This document discusses compensating transactions in service-oriented architectures. It begins by defining transactions and describing stateful and stateless transactions. It then discusses challenges with long-running processes and rollbacks. Compensation is introduced as a way to restore systems after errors through undo operations or tentative reservations. The document outlines a Try-Confirm-Cancel model and shows how it can be implemented using BPEL and WSO2 ESB to handle compensation in long-running processes.

1. Compensating Service Transactions
SOA Patterns Webinar Series
09/30/2014
Nuwan Bandara, Senior Lead Engineer
Nipun Suwandaratna, Senior Engineer

2. Agenda
o Transactions - basic overview
o Rollback Mechanism
o Long Running Processes
o What is Compensation
o Compensations methods
o Try-Confirm Cancel (TCC) Model
o Compensations with BPEL
o TCC with WSO2 ESB
o Demo

3. Transactions
What are Transactions ?
An action or a series of coordinated actions performed to fulfil a
business functionality (business Transaction)
Service B
Service C
Service D
Service A

4. Stateful and Stateless Transactions
and processes
o Stateless transactions do not track or maintain a record of the
occurred events
o Stateful transactions keep track of the current state of the
interaction. This information could be in memory or persisted
to a storage such as DB or File
o A Process could be either short transient or long running, and
made up of a collection of stateful and/or stateless
transactions

5. Error Situation
What if something Goes Wrong ?
Can we always Rollback the Process?

6. Rollback situation
๏Item purchase over an ecommerce site

7. Rollback Mechanism
o Services are atomic units that provide a clearly defined business
functionality – A service is responsible for handling errors when it is
invoked. But it’s not the job of a service to make sure the integrity of the
overall process (that would make the system tightly coupled)
o When services are grouped to form a process (composite service) the
Transaction Coordinator has to handle the undo operations when errors
occur.
o ACID transactions ensure integrity.
o Not practical in Long Running Processes since resources are blocked for a
long time
o More memory/resource consuming since all services need to preserve
their original states till a commit or rollback is notified

8. Long Running Processes
o Services are atomic units that provide a clearly defined business
functionality – A service is responsible for handling errors when it is
invoked. But it’s not the job of a service to make sure the integrity of
the overall process (that would make the system tightly coupled)
o A process that is suspended upon external input
o Inputs could be triggered by some event or by human interaction
o May be made up of a series of transactions

9. Long Running Process Example
Travel Booking Service Scenario
Book Hotel
Book Air Ticket
Book Taxi
Travel
Booking
Service

10. Travel Booking Service - Sequence

11. What is Compensation
Compensation is the process of restoring a system to it’s original
state upon the occurrence of an error or a situation where the
business transaction could not successfully proceed any further.

12. When is Compensation Needed
o In situations where multiple services that are not make up a
single transaction and the business process as a whole is not
ACID
o That is, a rollback of the current transaction/service does not
help to restore the integrity of the overall business
transaction.
o In Long-Running processes
*

13. Compensation as a regular Transaction
o This is when the compensation is performed as a regular
transaction
E.g: In a stock trading scenario
If a stock broker buys 5000 stocks of company A for the price
of $2.00 each by mistake the only way to compensate for it is
to sell the same for $2.00 each (or more)
But stock prices cannot be guaranteed

14. Using Undo Operations
o Each service should provide a fault operation
o Synchronous services – No issues, service returns fault
o Asynchronous – Mechanism should be in place for the caller to
wait for the response from the service
*

15. Undo Operations in the Travel Booking System
Taxi Booking Service
-Book Taxi
-Cancel Booking
Hotel Booking Service
-Book Hotel Room
-Cancel Booking
Travel
Booking
Service
Airline Booking Service
-Book Ticket
-Cancel Booking

16. Cons of Undo Operations
o Air Line is at a risk of loss if the
reservation is cancelled
o Hotel is at a risk of loss if booking is
cancelled
o Taxi service is at a loss if the taxi
booking is cancelled
o Customer or Travel Agent is at a risk of
loss if any of the above systems denies
cancellation or does not refund full
amount
o Someone is at the risk of losing money
if the resources that are reserved are
revenue generating resources

17. The TRY-CONFIRM/CANCEL Model
o TRY Operation – Reserves Airline Seat, but the booking is not
complete
o CONFIRM – Confirms the bookings if all other requirements
are fulfilled
o IF other requirements are not fulfilled send a CANCEL
request

18. Characteristics of the TRY-CONFIRM/CANCEL
Model
o Each request remains tentative until it’s Confirmed or
Cancelled
o Cancellation timeouts will be set by service provider
o Aggregates XA based (2PC) and Compensating transaction
models
o Uses a coordinating service
o Uses Http Verbs (POST, PUT) to carry out the transaction

19. Advantages of the TRY-CONFIRM/CANCEL
Model
o Leads to a loosely coupled transaction model
o Eliminates the need to have a compensating service for all
the forward actions
o Decouples Error handling
o Increases Autonomy

20. Compensation with BPEL
o Driven by partner links
o Each invocation has a compensation

21. TCC With WSO2 ESB

22. Thank You !!