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Compensating Service Transactions
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?
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
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
