Buildıng a tıme machine with event sourcıng Alper Hankendi @alper_hankendi github.com/alperhankendi

Agenda ➔ Getting Started Event Sourcing ➔ History of Event Sourcing ➔ Building Aggregates ➔ Projections ➔ Rolling Snapshot ➔ Related Concepts ➔ Evolution of Event Sourcing

Once upon a time...

Scalıng up...

Let’s go asynchronous

COMMAND SOURCING

COMMANDs >>> Event sourcing

COMMANDS vs events Command “Create Account” —> A request (imperative sentence) —> May fail —> May affect multiple Aggregates Rebuild Aggregate State From Commands

COMMANDS vs events Event “Account Created” —> Statement of fact (past tense) —> Never fails —> May affect a single Aggregates —> Events are immutable Rebuild Aggregate State From Events

What about reads?

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CQRS and Event sourcıng

Projections

What about event storage?

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Column Types Partition Keys Clustering Columns Columns Event Table RowId text AggregateId text Event Time timespam Event Data blob Event Mapper string RowId AggregateId EventTime EventData EventMapper b7913401-f237-4378-b400-4597d26838fd ABC-001 2019 May 3 00:25:07 { "Owner": "Alper Hankendi", "Name": "TL" } namespace.AccountCreated 220e344d-d16b-4a45-b9c6-71d023320a29 DEF-001 2019 May 5 00:45:15 { "Amount": 233.0, "Description": "" } namespace.Deposited

Event sourcing “Capture all changes to an application state as a sequence of events. “ M.Fowler

Advantages of event sourcing —> Split logic from events —> Stable record of events, rather than just the final state —> Change logic and re-apply events —> Emit events we don’t need —> Add Logic to them after the fact (events..) —> audit free, yey!

Event Sourcıng..use cases —> When the mode is complex —> Going to need additional behavior over time —> Need to re-run according to new rules Payroll , accounting, money/audit, banks and… “any state machine based business login”

More concepts...

A couple of concepts from ddd

Interactıon wıth aggregates

Commands Events Projections

EVENT STORE Bank Account Created Deposited Withdrawn Change Owner Deposited

EVENT STORE UPS! The bank says “Where ıs my commısıon ffs”? Bank Account Created Deposited Withdrawn Change Owner Deposited You can’t delete events...

EVENT STORE UPS! The bank says “Where ıs my commısıon ffs”? Bank Account Created Deposited Withdrawn Change Owner Deposited you can fıx the past wıth adding a new event WithdrewFix

How do I replay tons of events ? The problem is that there may be a large number of events between the beginning of time and the current point. You can imagine that an event stream with a million or many more events would be inefficient to load.

The answer is “rollıng snapshot” Snapshot is a projection of the current state of an aggregate at a given point. It presents the desired state when all events to that point have been replayed.

Gettıng and Savıng aggregates

Evolutıon of ddd -- cqrs & event sourcıng -- event storming

A couple of concepts from event stormıng

Story Challenges —> Migrating product review data from Legacy system —> Big bang is forbidden —> Mobile Application needs backward compatibility —> Scalability (80M visit on product page, 1K command per second) —> Eventual Consistency —> Business Validation on command side

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FIN @alper_hankendi github.com/alperhankendi source code https://github.com/alperhankendi/event-sourcing-demo ps: code puzzling in readme file, fork the code and crack it!