Safe Haskell | None |
---|
- class IResource a where
- keyResource :: a -> String
- readResourceByKey :: String -> IO (Maybe a)
- readResource :: a -> IO (Maybe a)
- writeResource :: a -> IO ()
- delResource :: a -> IO ()
- data Resources a b
- resources :: Resources a ()
Documentation
Must be defined for every object to be cached.
:: a | |
-> String | must be defined |
readResourceByKey :: String -> IO (Maybe a)Source
Implements the database access and marshalling of the object.
while the database access must be strict, the marshaling must be lazy if, as is often the case,
some parts of the object are not really accesed.
If the object contains DBRefs, this avoids unnecesary cache lookups.
This method is called inside atomically
blocks.
Since STM transactions retry, readResourceByKey may be called twice in strange situations. So it must be idempotent, not only in the result but also in the effect in the database
. However, because it is executed by safeIOToSTM
it is guaranteed that the execution is not interrupted.
readResource :: a -> IO (Maybe a)Source
writeResource :: a -> IO ()Source
To write into persistent storage. It must be strict.
Since STM transactions may retry, writeResource
must be idempotent, not only in the result but also in the effect in the database.
. However, because it is executed by safeIOToSTM
it is guaranteed that the execution is not interrupted.
All the new obbects are writeen to the database on synchromization,
so writeResource must not autocommit.
Commit code must be located in the postcondition. (see setConditions
)
Since there is no provision for rollback from failure in writing to
persistent storage, writeResource
must retry until success.
delResource :: a -> IO ()Source
Delete the resource. It is called syncronously. So it must tocommit
Resources data definition used by withSTMResources