SAGA
====
- long running transactions (distributed) across multiple microservices/DBs.
way to manage data consistency across microservices in distributed transaction
scenarios.
- alternative to the two-phase commit protocol for managing distributed transactions.
- atomic : all-or-nothing
- durable: the state of the saga is persisted after each transaction.
- responsive: each local transaction is executed within a bounded time interval.
- on activity/step failure:
- fault tolerance : retry policies; timeouts
- compensating actions are executed to undo the changes.
here compensation does not necessarily mean rollback, it can be anything that can
bring the system to a consistent state.
- compensating actions are executed in reverse order.
- being able to resume an execution from where it left off after a failure (system
crash, network partition, etc.)
Orchestration
------------- central component (orchestrator
- the orchestrator is responsible
next local transaction to invoke.
- the orchestrator is responsible
failure.
- the orchestrator is responsible
- the orchestrator is responsible
system failure.
| state machine) that coordinates the saga.
for invoking the local transactions and deciding the
for invoking the compensating actions in case of a
for persisting the state of the saga.
for resuming the saga from where it left off after a
Choreography
------------ sequence of local transactions. each local transaction updates the database and
publishes a message or event to trigger the next local transaction in the saga.
- each service is responsible for listening to events and deciding whether to react to
the event by invoking local transactions and emitting new events.
- each service is responsible for invoking the compensating actions in case of a
failure.
- each service is responsible for persisting the state of the saga.
- each service is responsible for resuming the saga from where it left off after a
system failure.
SAGA vs 2PC
----------- 2PC is a blocking protocol, whereas saga is non-blocking.
- 2PC is a binary protocol (all-or-nothing), whereas saga is a multi-phase protocol.
- 2PC is a synchronous protocol, whereas saga is an asynchronous protocol.
- 2PC is a pessimistic protocol, whereas saga is an optimistic protocol.
- 2PC is a centralized protocol, whereas saga is a decentralized protocol.
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