ES/CQRS the Akka way

Application is built in CQRS (Command Query Responsibility Segregation) fashion:

• Command side
  • Akka persistence
  • Cassandra as event store
• Query side
  • Akka persistence query for event store to view store transfer
  • Slick
  • Postgres as a view store

Application will constantly read from the event store and keep query side in sync. This model has Eventual consistency which means once the command is executed the view will eventually be updated.

Dependencies: Java 8 & sbt

• Run docker-compose -f docker-compose-essential.yml up
• From the root directory run sbt run
• Go to http://localhost:9001/ to test the application

Request:

curl -X POST \
  http://localhost:9001/api/note \
  -H 'Content-Type: application/json' \
  -d '{
	"title": "Some title",
	"published": false,
	"subTitle": "Some subtitle"
}'

Response:

{
    "noteId": "af8a7f3b-67ca-45cd-87d1-44438f1d3f58",
    "correlationId": "6238f89e05c205be"
}

Request:

curl -X GET http://localhost:9001/api/query/note/af8a7f3b-67ca-45cd-87d1-44438f1d3f58

Response:

{
    "note": {
        "id": "af8a7f3b-67ca-45cd-87d1-44438f1d3f58",
        "title": "Some title",
        "subTitle": "Some subtitle",
        "published": false
    },
    "correlationId": "aa1c3e6e633ee8a5"
}

All Akka persistence events are generated from .avsc files located in src/main/resources/avro/avsc.

All events that you see changed in avsc folder needs to be versioned properly if we want to avoid breaking changes in environments.

Tricky thing to do here is to take the version that was there before and copy it to avsc-history folder.

You need to make a change in Note object which is within NoteCreated event.

1. Go to src/main/resources/avro folder and find in which .avdl file is this event modelled
2. Change the record by adding the new field WITH default value. If default value isn't added old events won't be able to reply. There is no changing of current fields or types. Only addition.
3. Execute command sbt generateAvsc
4. Notice that in src/main/resources/avro/avsc there are multiple files changed NoteAvro, NoteCreatedAvro and NoteUpdatedAvro. NoteAvro.avsc isn't really used as standalone object so we leave it as is. NoteCreatedAvro and NoteUpdatedAvro both use it internally so we need to version these events.
5. We need to copy previous versions (ones before you ran command in step 3, just use history to fetch it) and copy it into src/main/resources/avro/avsc-history in a folder with JIRA number ie EP-1234. Just copy the previous versions there. Nothing else.
6. Execute command sbt compile and boom, new version of Avro events is generated.