Redis is still very simple and most features are orthogonal and completely isolated. Don't trust my words, check yourself, if you count the lines of code with "cloc" in Redis unstable, it is composed of 64890 lines of code. In any other system this amount of code is used, maybe, for the query parser. A single developer can still read the Redis source code in a few days and understand how it works and modify any part alone.
EDIT: I also want to stress on a fact, that at the same time in the newer releases there was a simplification attempt. For instance the Lua scripting side effects problem are going to be removed completely by simplifying how the replication of Lua scripts is performed. Redis 5 is already like that, but the dead code was not yet removed for safety. Redis 6 will completely remove many parts of code. So there is not just the stress on adding, but also on refactoring. ACLs themselves allowed to refactor authentication in separated functions inside acl.c to lower the overall complexity.
To be honest, the top level comment in this chain reads a bit like an attempt at a plug for FoundationDB than a legitimate criticism of Redis. You could s/Redis/<literally any other DB>/g
I can second that. It's been a few years since I actually did this, but within about two working days worth of reading, I had consumed and understood a good chunk of the code and overall control flow. I was able to make some reasonably invasive changes within a few hours each after that (initially adding "msg/trigger on expire" and check-and-set operations on hashes, then a bit later a change to memory management that was a few percent of global speedup, and a partial COW implementation for strings). Admittedly, I have fair amount of experience working on interpreters in C, which is pretty close to what plain Redis does, just simpler and hidden behind http.
A quick glance suggests that this would still be similarly possible today. Redis is by no means perfect, but it deserves it's reputation for being accessible!
PS: Thanks for the mention of the Lua side effects changes. I'll be curious to read up about your solutions to those puzzles.
Impressively small or large? It's tiny compared to most storage products/databases.
I would say that's mostly due to that when it gets down to brass tacks, Redis (ignoring the orchestration in Cluster) really is a fairly simple interpreter with simple memory management (largely delegated to the malloc implementation) that glues together a fair number of nice
data structure/algorithm implementations that have little codependence. That last point is really, really important in it's developer accessibility. (antirez called this modularity elsewhere I think.) It also helps keep code size down.
EDIT: I also want to stress on a fact, that at the same time in the newer releases there was a simplification attempt. For instance the Lua scripting side effects problem are going to be removed completely by simplifying how the replication of Lua scripts is performed. Redis 5 is already like that, but the dead code was not yet removed for safety. Redis 6 will completely remove many parts of code. So there is not just the stress on adding, but also on refactoring. ACLs themselves allowed to refactor authentication in separated functions inside acl.c to lower the overall complexity.