# Event Sourcing and CQRS

This post is a part of the Event Sourcing introduction series that I wrote for Event Store. All the articles are available in the Event Store Blog, along with other great articles about Event Sourcing.

CQRS stands for Command-Query Segregation Principle. Greg Young described (and named) the pattern thoroughly in 2010, but the idea existed way before that time. On a high level, CQRS states the fact that operations that trigger state transitions should be described as commands and any data retrieval that goes beyond the need of the command execution, should be named a query. Because the operational requirements for executing commands and queries are very often different, developers should consider using different persistence techniques for handling commands and queries, therefore segregating them.

## Operations vs Reporting

Most software systems need to persist data. Since for the scope of this guide we assume that the business logic of a software system is represented and executed by the Domain Model, we can say that application persistence focuses on persisting domain objects, like aggregates and entities. Because domain objects often have complex structures, developers might need to apply advanced persistence techniques to overcome the impedance mismatch. The Domain Model-oriented persistence is optimised for transactional performance, because the essential function of the Domain Model is to execute operations that trigger state transitions in the system, according to the logic and rules of the model.

Such an approach, however, could create challenges when it comes to retrieving data for reporting purposes.

Note: Don’t see reporting as an act of creating a PDF or CSV file. Each query to the database that aims to get some data in order to show it to the user or return the result of an API call is also reporting.

The challenge surfaces from the fact that reporting needs are often drastically different from the needs of executing transactional operations. In addition, many systems have a clear imbalance between the number of writes and reads. For example, a typical user-facing application has much less writes than reads, because users normally read the information provided by the system and rarely execute operations in the systems. Some systems, however, have the opposite imbalance with prevailing writes. Think of a back-office of your bank, or a vehicle tracking system as examples.

One of the first traces of the original discussion around the reporting issue when using the Domain Model pattern happened during the The challenge of executing DDD panel with Martin Fowler, Eric Evans and others, back in 2004. Also in 2004, Martin Fowler published the ReportingDatabase pattern in Bliki. The pattern suggests using a separate database for reporting purposes and to push changes from the operational database to that reporting database.

Source: Martin Fowler’s bliki

The Reporting Database pattern is similar to CQRS but not exactly the same. It aims to solve the similar issue, but gets confused with the term reporting. In the Reporting Database pattern context, reporting has a more traditional meaning.

The kick-off question of the aforementioned panel discussion was:

On the other hand business execs want / crave / require “ad-hoc” reporting, with the degree of “ad-hoc-ness” varying from simple reports to data warehouses. You’d like to simply write some SQL and stuff the results into a report, but this is impossible when you have the domain model supporting all of your complex logic. So, what do you do?

Here, reporting only covers something that is accessed now and then. Such a report is only actual at the moment it gets produced, just as a printed report.

CQRS, however, assumes that any query is some sort of report, whether it is moved, printed out, or shown to the user inside the application.

## Commands vs Queries

Developers often confuse CQRS with the Reporting Database pattern, although there is nothing in the name (Command-Query Responsibility Segregation) to suggest that.

The essence of CQRS is to separate command flow from the query flow, based on fundamental characteristics of those concepts.

### Commands

The main attribute of a command is that when the command gets successfully executed, the system transitions to a new state.

Another important attribute of a command is that it conveys the intent of the user. Requests like UpdateCustomer are not exactly commands according to CQRS. Within the application boundaries, handling a command should result in one transaction on one Aggregate.

As a consequence, the command flow in CQRS goes hand in hand with the Task-Based User Interface pattern. In contrast with the CRUD-based user interface, which has four basic operations available for users - Create, Edit, Save and Delete, a task-based UI makes each operation explicit, like Check Out, Add Item or Cancel Order. By doing this, it makes operations available to the user easily translatable to commands that the UI sends to the Domain Model via an API.

### Queries

Queries, as the name suggest, allow getting data from the persistent store, so it can be shown to the user, sent to another system or used for any other purpose.

Note: Although the image shows all the system elements stored in what looks like a single database, it is just an example and doesn’t have to be that way.

Unlike commands, queries do not need to involve the Domain Model because queries do not execute any operations and should not contain any business logic. In addition, queries have no side effects and are completely idempotent, so it doesn’t matter how many times the query gets executed - it will always return the same result, unless the system state changed in the meantime.

Therefore, when implementing a query handler, developers don’t need to use the same way to access data as they do when persisting domain objects. For example, if the Domain Model persistence uses some ORM framework that distribute state of domain objects across tables in a relational database, a query could fire an SQL statement that ignores the ORM and gets the data directly from those tables.

CQRS also suggests that queries target specific use cases and return a pre-composed data set that can be shown in its entirety on the screen or in a cohesive part of the screen.

## CQRS with Event Sourcing

Back in 2016 in his talk A Decade of DDD, CQRS, Event Sourcing Greg Young said the following:

Warning: You need to look at CQRS not as being the main thing. CQRS was a product of its time and meant to be a stepping stone towards the ideas of Event Sourcing.

It is not a coincidence that Greg proposed the CQRS pattern at the same time as he introduced Event Sourcing to the public. Unlike state-based persistence, where there might be a way to avoid using the domain model for queries, such an approach is hard to impossible to use in event-sourced systems. It is because there is no place where the state of a domain object is stored entirely.

As described in the previous article, domain entities in event-sourced systems are stored as event streams, essentially each entity is a sequence of events from the persistence point of view. There is more to it yet. Domain events in event store alone do not allow reconstructing the entity state without knowing the logic that the entity uses to rehydrate its own state from events and this logic is in the code of the Domain Model.

### Projections

We need a way to project events to an alternative store, which we can easily query. It could be a relational or document database, or cache, or any other type of persistence that is applicable for a specific use case.

The idea is that a software component called a Projection subscribes to the live event feed of the events database and when it receives an event, it could project the information in that event to a query model in a dedicated reporting database.

Note: In the context of CQRS you often hear write side, read side and read model. The write side is the command side, since that’s the place where state mutations, also referred as writes, take place. By comparison, the query side is often being referred as the read side, and the query model gets by name read model.

Because the query side of CQRS in event-sourced systems is more complicated than the command side, projections post is dedicated to this topic.