Sharpino 1.4.9

Sharpino

<img src="ico/sharpino.png" alt="drawing" width="50"/>

A little F# event-sourcing library

What is it?

Support for Event-sourcing in F#. No sensible data (GDPR) support.

Features

• Support in memory and Postgres storage. Support Eventstoredb (only for the LightCommandHandler).
• Support publishing events to Kafka.
• Example application with tests including Kafka subscriber.
• Contexts represent sets of collections of entities (e.g. a collection of todos, a collection of tags, a collection of categories, etc.) associated with events
• A specific practice to refactor context and test context refactoring

Projects

Sharpino.Lib.Core:

• Core.fs: Abstract definition of Events, Commands and Undoer (the reverse of a command to be used if storage lacks transaction between streams). Definition of EvolveUnForgivingErrors and "normal" Evolve. The former raises an error if there are some events that cannot be applied to the current state of the context. The latter just skip those events.

Sharpino.Lib:

• CommandHandler.fs: gets and stores snapshots, executes commands, and produces and stores events using the event store.
• LightCommandHandler.fs: gets and stores snapshots, executes commands, produces and stores events using an event store that supports pub/sub model ( Eventstoredb ).
• DbStorage.fs and MemoryStorage.fs: Manages persistency in Postgres or in-memory.
• Cache.fs. Cache current state.

Sharpino.Sample You need a user called 'safe' with password 'safe' in your Postgres (if you want to use Postgres as Eventstore).

It is an example of a library for managing todos with tags and categories. There are two versions in the sense of two different configurations concerning the distribution of the models (collection of entities) between the contexts. There is a strategy to test the migration between versions (contexts refactoring) that is described in the code (See: AppVersions.fs and MultiVersionsTests.fs ) .

• contexts (e.g. TodosContext) controls a partition of the collection of the entities and provides members to handle them.

• contexts members have corresponding events (e.g. TagsEvents) that are Discriminated Unions cases. Event types implement the Process interface.

• contexts are related to Commands (e.g. TagCommand) that are Discriminated Unions cases that can return lists of events by implementing the Executable interface.

Commands defines also undoers are functions that can undo the commands to reverse action in a multiple-stream operation for storage that doesn't support multiple-stream transactions (see LightCommandHandler).

• A Storage stores and retrieves events and snapshots.
• The api layer functions provide business logic involving one or more clusters by accessing their state, and by building one or more commands and sending them to the CommandHandler.
• An example of how to handle multiple versions of the application to help refactoring and migration between different versions: application versions.

Sharpino.Sample.tests

• tests for the sample application

Sharpino.Sample.Kafka

• scripts to setup a Kafka topics corresponding to the contexts of the sample application

How to use it

• You can run the sample application as a rest service by running the following command from Sharpino.Sample folder:

dotnet run

• You can run the client Fable/Elmish sample application by running the following command from the Sharpino.Sample.Client folder:

npm install
npm start

• Just use ordinary dotnet command line tools for building the solution. Particularly you can run tests of the sample application by using the following command:

dotnet test 

You can also run the tests by the following command from Sharpino.Sample.Tests folder:

dotnet run

In the latter case, you get the output from Expecto test runner (in this case the console shows eventual standard output/printf).

By default, the tests run only the in-memory implementation of the storage. You can set up the Postgres tables and db by using dbmate. In the Sharpino.Sample folder you can run the following command to set up the Postgres database:

dbmate -e up

(see the .env to set up the DATABASE_URL environment variable to connect to the Postgres database with a connection string). If you have Eventstore the standard configuration should work. (I have tested it with Eventstore 20.10.2-alpha on M2 Apple Silicon chip under Docker).

Tests on eventstoredb. EventStroreDb is not mantained that much at the moment

The following line needs to stay commented out.

        // (AppVersions.evSApp,                    AppVersions.evSApp,                 fun () -> () |> Result.Ok)

Sample application 2 is a problem of booking seats in two rows of five seats:

1. Booking seats among multiple rows (where those rows are aggregates) in an event-sourcing way.
2. Booking seats in a single row by two concurrent commands that singularly do not violate any invariant rule and yet the final state is potentially invalid.

Problem 1

I have two rows of seats related to two different streams of events. Each row has 5 seats. I want to book a seat in row 1 and a seat in row 2. I want to do this in a single transaction so that if just one of the claimed seats is already booked then the entire multiple-row transaction fails and no seats are booked at all.

Questions:

1. can you do this in a transactional way? Answer: yes because in-memory and Postgres event-store implementation as single sources of truth are transactional. The runTwoCommands in the Command Handler is transactional.
2. Can it handle more rows? up to tre. (runThreeCommands in the Command Handler)
3. Is feasible to scale to thousands of seats/hundreds of rows (even though we know that few rows will be actually involved in a single booking operation)? Not yet.
4. Is Apache Kafka integration included in this example? No.
5. Is EventStoreDb integration included in this example? Not yet (it will show the "undo" feature of commands to do rollback commands on multiple streams of events).

Problem 2

There is an invariant rule that says that no booking can end up in leaving the only middle seat free in a row. This invariant rule must be preserved even if two concurrent transactions try to book the two left seats and the two right seats independently so violating (together) this invariant.

Questions:

1. can you just use a lock on any row to solve this problem? Answer: yes by setting PessimisticLocking to true in appSettings.json that will force single-threaded execution of any command involving the same "aggregate"
2. can you solve this problem without using locks? Answer: yes. if I set PessimisticLocking to false then parallel command processing is allowed and invalid events can be stored in the eventstore. However they will be skipped by the "evolve" function anyway.
3. Where are more info about how to test this behavior? Answer: See the testList called hackingEventInStorageTest. It will simply add invalid events and show that the current state is not affected by them.
4. You also need to give timely feedback to the user. How can you achieve that if you satisfy invariants by skipping the events? Answer: you can't. The user may need to do some refresh or wait a confirmation (open a link that is not immediately generated). There is no immediate consistency in this case.

Faq:

• Why "Sharpino"?
  • It's a mix of Sharp and fino (Italian for "thin"). "sciarpino" (same pronunciation) in Italian means also "little scarf".
• Why another event-sourcing library?
  • I wanted to study the subject and it ended up in a tiny little framework.
• Why F#?
  • Any functional language from the ML family language in my opinion is a good fit for the following reasons:
    • Events are immutable, building the state of the context is a function of those events.
    • Discriminated Unions are suitable to represent events and commands.
    • The use of the lambda expression is a nice trick for the undoers (the under is returned as a lambda that retrieves the context for applying the undo and returns another lambda that actually can "undo" the command).
    • It is a .net language, so you can use everything in the .net ecosystem (including C# libraries).
• How to use it
  • add the nuget package Sharpino to your project (current version 1.4.1)
  • note: on my side, when I added Sharpino as a library into a web app, then I had to add the following line to the web app project file to avoid a false error (the error was "A function labeled with the 'EntryPointAttribute' attribute must be the last declaration")

  <GenerateProgramFile>false</GenerateProgramFile>
  

News:

• Version 1.4.8: streams of events can relate to proper aggregate identified by id (and not only context). I can run commands for an arbitrary number of aggregates of some specific type. See Sample3 (booking seats of rows where rows are aggregates of a context which is a stadium). Integration with Kafka for those "fine" aggregates identified by Id is not included in this version.

• Booking seat example: https://github.com/tonyx/seatsLockWithSharpinoExample (it shows some scalability issues that will be fixed in future releases)

• Version 1.4.7: contains sample app that builds state of contexts by using Kafka subscriber (receives and processes events to build locally the state of those contexts, despite can still access the "souce of truth", which is the db/event store, when something goes wrong in processing its state, i.e. out of sync events).

• Version 1.4.6: fix bug in Postgres AddEvents

• Version 1.4.5: Upgrade to net8.0

• Info: in adding new features I may risk breaking backward compatibility. At the moment I am handling in this simple way: if I change the signature of a function I add a new function with the new signature and I deprecate the old one. I will keep the old one for a while. I will remove it only if I am sure that nobody is using it.

• Version: 1.4.4 Postgres tables of events need a new column: kafkaoffset of type BigInt. It is used to store the offset/position of the event in the Kafka topic. See the new four last alter_ Db script in Sharpino.Sample app. This feature is Not backward compatible: You need your equivalent script to update the tables of your stream of events. (Error handling can be improved in writing/reading Kafka event info there). Those data will be used in the future to feed the "kafkaViewer" on initialization.

• From Version 1.4.1 CommandHandler changed: runCommand requires a further parameter: todoViewer of type (stateViewer: unit → Result<EventId * 'A, string>). It can be obtained by the CommandHandler module itself.getStorageStateViewera<'A, 'E> (for database event-store based state viewer. The one for Broker based is coming soon)

• new blog post

• Version 1.4.1: little change in Kafka consumer. Can use DeliveryResults to optimize tests

• Version 1.4.0: runCommand instead of Result<unit, string> returns, under result, info about event-store created IDs (Postgres based) of new events and eventually Kafka Delivery result (if Kafka is configured).

• Version 1.3.9: Repository interface changed (using Result type when it is needed). Note: the new Repository interface (and implementation) is not compatible with the one introduced in Version 1.3.8!

• Version 1.3.8: can use a new Repository type instead of lists (even though they are still implemented as plain lists at the moment) to handle collections of entities.

• Version 1.3.5: the library is split into two nuget packages: Sharpino.Core and Sharpino.Lib. the Sharpino.Core can be included in a Shared project in the Fable Remoting style. The collections of the entities used in the Sharpino.Sample are not lists anymore but use Repository data type (which at the moment uses plain lists anyway).

• Version 1.3.4 there is the possibility to choose a pessimistic lock (or not) in command processing. Needed a configuration file named appSettings.json in the root of the project with the following content:

    "SharpinoConfig": {
        "PessimisticLock": false // or true
    }

More documentation (a little bit out of date. Will fix it soon) (Sharpino gitbook)

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