A minimal library to work with versioned events in atomic database transactions.
Project description
Even History
Why do we need this?
Well, you probably don't. But I needed it for something personal that I was working on and thought it would be good to show it off so that maybe it can help someone else.
Usecases
The library is quite simple, you only have to have two classes that satisfy
the IEventHistoryService
and IEventService
protocols.
The IEventHistoryService
is a simple protocol, that has a save
method which
takes in only two arguments, one the object its trying to save, called event_object
(NOTE its not a keyword argument in the TransactionService
so the first position
argument would be expected to be event_object.) The IEventService
protocol is very
similar to it and has two methods, save
and update
methods. Both of which have the
same calling signature, the first argument is event_object
which is the object that
you want to save and the other arugment is a keyword argument called session
.
The session
is the ContextManager
provided by many database library for
ACID transactions. PyMongo provides it too, using the start_session
method of
pymongo.MongoClient
class.
What does it do
This library is very simple, in fact quite a dumb one to begin with but it does
its simple job well. The idea is to use the already existing services with your
business entity (models). The requirement for this library is quite simple too, all
you need is a service layer to save and update date in your desired database.
You would also need a context manager, either make one yourself (refer to the tests
directory to see how to make a simple one) or use the one provided with your database drivers/library.
After which, all you need to do is to initialise the TransactionService
class with
these services as arguments.
For example,
...
@dataclasses.dataclass
class Event:
...
# these two are required fields to be included in
# your model for TransactionService to work properly.
timestampt: datetime.datetime
version: int = 0
class EventService:
...
def save(self, event_object, session=None):
...
def update(self, event_object, session=None):
...
class EventHistoryService:
...
def save(self, event_object, session=None):
...
client = pymongo.MongoClient("url-to-some-replica-set")
your_event_object = Event(**some_data_dict)
transaction_service = event_history.TransactionService(
EventService(),
EventHistoryService(),
)
with client.start_session() as session:
transaction_service.save(your_event_object, session=session)
logging.info("All done! Now check your database.")
...
your_event_object = Event(**some_updated_dict)
with client.start_session() as session:
transaction_service.update(your_event_object, session=session)
loggin.info("It should be updated! Now check your database.)
This is a minimal example that shows that basic use of this Transaction service with
PyMongo sessions but as explained earlier, this library is designed to be context
agnostic and can handler it with any context manager you provide (as long as your
save
and update
methods in service classes is able to use them).
Finally, you can use this library if you want - however, I don't think this is an actual library since all its doing is abstracting away some of the burden of writing further abstractions on top of your service layers. So, if you can just look at the pattern and come up with your own simple abstraction, that would be better than adding a dependency (this sideproject) in your projects.
For a fully fledged example, take a look at the example
directory and the code inside.
It shows how it can be used with pymongo
library and how it can work very nicely
with it.
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