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CQRS Lite and Otherwise

george mauer - http://georgemauer.net

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I’m George

I’m Director of Development at SURGE Consulting

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I’m George

I’m Director of Development at SURGE Consulting

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CQS

It’s a

Bertrand Russel thing so you know it’s

right

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Separating Read and Write Objects is the Core Concept

CustomerService

void MakeCustomerPreferred(CustomerId)

Customer GetCustomer(CustomerId)

CustomerSet GetCustomersWithName(Name)

CustomerSet GetPreferredCustomers()

void ChangeCustomerLocale(CustomerId, NewLocale)

void CreateCustomer(Customer)

void EditCustomerDetails(CustomerDetails)

CustomerWriteService

void MakeCustomerPreferred(CustomerId)

void ChangeCustomerLocale(CustomerId, NewLocale)

void CreateCustomer(Customer)

void EditCustomerDetails(CustomerDetails)

​

CustomerReadService

Customer GetCustomer(CustomerId)

CustomerSet GetCustomersWithName(Name)

CustomerSet GetPreferredCustomers()

http://codebetter.com/gregyoung/2010/02/16/cqrs-task-based-uis-event-sourcing-agh/

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CQRS Evolved From DDD

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Udi Dahan

Greg Young

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CQRS a la Greg Young

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Guidance from Udi Dahan

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So how does The Mob define it?

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Task-based UI

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Message Bus

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Explicit Commands

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Explicit Events

And an audit log

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Event Sourcing

There is no database

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Split Read/Write Data Models

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Partitions & Multiple Processes

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Eventual Consistency & SLAs

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It’s Just Weird!

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CQRS Lite

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Which of these can we cut?

  • Task-based UI
  • Message Bus
  • Explicit Commands
  • Explicit Events / Audit Log
  • Event Sourcing
  • Read/Write Models
  • Eventual Consistency
  • Partition Tolerance

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Which of these can we cut?

  • Task-based UI
  • Message Bus
  • Explicit Commands
  • Explicit Events / Audit Log
  • Event Sourcing
  • Read/Write Models
  • Eventual Consistency
  • Partition Tolerance

Wishful Thinking�Simplify�OK�OK�Later�Conceptually, Yes. As Needed

Introduce as needed�Much Later

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Start with standard

n-tier MVC architecture

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Write ViewModels are Commands

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Write actions delegate to applicators and manage transactions

[HttpPost, Route("")]� public async Task<dynamic> Create(CreateTimeEntryCommand cmd) {� var id = await timeEntryModification.Apply(cmd);� await Db.SaveChangesAsync();� cmd.LogSuccess();� return Created($"api/time-entry/{id}", id);� }

public class TimeEntryModification :� IApplyCommand<DeleteTimeEntryCommand>,� IApplyCommand<EditTimeEntryCommand>,� IApplyCommand<AdminOverrideEditTimeEntryCommand>,� IApplyCommand<CreateTimeEntryCommand, Guid> {�� public async Task<Guid> Apply(CreateTimeEntryCommand cmd) {� // ..� return id;� }��}

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BONUS IDEA: Applicators can access internal state

Read-only properties enforce the pattern

Applicators are internal to entities

​

Probably not necessary for the pattern

¯\_(ツ)_/¯

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Applicators publish Events

which can be subscribed to

public async Task<Guid> Apply(LockPeriodCommand cmd) {� var tp = new TimePeriod(cmd.Start, cmd.End);� // yes, this is db-inefficient, but this is not a huge dataset and an infrequent operation� if ( (await db.LockedPeriods.ToListAsync()).Any(x => tp.OverlapsWith(x.TimePeriod())) )� throw new InvalidOperationException("Locked time periods cannot overlap");� var lp = new LockedPeriod(tp);� db.LockedPeriods.Add(lp);� await messages.Dispatch(new LockPeriodCommand.Occurred {

LockedPeriodId = lp.Id, TimePeriod = lp.TimePeriod() });� return lp.Id;� }

public class Adjustment :� IHandleEvent<LockPeriodCommand.Occurred> {�� public async Task Apply(LockPeriodCommand.Occurred ev) =>� await listItemsStore.Update(li => � li.IsUnlocked = false, await listItems.DataForAnyUser(queryModel(ev.TimePeriod, true))� );�}

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Just Implement your own �Message Bus

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Queries can be handled by a read service (can be a controller)

and are as simple as possible

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and use custom stores as needed

​

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To Summarize

  • Write models should be Commands
  • Each command handled in dedicated class
  • Successful command publishes an event
  • Use a simple in-process message bus
  • Events can be subscribed to
  • Reads are directly in controllers
  • And as simple as possible - use custom stores!

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Enable gradual transition

The goal:

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Questions?

gmauer@gmail.com

@togakangaroo

http://georgemauer.net

Slides:

http://bit.ly/2rBaixy