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Stuff About CQRS

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This document discusses Command Query Responsibility Segregation (CQRS). It begins by explaining that CQRS splits commands and queries into separate paths at an architectural level. This aligns with how people think about tasks (commands) and requesting information (queries) differently. The document then discusses how CQRS provides an eventually consistent architecture based on CAP theorem, similar to BASE. It explains that CQRS reads data from a separate read model than where commands write to the domain model. In summary, CQRS separates commands from queries in a way that matches how people think, and provides an eventually consistent architecture.

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Stuff About CQRS Jak Charlton @JakCharlton jak@thinkddd.com
Disclaimer: This Isn’t All About Command Query Responsibility Segregation
does anyone here even know what CQRS is anyway, or were the other sessions full?
Another Disclaimer: There won’t be any code, I don’t do code, just opinions
My Objective Today
My Objective Today is to Make You Think
My Objective Today is to Make You Think “Maybe There Is A Better Way?”
Why Don’t We Talk Like “Normal” People Do?
Why Don’t We Talk Like “Normal” People Do? Developers are usually techy
Why Don’t We Talk Like “Normal” People Do? Developers are usually techy Business people usually don’t understand “techy”
Why Don’t We Talk Like “Normal” People Do? Developers are usually techy Business people usually don’t understand “techy” So why do we try to make them think like we do?
Why Does Language Matter So Much?
Why Does Language Matter So Much? Without language, we cannot convey meaning
Why Does Language Matter So Much? Without language, we cannot convey meaning But when we talk different languages, we lose in the translation
Why Does Language Matter So Much? Without language, we cannot convey meaning But when we talk different languages, we lose in the translation Let’s agree on one
How Do “Normal” People See the World?
How Do “Normal” People See the World? People don’t think in terms of “data”
How Do “Normal” People See the World? People don’t think in terms of “data” People think “tasks”
How Do “Normal” People See the World? People don’t think in terms of “data” People think “tasks” People are objective based, only computers work on data
Otherwise Known As “Grid Abuse”
Otherwise Known As “Grid Abuse” Grids are a UI problem created by developers
Otherwise Known As “Grid Abuse” Grids are a UI problem created by developers People don’t see information in columns and rows
Otherwise Known As “Grid Abuse” Grids are a UI problem created by developers People don’t see information in columns and rows Even Excel doesn’t force you to think that way
If People Don’t Think In Grids, Why Force Data Into Grids?
If People Don’t Think In Grids, Why Force Data Into Grids? Developers are ingrained with RDBMS Rows and Columns
If People Don’t Think In Grids, Why Force Data Into Grids? Developers are ingrained with RDBMS Rows and Columns Business information doesn’t come in rows and columns
If People Don’t Think In Grids, Why Force Data Into Grids? Developers are ingrained with RDBMS Rows and Columns Business information doesn’t come in rows and columns We forced them to store and use it that way
Thinking About NoSQL Databases
Thinking About NoSQL Databases If users think about things like “my documents” maybe we should just store stuff that way?
Thinking About NoSQL Databases If users think about things like “my documents” maybe we should just store stuff that way? NoSQL databases let us concentrate on what to store, not how
Quick Summary
Quick Summary People need to communicate
Quick Summary People need to communicate People think Tasks
Quick Summary People need to communicate People think Tasks People think “stuff” or “documents” or “things”
What Does This Have to Do With CQRS ???
What Does This Have to Do With CQRS ??? Not much ...
What Does This Have to Do With CQRS ??? Not much ... Yet...
What Does This Have to Do With CQRS ??? Not much ... Yet... One more step... now we get a bit techy
CAP Theory
CAP Theory Consistency: The client perceives that a set of operations has occurred all at once.
CAP Theory Consistency: The client perceives that a set of operations has occurred all at once. Availability: Every operation must terminate in an intended response.
CAP Theory Consistency: The client perceives that a set of operations has occurred all at once. Availability: Every operation must terminate in an intended response. Partition tolerance: Operations will complete, even if individual components are unavailable.
CAP Theory Consistency: The client perceives that a set of operations has occurred all at once. Availability: Every operation must terminate in an intended response. Partition tolerance: Operations will complete, even if individual components are unavailable. You cannot have all three
ACID
ACID Atomic, Consistent, Isolated, Durable
ACID Atomic, Consistent, Isolated, Durable In CAP Theory terms ACID gives you:
ACID Atomic, Consistent, Isolated, Durable In CAP Theory terms ACID gives you: Consistency and Availability
ACID Atomic, Consistent, Isolated, Durable In CAP Theory terms ACID gives you: Consistency and Availability But Sacrifices Partition Tolerance
BASE
BASE Basically Available, Soft state, Eventually consistent
BASE Basically Available, Soft state, Eventually consistent In CAP Theory BASE gives you:
BASE Basically Available, Soft state, Eventually consistent In CAP Theory BASE gives you: Availability and Partition Tolerance
BASE Basically Available, Soft state, Eventually consistent In CAP Theory BASE gives you: Availability and Partition Tolerance But Sacrifices Consistency
Command Query Responsibility Segregation
Command Query Responsibility Segregation Bertrand Meyer principle of CQS: every method should either be a command that performs an action, or a query that returns data to the caller
Command Query Responsibility Segregation Bertrand Meyer principle of CQS: every method should either be a command that performs an action, or a��query that returns data to the caller At an architectural level this means: either issue commands, or issue queries, but never both
Command Query Responsibility Segregation Bertrand Meyer principle of CQS: every method should either be a command that performs an action, or a query that returns data to the caller At an architectural level this means: either issue commands, or issue queries, but never both And, query from a separate source from your domain commands
CQRS and BASE
CQRS and BASE CQRS gives us BASE at an architectural level
CQRS and BASE CQRS gives us BASE at an architectural level We may not always be consistent
CQRS and BASE CQRS gives us BASE at an architectural level We may not always be consistent But most data isn’t consistent in reality anyway, we just think it is
CQRS and BASE CQRS gives us BASE at an architectural level We may not always be consistent But most data isn’t consistent in reality anyway, we just think it is Eventually our architecture will be consistent
CQRS in a Picture Client Command Queries Domain Event Handlers Event Handlers Publish (synchronous no bus) Persist Update Domain Persistence Read Model
CQRS in a Simpler Picture Domain Events Read Model Co Os m m DT an ds Client
What Did All That Beginning Stuff Have to Do With CQRS ???
What Did All That Beginning Stuff Have to Do With CQRS ??? The way people think matters
What Did All That Beginning Stuff Have to Do With CQRS ??? The way people think matters CQRS splits actions from queries
What Did All That Beginning Stuff Have to Do With CQRS ??? The way people think matters CQRS splits actions from queries And people think about asking people to do things or asking them for information
Benefits of CQRS
Benefits of CQRS Simplified Architecture
Benefits of CQRS Simplified Architecture Closer Alignment with Users
Benefits of CQRS Simplified Architecture Closer Alignment with Users Performance
Benefits of CQRS Simplified Architecture Closer Alignment with Users Performance Scalability
Benefits of CQRS Simplified Architecture Closer Alignment with Users Performance Scalability Eliminates a leaky abstraction
My Objective Today was to Make You Think
My Objective Today was to Make You Think “Maybe There Is A Better Way?”
My Objective Today was to Make You Think “Maybe There Is A Better Way?” I Hope I Succeeded
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Stuff About CQRS

Editor's Notes

  1. CQRS is a technical approach that solves a lot of problems But it requires something more, a better way of working So this presentation is more about how to think differently
  13. You don’t ring your bank and say “I want you to view my customer record, edit the address fields and save it to your database” You do call your bank and say “I would like to change my correspondence address”
  14. You don’t ring your bank and say “I want you to view my customer record, edit the address fields and save it to your database” You do call your bank and say “I would like to change my correspondence address”
  15. You don’t ring your bank and say “I want you to view my customer record, edit the address fields and save it to your database” You do call your bank and say “I would like to change my correspondence address”
  16. Users think of lists of information ... my tweets, my bank account transaction, my facebook wall, my auctions, my Documents This abuse extends to many areas of a business - we have abused users into talking our way, not theirs
  17. Users think of lists of information ... my tweets, my bank account transaction, my facebook wall, my auctions, my Documents This abuse extends to many areas of a business - we have abused users into talking our way, not theirs
  18. Users think of lists of information ... my tweets, my bank account transaction, my facebook wall, my auctions, my Documents This abuse extends to many areas of a business - we have abused users into talking our way, not theirs
  22. Store data in a loose fashion Only store what you need In the format you want
  23. Store data in a loose fashion Only store what you need In the format you want