Channel your
 Fear of the Computer
 into confident engineering Maggie Zhou The Lead Developer New York 2018 

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todo: 
 photo of self Photo of sf Photo of slack 

We rely on so many so?ware systems to build our products. 

Change is required to make progress. 

But sometimes things break photo here. 

Comic: Alex Norris of webcomicname.com 

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Techniques for confidence • Measure your code. But how? • Ramp up your work incrementally. But why? 

Case Study 1 How to modify a black box with confidence • Instrument the code to understand your baseline • Ramp up gradually and enable ramping back down 

Instrument the code func connect_to_db(db_xyz): return connect(xyz) func connect_to_db(db_xyz): time = now() conn = connect(xyz) metric(now() - time, db_xyz) return conn 

Feature Flags! 

Why feature flags? • Gate user visible client-side features with server-side configuraOon • Do A/B tesOng • Merge code that isn’t ready for full-feature release • Incremental ramp-ups • Easy, well-socialized place to ramp down and turn it off 

Why feature flags for infrastructure changes? • Gate user visible client-side features with server-side configuraOon • Do A/B tesOng • Merge code that isn’t ready for full-feature release • Incremental ramp-ups • Easy, well-socialized place to ramp down and turn it off 

Build confidence From incremental ramp-ups and watching your metrics. 

Ramp up incrementally func connect_to_db(db_xyz): time = now() conn = connect_old_way(xyz) metric(now() - time, db_xyz) return conn 

Ramp up incrementally func connect_to_db(db_xyz): time = now() if (Feature::isOn(‘db_new’)): conn = connect_new_way(xyz) else: conn = connect_old_way(xyz) metric(now() - time, db_xyz) return conn 
 
 
 …
 … $config[‘db_new’] = array( ‘on’ => 1 
 # Ramp up to 1% of traffic ) … 

Ramp up incrementally This triggered out of memory errors!
 
 Why? 

Actually… A hidden recursive dependency on the ORM inside the Feature library! 

Ramp up incrementally func connect_to_db(db_xyz): time = now() conn = connect_old_way(xyz) metric(now() - time, db_xyz) return conn 

Ramp up incrementally func connect_to_db(db_xyz): time = now() if (host in rampup_list): conn = connect_new_way(xyz) else: conn = connect_old_way(xyz) metric(now() - time, db_xyz) return conn 

Confidence From knowing 
 where the off-switch is. 

Releasing to 100% should be boring. 

Minimize user impact by making it easy to ramp down your change. 

-Allison Kaptur, in ‘Love your Bugs’ “…if you’re wriOng a mobile or a desktop applicaOon: You need server-side feature gaOng and server-side flags. When you discover a problem and you don’t have server- side controls, the resoluOon might take days or weeks as you push out a new release or submit a new version to the app store. That’s a bad situaOon to be in.” 

Confidence from: • Ramping up my change incrementally • Knowing how to ramp down my change quickly • Knowing that my team has my back and that I’ve socialized how to turn off my change. • Understanding my baseline performance and watching metrics as my change is released. 

Case Study 2 Build confidence by understanding your changes. How? • Stare really hard at the code? 

Case Study 2 Build confidence by understanding your changes. How? • Stare really hard at the code? • Reduce risk by dogfooding • Have analyOcs available just for your dogfood group with the changes you made • Measure your yardsOcks 

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one API endpoint ms 

Segment your metrics by your ramp-up key to catch performance regressions early! 

But what’s going on? Measurements at the data access layer showed no performance regressions. 

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-Carlos Buenos in Mature OpBmizaBon “Measurement so?ware is just as likely to have bugs as anything else we write, and we should take more care than usual. A bug in user facing code results in a bad experience. A bug in measurement code results in bad decisions.” 

–Johnny Appleseed “Type a quote here.” 

Make big infrastructure changes with confidence • Instrument your code. • Understand your baseline. • Check your yardsOcks. • Segment your metrics into populaOons that match your ramp-ups. • Ramp up incrementally • Make it easy and well-understood how to config your changes off. 

Make big infrastructure changes with confidence. 

Maggie Zhou The Lead Developer New York 2018 Resources: bit.ly/zmagg_leaddev2018 Find me on the Internet @zmagg ✨Thank you ✨ to my teams at Slack and Etsy for all the lessons learned. Special thanks to Kiran BhaSaram, Kiné Camara, Julia Evans, DureW Hirpa, Andrew Morrison, Bhaskar Mookerji, Tracy Stampfil, Meri Williams and Lydia Wagner for your early feedback and support on dra]s of this talk .