Data persistence

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Data persistence
Repositories
Revo framework offers a number of facilities for working with entities and their persistence. Repositories are one of the core concepts in framework’s data persistence. There are two distinct types of repositories that a developer can use. Both of them come with a certain level of implementation abstraction.
Aggregate repository
In terms of domain-driven design, IRepository (defined in Revo.Infrastructure module) is the high-level repository that would be used in the command handlers for the write side of the application working with the domain model.
The implementation of IRepository interface is not bound to any specific database system backend (or an ORM library) and instead delegates most of the actual data-persistence related responsibilities to different aggregate stores - i.e. event sourced aggregate store or CRUD aggregate store. The repository itself is then just a thin wrapper over those aggregate stores, making it easier to work with aggregate stored in different persistence backends by providing a single unified API for them while also handing some of the concepts related to domain repositories – like the unit-of-work pattern and event publishing.

The repository declares all methods as generic, simplifying all repository interactions to just one universal interface. When querying for an entity, the repository itself then tries to deduce the aggregate store it belongs. When the correct aggregate store is found, the repository delegates the actual persistence work (loading, saving, querying…) to it.
To enforce aggregate consistency boundaries on compliance with DDD rules, repositories only allow working with aggregate roots, which is enforced by generic type constraints for all its generic methods (requiring any IAggregateRoot descendant). This prevents breaking the encapsulation of aggregates by querying and manually modifying single aggregate entities separately.
Following  abridged code snippet of IRepository interface definition shows some of the basic functionality it offers for working with aggregates.
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public interface IRepository : IUnitOfWorkProvider, IDisposable
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{
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	void Add<T>(T aggregate) where T : class, IAggregateRoot;
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	T FirstOrDefault<T>(Expression<Func<T, bool>> predicate) where T : class, 
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        IAggregateRoot, IQueryableEntity;
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	T First<T>(Expression<Func<T, bool>> predicate)
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        where T : class, IAggregateRoot, IQueryableEntity;
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	Task<T> FirstOrDefaultAsync<T>(Expression<Func<T, bool>> predicate)
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        where T : class, IAggregateRoot, IQueryableEntity;
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	Task<T> FirstAsync<T>(Expression<Func<T, bool>> predicate)
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        where T : class, IAggregateRoot, IQueryableEntity;
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	T Find<T>(Guid id) where T : class, IAggregateRoot;
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	Task<T> FindAsync<T>(Guid id) where T : class, IAggregateRoot;
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	IQueryable<T> FindAll<T>()
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        where T : class, IAggregateRoot, IQueryableEntity;
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	Task<IList<T>> FindAllAsync<T>()
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        where T : class, IAggregateRoot, IQueryableEntity;
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	T Get<T>(Guid id) where T : class, IAggregateRoot;
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	Task<T> GetAsync<T>(Guid id) where T : class, IAggregateRoot;
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	IQueryable<T> Where<T>(Expression<Func<T, bool>> predicate)
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        where T : class, IAggregateRoot, IQueryableEntity;
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	void Remove<T>(T aggregate) where T : class, IAggregateRoot;
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	void SaveChanges();
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	Task SaveChangesAsync();
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}
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Besides the common functionality of adding and removing aggregate roots and finding them by their ID, the repository also provides method for finding them using more advanced queries. These methods are, however, defined with additional generic method constraint and can only be used with entities that implement the IQueryableEntity interface (this interface is empty and serves just to signify what entities can be used in those queries). For example, this means while these methods will be available for aggregate roots stored in a RDBMS and accessed via Entity Framework (generally any BasicAggregateRoot-derived aggregate roots), they will not be available for event sourced aggregate roots because event store usually do not possess these kinds of querying features.
Aggregate changes are automatically persisted upon saving the repository. It is unnecessary to call the save method explicitly as the repository implements a unit of work provider, which means it gets automatically saved when the unit of work is committed at the end of the command processing (unless any unhandled exceptions are thrown). The mechanism for detecting changes in the entities depends on the actual aggregate store implementation (e.g. event sourced entities are saved when they published any new uncommitted events, while changes in entities persisted by Entity Framework rely on its own internal change tracking mechanism).
CRUD aggregate store
CRUD aggregate store is not tied to single specific database technology and is just a thin layer on top of ICrudRepository. CRUD data repository is the second type of repositories providing a more direct access to databases without the constraints of the domain and is in-depth discussed in chapter 5.5. All the persistence-specific features of their implementations (e.g. the mapping of entities to database) also apply to aggregates accessed via this aggregate store.
Event sourced aggregate store
Similar to the CRUD aggregate store, the event sourced aggregate store is not implemented using a single specific database technology and is internally using IEventStore which is the source of the actual database connector implementation. 
CRUD data repository
CRUD data repositories (implementations of ICrudRepository which is contained in Revo.DataAccess module) represent a way of a more direct way of accessing database. Unlike the aggregate repository (IRepository), it is not burdened with domain concepts like aggregate consistency boundaries and encapsulation and provides more flexibility when working with data. Where IRepository offered only basic functionality for working with entities to abstract from the underlying technology, ICrudRepository strives to do the opposite and offer maximum of the features that the used database (or an ORM) has while still maintaining some minimum level of abstraction in order to enable easy testing. Having said that, it is important to emphasize that they serve a completely different purpose – while the aggregate IRepository would usually be used on the write side of the application in command handlers to update the domain, the CRUD repository will mostly be used just for efficient access to read models in projectors and query handlers and in other places that are not encumbered by the complex business rules handled by the domain model. 
Under no circumstances it should be used to modify domain model data bypassing its business rules. However, in some scenarios, when it is unnecessary to maintain different write model and read model in the database it might be possible to reuse the domain model for queries.
The ICrudRepository interfaces relies heavily on the use of IQueryable<T> support of the .NET platform. This enables to perform arbitrary queries on the database without bloating the interface definition with a huge number of different query methods or a need for custom repositories for every specific, saving a lot of developer’s time, while still also providing type safety and the safety of compile-time checks to a certain degree.
Most of the read-related functionality of ICrudRepository is actually defined in a base type named IReadRepository. This base type does not allow any modifications to the entities (i.e. it has no save or add/remove methods), which means it is very suitable in situations when we want to limit the operations an object can do with entities in the repository – for example in a query handler that should always only performs reads of the database. It also potentially makes testing those entities easier.
Most of the CRUD repository implementations will also define its own repository interfaces derived from ICrudRepository in order to facilitate the use of the features specific to its technology (for example, IEFCoreCrudRepository making it possible to directly work with some of the EF Core ORM features).
Both types of repositories also offer an in-memory implementation of their interfaces that are suitable for testing purposes. For more information about this, see Testing.
Repository filters
All repositories in Revo framework also support the concept of repository filters. Repository filters allow decorating of the behavior of repositories with cross-cutting concerns like authorization or multi-tenancy. These filters can control what entities are queried and saved by the repository. Repository filters can be registered to be implicitly enabled for all new repositories. By default, Revo currently registers two repository filters:
tenant repository filter (see multi-tenancy support),
authorization repository filter (see how to implement authorization).

Event stores
SQL event store
TODO
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