README.adoc

CH-22 Event Sourcing

Table of Contents

• RavenDB
  • Screenshots
  • Result Run
• SQLite
  • Problems
  • Schema
  • Screenshots
  • Result Run
• EventStoreDB
  • Problems
  • Screenshots
  • Result Run
• Running Queries on EventStoreDB
  • Problems
  • Screenshots
• ToDon’t List

RavenDB

I ran RavenDB from docker. I need to manually create a database.

link:assets/docker-compose.yaml[role=include]

Screenshots

Screenshots from the application run.

Figure 1. stream was created

Figure 2. AccountCreated event

Figure 3. PhoneCallCharged event (first time)

Figure 4. CreditSatisfiesFreeCallAllowanceOffer event

Figure 5. CreditAdded event

Figure 6. PhoneCallCharged event (second time)

Figure 7. all events after the application run

Result Run

See ESRavenProgram

application logs

Create account # this leads to AccountCreated event

Record call # this leads to PhoneCallCharged event

Top up credit # this leads to CreditSatisfiesFreeCallAllowanceOffer and  CreditAdded events

Press any key to continue...
Record call # this leads to PhoneCallCharged event

SQLite

Problems

Jackson LocalDateTime support

The exception:

Caused by: com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Java 8 date/time type `java.time.LocalDateTime` not supported by default: add Module "com.fasterxml.jackson.datatype:jackson-datatype-jsr310" to enable handling (through reference chain: sqliteeventstore.model.PayAsYouGo.PhoneCallCharged["phoneCall"]->sqliteeventstore.model.PayAsYouGo.PhoneCall["callStart"])

Fixed by registering the module.

objectMapper.registerModule(new JavaTimeModule());

Jackson tries tp deserialize Domain methods

In PhoneNumber, Jackson tried to deserialize boolean checks e.g. isUKLandlineOrMobile. However, it’s not a field. Adding JsonIgnore solves the problem. But it brings a bit of infrastructure into the model.

Jackson serialization/deserialization

Jackson needs no-arg constructors, getters and setters. Which makes classes a bit of boilerplate-ish.

Maintaining versions

RavenDB works as UnitOfWork and tracks changes of aggregates. Here I don’t use UnitOfWork pattern, that’s why I "manually" update event_sources table. See EventStore.appendEventsToStream.

Schema

I use SQLite, the schema is below.

link:assets/sqlite/schema.sql[role=include]

Screenshots

Screenshots from the application run. There are almost the same as for RavenDB. I keep them just to demonstrate that the application works.

Figure 8. stream was created

Figure 9. AccountCreated event

Figure 10. PhoneCallCharged event (first time)

Figure 11. CreditSatisfiesFreeCallAllowanceOffer and CreditAdded events

Figure 12. PhoneCallCharged event (second time)

Figure 13. stream version after the application run

Result Run

See EsSqliteProgram

application logs

Create account # this leads to AccountCreated event

Record call # this leads to PhoneCallCharged event

Top up credit # this leads to CreditSatisfiesFreeCallAllowanceOffer and  CreditAdded events

Press any key to continue...
Record call # this leads to PhoneCallCharged event

EventStoreDB

By default EventStoreDB is non-blocking. Each call returns CompletableFuture. I used "blocking" calls. To achieve that, I added a helper method that waits for completions. See EventStoreD.await.

I run EventStoreDB from docker.

link:assets/docker-compose.yaml[role=include]

Problems

Integer Overloading

In EventStore.getStream, we calculate amount. It can lead to integer overload.

@Override public List<DomainEvent> getStream(String streamName, int fromVersion, int toVersion) {
  // if fromVersion = 0, toVersion = Integer.MAX_VALUE
  // we have interger overload
  var amount = (toVersion - fromVersion) + 1;
}

Getting the end of a stream

ReadStreamOptions.get().fromEnd() leads to the exception below.

exception

java.lang.RuntimeException: java.util.concurrent.ExecutionException: io.grpc.StatusRuntimeException: UNKNOWN: Unexpected ReadStreamResult: Error

ReadStreamOptions.get().fromEnd().backwards() works.

Different ObjectMappers

EventStoreDB creates ObjectMapper ad hoc, that’s why I can’t even register a module.

com.eventstore.dbclient.EventDataBuilder

public static <A> EventDataBuilder json(UUID id, String eventType, A eventData) {
    try {
        JsonMapper mapper = new JsonMapper(); // <--- HERE
        return json(id, eventType, mapper.writeValueAsBytes(eventData));
    } catch (JsonProcessingException e) {
        throw new RuntimeException(e);
    }
}

E.g. I can’t use LocalDateTime, because this mapper doesn’t have jackson-datatype-jsr310 module. And I can’t add it.

I can’t register module

Caused by: com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Java 8 date/time type `java.time.LocalDateTime` not supported by default: add Module "com.fasterxml.jackson.datatype:jackson-datatype-jsr310" to enable handling (through reference chain: sqliteeventstore.model.PayAsYouGo.PhoneCallCharged["phoneCall"]->sqliteeventstore.model.PayAsYouGo.PhoneCall["callStart"])

I switched LocalDateTime to String.

When we save, we serialize data with ObjectMapper, that "embedded" into EventStoreDB. When we load, we deserialize data with own ObjectMapper. Data is serialized and deserialized by different ObjectMapper. It’s not good.

Screenshots

We start from a clean EventStoreDB.

Figure 14. clean eventstore

Then we create a steam and the first event

Figure 15. stream created

Figure 16. AccountCreated event

Figure 17. AccountCreated event data

Figure 18. PhoneCallCharged event

Figure 19. PhoneCallCharged event data

Top up account leads to two events: CreditSatisfiesFreeCallAllowanceOffer and CreditAdded.

Figure 20. top up account

Figure 21. CreditSatisfiesFreeCallAllowanceOffer event data

Figure 22. CreditAdded event data

The last phone call

Figure 23. PhoneCallCharged event

Figure 24. PhoneCallCharged event data

Result Run

See ESDBProgram. But in logs, there is nothing interesting. Most interesting is screenshots.

application logs

Create account

Record call

Top up credit

Press any key to continue...
Record call

Running Queries on EventStoreDB

I do it on empty EventStoreDB. To populate data in EventStoreDB, run ImportTestData.

Problems

Money Calculations

I was getting MoreThanTwoDecimalPlacesInMoneyValueException while importing data. I think it’s because numbers in ImportTestData and because of usage of double. To save time, I deleted this check.

Inheritance

FreePhoneCallCosting extends PhoneCallCosting. PhoneCallCosting was a record but to make this inheritance I converted it to class.

Querying categories

Queries fromCategory didn’t return anything at all. I found a similar problem on StackOverflow. I went to: Projections → $by_category → Press "Start". It helped.

Emit is not allowed

It wasn’t clear how to create projections. I was getting the

'emit' is not allowed by the projection/configuration/mode

error message many times. Eventually I figured out. See screenshots in the chapters below.

Screenshots

Figure 25. populate data

Total minutes for a Day for a Customer

query

fromStream('{PayAsYouGoAccount}-{a1af9c2612e248529c5ea73ab196e46d}')
.when({
    // initialize the state
    $init : function(s,e) {
        return {
            minutes : 0
        }
    },
    "eventstoredb.model.PayAsYouGo.PhoneCallCharged" : function(s,e) {
        var dateOfCall = e.data.phoneCall.callStart;
        var june4th = '2014-06-04';
        if (dateOfCall.substring(0, 10) === june4th) {
            s.minutes += e.data.phoneCall.minutes.number;
        }
    }
 });

Figure 26. result

Minutes per Day for a Customer

query

fromStream('{PayAsYouGoAccount}-{a9b540ef309d49fd987bb05cb31634a6}')
.when({
        // initialize the state
        $init : function(s,e) {
            return {
                june3rd: 0,
                june4th: 0,
                june5th: 0
            }
        },
         "eventstoredb.model.PayAsYouGo.PhoneCallCharged" : function(s,e) {

            const dateOfCall = e.data.phoneCall.callStart;
            const june3rd = '2014-06-03';
            const june4th = '2014-06-04';
            const june5th = '2014-06-05';

            if (dateOfCall.substring(0, 10) == june3rd) {
                s.june3rd += e.data.phoneCall.minutes.number
            }
            if (dateOfCall.substring(0, 10) == june4th) {
                s.june4th += e.data.phoneCall.minutes.number
            }
            if (dateOfCall.substring(0, 10) == june5th) {
                s.june5th += e.data.phoneCall.minutes.number
            }
        }
 // handle other types of event and update the state accordingly
});

Figure 27. result

Minutes per Day for All Customers

query

fromCategory('{PayAsYouGoAccount}')
.when({
    // initialize the state
    $init : function(s,e) {
        return {
            june3rd: 0,
            june4th: 0,
            june5th: 0
        }
    },
    "eventstoredb.model.PayAsYouGo.PhoneCallCharged" : function(s,e) {
        const dateOfCall = e.data.phoneCall.callStart;
        const june3rd = '2014-06-03';
        const june4th = '2014-06-04';
        const june5th = '2014-06-05';

        if (dateOfCall.substring(0, 10) == june3rd) {
            s.june3rd += e.data.phoneCall.minutes.number;
        }
        if (dateOfCall.substring(0, 10) == june4th) {
            s.june4th += e.data.phoneCall.minutes.number;
        }
        if (dateOfCall.substring(0, 10) == june5th) {
            s.june5th += e.data.phoneCall.minutes.number;
        }
    }
// handle other types of event and update the state accordingly
});

Figure 28. result

Create a Projection

Go to Projections → New Projection.

projection query

fromCategory('{PayAsYouGoAccount}')
.when({
    "eventstoredb.model.PayAsYouGo.CreditAdded": function(s, event) {
        linkTo('AllTopUps', event);
    }
});

Figure 29. create projection UI

After pressing "Create", you will be redirected to projection details page.

Figure 30. projection details

you can also see the projection in streams

Figure 31. our projection

we can explore the projection.

Figure 32. projection is a stream

Figure 33. projection event data

And we can query projections

query from projection

fromStream('$projections-AllTopUps')
.when({
     $init : function(state, event) {
        return {
            count: 0
        };
    },
    $any: function(state, event) {
        state.count += 1;
    }
});

Figure 34. result

ToDon’t List

• ❏ RavenDB: write tests for snapshot logic

• ❏ SQLite: write tests for snapshot logic

• ❏ SQLite: add UnitOfWork

• ❏ SQLite: add transactions

• ❏ Try MongoDB as eventstore

• ❏ Try Kafka as eventstore

• ❏ Try MartenDB as eventstore