Reference
- The isolation property of ACID transactions ensures that the outcome of executing multiple transactions concurrently is the same as if they were executed in some serial order
- Isolation makes it a lot easier to write business logic that executes concurrently
- The challenge with using sagas is that they lack the isolation property of ACID transactions.
- Problem
- other sagas can change the data accessed by the saga while it’s executing
- other sagas can read its data before the saga has completed its updates, and consequently can be exposed to inconsistent data
- saga to be ACD:
- Atomicity—The saga implementation ensures that all transactions are executed or all changes are undone.
- Consistency—Referential integrity within a service is handled by local databases. Referential integrity across services is handled by the services.
- Durability—Handled by local databases.
- Strategy to deal with the lack of isolation: countermeasures
Overview of anomalies
- Lost updates: Update a data and another Saga override the same data right away
- Dirty reads: Read a data while another Saga update that data but not completed
- Fuzzy/nonrepeatable reads: Different step of Saga read same data get different result because other saga has made update
Countermeasures for handling the lack of isolation
- Semantic lock: application level lock
- Commutative updates: Design update operations to be executable in any order
- Pessimistic view: Reorder of saga steps to minimize business risk due to dirty read
- Reread value: Reread data to verify it's unchanged before overwriting it
- Version file: Record the update to a record so that they can be reordered. Ex. record the operations as they arrive and then execute them in the correct order
- By value: Use request's business risk to dynamically to select the concurrency mechanism. Ex. transfer money, use distributed transaction. Simple status update, use Sagas.
THE STRUCTURE OF A SAGA
a saga consists of three types of transactions (One complete flow may have every kinds of transactions):
- Compensatable transactions: can potentially be rolled back. Ex. Delete a created Order
- Pivot transaction: The go/no-go point in a saga. Ex. verify before proceed, go or no-go after verify.
- Retriable transactions: Follow pivot transaction but guarantee to succeed. Ex. Can retry when fail.
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