add saga init dsc

saga/README.md

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+---
+layout: pattern
+title: Saga
+folder: Communication
+permalink: /patterns/saga/
+categories: Behavioral
+tags:
+ - Java
+ - Difficulty-Expert
+ - Idiom
+ - Distributed communication
+---
+
+## Also known as
+This pattern has a similar goal with two-phase commit (XA transaction)
+
+## Intent
+This pattern is used in distributed services to perform a group of operations atomically.
+This is an analog of transaction in a database but in terms of microservices architecture this is performed in a distributed environment
+
+## Explanation
+A saga is a sequence of local transactions in a certain context. If one transaction fails for some reason, 
+the saga executes compensating transactions(rollbacks) to undo the impact of the preceding transactions.
+There are two types of Saga:
+
+- Choreography-Based Saga. 
+In this approach, there is no central orchestrator. 
+Each service participating in the Saga performs their transaction and publish events. 
+The other services act upon those events and perform their transactions. 
+Also, they may or not publish other events based on the situation.
+
+- Orchestration-Based Saga
+In this approach, there is a Saga orchestrator that manages all the transactions and directs 
+the participant services to execute local transactions based on events. 
+This orchestrator can also be though of as a Saga Manager.
+
+## Applicability
+Use the Saga pattern, if:
+- you need to perform a group of operations related to different microservices atomically
+- you need to rollback changes in different places in case of failure one of the operation
+- you need to take care of data consistency in different places including different databases
+- you can not use 2PC(two phase commit)
+
+## Credits
+- [pattern description](https://microservices.io/patterns/data/saga.html)