UPDATE your VIEW on DELETE The benefits of Event Sourcing - - PowerPoint PPT Presentation

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UPDATE your VIEW on DELETE The benefits of Event Sourcing - - PowerPoint PPT Presentation

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UPDATE your VIEW on DELETE The benefits of Event Sourcing Sebastian von Conrad - @envato - @vonconrad Sebastian von Conrad - @envato - @vonconrad Sebastian von Conrad - @envato - @vonconrad Sebastian von Conrad - @envato -

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Sebastian von Conrad - @envato - @vonconrad

UPDATE your VIEW on DELETE

The benefits of Event Sourcing

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Sebastian von Conrad - @envato - @vonconrad

  

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In the top 300 of the world’s largest websites.

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Event Sourcing.

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(With an appearance

  • f CQRS architecture.)
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What is Event Sourcing?

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What is an Event?

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This poor bugger is horrendously

  • verloaded.
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An event is a business fact…

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…that happened at a particular time.

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  • Appointment Scheduled
  • Appointment Rescheduled
  • Appointment Location Moved
  • Appointment Cancelled
  • Invitation Extended
  • Invitation Notification Sent
  • Invitation Accepted
  • Invitation Declined
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Sebastian von Conrad - @envato - @vonconrad

  • Item Version Submitted
  • Item Version Approved
  • Item Added To Cart
  • Item Removed From Cart
  • Item Licence Purchased
  • Item Support Purchased
  • Withdrawal Request Submitted
  • Withdrawal Completed
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Sebastian von Conrad - @envato - @vonconrad

Event Sourcing, in this context.

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Standard practice is store the current state

  • f an object in a

database using an ORM.

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When state changes, the DB representation changes.

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John wants to take his partner’s last name.

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ORM

#<User id: 216, first_name: “John”, last_name: “Reed” ...> + --- + ---------- + --------- + --- + | id | first_name | last_name | ... | | --- | ---------- | --------- | --- | | 216 | John | Reed | ... | | ... | ... | ... | ... | + --- + ---------- + --------- + --- +

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ORM

@user.update(last_name: “Hill”) + --- + ---------- + --------- + --- + | id | first_name | last_name | ... | | --- | ---------- | --------- | --- | | 216 | John | Hill | ... | | ... | ... | ... | ... | + --- + ---------- + --------- + --- +

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Event Sourcing doesn’t do that.

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An append-only set

  • f immutable events

as source of truth.

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Derive everything else from the events.

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Source the current state by replaying events.

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{ user_id: 216, event_type: “signed_up”, body: { first_name: “John”, last_name: “Reed” } } #<User id: 216, first_name: “John”, last_name: “Reed”, ...>

Event Sourcing

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@user.change_name(last_name: “Hill”) { user_id: 216, event_type: “name_changed”, body: { last_name: “Hill” } }

Event Sourcing

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{ event_type: “sign_up”, body: { first_name: “John”, last_name: “Reed” } }, { event_type: “name_changed”, body: { last_name: “Hill” } } #<User id: 216, first_name: “John”, last_name: “Hill”, ...>

Event Sourcing

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Make everything else completely disposable.

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Including current state.

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Language agnostic.

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Why Event Sourcing?

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DELETE is evil.

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Every UPDATE is a DELETE.

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…so UPDATE is evil too.

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Business concepts at the heart of the system.

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It’s tried and tested. (For centuries.)

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You’re using it already and would refuse to do it any

  • ther way.
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Current state is hard to distribute.

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Append-only streams

  • f events are easy

to distribute.

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Last but not least: free time machine!

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CQRS

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CQS: methods can read (queries) or write (commands) but not both.

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CQRS: objects have queries or commands but not both.

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In our world we take it one step further.

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Client Command Handler Query Handler Event Store

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Client Command Handler Query Handler Event Store

Command Event

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Commands represent user intent.

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Commands can be rejected.

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Events are created when commands are accepted.

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Client Command Handler Query Handler Event Store

<= Queries => DTOs Projections

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Projectors process Events in order.

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Projectors maintain denormalised Projections.

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Projectors and Projections are 1:1.

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One Projection per screen/endpoint.

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Query Handlers query projections.

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Query Handlers return DTOs to clients.

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Client Command Handler Query Handler Event Store

<= Queries => DTOs (Sync) Projections (Async) Command (Sync) Event (Sync)

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Eventual Consistency.

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(That thing we’re not supposed to talk about.)

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The world is eventually consistent.

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Is a nanosecond okay? What about a month?

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Risk is always a function of time.

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Downstream Event Processors (aka DEPs).

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Sebastian von Conrad - @envato - @vonconrad Client Command Handler Query Handler Event Store

DEPs

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DEPs

Processes Events Emits Events External Service Triggers Behaviour

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DEPs process events like projectors.

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Can react by emitting events back to the Event stream.

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Can react by triggering external behaviour.

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Encourages clean separation of concerns.

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Case study.

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Country Conflicts.

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Business problem?

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Detect and investigate conflicting information regarding a user’s physical location.

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Sebastian von Conrad - @envato - @vonconrad Product A Product B Single Sign-On

Country Conflicts

Evidence Evidence Evidence Product A Product C Conflicts Conflicts

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Gather evidence and store as events.

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DEP looks at evidence and raises ConflictDetected.

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(Another) DEP sees conflict event and emails a notification.

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(Another) DEP might auto-resolve the conflict based on more evidence.

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Sebastian von Conrad - @envato - @vonconrad Product A Product B Single Sign-On

Country Conflicts

Evidence Evidence Evidence Product A Product C Conflicts Conflicts Admin UI

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Admins issue Commands to manually resolve conflicts.

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After 30 days, GracePeriodExpired is raised.

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DEP checks whether it is. If not, raises ConflictActivated.

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  • Evidence Provided
  • Conflict Detected
  • Conflict Notification Email Sent
  • Conflict Automatically Resolved
  • Conflict Manually Resolved
  • Conflict Grace Period Expired
  • Conflict Activated
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Other systems are querying conflicts through projections.

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Small, well-defined, and simple.

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Other systems are substantially larger.

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So why this CQRS architecture?

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Encourages Single Responsibilities.

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Command and Query Handlers can scale independently.

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Writes are fast.

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Reads are faster.

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Projections can be thrown away when no longer needed.

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Separating recording from interpreting what happened.

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Limit blast radius

  • f changes.
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Reduces fear and enables rapid change.

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Keep the cost of change lower for longer.

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The heart of a system is far more stable than the edges.

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Should you use it?

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Well, maybe.

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Not appropriate for every problem.

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Works better for read-heavy systems.

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Well suited for commerce-oriented domains, for example.

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Or anywhere where history is important.

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Competitive advantage Complexity

ES? Buy WTF KISS Strategic Design*

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It’s a great tool to have in your toolbox.

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Sebastian von Conrad - @envato - @vonconrad

Thank you. Questions?