py-saga is an asynchronous open-source python implementation of the saga pattern.
Sagas are a way to execute complex transactions across distributed systems while guaranteeing eventual consistency.
In short, they are transactions made up of possibly interdependent steps. Those steps are executed in paralell across a distributed system. If one of them fails, all other steps are undone through a compensation.
For example: imagine you have a scenario where a user is trying to purchase a trip package to Rio. This package includes roundtrip airplane tickets, a hotel booking and tickets for the Christ the Redeemer statue.
In this case, your buy package saga is composed of three different steps:
- book plane ticket
- book hotel room
- buy tourist tickets
In case any of those steps fails, you must undo the other ones, because the package can only be sold as a unit. For this reason, you also have to define compensations for each step. Your steps then look like this:
| Step | Compensation |
|---|---|
| book plane ticket | cancel plane ticket booking |
| book hotel room | cancel hotel room reservation |
| buy tourist tickets | cancel ticket reservation |
Let's execute our buy package saga then!
When Maria tries to buy her package, everything goes according to plan, and the saga is executed as planned:
❗️️ begin saga
✔️ book plane ticket
✔️ book hotel room
✔️ buy tourist tickets
❗️️ all steps succeeded
️✔️ end saga (success)
However, when Kevin tries to buy his packages, the hotel informs there aren't any available rooms. The saga is then executed as follows:
❗️️ begin saga
✔️ book plane ticket
❌ book hotel room (no available hotel rooms)
✔️ buy tourist tickets
❗️️ step failed: reverting
✔ ️cancel plane ticket booking
✔ cancel tourist ticket reservations
❌ end saga (no available hotel rooms)
Notice that, for both cases, the final system state is coherent, even though in one case our distributed transaction failed.