My current problem is to track the last 250 actions of each user using an app - these need to be stored in some kind of database, with reasonably fast access and fairly fast writes.
Up till now, I have just saved all actions of all users to a database, but the size of the database is completely blowing up. All messages older than 250 messages (per user) can be discarded, but SQL based libraries do not provide a reasonable way to do this.
Optimally, the database system would use circular buffers of non-fixed length. The problem with pre-allocating all the space required will be impossible (over 90% of users only ever perform < 100 actions, meaning pre-allocated space would be unfeasible for memory reasons). Additionally, for memory reasons, the size of entries needs to be dynamic, since allocating the maximum message length for each message will cause alot of empty space.
I was considering writing such a database system myself, using small (256byte) linked equally-sized chunks of data and keeping track of empty spaces. Obviously I would prefer a pre-made solution, since writing an own database system is time-consuming and error-prone.
Is there any system that - more or less - does what I intend to do? If not, what approach is the best towards writing such a database system myself?