Schema

Traditional (relational) or schemaless

Users
id
name

Puzzles
id
name
data (grid, clues)
metadata (author, date)

Users ↔ Puzzles
user.id
puzzle.id
start (join) datetime
solve time
solve percentage

Key-value store (redis)

Rooms
id
puzzle.id
start time
clients
join history
grid, grid owners
chat log