Command Query Responsibility Segregation design pattern

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27
votes
6answers
3k views

Better to have 2 methods with clear meaning, or just 1 dual use method?

To simplify the interface, is it better to just not have the getBalance() method? Passing 0 to the charge(float c); will give the same result: public class Client { private float bal; float ...
14
votes
5answers
6k views

Is it bad practice for services to share a database in SOA?

I have recently been reading Hohpe and Woolf's Enterprise Integration Patterns, some of Thomas Erl's books on SOA and watching various videos and podcasts by Udi Dahan et al. on CQRS and Event Driven ...
12
votes
4answers
5k views

DDD, Saga & Event-sourcing: Can a Compensate Action simply be a delete on the event store?

I realize the above question probably raises a few 'what??'s, but let me try to explain : I'm trying to wrap my head on a couple of related concepts, basically the Saga-pattern ( http://www....
12
votes
3answers
2k views

Are Domain Objects in Domain Driven Design only supposed to be write-only?

I've been reading about Domain Driven Design for almost two years and have been cautiously been introducing some concepts in to my daily work or at least making plans for how things I do regularly ...
9
votes
3answers
2k views

I have to compromise: DRY, or Command-Query-Separation?

I was recently refactoring a method that was both a command and a query method. After separating it into a one command method and one query method, I found that there are now multiple places in the ...
8
votes
2answers
10k views

Can SQL Server and Mongo be used together?

We have a large news-oriented site that has high web traffic. The architecture is your often seen DB - Repo Layer - Services Layer - Asp.Net MVC. The problem that we've been seeing is around read ...
8
votes
2answers
4k views

Command handlers and DDD

I have an ASP.NET MVC application, that uses a query service to get data and a command service to send commands. My question is about the command part. If a request comes in, the command service uses ...
7
votes
2answers
6k views

Why separate class CommandHandler with Handle() instead of handling method in Command itself

I have a part of CQRS pattern implemented using S#arp Architecture like this: public class MyCommand { public CustomerId { get; set; } // some other fields } public class MyCommandHandler&...
7
votes
4answers
491 views

Where should an object in CQRS+ES be fully initialized: in the constructor, or when applying the first event?

There appears to be widespread agreement in the OOP community that the class constructor should not leave an object partly or even fully uninitialized. What do I mean by "initialization"? Roughly ...
7
votes
3answers
451 views

Microservices and isolated data storage models [duplicate]

I have been looking at Microservices for a while now. The concept is not new but it's communicated in a lightweight manner. So, I am very excited about this. However, there is a question that I am ...
6
votes
4answers
863 views

Command-Query-Separation and multithreading safe interfaces

I like the command query separation pattern (from OOSC / Eiffel - basically you either return a value or you change the state of the class - but not both). This makes reasoning about the class easier ...
6
votes
1answer
1k views

CQRS without using others patterns

I would like to explain CQRS to my team of developers. I just can't figure out how to explain it in the simplest way so they can implement the pattern rapidly without any others frameworks. I've read ...
6
votes
2answers
347 views

Is it necessary anymore to extract logic from API controllers?

I've been working in MVC web frameworks since its they started getting popular with RoR and ASP.NET MVC. I have always been careful to never put "business logic" on my controllers since that couples ...
5
votes
3answers
114 views

How do you deal with legacy data integrity issues when rewriting software?

I am working on a project which is a rewrite of an existing legacy software. The legacy software primarily consists of CRUD operations (create, read, update, delete) on an SQL database. Despite the ...
5
votes
1answer
2k views

CQRS + Event Sourcing: (is it correct that) Commands are generally communicated point-to-point, while Domain Events are communicated through pub/sub?

I'm basically trying to wrap my head around the concept of CQRS and related concepts. Although CQRS doesn't necessarily incorporate Messaging and Event Sourcing it seems to be a good combination (as ...
5
votes
3answers
1k views

What is the best way to scale a CQRS system?

In the previous months I have been asking a lot of questions concerning the architecture of an application I'm working on. Thanks to the answers the architectural design has changed - in fact, it has ...
5
votes
1answer
2k views

How Add/Create* commands should be handled in CQRS + Event Sourcing architecture

I want to implement my first application using CQRS pattern along with Event Sourcing. I am wondering how creation of aggregate roots should be handled properly. Let's say someone sends CreateItem ...
5
votes
2answers
430 views

CQRS: How to restore read model

It's very interesting how to restore read model in system based on CQRS. In regular mode system processes commands, creates domain events and posts them to message bus. Then another part of system (...
5
votes
1answer
191 views

How to query Aggregate Root to react to event from other AR

I want to create a little GPS tracking program. Simplified: Users can create Tracks. To make things scale Track and User are two separate AR's. Track contains an AuthorUser which it refers to by ...
5
votes
2answers
210 views

CQRS and validations

I'm starting to introduce myself in CQRS concepts, but I get stucked with the following situation: Supouse you have an entity that must have an unique name. In order to verify that, prior to create ...
5
votes
1answer
2k views

.NET CQRS Frameworks nCQRS vs Lokad.CQRS

For the last couple of weeks I've been reading up and watching any resources I can find on CQRS. I really like the concept and am keen to start delving deeper. I have only found 2 major .NET ...
5
votes
1answer
394 views

DDD CQRS - per-query and per-command authorization

Summary Should authorization in CQRS/DDD be implemented per-command/query or not? I am developing for the first time an online application using more or less strictly the DDD CQRS pattern. I bumped ...
5
votes
2answers
781 views

Inheritance, commands and event sourcing

In order not to redo things several times I wanted to factorize common stuff. For Instance, let's say we have a cow and a horse. The cow produces milk, the horse runs fast, but both eat grass. ...
4
votes
3answers
67 views

In CQRS/ES, can a command create another command?

In CQRS/ES, a command is sent from the client to the server and routed to the appropriate command handler. That command handler loads an aggregate from its repository, and calls some method on it, and ...
4
votes
1answer
675 views

Distinguishing between UI command & domain commands

I am building a WPF client application using the MVVM pattern that provides an interface on top of an existing set of business logic residing in a library which is shared with other applications. The ...
4
votes
2answers
453 views

Domain Driven Design - designing Aggregate Roots

Imagine you're designing an application for organizing Workshops. Workshop itself is quite complicated - it acts as a state machine with multiple possible states and transitions between them. We're ...
4
votes
4answers
539 views

Typed search over complex model. Possible CQRS / NoSQL solution

Given the following relational model: I need to implement a typed search for hotels, which should be able to query on name, city, country, category, hoteltype, roomprice, customtype, room custom ...
4
votes
2answers
1k views

Distributed application using RabbitMQ

I am on my way to create an application with 4 bounded context using CQRS & event sourcing. In order to make these bounded context talk to each other I was planning on using Rabbit MQ. My ...
3
votes
1answer
2k views

Is there a mature, easy-to-use CQRS framework that is not Azure-specific, not outdated, or not well adopted?

I've been doing research on .net implementations with CQRS. Either everything I've seen so far is do-it-yourself, azure-specific, outdated (i think), or not well adopted Problems: Scritchy - is not ...
3
votes
2answers
3k views

REST Service and CQRS

I am struggling with architecture on a new project. I am using the following patterns/technology. CQRS - anything going in goes through a command REST - using WebAPI MVC - asp.net mvc Angular - ...
3
votes
2answers
3k views

Domain Services vs. Factories vs. Aggregate Roots

After dealing with DDD for months now, I'm still confused about the general purposes of domain services, factories and aggregate roots in relation to each other, e.g. where they overlap in their ...
3
votes
2answers
678 views

Where to put format validation in a CQRS “stylish” domain model?

It feels right to put format validation inside the domain objects (VOs or entities) because it is the natural place for high cohesion and the domain knows best what every domain description/attribute/...
3
votes
1answer
769 views

Pushing complete notifications to client

So with cqrs, we accept that consistency is eventual. However, that doesn't mean that the user has to continually poll, or that eventual means an update has to take more then 500ms to sync. For ...
3
votes
1answer
725 views

CQRS and cache invalidation (while load balancing)

I'm slowly trying to migrate our web application to use CQRS principals. In particular, I have separated "write" and "read" interfaces into commands and queries. I have implemented a "reader" ...
3
votes
2answers
728 views

CQRS applicability when some commands need to block the UI

I am working on an app which I would dearly love to transition from a fairly traditional layered architecture to CQRS, for a number of reasons, not least fo which is that having a robust event log ...
2
votes
3answers
252 views

When using DDD and CRQS, should be exactly one event per command?

I am looking for a way to design a ddd application with convention over configuration. Say an aggregate "Client" has a command defined "FillProfile". It will logically raise an event "ProfileFilled". ...
2
votes
1answer
331 views

What is eventual consistency?

I've heard people talking about "eventual consistency" (for example in this question and this answer). What is "eventual consistency"? How does it compare to "transactional consistency"? When does it ...
2
votes
1answer
107 views

Event sourcing – finding aggegate by different property than ID

Is there any mechanism or best practise for finding aggregates by different property than aggregate ID? I'll try to explain my question on example. There is Cart aggregate (event-sourced) and ...
2
votes
3answers
306 views

CQRS and DDD terminology

Since CQRS and DDD do not imply one another I was curious what terms belong to DDD and what terms belong to CQRS. Because the two concepts are so much used together in the literature I usually mix ...
2
votes
1answer
159 views

Can “REST without PUT” support optimistic locking using ETags?

Rest without PUT models immutable, append-only (event-sourced) state changes by favouring POST-ing new reified change resources over PUT-ing updates to original. Example: making and then changing a ...
2
votes
2answers
579 views

Managing non-domain application behaviour in CQRS

Here's a scenario. I am building QueueUnderflow, a community-edited Q&A site for people who haven't yet grasped the basics of data structures. My users don't like it when people edit their posts ...
2
votes
1answer
346 views

Event Sourcing , sagas, bus and eventual consistency

I am currently learning about Event Sourcing via the book Microsoft .NET - Architecting Applications for the Enterprise. Event sourcing is, in my own words, an architectural pattern of storing "events"...
2
votes
1answer
112 views

Encapsulate one business story/use case in one class?

I don't like classes like *Service, with bunch of business methods - aka anemic model. I was thinking, instead, to have a class per use case. This was inspired (if not the same) from CQRS. Even if we ...
2
votes
2answers
334 views

Version hash to solve Event Sourcing problems

The basic examples I have seen about Event Sourcing do not deal with out of order events, clock offsets in different systems and late events from system partitions. I am wondering if more polished ...
2
votes
2answers
616 views

How does a search functionality fit in DDD with CQRS?

In Vaughn Vernon's book Implementing domain driven design and the accompanying sample application I found that he implemented a CQRS approach to the iddd_collaboration bounded context. He presents ...
2
votes
1answer
57 views

How to update an event log in case of new events

In CQRS with Event Sourcing, if the current domain lacks a feature so it has to be extended with a new command (sent from a new saga) and an event (sent by the aggregate handler of that command), how ...
2
votes
2answers
134 views

Deleting subtrees in hierarchical agreggates

I am working on a file-system-like application where I use CQRS/event-sourcing to handle the state of folders and files (creating, moving, renaming, etc). Each node (folder or file) is represented by ...
2
votes
0answers
115 views

CQRS “private” command

In CQRS, if a saga (or more precisely a process manager according to the CQRS Journey book) sends out a new command to change aggregates how can I ensure that this command is not possible to send from ...
2
votes
0answers
266 views

Entities (DDD) when using CQRS on Akka

When using Akka, CQRS style, is still there a place for Entities? Or does everything now go to Aggregates, implemented as Actors + Value Objects. I notice that most entities are written as mutable ...
1
vote
3answers
104 views

Event Sourcing and cross-context aggregate

I had this idea of different write models being used in different bounded contexts, but both being the same aggregate instance (the same events). For example consider a User aggregate that is used ...