Changes in 0.3.*:
Changes in 0.2.*:
Changes in 0.1.*:
This library provides a thread-safe (STM) frontend for finite map types together with a
backend interface for persistent storage. The TMap data type
is thread-safe, since all access functions run inside an STM monad . Any type
instantiating Data.Edison.Assoc.FiniteMapX
(see EdisonAPI) can be used as a map type.
When a TMap is modified within an STM transaction, a corresponding
backend IO-request is added using the onCommit hook (cf. stm-io-hooks
package). To ensure consistency,
the (Adv)STM monad runs these requests iff the transaction commits.
Additional backends (e.g. HDBC) can be added by instantiating the class
Backend
.
Example:
Thread 1:
atomically $ do
isMemb <- member 1 tmap
when (not isMemb) $
insert 1 "john doe" tmap
Thread 2:
atomically $ do
v <- lookup 1 tmap
-- ... do something with 'v'
adjust (\_ -> "jd") 1 tmap
The function member
will first check whether the key value '1' is in the map;
if not, it sends a lookup-request to the persistent backend
and then retries the transaction. Note that "sending a lookup-request"
essentially means adding a call to the corresponding IO-function of the backend
to the list of retry-IO actions. (This is done using the retryWith
IO hook of the stm-io-package.)
If the value does not yet exist, function insert
adds the key-value mapping to the
TMap and sends an insert-request to the backend using the
onCommit
hook of the stm-io-package. Note that onCommit
guarantees that
the backend IO action is only executed iff the transaction commits. Any
changes that were made to the TMap are invisible to other threads until
the onCommit actions are run. Therefore, the threads will always observe a
consistent state.
The module Data.TStorage
provides a high level interface to TMap inspired
by the TCache package ((C) Alberto Gomez Corona). It
can store any type that has a key (i.e. is an instance of type class HasKey
).
See file Sample.hs for an example on how to use it.
Warning: This package is very experimental and the interface will
probably change.