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## Concurrency Check |
## Concurrency Check |
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TODO |
### Introduction |
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Concurrency Check (also known as **Concurrency Control**) refers to specific mechanisms used to ensure data consistency in the presence of concurrent changes (multiple processes, users access or change the same data in a database at the same time). |
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There are two commonly used concurrency control mechanisms/approaches: |
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* **Optimistic Concurrency Control**: Optimistic Concurrency Control allows multiple users to attempt to **update** the same record without informing the users that others are also attempting to **update** it. |
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* If a user successfully updates the record, the other users need to get the latest changes for the current record to be able to make changes. |
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* ABP's concurrency check system uses the **Optimistic Concurrency Control**. |
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* **Pessimistic Concurrency Control**: Pessimistic Concurrency Control prevents simultaneous updates to records and uses a locking mechanism. For more information please see [here](https://www.martinfowler.com/eaaCatalog/pessimisticOfflineLock.html). |
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### Usage |
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#### `IHasConcurrencyStamp` Interface |
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To enable **concurrency control** to your entity class, you should implement the `IHasConcurrencyStamp` interface, directly or indirectly. |
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```csharp |
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public interface IHasConcurrencyStamp |
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{ |
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public string ConcurrencyStamp { get; set; } |
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} |
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``` |
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* It is the base interface for **concurrency control** and only has a simple property named `ConcurrencyStamp`. |
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* While a new record is **creating**, if the entity implements the `IHasConcurrencyStamp` interface, ABP Framework automatically sets a unique value to the **ConcurrencyStamp** property. |
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* While a record is **updating**, ABP Framework compares the **ConcurrencyStamp** property of the entity with the provided **ConcurrencyStamp** value by the user and if the values match, it automatically updates the **ConcurrencyStamp** property with the new unique value. If there is a mismatch, `AbpDbConcurrencyException` is thrown. |
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**Example: Applying Concurrency Control for the Book Entity** |
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Implement the `IHasConcurrencyStamp` interface for your entity: |
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```csharp |
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public class Book : Entity<Guid>, IHasConcurrencyStamp |
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{ |
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public string ConcurrencyStamp { get; set; } |
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//... |
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} |
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``` |
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Also, implement your output and update the DTO classes from the `IHasConcurrencyStamp` interface: |
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```csharp |
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public class BookDto : EntityDto<Guid>, IHasConcurrencyStamp |
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{ |
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//... |
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public string ConcurrencyStamp { get; set; } |
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} |
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public class UpdateBookDto : IHasConcurrencyStamp |
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{ |
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//... |
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public string ConcurrencyStamp { get; set; } |
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} |
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``` |
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Set the **ConcurrencyStamp** input value to the entity in the **UpdateAsync** method of your application service, for that purpose you can use the `SetConcurrencyStampIfNotNull` method as below: |
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```csharp |
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public class BookAppService : ApplicationService, IBookAppService |
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{ |
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//... |
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public virtual async Task<BookDto> UpdateAsync(Guid id, UpdateBookDto input) |
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{ |
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var book = await BookRepository.GetAsync(id); |
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book.SetConcurrencyStampIfNotNull(input.ConcurrencyStamp); |
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//set other input values to the entity ... |
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await BookRepository.UpdateAsync(book); |
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} |
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} |
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``` |
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* After that, when multiple users try to update the same record at the same time, the concurrency stamp mismatch occurs and `AbpDbConcurrencyException` is thrown. |
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#### Base Classes |
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[Aggregate Root](./Entities.md#aggregateroot-class) entity classes already implement the `IHasConcurrencyStamp` interface. So, if you are deriving from one of these base classes, you don't need to manually implement the `IHasConcurrencyStamp` interface: |
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- `AggregateRoot`, `AggregateRoot<TKey>` |
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- `CreationAuditedAggregateRoot`, `CreationAuditedAggregateRoot<TKey>` |
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- `AuditedAggregateRoot`, `AuditedAggregateRoot<TKey>` |
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- `FullAuditedAggregateRoot`, `FullAuditedAggregateRoot<TKey>` |
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**Example: Applying Concurrency Control for the Book Entity** |
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You can inherit your entity from one of [the base classes](#base-classes): |
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```csharp |
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public class Book : FullAuditedAggregateRoot<Guid> |
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{ |
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//... |
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} |
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``` |
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Then, you can implement your output and update the DTO classes from the `IHasConcurrencyStamp` interface: |
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```csharp |
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public class BookDto : EntityDto<Guid>, IHasConcurrencyStamp |
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{ |
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//... |
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public string ConcurrencyStamp { get; set; } |
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} |
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public class UpdateBookDto : IHasConcurrencyStamp |
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{ |
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//... |
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public string ConcurrencyStamp { get; set; } |
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} |
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``` |
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Set the **ConcurrencyStamp** input value to the entity in the **UpdateAsync** method of your application service, for that purpose you can use the `SetConcurrencyStampIfNotNull` method as below: |
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```csharp |
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public class BookAppService : ApplicationService, IBookAppService |
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{ |
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//... |
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public virtual async Task<BookDto> UpdateAsync(Guid id, UpdateBookDto input) |
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{ |
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var book = await BookRepository.GetAsync(id); |
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book.SetConcurrencyStampIfNotNull(input.ConcurrencyStamp); |
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//set other input values to the entity ... |
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await BookRepository.UpdateAsync(book); |
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} |
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} |
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``` |
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After that, when multiple users try to update the same record at the same time, the concurrency stamp mismatch occurs and `AbpDbConcurrencyException` is thrown. You can either handle the exception manually or let the ABP Framework handle it for you. |
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ABP Framework shows a user-friendly error message as in the image below, if you don't handle the exception manually. |
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