Database Transactions

• A Database Transaction is sequence of modification operations that change the state of the database
  • Implicitly short term as opposed to long-term
• Database community has assigned certain properties to transactions to make them reliable
  • These properties are referred to by the acronym ACID

ACID

A = Atomic Transaction operations are all performed or none are performed C = Consistent WRT some defined set of constraints Constraints define what consistent means

I= Isolated Each transaction sees the effects of all preceding transactions, but not the effect of overlapping transactions Parallel transactions must be serializable

D = Durable Each committed transaction is guaranteed to be applied to the database.

Distributed ACID

• Independent databases with their own local transactions (Li)
• Single global transaction
• Want it to have ACID properties:  _CID(Li) => _CID(G0)
  • C(L1),C(L2),C(L3) => C(G0)
  • I(L1),I(L2),I(L3) => I(G0)
  • D(L1),D(L2),D(L3) => D(G0)
• Atomicity is the problem
  • Abort(L1),Commit(L2),Commit(L3)

Distributed Atomicity vid 2-Phase Commit (2PC)

• Solution: add Prepare (-to-commit) operation
• Need a coordinator/representative for the global transaction (= coordinator)
• Coordinator asks each local transaction in turn to:
  1. Prepare
  2. Commit or Abort
• If any transaction aborts, global transaction aborts

Two Phase Commit Protocol

Examine Some Code

• Kandula Apache Implementation Web Page
  • Two versions: one for Axis 1 and one for Axis 2
  • Kandula for Axis 1 Javadocs
• Ws-AtomicTransaction WSDL
  • Note that there are two parallel sets of defined operations
  • One is for syncronous (RPC) invocation
  • One is for asyncronous invocation
• Ws-AtomicTransaction XSD
  • Defines the types used by the WSDL
  • Mostly enumerations

Related WS-* Specifications

• WS-Coordination
  • CreateCoordinationContext
  • Register(String protocol, Endpoint endpoint)
• WS-Addressing
  • URI's are not considered sufficiently "soap-ish"
  • XML wrapper around URI;s that support additional information (synch vs asynch, msg ids, etc)
  • Term Endpoint is used for this XML structure
• WS-Transaction
  • Extend AtomicTransaction to include Long transactions
  • ACID properties are not guaranteed
  • Atomic abort may not be possible
  • Only known solution is compensating transactions
    • Require knowledge of the semantics of the transactions
  • Example: Transaction is buy-book
    • When book is returned, it is not possible to undo
      => credit purchaser and add book back to inventory

References

• IBM has a site that has many of the WS-* specifications, WSDL, and XSDs
  • WS-Coordination specification
  • WS-AtomicTransaction specification
  • WS-BusinessActivity specification
  • WS-Coordination WSDL
  • WS-Coordination XSD
  • WS-AtomicTransaction WSDL
  • WS-AtomicTransaction XSD
  • http://www-128.ibm.com/developerworks/library/specification/ws-tx/

• Web Services Transaction Specification
  http://dev2dev.bea.com/pub/a/2004/01/ws-transaction.html
  Apache Kandula Implementations of Coordinator, Transaction, Addressing, AtomicTransaction
  http://ws.apache.org/kandula
• Articles
  • Transactions in the world of Web services (Parts 1 and 2)
    • http://www-128.ibm.com/developerworks/library/ws-wstx1/
    • http://www-128.ibm.com/developerworks/library/ws-wstx2/
  • The hidden impact of WS-Addressing on SOAP
    http://www-128.ibm.com/developerworks/webservices/library/ws-address.html