[Haskell-cafe] acid state

[Anatoly Yakovenko]

Ah, i think i get it.

On Tue, Dec 13, 2011 at 12:48 AM, Felipe Almeida Lessa
<felipe.lessa at gmail.com> wrote:
> 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.