I have read alot about Software Transactional Memory, especially in relaiton to Haskell but I am trying to figure how it is different from database transactions? Are there some advantages I do not understand with STM?
+2
A:
STM is mostly used for concurrency, while database transactions are about data consistency.
so stms are not consistent?
Zubair
2010-03-23 07:21:04
In databases there are explicit invariants that can be specified and maintained, mostly about preserving mappings between key fields across tables. If I have tables "Customer" and "Address" then I can specify that every Address must have a matching Customer, and the database will maintain this. Haskell STM doesn't do this.
Paul Johnson
2010-03-23 08:05:13
@Paul in my opinion, this kind of consistency is not exclusive of databases. You can always enforce an invariant inside a STM transaction.
Vicente Botet Escriba
2010-05-18 22:06:33
+5
A:
The idea of a "transaction" in software transactional memory is explicitly borrowed from databases. The difference is where the transactions are implemented and how they are used.
STM is a language-level concept: a sequence of operations does not take effect until a transaction is committed. Typically this means that the values of some global/shared variables only change when a transaction succeeds. The property is enforced by the language runtime. There is no inherent notion of persistence: the variables involved in a transaction may be purely dynamic in nature (e.g., the size of a work queue).
Database transactions are an application-level concept: a sequence of data operations do not take effect until the transaction is committed. Since this is a database, persistence is fundamental: the meaning of "taking effect" inside of a database is that the data is saved in some persistent store.
You could potentially use a database and database transactions to implement a STM-style algorithm, but you'd lose the ease and convenience (and probably in most cases the performance) of a language-level implementation.
Chris Conway
2010-03-23 00:02:59
Yes, exactly. In STM, the effects of the code within the transaction (e.g., assignments to variables, I/O) aren't visible until the transaction is "committed".
Chris Conway
2010-03-24 13:43:40