Prescriptive Software Practices: Code Re-use Edition

Individual software practices don’t exist in a vacuum, and need to be viewed collectively.

Today I saw this tweet, that I initially violently agreed with, before realising the answer is really more “it depends”.

Now I fully agree that demanding that people write the abstraction layer before they’ve even written the first component to use the underlying tool, is a folly that leads to bad libraries. You don’t know how to best use the underlying API, and you don’t know how you want to use it, and which of the methods you want to wrap or enhance.

The requirement to wrap every ‘method’ is the main reason I dislike intermediate libraries, one time I asked “are we using this new AWS feature that’s perfect for our use case?” The answer: “No, we can’t because Terraform doesn’t support it yet.”

Any time you put something in-between you and the underlying service you’re introducing a potential roadblock. I’ll explain later how I think you can minimise this.

The main reason I think code-reuse/libraries are hard to get right is a conflict at the core of them:

  • A trivial library can be simple to use, but if the functionality is simple, what is it really adding?
  • A feature-filled library is usually (but not always) harder to make use of, and if most people only use a fraction of it, what makes it worth they overhead?

Things don’t exist in isolation…

Warning, inbound analogy: Very often “we” like to look to other counties and cite how wonderfully they do a thing. An example from the UK is that we’re often told that “people shouldn’t mind renting flats because Germany people tend to buy later.”

Which sounds great, but when you point out that Germany has a bunch of related things – longer leases, more freedom to decorate/change properties, and that they consistently build houses to maintain far more modest house-price rises – people tend to go quiet.

Returning to software, everything is similarly related and supported by other practices. If you don’t fully understand a problem, you can’t cleanly decompose it in a sensible collection of services, and only when you’ve done that will sensible opportunities for code re-use/libraries emerge. (At this point you’re welcome to argue that if you’ve decomposed your system properly then you should need to reimplement functions).

XP/Agile/Clean Code/BDD/TDD/… can become quasi-religious in how much you must adhere to all of their tenets. I suspect very few people are fully compliant with any one tribe, and to be effective as teams you need to view things are recommendations or possibilities, and not commandments that thou shall obey.

How to do code re-use right…

This is just my experience, but a few questions to ask or points that I’ve found have worked for the people I’ve worked with in the past:

  • Avoid needing them in a first place: if your transaction volume is low enough just have a dedicated service that does the particular thing… A single point of truth is the easiest way, but that isn’t always possible due to latency or cost concerns
  • Consider Security/Auth/Data-protection first: These are things that you need to create decent libraries/patterns for, because if the easiest thing is the right thing, you’re going to be making fewer critical mistakes, and it can make patching easier if you’re exposing a consistent interface but have to update an underlying library with breaking changes
  • Judge the demand: While many times people can be “wow, I didn’t realise I needed x until it appeared” unless it’s really obvious that lots of people have the exact problem, do you really need to write a library?
  • Understand it before you abstract it: Don’t write them first. My ideal preference is that when you have a few teams working in the domain, let them create distinct implementations. Afterwards, regroup and use that learning as the basis for a library. This is more work, but the result will be much better
  • Keep the library fresh: Is it one person’s job? Is it a collective whole-team effort? A library needs to be a living thing as the systems it interacts with will change. Developers will rightly shy away from using a clunky piece of abandoned code
  • Layer up in blocks: a client has a back-end system with specific authentication requirements and has been building out client libraries. There are 3 distinct libraries: connection management, request building and result parsing. You didn’t have to use all of these, and can just use the connection library if you want more direct access
  • Make your library defensive but permissive: TypeScript has got me back into typing, but previous experience makes me nervous. In micro-services environments a library update can require many unrelated deployments, when only be two components are functionally involved. Errors because enums aren’t valid can be useful, but can you expose the error when that property is accessed rather than parsed?

In summary…

Teams need to find their own path, and find where on the line between “Don’t Repeat Yourself” and “Just Copy-Paste/Do Your Own Thing” they lie. It is highly unlikely to be at either extreme.

“It Depends” isn’t a particularly enlightening answer, but like so many things about building decent products, it is what it is.

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