Build a bounded buffer pool

The room every page passes through, with a pin count and a usage dial on each desk. This lesson builds that room in about two hundred lines of Go and breaks it on purpose.

Before you start. The Buffer Pool explained the room: frames, pinned and dirty pages, the clock sweep, and the small ring a big scan gets. Pages built a pager whose cache is a map that only grows, and said so. This lesson replaces that map with a pool of fixed size. You need to read Go, nothing more; the code is in teaching/code/database-internals/01-bufferpool.

The idea in 30 seconds

A bounded buffer pool is four answers: a map from page to frame, a victim rule, a pin count that makes a frame untouchable, and a dirty flag that forces a write before reuse.
The clock sweep gives each frame a usage dial from 0 to 5; a hit turns it up, the hand turns it down, and the first unpinned frame at zero is the victim.
When every frame is pinned there is no victim, and the pool must report that as an error.
A large sequential scan, given no limit, takes frames from the pages other work depends on; a small private ring for the scan stops that.
In the demo, the regular pages kept 200 of 300 hits after an unringed scan and all 300 after a ringed one, from the same 1,900 disk reads by the scan.
Writing a dirty page back at eviction, not at every change, is what lets many changes to one page cost one write.

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