saga_py

Create a series of dependent actions and roll everything back when one of them fails.

Install

$ pip install saga_py

Usage

Simple example

from saga import SagaBuilder

counter1 = 0
counter2 = 0

def incr_counter1(amount):
    global counter1
    counter1 += amount

def incr_counter2(amount):
    global counter2
    counter2 += amount

def decr_counter1(amount):
    global counter1
    counter1 -= amount

def decr_counter2(amount):
    global counter2
    counter2 -= amount

SagaBuilder \
    .create() \
    .action(lambda: incr_counter1(15), lambda: decr_counter1(15)) \
    .action(lambda: incr_counter2(1), lambda: decr_counter2(1)) \
    .build() \
    .execute()

# if every action succeeds, the effects of all actions are applied
print(counter1)  # 15
print(counter2)  # 1

An action fails example

If one action fails, the compensations for all already executed actions are run and a SagaError is raised that wraps all Exceptions encountered during the run.

from saga import SagaBuilder, SagaError

counter1 = 0
counter2 = 0

def incr_counter1(amount):
    global counter1
    counter1 += amount

def incr_counter2(amount):
    global counter2
    counter2 += amount
    raise BaseException('some error happened')

def decr_counter1(amount):
    global counter1
    counter1 -= amount

def decr_counter2(amount):
    global counter2
    counter2 -= amount

try:
    SagaBuilder \
        .create() \
        .action(lambda: incr_counter1(15), lambda: decr_counter1(15)) \
        .action(lambda: incr_counter2(1), lambda: decr_counter2(1)) \
        .build() \
        .execute()
except SagaError as e:
    print(e)  # wraps the BaseException('some error happened')

print(counter1)  # 0
print(counter2)  # 0

An action and a compensation fail example

Since the compensation for action2 fails, the compensation effect is undefined from the framework's perspective, all other compensations are run regardless.

from saga import SagaBuilder, SagaError

counter1 = 0
counter2 = 0

def incr_counter1(amount):
    global counter1
    counter1 += amount

def incr_counter2(amount):
    global counter2
    counter2 += amount
    raise BaseException('some error happened')

def decr_counter1(amount):
    global counter1
    counter1 -= amount

def decr_counter2(amount):
    global counter2
    raise BaseException('compensation also fails')

try:
    SagaBuilder \
        .create() \
        .action(lambda: incr_counter1(15), lambda: decr_counter1(15)) \
        .action(lambda: incr_counter2(1), lambda: decr_counter2(1)) \
        .build() \
        .execute()
except SagaError as e:
    print(e)  #

print(counter1)  # 0
print(counter2)  # 1

Passing values from one action to the next

An action can return a dict of return values. The dict is then passed as keyword arguments to the next action and it's corresponding compensation. No values can be passed between compensations.

from saga import SagaBuilder, SagaError

counter1 = 0
counter2 = 0

def incr_counter1(amount):
    global counter1
    counter1 += amount
    return {'counter1_value': counter1}

def incr_counter2(counter1_value):
    global counter2
    counter2 += amount

def decr_counter1(amount):
    global counter1
    counter1 -= amount

def decr_counter2(counter1_value):
    global counter2
    counter2 -= amount

SagaBuilder \
    .create() \
    .action(lambda: incr_counter1(15), lambda: decr_counter1(15)) \
    .action(incr_counter2, decr_counter2) \
    .build() \
    .execute()

print(counter1)  # 15
print(counter2)  # 15