comparison

Sometimes I need to divide my knowledge in phases: when I started to program, I felt that C/C++ were the best languages to make a software. Then, I’ve began to work with Ruby and learned Test-Driven Development. I divide my knowledge “before TDD” and “after TDD”, because it literally changed the way I could think about software and tests in general. Now, I’ll probably make the same division: “before functional programming” and “after FP”, and I only discovered that things have changed this week: I just discovered I don’t know how to do object oriented programming anymore – and to be honest, it’s not really a bad thing: I also saw in practice how object-oriented programming expects you to write lots of boilerplate in most situations.

This realization came to me when I was working in a peculiar kind of problem – it’s basically a ruby code that needs to connect on messaging system and, when it receives a message, needs to persist it in a relational database. The problem is that this specific system have a strong consistency requirement, so the message could only be updated on DB before a specific event happens: after that, we can’t change it anymore, except for some specific fields. Also, as we’re working with messaging, we can receive the message multiple times, out of order, and all of those strange things that happens on Internet-land. So, to summarize:

1. The system will receive a message
2. It’ll UPSERT a record on DB, just with some identification fields (they’ll never change, and if they do, they characterize another message to be persisted – think about an external id)
3. It’ll BEGIN a transaction and SELECT ... FOR UPDATE my just upserted message
4. It’ll find, on another table, the rest of the fields on the message
   1. If this record doesn’t exist, we’ll create it (and log)
   2. If it does exist, we’ll check additional info, and decide: or we ignore it (and log), or we upsert (and log), or raise an error

Now, for the implementation. Because consistency is a must have on this system, I don’t want to expose for future programmers (be it myself or some of my friends) some code that’ll induce me to errors: after all, who wants to do with_consistency_check(msg) { Message.upsert!(msg) } when I can just do Message.upsert!(msg), right? So, I don’t want to allow people to be able to modify, find, update, or anything else outside of the with_consistency_check (or whatever name I decide). So, how to do it?
(more…)