The core for asynchronous programming are the objects Task and Task<T>. Both of them are compatible with the keywords async and await.

First of all we need to identify if the code’s I/O-bound or CPU-bound.

• the code’s limited for external operations and waits for something a lot of time. Examples of this are DDBB calls, or a server’s response. In this case we have to use async/await to free the thread while we wait
• the code does a CPU-intensive operation. Then we move the work to another thread using Task.Run() so we don’t block the main thread.

async code vs parallel code

(!) Asynchronous code is not the same as parallel code (!)

• In async code you are trying to make your threads do as little work as possible. This will keep your app responsibe, capable to serve many requests at once and scale well.
• In parallel code you do the opposite. You use and keep a hold on a thread to do CPU-intensive calculations

async code

The importante of async programming is that you choose when to wait on a task. This way, you can start other tasks concurrently

In async code, one single thread can start the next task concurrently before the previous one completes.
(!) async code doesn’t cause additional threads to be created because an async method doesn’t run on its own thread. (!) It runs on the current synchronization context and uses time on the thread only when the method is active.

parallel code

For parallelism you need multiple threads where each thread executes a task, and all of those tasks are executed at the same time

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The following are code examples to test several scenarios.

(check this project to see more testing code examples)

Test a controller

• GivenCorrectDate_WhenGetAvailability_ThenAssertCorrectReturnValue asserts a controller returns 200 when everything goes right
• GivenWrongDate_WhenGetAvailability_ThenAssert400ReturnValue assert a controller returns 400 with specific error message
• GivenServiceThrowsException_WhenReserveSlot_ThenAssertExceptionCaught assert method throws an exception, but it is correctly caught

public class SlotsControllerTest
{
	// class we're testing
	private SlotsController _controller;

	private Mock<ISlotsService> _slotsServiceMock;
	
	private Mock<IOptions<CoreConfig>> _iOptConfigMock;
	private Mock<CoreConfig> _configMock;
	
	[SetUp]
	public void SetUp()
	{
		_configMock = new Mock<CoreConfig>();
		_iOptConfigMock = new Mock<IOptions<CoreConfig>>();
		_iOptConfigMock.Setup(iOpt => iOpt.Value).Returns(_configMock);
		
		_slotsServiceMock = new Mock<ISlotsService>();
		_controller = new SlotsController(_slotsServiceMock.Object, _iOptConfigMock.Object);
	}

[Test]
public async Task GivenWrongDate_WhenGetAvailability_ThenAssert400ReturnValue()
{
	// given
	string date = "";
	string errorMessage = "oh no! something went wrong!";
	
	var errorMessages = new ErrorMessages
	{
		GeneralErrorMessage = errorMessage
	};
	
	_configMock.Setup(conf => conf.GeneralErrorMessage).Returns(errorMessage);
	
	// when
	var result = await _controller.GetAvailability(date) as BadRequestObjectResult;
	
	// then
	result.Should().NotBeNull();
	result.StatusCode.Should().Be(400);
	result.Value.ToString().Should().Contain(errorMessage);
}

[Test]
public async Task GivenCorrectDate_WhenGetAvailability_ThenAssertCorrectReturnValue()
{
	// given
	string date = "20241012";
	var parsedDate = new DateOnly(2024, 10, 12);
	
	string dateFormat = "yyyyMMdd";
	_configMock.Setup(conf => conf.InputDateFormat).Returns(dateFormat);
	
	var dto = new WeekAvailabilityResponse();
	_slotsServiceMock.Setup(service => service.GetWeekSlotsAsync(parsedDate)).ReturnsAsync(dto);
	
	// when
	var result = await _controller.GetAvailability(date) as OkObjectResult;
	
	// then
	result.Should().NotBeNull();
	result.StatusCode.Should().Be(200);
	result.Value.Should().Be(dto);
}

[Test]
public async Task GivenServiceThrowsException_WhenReserveSlot_ThenAssertExceptionCaught()
{
	// given
	var request = new ReserveSlotRequest();

	string errorMessage = "error when throw exception";
	var errorMessages = new ErrorMessages
	{
		GeneralErrorMessage = errorMessage
	};
	
	_configMock.Setup(conf => conf.GeneralErrorMessage).Returns(errorMessage);
	_slotsServiceMock.Setup(service => service.ReserveSlotsAsync(parsedDate)).ThrowAsync(new HttpRequestException(errorMessage));

	// when
	var result = await _controller.ReserveSlot(request) as BadRequestObjectResult
	
	// then
	result.Should().NotBeNull();
	result.StatusCode.Should().Be(400);
	result.Value.ToString().Should().Contain(errorMessage);
}

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Para documentar metodos en .NET es comun utiliar comentarios XML, mediante los cuales se puede generar documentacion externa usando herramientas como DocFX o Sandcastle.
Esto permite que tu API o biblioteca tenga una descripcion completa de cada método.

Ejemplo de comentario

public class ProductsController : ControllerBase
{
	/// <summary>
	/// Get a specific product by its ID.
	/// </summary>
	/// <param name="id">The ID from the product to retrieve</param>
	/// <returns>Returns the product if it's found. Otherwise it returns HTTP 404.</return>
	/// <response code="200">If the product is found.</response>
	/// <response code="404">If the product is not found.</response>
	[HttpGet("{id}")]
	public IActionResult GetProductById(int id)
	{
		var product = // get product from a service
		if(product is null)
		{
			return NotFound();
		}
		
		return Ok(product);
	}
}

For many apps, a basic health probe configuration that reports the app’s availability to process request is sufficient to discover the status of the app

At Startup.cs we add the following

public void ConfigureServices(IServiceCollection services)
{
	// ... other configuration
	services.AddScoped<IUserService, UserService>();

	// add health checks
	services.AddHealthChecks();
}

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
	// ... other configuration

	// map health checks to an specific endpoint
	app.UseHealthChecks("/beat");
}

This endpoint will now be available at the service’s root. If this publish f.e. at port 5000, we can now call the following endpoint

http://localhost:5000/beat

// response
status 200, Healthy

The problem with this basic check is that if something fails inside the constructor of a controller or a service, this still returns status 200, Healthy.

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Ambas son dos maneras de reutilizar código para crear nuevas clases

Herencia

Permite que una subclase herede propiedades y métodos de su superclase o clase base. Facilita la reutilización de código y creación de relaciones jerárquicas entre clases.

Sin embargo puede llevar a estructuras de clases rígidas y complejas, y a problemas de acoplamiento.

Composición

En lugar de heredad de una clase base, la clase se “compone” de otras clases incluyendo instancias de otras clases como campos.

Esto promueve un diseño más modular y flexible ya que permite cambiar el comportamiento en tiempo de ejecución.

Depende del problema específico pero la composición suele ser preferida por su flexibilidad y capacidad para evitar problemas comunes de herencia.

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These principles establish practices that helps maintain and extend software as it grows.

S - single responsibility
O - open/closed
L - liskov substitution
I - interface segregation
D - dependency inversion

Single responsibility

A class should have only one job - it should have only one reason to change

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(This is an implementation example. For an explanation on this, please check my other post: SQL Triggers & Stored Procedures)

Function example

-- example of function that triggers when an entry is inserted into a table
--   and manages this data inserting data as needed in another table
CREATE OR REPLACE FUNCTION your_schema.my_function_name(arg1 character varying, data character varying)
	RETURNS character varying
	LANGUAGE plpgsql
AS $function$
BEGIN

-- check if already exists in the other table
IF
	(SELECT COUNT(*) FROM your_schema.other_table WHERE name=arg1) > 0) THEN
		RETURN 'Error: 1210. data already exists';
END IF;

-- insert and manage data
INSERT INTO your_schema.other_table (name, data) VALUES (arg1, data);
RETURN 'Success: 1200';

END $function$;

How to Debug in DBeaver

There are two options, logs or break the function with an exception.

If logs are enough, you just write the message to output per console.

RAISE NOTICE 'this is null';

To see logs in DBeaver click here. Then you may execute the function to see the logs.

If you want to break the function runtime with an exception, you write the following instead

RAISE EXCEPTION SQLSTATE '90001' USING MESSAGE = 'error. this already exists';

SQL View

A view in SQL is essentially a virtual table. It doesn’t store data physically. Instead it presents data from one or more underlying tables through a predefined SQL query. Think of it as a saved query that you can treat like a table.

• Views don’t hold data themselves. When you query a view, the database executes the underlying query to fetch data in real-time.
• Views can simplify complex queries by encapsulating them. Instead of writing a complex JOIN or subquery each time, you select from the view.
• Views can restrict user access to specific rows or columns, enhancing security by exposing only necessary data.
• Since views are generated on the fly, they always reflect the current state.

CREATE OR REPLACE VIEW active_customers AS
SELECT customer_id, name, email
FROM customers
WHERE status = 'active';

When to use a view

• Use it to simplify complex queries that you use frequently and you need the most current data every time.
• Restrict user access to specific data by exposing only certain columns or rows through a view

Materialized View

A materialized view is like a regular view, but it stores the query result’s phisically and it doesn’t involve executing the underlying query each time.

• Since data is precomputed and stored, querying a materialized view is faster for complex queries over large datasets.
• Because of this, data in a materialized view can become outdated and needs to be refreshed periodically.

CREATE OR REPLACE MATERIALIZED VIEW sales_summary AS
SELECT product_id, SUM(quantity) AS total_quantity
FROM sales
GROUP BY product_id;

When to use a materialized view

• Is ideal for speeding up complex queries that are resource-intensive and slow to execute.
• Suitable for scenarios where data doesn’t change frequently and fast read performance is needed.
• You need to tolerate data that’s not always up-to-date

SQL Triggers

A SQL trigger is a code block that executes automatically when a specified event occurs on a table or view, such as an insert, update or delete.

A trigger can be used to perform actions before or after the event such as checking data integrity or spread data changes between tables.

Such an example would be to create a trigger that prevents users from inserting or updating data in a table if the data violates a rule, such as a maximum length or a required field.

CREATE TRIGGER trigger_name
BEFORE/AFTER event
ON table_name
FOR EACH ROW
BEGIN
	-- trigger code or call to procedure
END;

SQL Stored Procedures

A SQL Procedure is a code block that performs one or more tasks and can be called by other programs or queries. A procedure can accept parameters, return values and use variables, loops and conditional statements.

A procedure can be used to encapsulate complex logic or reuse code.

CREATE PROCEDURE procedure_name (parameters)
BEGIN
	-- procedure code
END;

Best practices

• Use descriptive and consistent names.
• Document your code with comments and explain the purpose and logic
• Avoid using too many or complex procedures or functions. This may affect performance or reliability of your database. They really increase difficulty to follow an operation.

Reference(s)

https://www.linkedin.com/advice/3/how-do-you-use-sql-triggers-procedures-functions?lang=en

Before raw string literals

The main problems with long string literals were:

• strings that include double quotes tend to be unreadable
• identation looks messy in multiline strings

we have the following json we want to put into a string literal

{
	"number": 42,
	"text": "Hello, world",
	"nested": { "flag": true}
}

an ordinary quoted string literal looks like this:

string json = "{\r\n  \"number\": 42,\r\n  \"text\": \"Hello, world\",\r\n  \"nested\": { \"flag\": true }\r\n}"

verbatim string literals work slightly better here but will be missaligned when used over nested code. Also quotes look different as they still need to be scaped.

foreach(var item in list)
{
	if(Check(item))
	{
		string json = @"{
  ""number"": 42,
  ""text"": ""Hello, world"",
  ""nested"": { ""flag"": true }
}";
	}
}

Using raw string literals

Here’s how this example looks using raw literals.

foreach(var item in list)
{
	if(Check(item))
	{
		string json = """
		{
			"number": 42,
			"text": "Hello, world",
			"nested": { "flag": true }
		}
		""";
	}
}

Inside raw literals we don’t need to scape chars. Also we’re able to indent our code.

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Named method vs anonymous method

A named method is a method that can be called by its name:

// named method declaration
public string Join(string s1, string s2)
{
	return s1+s2;
}

// named method usage
var result = Join("this is ", "a joined string");

An anonymous method is a method that is passed as an argument to a function, without the need for its name. These methods can be constructed at runtime or be evaluated from a lambda expression.

// declaration
public void ProcessBook(Action<Book> process, List<Book> books)
{
	foreach (Book b in books)
	{
		process(b);
	}
}

// usage - print book titles
ProcessBook(bookList, book => Console.WriteLine(book.Title))

Here book => Console.WriteLine(book.Title) is the lambda expression and it’s result is an anonymous function that will be run by the method ProcessBook

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