Command Query Responsibility Segregation design pattern

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23
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6answers
2k 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 ...
1
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3answers
77 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 ...
5
votes
3answers
188 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 ...
-1
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2answers
43 views

“Separate Query from Modifier” and return values [closed]

I've heard it said that you should keep query and modifier functions separate from one another, and I agree with the sentiment. When trying to apply it, I often run into two issues/questions: What, ...
2
votes
3answers
140 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". ...
1
vote
3answers
150 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
109 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 ...
1
vote
1answer
44 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 ...
1
vote
0answers
84 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 ...
5
votes
1answer
197 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 ...
4
votes
1answer
109 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 ...
2
votes
2answers
83 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
1answer
175 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 ...
5
votes
2answers
241 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 ...
2
votes
1answer
73 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 ...
0
votes
4answers
302 views

CQRS: Query write model

Context: User wants to edit entity details, say user name. Assuming the fact that the Read Model is eventually consistent with the Write Model is it conceptually wrong to query write model in order ...
1
vote
1answer
62 views

Loading Aggregate Root from Database with Validations

Here is a similar question Where to validate domain model rules that depend on database content? I am asking this new question because I have more descriptions and I don't want to change the ...
6
votes
2answers
310 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 ...
3
votes
2answers
385 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 ...
1
vote
2answers
68 views

What layer generates commands for dependent services?

Our business works with truck drivers making pickups/deliveries of containers. The location of containers needs to be tracked. The drivers use mobile devices to generate a "DriverReport" (a log of ...
1
vote
1answer
349 views

How shall I project CQRS command and query services through WCF

I got directed to this site for more conceptual questions. Here is my issue: Im designing an application that uses CQRS to expose functionality to higher levels in the stack. This looks like below ...
1
vote
1answer
62 views

How to model parallel processes (in a Bounded Context) with the same data store?

The situation. Let's say a background process puts stuff into data containers. It runs all the time, observes system events, and populates the data accordingly. The user may launch a control panel ...
1
vote
1answer
251 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
108 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 ...
1
vote
1answer
250 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
488 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 ...
3
votes
2answers
568 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 ...
1
vote
0answers
238 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 ...
3
votes
2answers
2k 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 - ...
1
vote
2answers
1k views

Designing a social network with CQRS, graph databases and relational databases in mind

I have done quite an amount of research on the topic so far, but i couldn't come up with a conclusion to make up my mind. I am designing a social network and during my research i stumbled upon graph ...
1
vote
2answers
482 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 ...
0
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
1answer
1k 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
4answers
399 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 ...
4
votes
2answers
5k 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 ...
4
votes
4answers
491 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 ...
3
votes
1answer
705 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 ...
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 ...
4
votes
2answers
196 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 ...
1
vote
2answers
2k views

Audit trails and recording actions

Background A discussion that has come up at work recently is how we handle audit logging and the recording of events. We are integrating with a 3rd party app so triggers are a no no from the off so ...
1
vote
1answer
1k views

CQRS with Repository pattern and Inversion of Control (with DI)

I assigned a POC project to someone where I asked to implement both Command Query Responsibility Segregation, Inversion of Control (with Dependency Injection) and Repository pattern. “Someone” gave ...
3
votes
1answer
612 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 ...
1
vote
1answer
588 views

Separation versus Segregation

The term "segregation" is used in the CQRS pattern. What's the difference between segregation versus separation in this case?
4
votes
2answers
727 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
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 ...
4
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?

Didn't know how to shorten that title. I'm basically trying to wrap my head around the concept of CQRS ( http://en.wikipedia.org/wiki/Command-query_separation) and related concepts. Although CQRS ...
9
votes
2answers
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 ( ...
1
vote
2answers
357 views

Designing a hierarchical structure with lots of reads and writes?

I am in the process of working on a video on demand system part of it involves the management of a hierarchical tree structure (think windows explorer) which allows users to upload videos, move ...
6
votes
4answers
826 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 ...
3
votes
1answer
666 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" ...