“We have a stream of data, but need to replace specific values with others: a charge code, for example, with its department.”
“OK, that's easy, we'll save the codes and values in a map.”
“There may be hundreds of thousands of them.”
“A database might be better, then. Can we load the input data into a table and produce the output with a join?”
“Databases are too slow; we're dealing with thousands of messages a second.”
This interchange sounds like something that might come from an interview. Perhaps it's a “Kobayashi Maru” question, designed to test how the interviewee behaves under pressure, in which there is no correct answer and the interviewer piles on the constraints until the problem is insolvable.
Or perhaps it's the mental process of a software engineer working through the constraints of an actual problem, and testing her design against those constraints. In my experience, there are two big differences between the real world and the interview.
The first is that you might not know all of your constraints at design time. You might build your system using a high-performance messaging system, and later learn that it has an easily-reproducible but difficult-to-fix bug — one that you have to deal with in production.
The second is that real-world design is all about ranking the constraints, and making tradeoffs based on those rankings. If you absolutely need the speed of a large in-memory lookup table, then that constraint has to rank higher than hardware cost. This is often distasteful to your users, but it's unavoidable; a good engineer has to recognize that all constraints do not have the same priority, and push users to the same understanding.
It is occasionally possible to “reprogram the simulation” in the real world, and sometimes that is the only way to resolve your constraints. Perhaps the input is structured such that you don't need to keep the entire lookup table in memory at once. Or perhaps you don't actually need to perform the substitution in real-time. Always look for these opportunities.
A third difference is that you have to live with your designs in the real world. For that reason, I recommend asking yourself as many hard questions as you can.