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Testing a Build Compared to the Last Build

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A long time ago in this tutorial, we talked about Blackfire's truly awesome "comparison" feature. If you profile a page, make a change, then profile it again, you can compare those two profiles to see exactly how that change impacted performance.

When you use the build system, you can do the exact same thing... and you can even write "tests" that compare a build to the previous build. For example, you could say:

Yo! If the wall time on the homepage is suddenly 30% slower than the previous build, I want this build to fail.

Adding a Comparison Test with percent()

How can we do that? It's dead simple. Add a new global metric - how about "Pages are not suddenly much slower" - and set this to run on every page: path: /.*. For the assertion, we can use a special function called percent: percent(main.wall_time) < 30%:

27 lines | .blackfire.yaml
// ... lines 2 - 5
"Pages are not suddenly *much* slower":
path: "/.*"
- "percent(main.wall_time)
// ... lines 10 - 27

That's it! There's also a function called diff(). If you said diff(metrics.sql.queries.count) < 2 it means that the difference between the number of SQL queries on the new profile minus the old profile should be less than 2.

Let's see what this looks like! Find your terminal and commit these changes:

git status
git add .
git commit -m "adding global wall time diff assert"

Now... deploy!

symfony deploy --bypass-checks

Comparison Tests: Not for Manual Builds

But... bad news. If we waited for that to finish deploying... and then triggered a new custom build... that test would not run. In fact, I want you to see that. Wait for the deploy to finish - okay, good - then move back over and start a build.

This does what we expect: it executes our scenario and creates 2 profiles. Look at the 3 successful constraints for the homepage: we see the other global test about "HTTP requests should be limited"... but we don't see the new one. What gives?

So... when you create a build, you can specify a "previous" build that it should be compared to by using an internal "build id". Our project is too new to see it, but this happens automatically with "periodic" builds: our comparison assertion will execute on periodic builds.


Triggering builds via a webhook requires an Enterprise plan.

But when we create a manual build... there's no way to specify a "previous" build... which is why the comparison stuff doesn't work. Fortunately, since I don't want to wait 12 hours to see if this is working, there is another way to trigger a build: through a webhook. Basically, if you want to create a build from outside the Blackfire UI, you can do that by making a request to a specific URL. And when you do that, you can optionally specify the "previous build" that this new build should be compared to.

Automatic Build on Deploy

This webhook-triggered-build is especially useful in one specific situation: creating a build each time you deploy. If you did that correctly, your comparison assertion would compare the latest deploy to the previous deploy... which is pretty awesome.

Because we're using SymfonyCloud, this is dead-simple to set up.

Find the Blackfire SymfonyCloud documentation and, down here under "Builds", I'll select our environment. Basically, by running this command, we can tell SymfonyCloud to send a webhook to create a Blackfire build each time we deploy.

Copy it, move over to your terminal and... paste:

symfony integration:add --type=webhook --url='https://USER:PASS@blackfire.io/api/v2/builds/env/aaaabbee-abcd-abcd-abcd-c49b32bb8f17/symfonycloud'

Hit enter to report all events and enter again to report all states. For the environments - this is asking which SymfonyCloud environments should trigger builds. Answer with just master - I'll explain why soon.

And... done! Let's redeploy our app. Oh, but before we do, refresh our builds page. Ok, we have 5 builds right now. Now run:

symfony redeploy --bypass-checks

This should be pretty quick. Then... go refresh the page. Yes! A new build - number 6 - triggered by SymfonyCloud. And it passes. Awesome! Let's redeploy again:

symfony redeploy --bypass-checks

When that finishes... there's build 7! But to see the comparison stuff in action, I need to do a real deploy so that the next build is tied to a new Git sha. I'll do a meaningless change, commit, then deploy:

git commit -m "triggering deploy" --allow-empty
symfony deploy --bypass-checks

Seeing the Compared Builds

Actually, I could have skipped changing any files and committed with --allow-empty to create an empty commit. When this finishes... no surprise! We have build 8!

On this build, it's super cool: each profile has a "Show Comparison" link to open the "comparison" view of that profile compared to the same profile on the build from the last deploy - which - if you click "latest successful build" - is build 7.

Back on build 8, click the "Show 4 successful constraints" link. There it is! We can see our "Pages are not suddenly much slower" assertion! It's comparing the wall time of this profile to the one from the last build.

Click to open up the profile... and make sure you're on the Assertions tab. I love this: 2 page-specific assertions from the scenario, and 2 global assertions: one using the percent() function.

The "Recommendations" also got a bit better: Blackfire automatically has some built-in recommendations using diff: this recommends that the new profile should have less than 2 additional queries compared to the last build. It looks like it failed... but that's just because the other part of this recommendation - not making more than 10 total queries - failed.

Next: what about running builds on your staging server so you can catch performance issues before going to production? Or what about executing Blackfire builds on each pull request? We can totally do that - with a second environment.