Logging and Correlation

public sealed class EmployeeService(
    HrDbContext db,
    ILogger<EmployeeService> logger) : IEmployeeService
{
    public async Task<EmployeeResponse> CreateAsync(
        CreateEmployeeRequest request,
        CancellationToken cancellationToken)
    {
        Employee employee = new()
        {
            FullName = request.FullName,
            WorkEmail = request.WorkEmail,
            DepartmentId = request.DepartmentId,
        };

        db.Employees.Add(employee);
        await db.SaveChangesAsync(cancellationToken);

        logger.LogInformation(
            "Created employee {EmployeeId} in department {DepartmentId}",
            employee.Id,
            employee.DepartmentId);

        return EmployeeResponse.From(employee);
    }

    public async Task<EmployeeResponse?> FindAsync(int id, CancellationToken cancellationToken)
    {
        Employee? employee = await db.Employees.FindAsync([id], cancellationToken);

        if (employee is null)
        {
            // Not an error. A caller asking for a record that is gone is normal.
            logger.LogDebug("Employee {EmployeeId} not found", id);
            return null;
        }

        return EmployeeResponse.From(employee);
    }
}

public sealed class CorrelationIdMiddleware(RequestDelegate next)
{
    private const string HeaderName = "X-Correlation-Id";

    public async Task InvokeAsync(HttpContext context, ILogger<CorrelationIdMiddleware> logger)
    {
        // Reuse the caller's id if there is one, so a chain of calls shares it.
        string correlationId =
            context.Request.Headers.TryGetValue(HeaderName, out var supplied)
            && !string.IsNullOrWhiteSpace(supplied)
                ? supplied.ToString()
                : Activity.Current?.TraceId.ToString() ?? Guid.NewGuid().ToString("N");

        context.Response.Headers[HeaderName] = correlationId;

        // Everything logged inside this using block carries the id.
        using (logger.BeginScope(new Dictionary<string, object>
        {
            ["CorrelationId"] = correlationId,
            ["RequestPath"] = context.Request.Path.ToString(),
        }))
        {
            await next(context);
        }
    }
}

// Program.cs — early, so the scope covers as much of the request as possible
app.UseMiddleware<CorrelationIdMiddleware>();