requests.
@giacomozecchini
#brightonSEO
https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/GET

But doesn’t cache POST
requests.
@giacomozecchini
#brightonSEO
https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/POST

Say welcome to the shiny new
Search Engine Rendering
Errors Logging framework!
@giacomozecchini
#brightonSEO

@giacomozecchini
#brightonSEO
Crawler
WRS
Cache
SEARCH ENGINE YOUR WEBSITE
Search Engines download or use the cache of
the resources they need to render your pages.

@giacomozecchini
#brightonSEO
CHROMIUM INSTANCE
SEARCH ENGINE
Crawler
INTERNET
During the rendering the website, WRS executes
Javascript and downloads additional resources
a website might need or request.

@giacomozecchini
#brightonSEO
CHROMIUM INSTANCE
SEARCH ENGINE
Crawler
SERVER
What if one of those Javascript sends a non
cacheable POST request to an external server?!
POST
REQUEST

@giacomozecchini
#brightonSEO
There are multiple ways of
sending POST requests in JS:
Fetch API
https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch
Navigator.sendBeacon()
https://developer.mozilla.org/en-US/docs/Web/API/Navigator/sendBeacon
XMLHttpRequest.send()
https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/send

{
"page":"https://www.example.com",
"timestamp": 1592568000000,
"category": "Fetch",
"error": "https://www.example.com/style.css"
}
@giacomozecchini
#brightonSEO
The message (or beacon) contains the information you want to store
in your database.

@giacomozecchini
#brightonSEO
TIME URL CATEGORY ERROR
25/10/1985 09:00:00 https://www.example.com Fetch https://www.example.com/style.css
21/10/2015 07:28:00 https://www.example.com/about.html Fetch https://www.example.com/app.js
12/11/1955 06:38:00 https://www.example.com Javascript File: https://www.example.com/app.js Line: 3 Col: 2
Error: Uncaught ReferenceError: APP is not defined
When you have everything in a database you can query the tables
and do all your analysis. You can also have automatic alerts, etc.

Debugging in
practice
@giacomozecchini
#brightonSEO

!! Warning !!
Don’t use this code on your
website, these are just (bad)
examples.
@giacomozecchini
#brightonSEO

Debugging example #1
Check if a page has been
rendered
@giacomozecchini
#brightonSEO

<html>
…
<script>
sendMessageToServer();
</script>
…
</html>
@giacomozecchini
#brightonSEO
When the WRS executes the script, the
function sends a message back to the
server.

Debugging example #2
Know if there is a problem
downloading CSS or JS ﬁles
@giacomozecchini
#brightonSEO

<html>
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1">
<script>
…
window.addEventListener('error', function(err) {
if (isDownloadError(err)){
sendMessageToServer(err);
}
}, true);
…
</script>
…
</head>
…
</html>
@giacomozecchini
#brightonSEO
If there is an error and it's a CSS or
JS load error you can send a
message back to the server. This
works for HTTP/DNS/Network errors,
Robots.txt, fetch timeouts, etc.

Caveats
@giacomozecchini
#brightonSEO

There are some products out
there but all of them focus on
users and not on search
engines.
@giacomozecchini
#brightonSEO

Search engines are different
and you need to solve
different problems.
@giacomozecchini
#brightonSEO

You should be careful adding
new code to your website!
@giacomozecchini
#brightonSEO

Web Performance issues
You don’t want to slow down
the user experience with
something you need only for
search engines.
@giacomozecchini
#brightonSEO

Web Performance issues
Check for the User-Agent and
run the script only for search
engines.
@giacomozecchini
#brightonSEO

Crawl budget
You don’t want to consume
your crawl budget on these
requests.
@giacomozecchini
#brightonSEO

Crawl budget
Host your debugging server on
a different domain or
subdomain.
@giacomozecchini
#brightonSEO

There are many other possible
problems, you just need to ﬁnd
a solution for them.
@giacomozecchini
#brightonSEO

Conclusions
@giacomozecchini
#brightonSEO

The simpler a page is, the
more chances it will render
correctly. The majority of
pages are just ﬁne.
@giacomozecchini
#brightonSEO

If you work on big or complex
websites you may encounter
rendering problems.
@giacomozecchini
#brightonSEO

Debugging rendering
problems is a very time
consuming task..
@giacomozecchini
#brightonSEO

..but, if you use the right
approach you can cut down
the time it takes.
@giacomozecchini
#brightonSEO

You can use this approach as
a one time debugging script to
get more information or as a
monitoring system.
@giacomozecchini
#brightonSEO

Thank You!
Got questions? DM me on Twitter.
@giacomozecchini

Debugging rendering problems at scale

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