[Haskell-cafe] acid state

[Felipe Almeida Lessa]

On Tue, Dec 13, 2011 at 4:55 AM, Anatoly Yakovenko
<aeyakovenko at gmail.com> wrote:
> 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.