Thursday, June 20, 2019

Modern Programming: never use Inheritance; use Composition instead

Inheritance vs Composition is an age old debate. The world has evolved enough that it's time to put this discussion to rest. There is no good reason to ever use inheritance in new code. Composition is functionally identical to Inheritance, produces superior outcomes, flatter class hierarchies and more flexible code than inheritance. Composition also does not violate encapsulation and avoids classes of issues produced by unexpected polymorphic dispatch to implementations. We get better class design for free as well. 

In short, never use inheritance - always express the same code re-use through composition and be happy. Let's go through each of the points one by one:

1. Composition is functionally identical to inheritance.
This one is easy to work with. When inheriting implementations, all subclass methods have an implicit parameter (an instance of the superclass). Composition just makes this parameter explicit as a constructor argument and takes away the superclass. Code reuse through calling the superclass method can be done by just calling the same method on the composed object.

2. Composition produces superior outcomes
When operating in an inheritance class hierarchy, a code dependency and a type dependency become coupled. Very often, this is not necessary. For example, a 2DSurface class may have a computeArea method that you want to re-use, however your class is not a 2DSurface. With inheritance, the 2DSurface type and the code reuse are coupled. With composition, the two are separate. This is advantageous when you want to introduce code between the levels of the hierarchy. Eg. You want to add a "TimingClass" that wraps calls into to the base class. Without composition, the inheritance hierarchy deepens and a TimingClass also ends up being a 2DSurface.

3. Composition produces flatter class hierarchies
When operating within an inheritance hierarchy, the more variants or ways the code gets reused, the levels of the hierarchy grow deeper with "leaf" level code overriding the base class code. Code with more than 2 layers of hierarchy is very difficult to manage since you have to trace up and down layers of code that are mutually calling each other. Composition forces a single, clean extension point in the code and adding in classes like the "TimingClass" above do not increase the depth of the inheritance hierarchy.

4. Composition does not violate encapsulation a.k.a. friendly refactoring
Changing a base class method's access and mutations of a base class member is virtually impossible in a non trivial class hierarchy (because subclasses may rely on protected methods or direct access to state which violates encapsulation). The solution is to solely depend on the public api methods of a class (so that internal state may be refactored without affecting behavior). Composition forces this in a direct manner (enforced by the compiler).

5. Composition prevents polymorphic dispatch bugs
A common (bad) design pattern is to have an abstract base class call the abstract method that must be implemented by subclasses. This produces bugs when code is called across different levels of the hierarchy. Check out the first example on this page.

6. Polymorphic dispatch can be implemented better through generics and type bounds
The common process of dispatching through a single "super-class" hierarchy is antiquated. Generics with type bounds provide a much better substitute. Eg. Shape.getArea() doesn't need to be implemented by writing all code against the abstract Shape class with the Shape class providing .getLength() and .getWidth() that get overridden. A much better way is to implement finer grained interfaces (traits) of the form: ShapeWithArea (.getArea()), RectilinearShape  (.getLength(), .getWidth()) . Composition can then fully express just the needed dependencies (eg. Class<T extends ShapeWithArea> if it just needs a ShapeWithArea but doesn't need a RectilinearShape and if it needs both, the type parameter becomes Class<T extends ShapeWithArea, RectilinearShape>). 

7. Dependency injection / Testability is much easier with composition
In composition, the superclass is just another dependency of the class and thus can be mocked and faked. This is much harder to do when trying to mock out the superclass itself in the type hierarchy if inheritance is used. Even if we tried to stub out the superclass methods, we wouldn't know if new ones got added.

Overall, all the advancements in our understanding of type theory, category theory point to composition being the "right" abstraction for composing code. Inheritance is a special case of composition (a trivial subset), has worse properties around unit-testing, dependency injection and code maintenance. Given the above, inheritance from concrete or abstract classes (especially those with any form of state) should be strongly avoided. Instead, implementation of interfaces + composition should be the strongly preferred approach. 

Tuesday, May 21, 2019

Managing Humans with Math

I was in a funky corner of the internet, reading about reinforcement learning when I chanced upon this article that compares a number of reinforcement learning algorithms. Since we as humans are glorified neural networks amenable to reinforcement learning and companies are nothing but hierarchical relationships between humans, it was very interesting to go through that article with a business point of view.

Long story short, the article figures out various conditions in which the algorithms are able to break down a task recursively, the optimality convergence conditions and the "knowledge" required of the lowest level agents to learn successfully. From my reading of the article, I'm most drawn to the "Options" style of management: work with high quality people, give them maximum freedom to act and step in only at decision points.

This is the style of management that I'd grown used to at Google (and that I've seen work really well there) and is quite in contrast to other managements that I have experienced. Not to say that the other managements were ineffective, they just took different routes to get to the same goals.

In summary: prefer the "Options" style of management, work with smart people, give them all the freedom they want and deserve, provide a strong learning function and clear feedback on how they're performing. Step in only at "Choice" points and then too defer to people with context of the problem. Most decisions are reversible, so just take one and let's keep going!

Wednesday, February 13, 2019

Hermetic MySQL in the modern world

So you're looking to run a MySQL docker instance without any environmental dependencies? Here's how to do that:

$ docker run --name mysql-test -e MYSQL_ALLOW_EMPTY_PASSWORD=true -e MYSQL_DATABASE=testingdb -e MYSQL_USER=scott -e MYSQL_PASSWORD=tiger -p 3306:3306  mysql/mysql-server:latest

Documentation on the various environment variables are located here:

To connect to your MySQL instance without depending on my.cnf:

$ mysql --no-defaults --host --port 3306 --user scott --password --protocol tcp testingdb

That's it. You have a running instance of mysql from a docker image and you're connecting to it with a generic mysql command line client. Have fun.

Saturday, February 02, 2019

Alerts should be actionable a.k.a.: Do not email on success!

Following the Unix philosophy: Do one thing, do it well and be quiet about it.

In software engineering, if you're writing a system that's useful and suddenly, one day, you think it's nice to notify users via email that their useful thing is being done, you're making a mistake. 

Emails from software systems should be actionable: if a system is sending an email to a user, it should be helpful, provide enough context about who it is, where it's running, who owns / runs it and what the problem is that requires human attention. Ideally, the alert email should clearly specify the next steps and the dashboards that can be used to ensure that the problem is fixed.

The worst offense of the system is to send out success emails. This fails on 2 counts:
1. Success emails are not actionable - if I read a success email, I am informed and I promptly create a Gmail filter to never see another success email from the system again. The system made me do active work to ignore it.
2. Success emails are not trackable - if I want to see what's the ratio of success to failures of the system, Gmail is a terrible way to do it. From first hand experience, measuring alert volume over time in Gmail is a time sink. Please build a dashboard and make the world a better place. Your future self will thank you.

The best alert emails are those that just tell me the commands to fix the problem. Oh happy me, I don't have to read a runbook, talk to people, poke around dashboards to see what the problem is. Run a few commands and presto, it's fixed.

Take some time and make your life better, don't send success emails and send actionable emails on failure.


Thursday, January 31, 2019

How to debug a crashing docker container

If want to run your docker process with some tweaks because it's crashing in your docker container and causing the container itself to stop (without giving you a way to inspect the files on the image), here's the magic command to start it with just bash.

(I found this after quite a bit of hunting on the internet, the magic flag is --entrypoint and don't forget the -s at the end)

Here's a sample command:
docker run -it --entrypoint /bin/bash  $IMAGE -s

Sourced from:

Wednesday, January 23, 2019

How to be an Effective Engineer?

Read this book by Edmond Lau: 

Highly recommended. His experiences parallel my own. The book is written engagingly, quick to read and succinct in its delivery. 

Friday, January 04, 2019

Understanding Divye

I typically share the contents of this document the first time I meet someone I'm going to be working with for a long time.

Style of communication:
Direct and clear. I don't do well reading between the lines. If you would like me to know something, please say something.

What is important to me:
High quality code and a highly productive, functional team. I don't believe in posturing. I believe in commitments and delivery. Delivering more and talking less is a good way to show impact.

What kind of leader I'm trying to become:
One who can grow people. I'm driven by Mission, Vision, Values.

What I value in an organization:
Transparency, Trust and Integrity. If you have a problem, speak up. Don't ever go behind someone's back. Bring problems up together so that I can see both sides represented fairly. Be direct.

What I seek in people I work with:
Ability to execute and potential to grow. I will endeavor to make you a better engineer or leader.

How I believe our industry operates:
Our industry is a factory - our deliverables are software and running services. Computer Science is a predictable industry with unpredictable schedules. The when might be unclear but the deliverable should be rock solid.

My planning philosophy:
Plan your work and deliver on your own benchmarks. Look out and adjacent.

My operating style:
I will work with you and always give you the "why". On the rare occasion where "because I say so" is required, I will let you know so.

My background:
Born and brought up in India. Son of two doctors, have a sister who is a doctor. Small town guy.

My favorite quotes:
"If I do what I say I will do, I guess that means you can trust me. If I don't, I guess that means you can't."

My biggest weakness:
I evolve, I don't let you know that fast enough.
If you talk to me and I tell you I support X and 24 hours later, I'm supporting !X, it means that I've gained new information that has changed my mind. Talk to me and clarify what changed. I have "strong opinions, loosely held".

  • What's working?
  • What's not working?
  • What's needed?

Things I would like you to know: