On Tue, Dec 13, 2011 at 4:55 AM, Anatoly Yakovenko
So I am trying to understand how acid state works. The HelloWorld example has a
type Message = String data Database = Database [Message]
$(deriveSafeCopy 0 'base ''Database)
-- Transactions are defined to run in either the 'Update' monad -- or the 'Query' monad. addMessage :: Message -> Update Database () addMessage msg = do Database messages <- get put $ Database (msg:messages)
It seems to me that since the Dababase is a list of messages every update would require acid-state to rewrite the list into the file, so each update would get slower as the list gets bigger, but what I am seeing is that updates are constant time regardless of the size of the list. So how does it work?
acid-state doesn't write the whole thing to the disk every time there's a transaction. Instead, it just writes the transaction on a transaction log. So it will just write something like "AddMessage msg" to the disk. Periodically, checkpoints are created which *do* have all your data inside them (but even so, checkpoints are written asynchronously). Cheers, -- Felipe.