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3 days of afterparty

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Hello Wroclaw.rb

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Fist time in Poland

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What I know now

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Pierogi

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Pelmeni -> Pierogi z mięsem Varenyky -> Pierogi ruskie

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Best state machine implementation

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class State def method_missing(method, *args, &block) eval(method.to_s.capitalize).send(:new) end end class Created < State; end class Open < State; end class Closed < State; end

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Event Sourcing is popular

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Memes is not

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Autoload is still

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Awesome people and community

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Anton Davydov github.com/davydovanton
 twitter.com/anton_davydov davydovanton.com

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• Software developer at Hippo • OpenSource evangelist • Hanami core developer • Try to make own product • Technical consulting

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Stickers

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• Coffee • Beer • Psychology • SW-2039-0208-4228 • How to draw images for presentations • Bad stories

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• Coffee • Beer • Psychology • SW-2039-0208-4228 • How to draw images for presentations • Bad stories

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Tried to be writer and make a story

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expectations

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reality

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But I got help from my friend

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Meet George

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He is quiet and love ask questions

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Events

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Git

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Each commit - something which was happened

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Each commit contain description and changes (payload)

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What happened

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Payload

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List of cart items which user deleted and added

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Activity audit

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Don’t delete deleted data

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deleted_at

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But usually we’re working with current state of application

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actions -> events data in DB -> current state

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actions -> events data in DB -> current state

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Knok knok ▼ George

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… ▼ George

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What will happen if we change it? ▼ George

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Store events ▼ George

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And calculate current state when we need it ▼ George

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Event Sourcing

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Store what was happened instead current state

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And get state when you need

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Where to store?

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Event storage

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Can be anything

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Rules

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Event was happened

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Immutable events

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Can’t delete or update event

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Only valid data in the events

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Data evolution

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Stop ▼ George

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We can break the app! ▼ George

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>_< ▼ George

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Pro tips: optional fields, binary data structures

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more in the book

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How to get state from event store?

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We need to calculate it using current event list

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Projections

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project(projection, events, init_state) -> state

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project(projection, events, init_state) -> state same as a block in ruby’s enumerator

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project(projection, events, init_state) -> state pure function

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We’re compile current state for all events each time

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… ▼ George

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We can’t store all data for last 10 years ▼ George

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And will have a performance issues for getting current state ▼ George

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How to fix it?

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Streams!

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Event bus#1 Event bus#1 Event bus#2 Event bus#1 Event bus#2 Event bus#2

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Event stream: bus#1 Event stream: bus#1 Event stream: bus#2 Event stream: bus#1 Event stream: bus#2 Event stream: bus#2

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Stream: bus#1
 Event stream: bus#1 Event stream: bus#1 Event stream: bus#1

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Stream: bus#2
 Event stream: bus#2 Event stream: bus#2 Event stream: bus#2

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Pros: less events for processing

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Cons: one more abstraction

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Add snapshot for the current data state!

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project(function, events, state) -> state

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project(function, events, state) -> state {}

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project(function, events, state) -> state { users: […] }

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You can use any DB as a cache

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Idea: use more that one DB for cache

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Relation DB for search and index page

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Document oriented DB for show page

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Real world

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Where it can be useful

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E-comerse systems

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Order and checkout flow

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Something where you need version control (wiki, docs, group editing)

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Domain depends on events (tracking systems)

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redux? ▼ George

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Event sourcing and event driven architecture are different things

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And they use events in different ways

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ES -> event happened and I store it ED -> event happened and I say everyone it

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Blockchain! ▼ George

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pros

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Easier to communicate with domain experts

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• Logging out from box • Time traveling • Restore a state of system

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• You don’t work with tables you work with events and changes • Experemental data structures • Easy to change database implementations

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cons

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• Hard for understand idea and complicated abstraction • Not popular in ruby • Developers need deprogramming

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• Harder to get state than CRUD • Hard to understand the whole chain of events • Another architecture type
 with different DB structure

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• Versions and versions compatibility • Updating or deleting events • Eventual Consistency

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Advanced topics

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How to display created data ASAP?

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Solution 1

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cheat user display real data

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solution 2

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WebSockets and Long Polling

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How to make right events order?

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distributed systems

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more here

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And here

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How to delete user related data

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Retroactive Event

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Lose encrypted data key

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How to use ES in ruby

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Simple way

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gems

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github.com/zilverline/sequent

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eventide-project.org

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Brave way

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Build all stuff by self

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hanami-events

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Event driven transport layer with event versions and types

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Connection without global state

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Hanami::Events.new(:memory_sync) Hanami::Events.new(:memory_async) Hanami::Events.new(:redis)

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Hanami::Events::Adapter.register(:google_pubsub) do Google::PubSub end Hanami::Events.new(:google_pubsub) # => event instance with your google pubsub adapter

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events = Hanami::Events.new(:memory_sync) events.broadcast('user.created', user: user) Broadcaster

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events.subscribe('user.created') do |payload| puts payload end events.subscribe('user.*') do |payload| puts payload end events.subscribe(/\Auser\..*/) do |payload| puts payload end Subscriber

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Event sourcing looks cool. ▼ George

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But I have legacy app. How can I migrate? ▼ George

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User add something to cart

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module Web::Controllers::CartItem class Create include Web::Action def call(params) halt(400) unless params.valid? if cart_repo.item_exist?(params[:item]) item = CartItemRepository.new.create_copy(params[:item]) else item = CartItemRepository.new.create(params[:item]) end analytics.call('Item Added') redirect_to routes.order_path(params[:order_id]) end end end

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Step 1: Event Storming

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Why? Detect important business events in the system

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Orders::Events::ItemAdded Orders::Events::CoppyItemAdded

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Step 2: store (apply) event

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module Web::Controllers::CartItem class Create include Web::Action def call(params) halt(400) unless params.valid? event_store.apply(Order::Events::ItemAdded.new(payload)) if cart_repo.item_exist?(params[:item]) item = CartItemRepository.new.create_copy(params[:item]) else item = CartItemRepository.new.create(params[:item]) end analytics.call('Item Added') redirect_to routes.order_path(params[:order_id]) end end end

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How to validate item?

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module Web::Controllers::CartItem class Create include Web::Action def call(params) halt(400) unless params.valid? event_store.apply(Orders::Events::ItemAdded.new(payload)) if cart_repo.item_exist?(params[:item]) item = CartItemRepository.new.create_copy(params[:item]) else item = CartItemRepository.new.create(params[:item]) end analytics.call('Item Added') redirect_to routes.order_path(params[:order_id]) end end end

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Step 3: calculate state

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module Web::Controllers::CartItem class Create include Web::Action def call(params) halt(400) unless params.valid? events = event_store.get_stream(params[:order_id]) items = @project.call(Order::Projections::ItemList, events) if items.include?(params[:item]) event_store.apply(Orders::Events::ItemAdded.new(payload)) else event_store.apply(Orders::Events::CoppyItemAdded.new(payload)) end analytics.call('Item Added') redirect_to routes.order_path(params[:order_id]) end end end

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module Web::Controllers::CartItem class Create include Web::Action def call(params) halt(400) unless params.valid? events = event_store.get_stream(params[:order_id]) items = @project.call(Order::Projections::ItemList, events, {}) if items.include?(params[:item]) event_store.apply(Orders::Events::ItemAdded.new(payload)) else event_store.apply(Orders::Events::CoppyItemAdded.new(payload)) end analytics.call('Item Added') redirect_to routes.order_path(params[:order_id]) end end end

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module Projections class AllTask def call(event, state) case event when Orders::Events::ItemAdded state[:orders] ||= [] state[:orders] << event.payload when Orders::Events::ItemRemoved # ... end state end end end

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module Projections class AllTask def call(event, state) case event when Orders::Events::ItemAdded state[:orders] ||= [] state[:orders] << event.payload when Orders::Events::ItemRemoved # ... end state end end end

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module Projections class AllTask def call(event, state) case event when Orders::Events::ItemAdded state[:orders] ||= [] state[:orders] << event.payload when Orders::Events::ItemRemoved # ... end state end end end

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module Web::Controllers::CartItem class Create include Web::Action def call(params) halt(400) unless params.valid? events = event_store.get_stream(params[:order_id]) items = @project.call(Order::Projections::ItemList, events, {}) if items.include?(params[:item]) event_store.apply(Orders::Events::ItemAdded.new(payload)) else event_store.apply(Orders::Events::CoppyItemAdded.new(payload)) end analytics.call('Item Added') redirect_to routes.order_path(params[:order_id]) end end end

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Step 4: subscribers

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# analytics subscribers event_store.subscribe(Orders::Events::ItemAdded) do |event| analytics.call('Item removed', event.payload) end event_store.subscribe(Orders::Events::CoppyItemAdded) do |event| analytics.call(‘Copy of item added’, event.payload) end event_store.subscribe(Orders::Events::ItemRemoved) do |event| analytics.call(‘Item removed', event.payload) end

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module Web::Controllers::CartItem class Create include Web::Action def call(params) halt(400) unless params.valid? events = event_store.get_stream(params[:order_id]) items = @project.call(Order::Projections::ItemList, events, {}) if items.include?(params[:item]) event_store.apply(Orders::Events::ItemAdded.new(payload)) else event_store.apply(Orders::Events::CoppyItemAdded.new(payload)) end redirect_to routes.order_path(params[:order_id]) end end end

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Step 4: move logic to domain

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PROFIT

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pro tip: you can use ES only as a part of your system

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What next?

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SAGA PATTERN

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Long business transactions

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CQRS
 
 (Command Query Responsibility Segregation)

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– Martin Fowler “You can use a different model to update information than the model you use to read information”

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Looking outside ruby world

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github.com/heynickc/ awesome-ddd

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github.com/davydovanton/event_sourcing_ruby

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DDD

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Again? ▼ George

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Detect and store business events

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Split system by domains

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Distributed systems

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more here

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Conclusions

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Sometimes event base architecture win

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Sometimes not

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EventSourcing is simple as conception

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But introduce a lot of practical problems
 (data evolution, event order, etc)

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ES expensive

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It’s not about distributed systems and communication across services

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ES will be a trend in the future

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Do you don’t trust?

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ES will be a trend in the future

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twitter.com/anton_davydov github.com/davydovanton [email protected] davydovanton.com Thank you ❤