CQRS stands for Command Query Responsibility Segregation. CQRS guides us to separate our logical implementations into 2 categories like 'Commands', 'Query'. The 'Commands' specifies the operations like creation or updating of data into the data source(database). The 'Query' specifies the operations to fetch the data.
In CQRS models(Request/Response classes) are independent or owned by a single operation, which means model classes can not be shared between the different 'Commands' or different 'Queries' or between a 'Command' and 'Query'.
From the diagram one thing to observe Request/Response model(optional), that's because some times we will use query parameters or return a simple scalar type in those cases we won't create models.Create .Net5 Web API:
To implement the CQRS pattern let's create a sample .Net5 Web API application.
Configure Entity Framework Core Database Context:
For this demo, I had created a table like 'Products'. So to establish communication between our API project and database, here I will implement Code First Technique with an existing database.
Let's install entity framework core NuGet.
Package Manager Command:
Install-Package Microsoft.EntityFrameworkCore -Version 5.0.4
.Net CLI Command:
dotnet add package Microsoft.EntityFrameworkCore --version 5.0.4
Let's install the SQL server extension library for the entity framework core.
Package Manager Command:
Install-Package Microsoft.EntityFrameworkCore.SqlServer -Version 5.0.4
.Net CLI Command:
dotnet add package Microsoft.EntityFrameworkCore.SqlServer --version 5.0.4
Let's add the 'Products' table entity. Let's create a folder like 'Data' and inside create a subfolder like 'Entities' and add a file like 'Products.cs'Data/Entities/Products.cs:
namespace API.CQRS.Sample.Data.Entites { public class Products { public int ProductId { get; set; } public string Name { get; set; } public string Manufacturer { get; set; } public string Description { get; set; } public decimal Price{get;set;} } }Inside the 'Data' folder create a database context file like 'MyWorldDbContext.cs'
Data/MyWorldDbContext.cs:
public class MyWorldDbContext : DbContext { public MyWorldDbContext(DbContextOptions<MyWorldDbContext> options) : base(options) { } public DbSet<Products> Products { get; set; } protected override void OnModelCreating(ModelBuilder modelBuilder) { modelBuilder.Entity<Products>(build => { build.HasKey(_ => _.ProductId); }); } }In the JSON config file add the connection string
appsettings.Development.json:
"ConnectionStrings":{ "MyWorldDbConnection":"Your_connection_string" }Now register database context in the 'Startup.cs' file.
Startup.cs:
services.AddDbContext<MyWorldDbContext>(options => { options.UseSqlServer(Configuration.GetConnectionString("MyWorldDbConnection")); });
CQRS Folder Structure:
Let's have a look at the CQRS pattern folder structure.
Create Command Handler To Save A New Product:
So let's begin our sample by creating a command handler to save a new 'Product' into the table. So we have to create a 'RequestModel'(payload class). So in folder hierarchy like 'RequestModels/CommandRequestModels' add a file 'SaveProductRequestModel.cs'.
RequestModels/CommandRequestModels/SaveProductRequestModel.cs:
namespace API.CQRS.Sample.RequestModels.CommandRequestModels { public class SaveProductRequestModel { public string Name { get; set; } public string Manufacturer { get; set; } public string Description { get; set; } public decimal Price{get;set;} } }Now let's create an interface with an abstract method to save new products. So folder hierarchy like 'Contracts/CommandHandlers' and add a file like 'ISaveProductCommandHandler.cs'
Contracts/CommandHandler/ISaveProductCommandHandler.cs:
using System.Threading.Tasks; using API.CQRS.Sample.RequestModels.CommandRequestModels; namespace API.CQRS.Sample.Contracts.CommandHandlers { public interface ISaveProductCommandHandler { Task<int> SaveAsync(SaveProductRequestModel requestModel); } }
- From this method definition, we conclude that this command had only 'RequestModel'(SaveProductRequestMode) but no 'ResponseModel' because its return type is scalar type Integer.
Handlers/CommandHandlers/SaveProductCommandHandler.cs:
using System.Threading.Tasks; using API.CQRS.Sample.Contracts.CommandHandlers; using API.CQRS.Sample.Data; using API.CQRS.Sample.Data.Entites; using API.CQRS.Sample.RequestModels.CommandRequestModels; namespace API.CQRS.Sample.Handlers.CommandsHandlers { public class SaveProductCommandHandler: ISaveProductCommandHandler { public readonly MyWorldDbContext _myWorldDbContext; public SaveProductCommandHandler(MyWorldDbContext myWorldDbContext) { _myWorldDbContext = myWorldDbContext; } public async Task<int> SaveAsync(SaveProductRequestModel requestModel) { var newProducts = new Products { Description = requestModel.Description, Manufacturer = requestModel.Manufacturer, Name = requestModel.Name, Price = requestModel.Price }; _myWorldDbContext.Products.Add(newProducts); await _myWorldDbContext.SaveChangesAsync(); return newProducts.ProductId; } } }Register CommandHandler in 'Startup.cs' file.
Startup.cs:
services.AddScoped<ISaveProductCommandHandler, SaveProductCommandHandler>();Let's create an endpoint to save our 'Product'. Let's create a new controller like 'ProductController.cs'.
Controller/ProductController.cs:
using System.Threading.Tasks; using API.CQRS.Sample.Contracts.CommandHandlers; using API.CQRS.Sample.RequestModels.CommandRequestModels; using Microsoft.AspNetCore.Mvc; namespace API.CQRS.Sample.Controllers { [ApiController] [Route("[controller]")] public class ProductController : ControllerBase { private readonly ISaveProductCommandHandler _saveProductCommandHandler; public ProductController(ISaveProductCommandHandler saveProductCommandHandler) { _saveProductCommandHandler = saveProductCommandHandler; } [HttpPost] [Route("create")] public async Task<IActionResult> SaveProductAsync(SaveProductRequestModel requestModel) { var result = await _saveProductCommandHandler.SaveAsync(requestModel); return Ok(result); } } }
Create QueryHandler To Fetch All Products:
Let's begin creating a QueryHandler to fetch all 'Products'. Let's begin by creating a 'ResponseModel'. In the folder hierarchy 'ResponseModels/QueryResponseModels' add a file like 'AllProductsResponseModel'
ResponseModels/QueryResponseModels/AllProductsResponseModel.cs:
namespace API.CQRS.Sample.ResponseModels.QueryResponseModels { public class AllProductsResponseModel { public int ProductId { get; set; } public string Name { get; set; } public string Manufacturer { get; set; } public string Description { get; set; } public decimal Price { get; set; } } }Now let's declare a abstract method definition in query handler interface to fetch the all 'Products'. In folder hierarchy like 'Contracts/QueryHandlers' add a file 'IAllProductsQueryHandler.cs'.
Contracts/QueryHandlers/IAllProductQueryHandler.cs:
using System.Collections.Generic; using System.Threading.Tasks; using API.CQRS.Sample.ResponseModels.QueryResponseModels; namespace API.CQRS.Sample.Contracts.QueryHandlers { public interface IAllProductsQueryHandler { Task<List<AllProductsResponseModel>> GetListAsync(); } }
- Here we can observe that our query handler don't have a 'RequestModel', it had 'ResponseModel'(AllProductResponseModel).
Handlers/QueryHandlers/AllProductsQueryHandler.cs:
using System.Collections.Generic; using System.Linq; using System.Threading.Tasks; using API.CQRS.Sample.Contracts.QueryHandlers; using API.CQRS.Sample.Data; using API.CQRS.Sample.ResponseModels.QueryResponseModels; using Microsoft.EntityFrameworkCore; namespace API.CQRS.Sample.Handlers.QueryHandlers { public class AllProductsQueryHandler:IAllProductsQueryHandler { private readonly MyWorldDbContext _myWorldDbContext; public AllProductsQueryHandler(MyWorldDbContext myWorldDbContext) { _myWorldDbContext = myWorldDbContext; } public async Task<List<AllProductsResponseModel>> GetListAsync() { return await _myWorldDbContext.Products .Select(_ => new AllProductsResponseModel{ Description = _.Description, ProductId = _.ProductId, Manufacturer = _.Manufacturer, Name = _.Name, Price = _.Price }).ToListAsync(); } } }Now register our query handler in the 'Startup.cs' file
Startup.cs:
services.AddScoped<IAllProductsQueryHandler,AllProductsQueryHandler>();Now inject our query handler into the 'ProductController.cs'.
Controllers/ProductController.cs:
private readonly ISaveProductCommandHandler _saveProductCommandHandler; private readonly IAllProductsQueryHandler _allProductsQueryHandler; public ProductController( ISaveProductCommandHandler saveProductCommandHandler, IAllProductsQueryHandler allProductsQueryHandler) { _saveProductCommandHandler = saveProductCommandHandler; _allProductsQueryHandler = allProductsQueryHandler; }Let's add our new endpoint that fetches all 'Products'.
Controllers/ProductController.cs:
[HttpGet] [Route("all")] public async Task<IActionResult> AllProducts() { var result = await _allProductsQueryHandler.GetListAsync(); return Ok(result); }
Create Query Handler To Filter Products By Prices:
For better understanding let's create one more query handler that fetches the 'Products' by price range. Let's create 'ResponseModel' in the folder hierarchy like 'ResponeModels/QueryResponeModels' add a file like 'PriceRangeProductsResponseModel.cs'.
ResponseModels/QueryResponseModels/PriceRangeProductsResponseModel.cs:
namespace API.CQRS.Sample.ResponseModels.QueryResponseModels { public class PriceRangeProductsResponseModel { public int ProductId { get; set; } public string Name { get; set; } public decimal Price { get; set; } } }Now let's define abstract method in our query handler interface. In the folder hierarchy like 'Contracts/QueryHandlers' add a file like 'IPriceRangeProductsQueryHandler.cs'
Contracts/QueryHandlers/IPriceRangeProductsQueryHandler.cs:
using System.Collections.Generic; using System.Threading.Tasks; using API.CQRS.Sample.ResponseModels.QueryResponseModels; namespace API.CQRS.Sample.Contracts.QueryHandlers { public interface IPriceRangeProductsQueryHandler { Task<List<PriceRangeProductsResponseModel>> PriceRangeProductsAsync(int minPrice, int maxPrice); } }Now let's implement the 'PriceRangeProductsAsync' method. In the folder hierarchy like 'Handlers/QueryHandlers' add a file like 'PriceRangeProductsQueryHandler.cs'.
Handlers/QueryHandlers/PriceRangeProductsQueryHandler.cs:
using System.Collections.Generic; using System.Linq; using System.Threading.Tasks; using API.CQRS.Sample.Contracts.QueryHandlers; using API.CQRS.Sample.Data; using API.CQRS.Sample.ResponseModels.QueryResponseModels; using Microsoft.EntityFrameworkCore; namespace API.CQRS.Sample.Handlers.QueryHandlers { public class PriceRangeProductsQueryHandler : IPriceRangeProductsQueryHandler { private readonly MyWorldDbContext _myWorldDbContext; public PriceRangeProductsQueryHandler(MyWorldDbContext myWorldDbContext) { _myWorldDbContext = myWorldDbContext; } public async Task<List<PriceRangeProductsResponseModel>> PriceRangeProductsAsync(int minPrice, int maxPrice) { return await _myWorldDbContext.Products .Where(_ => _.Price >= minPrice && _.Price <= maxPrice) .Select(_ => new PriceRangeProductsResponseModel { Name = _.Name, ProductId = _.ProductId, Price = _.Price }).ToListAsync(); } } }Register the query handlers in the 'Startup.cs'.
Startup.cs:
services.AddScoped<IPriceRangeProductsQueryHandler, PriceRangeProductsQueryHandler>();Now inject the query handler into the 'ProductController.cs'
Controllers/ProductController.cs:
[HttpGet] [Route("price-range")] public async Task<IActionResult> PriceRangeProducts(int minPrice, int maxPrice) { var result = await _priceRangeProductsQueryHandler.PriceRangeProductsAsync(minPrice, maxPrice); return Ok(result); }
Finally we have completed our sample by implementing few Query/Command handlers. So our project folder structure will looks like:
MediatR:
MediatR is another design pattern. MediatR builds with handlers that have the capability to work on Commands and Queries.
MediatR reduces a lot of boilerplate code like injecting multiple services into the controller. MediatR provides a single entry point that expects a RequestModel, so based on the RequestModel corresponding Handler gets invoked. It works as a centralized communication hub because from the controller any handler gets invoked only through it. So it's the most common recommendation to use MediatR with CQRS.
From the diagram, we have to understand MediatR should have both RequestModel and ResponseMode. In MediatR 'RequestModel' always be a user-defined class whereas 'ResponseModel' either a user-defined class or scalar type.Install MediatR NuGet:
Let's install the MediatR package for dotnet.
Package Manager Command:
Install-Package MediatR -Version 9.0.0
.Net CLI Command:
dotnet add package MediatR --version 9.0.0
Let's install the Dependency Injection extension library for MediatR.
Package Manager Command:
Install-Package MediatR.Extensions.Microsoft.DependencyInjection -Version 9.0.0
.Net CLI Command:
dotnet add package MediatR.Extensions.Microsoft.DependencyInjection --version 9.0.0
Register And Inject MediatR:
Let's register MediatR service in 'Startup.cs'
Startup.cs:
services.AddMediatR(Assembly.GetExecutingAssembly());Now inject the 'IMediator' into the 'ProductController' constructor.
Controllers/ProductController.cs:
private readonly ISaveProductCommandHandler _saveProductCommandHandler; private readonly IAllProductsQueryHandler _allProductsQueryHandler; private readonly IPriceRangeProductsQueryHandler _priceRangeProductsQueryHandler; private readonly IMediator _mediator; public ProductController( ISaveProductCommandHandler saveProductCommandHandler, IAllProductsQueryHandler allProductsQueryHandler, IPriceRangeProductsQueryHandler priceRangeProductsQueryHandler, IMediator mediator) { _saveProductCommandHandler = saveProductCommandHandler; _allProductsQueryHandler = allProductsQueryHandler; _priceRangeProductsQueryHandler = priceRangeProductsQueryHandler; _mediator = mediator; }
Update Logic In Command Handler To Save A New Product Using MediatR:
In MediatR pattern 'RequestModel' required, based on the 'RequestModel' corresponding handler will be automatically invoked by the MedaitR. The 'RequesModel' should always be a user-define class. In folder hierarchy 'RequestModels/CommandRequestModels' update the 'SaveProductRequestModel.cs' like below:
RequestModels/CommandRequestModels/SaveProductRequestModel.cs:
RequestModels/CommandRequestModels/SaveProductRequestModel.cs:
using MediatR; namespace API.CQRS.Sample.RequestModels.CommandRequestModels { public class SaveProductRequestModel:IRequest<int> { public string Name { get; set; } public string Manufacturer { get; set; } public string Description { get; set; } public decimal Price{get;set;} } }
- In mediatr pattern our RequestModels(eg: SaveProductRequestModel) should inherit 'IRequest<out ResponseModel>'. The 'IRequest<out T>' 'T' model should be the 'ResponseModel' our handler.
Now we have to update the logic of our command handler 'SaveProductCommandHandler.cs'.
Handlers/CommandHandlers/SaveProductCommandHandler.cs:
using System.Threading; using System.Threading.Tasks; using API.CQRS.Sample.Contracts.CommandHandlers; using API.CQRS.Sample.Data; using API.CQRS.Sample.Data.Entites; using API.CQRS.Sample.RequestModels.CommandRequestModels; using MediatR; namespace API.CQRS.Sample.Handlers.CommandsHandlers { public class SaveProductCommandHandler : IRequestHandler<SaveProductRequestModel, int> { public readonly MyWorldDbContext _myWorldDbContext; public SaveProductCommandHandler(MyWorldDbContext myWorldDbContext) { _myWorldDbContext = myWorldDbContext; } public async Task<int> Handle(SaveProductRequestModel request, CancellationToken cancellationToken) { var newProducts = new Products { Description = request.Description, Manufacturer = request.Manufacturer, Name = request.Name, Price = request.Price }; _myWorldDbContext.Products.Add(newProducts); await _myWorldDbContext.SaveChangesAsync(); return newProducts.ProductId; } } }
- By using MediatR, we no need to create interfaces to our Command/Query handlers. So we can delete all our interface classes inside of the 'Contracts' folder in our project.
- Our Command Handler inherits 'MediatR.IRequestHandler<in TRequest, TResponse>'. The 'TRequest' means our 'RequestModel'(only user defined class), 'TResponse' means our 'ResponseModel'(either user defined class or scalar type).
- Our Command Handler should implement 'Handle' as a public method. So first input parameter of the 'Handle' is 'RequestModel'. The 'Handle' method return type is our 'ResponseModel'.
Let's update our endpoint to use MediatR to trigger our Command Handler.
Controllers/ProductController.cs:
[HttpPost] [Route("create")] public async Task<IActionResult> SaveProductAsync(SaveProductRequestModel requestModel) { //var result = await _saveProductCommandHandler.SaveAsync(requestModel); var result = await _mediator.Send(requestModel); return Ok(result); }
- So here 'Send' method of 'IMediator' that takes our RequestModel(eg: SaveProductRequestModel) as input value. So 'Send' method will invoke the appropriate Command Handler(eg: SaveProductCommandHandler) with the help of 'RequestModel'(eg: SaveProductRequestModel).
Update Logic In QueryHandler To Fetch All Products Using MediatR:
Currently 'AllProductsQueryHandler' class has method 'GetListAsync' that don't have 'RequestModel'. But we know MediatR fully depends on the 'RequestModel'. So we have to create an empty 'RequestModel' to make it work with 'MediatR'. In folder hierarchy like 'RequestModel/QueryRequestModels' add a new file like 'AllProductsRequestModel.cs'
RequestModel/QueryRequestModels/AllProductsRequestModel.cs:
using System.Collections.Generic; using API.CQRS.Sample.ResponseModels.QueryResponseModels; using MediatR; namespace API.CQRS.Sample.RequestModels.QueryRequestModels { public class AllProductsRequestModel:IRequest<List<AllProductsResponseModel>> { } }
- In mediatr pattern our RequestModels(eg: AllProductsRequestModel) should inherit 'IRequest<out ResponseModel>'. The 'IRequest<out T>' 'T' model should be the 'ResponseModel'(eg: List<AllProductsResponseModel>) of our handler.
Handlers/QueryHandlers/AllProductsQueryHandler.cs:
using System.Collections.Generic; using System.Linq; using System.Threading; using System.Threading.Tasks; using API.CQRS.Sample.Contracts.QueryHandlers; using API.CQRS.Sample.Data; using API.CQRS.Sample.RequestModels.QueryRequestModels; using API.CQRS.Sample.ResponseModels.QueryResponseModels; using MediatR; using Microsoft.EntityFrameworkCore; namespace API.CQRS.Sample.Handlers.QueryHandlers { public class AllProductsQueryHandler : IRequestHandler<AllProductsRequestModel, List<AllProductsResponseModel>> { private readonly MyWorldDbContext _myWorldDbContext; public AllProductsQueryHandler(MyWorldDbContext myWorldDbContext) { _myWorldDbContext = myWorldDbContext; } public async Task<List<AllProductsResponseModel>> Handle(AllProductsRequestModel request, CancellationToken cancellationToken) { return await _myWorldDbContext.Products .Select(_ => new AllProductsResponseModel { Description = _.Description, ProductId = _.ProductId, Manufacturer = _.Manufacturer, Name = _.Name, Price = _.Price }).ToListAsync(); } } }
- Our Query Handler inherits 'MediatR.IRequestHandler<in TRequest, TResponse>'. The 'TRequest' means our 'RequestModel'(only user defined class), 'TResponse' means our 'ResponseModel'(either user defined class or scalar type).
- Our Query Handler should implement 'Handle' as a public method. So first input parameter of the 'Handle' is 'RequestModel'(eg: AllProductsRequestModel). The 'Handle' method return type is our 'ResponseModel'(eg: List<AllProductsResponseModel>).
Update the endpoint logic to use Mediator to invoke the query handler.
Controllers/ProductController.cs:
[HttpGet] [Route("all")] public async Task<IActionResult> AllProducts() { //var result = await _allProductsQueryHandler.GetListAsync(); var result = await _mediator.Send(new AllProductsRequestModel()); return Ok(result); }
- So here 'Send' method of 'IMediator' that takes our RequestModel(eg: AllProductsRequestModel) as input value. So 'Send' method will invoke the appropriate Query Handler(eg: AllProductsQueryHandler) with the help of 'RequestModel'(eg: AllProductsRequestModel).
Update Logic In Query Handler To Filter Products By Prices Using MediatR:
Currently 'PriceRangeProductsQueryHandler' has method 'PriceRangeProductsAsync' that contains 2 input parameters like 'minPrice' and 'maxPrice'. Now we have to create 'RequestModel' with those input properties to support MediatR. In the folder hierarchy like 'RequestModel/QueryRequestModels' add a new file like 'PriceRangeProductsRequestModel.cs'
RequestModel/QueryRequestModels/PriceRangeProductsRequestModel.cs:
using System.Collections.Generic; using API.CQRS.Sample.ResponseModels.QueryResponseModels; using MediatR; namespace API.CQRS.Sample.RequestModels.QueryRequestModels { public class PriceRangeProductsRequestModel:IRequest<List<PriceRangeProductsResponseModel>> { public int MinPrice { get; set; } public int MaxPrice { get; set; } } }
- In mediatr pattern our RequestModels(eg: PriceRangeProductsRequestModel) should inherit 'IRequest<out ResponseModel>'. The 'IRequest<out T>' 'T' model should be the 'ResponseModel'(eg: List<PriceRangeProductsResponseModel>) of our handler.
Handlers/QueryHandlers/PriceRangeProductsQueryHandler.cs:
using System.Collections.Generic; using System.Linq; using System.Threading; using System.Threading.Tasks; using API.CQRS.Sample.Contracts.QueryHandlers; using API.CQRS.Sample.Data; using API.CQRS.Sample.RequestModels.QueryRequestModels; using API.CQRS.Sample.ResponseModels.QueryResponseModels; using MediatR; using Microsoft.EntityFrameworkCore; namespace API.CQRS.Sample.Handlers.QueryHandlers { public class PriceRangeProductsQueryHandler : IRequestHandler<PriceRangeProductsRequestModel, List<PriceRangeProductsResponseModel>> { private readonly MyWorldDbContext _myWorldDbContext; public PriceRangeProductsQueryHandler(MyWorldDbContext myWorldDbContext) { _myWorldDbContext = myWorldDbContext; } public async Task<List<PriceRangeProductsResponseModel>> Handle(PriceRangeProductsRequestModel request, CancellationToken cancellationToken) { return await _myWorldDbContext.Products .Where(_ => _.Price >= request.MinPrice && _.Price <= request.MaxPrice) .Select(_ => new PriceRangeProductsResponseModel { Name = _.Name, ProductId = _.ProductId, Price = _.Price }).ToListAsync(); } } }
- Our Query Handler inherits 'MediatR.IRequestHandler<in TRequest, TResponse>'. The 'TRequest' means our 'RequestModel'(only user defined class), 'TResponse' means our 'ResponseModel'(either user defined class or scalar type).
- Our Query Handler should implement 'Handle' as a public method. So first input parameter of the 'Handle' is 'RequestModel'(eg: PriceRangeProductsRequestModel). The 'Handle' method return type is our 'ResponseModel'(eg: List<PriceRangeProductsResponseModel>).
Now update the endpoint logic to use the MediatR.
Controllers/ProductController.cs:
[HttpGet] [Route("price-range")] public async Task<IActionResult> PriceRangeProducts([FromQuery]PriceRangeProductsRequestModel requestModel) { //var result = await _priceRangeProductsQueryHandler.PriceRangeProductsAsync(minPrice, maxPrice); var result = await _mediator.Send(requestModel); return Ok(result); }
- So here 'Send' method of 'IMediator' that takes our RequestModel(eg: PriceRangeProductsRequestModel) as input value. So 'Send' method will invoke the appropriate Query Handler(eg: PriceRangeProductsQueryHandler) with the help of 'RequestModel'(eg: PriceRangeProductsRequestModel).
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Wrapping Up:
Hopefully, I think this article delivered some useful information on CQRS and MediatR patterns. I love to have your feedback, suggestions, and better techniques in the comment section below.
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