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Compensating Service Transactions

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Compensating Service Transactions

  1. 1. Compensating Service Transactions SOA Patterns Webinar Series 09/30/2014 Nuwan Bandara, Senior Lead Engineer Nipun Suwandaratna, Senior Engineer
  2. 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. 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. 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. 5. Error Situation What if something Goes Wrong ? Can we always Rollback the Process?
  6. 6. Rollback situation ๏Item purchase over an ecommerce site
  7. 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. 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. 9. Long Running Process Example Travel Booking Service Scenario Book Hotel Book Air Ticket Book Taxi Travel Booking Service
  10. 10. Travel Booking Service - Sequence
  11. 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. 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. 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. 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. 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. 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. 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. 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. 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. 20. Compensation with BPEL o Driven by partner links o Each invocation has a compensation
  21. 21. TCC With WSO2 ESB
  22. 22. Thank You !!

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