WRITING MAINTAINABLE SOFTWARE ANDREW GODWIN // @andrewgodwin AT SCALE

Andrew Godwin / @andrewgodwin Hi, I’m Andrew Godwin • Principal Engineer at Astronomer (w/on Airflow) • Django Migrations, Channels & Async • Doing bad things with Python since 2005

Andrew Godwin / @andrewgodwin

Andrew Godwin / @andrewgodwin Why am I still writing Python? 17 years is quite a long time, though not as long as some

Andrew Godwin / @andrewgodwin I mostly work on large new projects The kind where you only know roughly what you want

Andrew Godwin / @andrewgodwin I rarely know exactly where I'm going …and so I use Python, as it's really good at that

Andrew Godwin / @andrewgodwin A lot of maintenance advice is "do it right first time"

Andrew Godwin / @andrewgodwin But we will all get it wrong sometimes So, how do we build knowing we can make mistakes?

Andrew Godwin / @andrewgodwin Software is almost never complete It will spend far longer running and being maintained than you spend creating it

Andrew Godwin / @andrewgodwin Nobody knows what they want Which is also why we're safe from AIs writing code, by the way

Andrew Godwin / @andrewgodwin This is mostly about systems, not libraries (though some of the lessons are similar!)

Andrew Godwin / @andrewgodwin Writing code Understanding code

Andrew Godwin / @andrewgodwin What makes maintenance hard? There's some obvious ones, of course…

Andrew Godwin / @andrewgodwin Obfuscation Or to echo a bad joke, "job security"

Andrew Godwin / @andrewgodwin q=lambda x,_=('c9*6iv"&s1[Y/`Oh7_|pEW:=!uT4+zeNl;Im\'X\\<-wabDMZ8ykgR@{r>B)~qKFd C3H0Q%,S}xVG](?^2#oPjJL.A$U fnt5'): type('').__dict__['}VJ|(@J}>'. translate(('_'*32+_+('__'*len(_))[:(1<<2)-1-(1<<8-2)]))](x,('_'*32+_+(' __'* len(_))[:(1<<2)-1-(1<<8-2)]));globals()[q('11$C3qV}11')] = lambda _,q=q,__builtins__=__builtins__: __builtins__.__dict__[q('11$C3qV}11')](q(_));q2=( lambda globals=(lambda q=(lambda x: getattr(__import__('q('),x)): q): (lambda os, __import__=q: (globals()(__import__(os)))))(); q=[q2('q3>|')(__import__( '(P(').__dict__[q('JVR%')][0],0),q];q2('^93o')(3,0);raw_input();input()

Andrew Godwin / @andrewgodwin No documentation The code! It's self-documenting!

Andrew Godwin / @andrewgodwin One giant file First one to 100,000 lines wins

Andrew Godwin / @andrewgodwin But what are the root causes? At least, the three I think are most important

Andrew Godwin / @andrewgodwin What Is this tech/design common? Can I research elsewhere? Where What's the flow of execution? How do I find side effects? Why What was the context? The other options?

Andrew Godwin / @andrewgodwin The What

Andrew Godwin / @andrewgodwin Boring technology is good! I am very proud to call Django "boring" these days

Andrew Godwin / @andrewgodwin Don't overdesign from the start It is very likely you do not need to work at "Google scale"

Andrew Godwin / @andrewgodwin You probably don't have big data If it is less than 20TB, it fits on a single machine

Andrew Godwin / @andrewgodwin The Principle Of Least Surprise Write it the more obvious way, even if it's 1% slower

Andrew Godwin / @andrewgodwin t = sum([p for g, es in gs for e, p in es.items()]) total = 0 for group, entries in gs: for entry, price in entries.item(): total += price

Andrew Godwin / @andrewgodwin The Where

Andrew Godwin / @andrewgodwin Ever opened 20 files following an execution path? Isn't it the worst?

Andrew Godwin / @andrewgodwin It's OK to repeat yourself! Only factor things out into a library if it makes it easier to read

Andrew Godwin / @andrewgodwin Code should be easy to follow More people will read it and understand it than will write it

Andrew Godwin / @andrewgodwin def sell_ticket(): if check_availability(): amount = calculate_cost() auth_code = preauthorize_payment(amount) allocate_inventory() send_confirmation() complete_payment(auth_code) else: raise CannotSellError() def check_availability(): ...

Andrew Godwin / @andrewgodwin class TicketSeller(PaymentMixin, EmailMixin, AvailMixin): def sell(self): super().sell() callback = getattr(self, f"start_{self.type}").call() self.allocate() callback()

Andrew Godwin / @andrewgodwin def sell_ticket(): if check_availability(): amount = calculate_cost() auth_code = preauthorize_payment(amount) allocate_inventory() send_confirmation() complete_payment(auth_code) else: raise CannotSellError() def check_availability(): ...

Andrew Godwin / @andrewgodwin Never use multiple inheritance OK, there are uses, but don't do anything that depends on understanding MRO

Andrew Godwin / @andrewgodwin Never use multiple inheritance OK, there are uses, but don't do anything that depends on understanding MRO Try not to

Andrew Godwin / @andrewgodwin Microservices are almost never worth it Unless there's matching ownership… more on that later

Andrew Godwin / @andrewgodwin The Why

Andrew Godwin / @andrewgodwin Reading what code does is mostly easy Unless it's some of my code from 2005-2006

Andrew Godwin / @andrewgodwin Guessing why it works that way is not There is rarely just one way to do something

Andrew Godwin / @andrewgodwin # Loop forever while True: # Check to see if we should run allocate if self.allocate_timer.check(): self.allocate() # Check to see if we should run clean if self.clean_timer.check(): self.clean() # Sleep for 0.01 seconds time.sleep(0.01)

Andrew Godwin / @andrewgodwin # Main loop - only exit when Ctrl-C or SIGTERM is received while True: # Run workloads if it's time if self.allocate_timer.check(): self.allocate() if self.clean_timer.check(): self.clean() # Sleep so we don't busy-loop if nothing is ready time.sleep(0.01)

Andrew Godwin / @andrewgodwin If it took you ages to work out, write why! You'll help someone else or maybe even your future self

Andrew Godwin / @andrewgodwin Software Engineering is more than coding It's the skill of making something people want or need

Andrew Godwin / @andrewgodwin Splitting Up Problems Communication Documentation Decision-making

Andrew Godwin / @andrewgodwin Big projects must have multiple teams Unless you want them to take years to ship

“ Andrew Godwin / @andrewgodwin Conway's Law Organizations, who design systems, are constrained to produce designs which are copies of the communication structures of these organizations.

Andrew Godwin / @andrewgodwin It works in reverse, too Don't try to build a system that goes against your organisational structure.

Andrew Godwin / @andrewgodwin One service/domain per team … and microservices only work when every service has a defined owner

Andrew Godwin / @andrewgodwin Reduce communication overheads! Don't make every team talk to every other team every day

Andrew Godwin / @andrewgodwin Consider your incentives If you only promote for launching new things, you'll never get anything maintained.

Andrew Godwin / @andrewgodwin Iteration speed is king The cost of improving and fixing matters more than of launching

Andrew Godwin / @andrewgodwin Progressive Typing Add types as you are more sure of code layouts Progressive Testing Tests are really helpful for refactors, but can be fragile early on Don't Optimise Early Until you know what the bottlenecks are

Andrew Godwin / @andrewgodwin The most important metric is "How long from code fix to production?"

Andrew Godwin / @andrewgodwin But Andrew! I work on planes/reactors/life support! Well, first of all, thank you! That's not easy! Also… have you considered Rust?

Andrew Godwin / @andrewgodwin You can't always iterate in production It's a careful balance as to how much you can get wrong

Andrew Godwin / @andrewgodwin All software needs maintenance If you somehow find a perfectly solvable problem, do let me know

Andrew Godwin / @andrewgodwin Simple, understandable designs It makes it much easier for others to work with Capture your context and reasons If the context changes, people know it's safe to change your code! Design for iteration It is never, ever done after the initial launch.

Andrew Godwin / @andrewgodwin Thank your maintainers. Coworkers. Open Source. Real-world infrastructure.

Aitäh! Andrew Godwin @andrewgodwin // aeracode.org