Instruction file imported from hieumh/tracking-finance-stock (.github/instructions/logging-backend-go.instructions.md). Copyright stays with the author.

DDD Backend Logging Instructions for Go - VS Code Copilot

Overview

This document provides comprehensive instructions for implementing logging best practices in Domain-Driven Design (DDD) backend systems using Go. Follow these guidelines when generating code suggestions for logging in DDD architectures.

Core Logging Principles

1. Layer-Specific Logging Rules

Domain Layer

NEVER log infrastructure concerns (database connections, HTTP details)
ALWAYS log business-critical events and domain state changes
LOG aggregate creation, modification, and business rule violations
INCLUDE aggregate ID, correlation ID, and business context
AVOID logging sensitive domain data in plain text

// ✅ GOOD - Domain logging
log.Info("Order created",
    slog.String("order_id", order.ID),
    slog.String("customer_id", order.CustomerID),
    slog.Float64("total_amount", order.TotalAmount),
    slog.String("correlation_id", ctx.Value("correlation_id").(string)))

// ❌ BAD - Infrastructure concerns in domain
log.Debug("Database connection established for order creation")

Application Layer

LOG use case/command execution start and completion
LOG authorization and validation results
LOG transaction boundaries and performance metrics
INCLUDE execution time, user context, and command details
AVOID logging sensitive data directly (use masking functions)
AVOID logging duration executed commands unless necessary for performance monitoring

start := time.Now()
// ... command execution
duration := time.Since(start)
log.Debug("CreateOrderCommand duration",
    slog.String("command_id", cmd.ID),
    slog.Duration("duration", duration))

Infrastructure Layer

LOG external service calls and their outcomes
LOG database operations, connections, and performance
LOG message queue operations and failures
INCLUDE connection details, retry attempts, and error details

2. Structured Logging Format

Required Fields

Always include these fields in log entries:

timestamp - RFC3339 format (handled by slog automatically)
level - Log level (DEBUG, INFO, WARN, ERROR)
message - Human-readable message
correlation_id - Request/operation correlation ID
service_id - Microservice or bounded context identifier
user_id - Acting user identifier (when available)

Contextual Fields (when applicable)

aggregate_id - Domain aggregate identifier
aggregate_type - Type of aggregate being operated on
domain_event - Domain event type being processed
use_case_type - Application use case or command type
transaction_id - Database transaction identifier
duration - Operation execution time

3. Go Logging Setup

Recommended Logger Configuration

package logging

import (
    "log/slog"
    "os"
    "context"
)

type Logger struct {
    *slog.Logger
}

func NewLogger(serviceName string) *Logger {
    opts := &slog.HandlerOptions{
        Level: slog.LevelDebug,
        AddSource: true,
    }

    handler := slog.NewJSONHandler(os.Stdout, opts)
    logger := slog.New(handler).With(
        slog.String("service", serviceName),
    )

    return &Logger{Logger: logger}
}

// WithContext adds correlation ID and other context info
func (l *Logger) WithContext(ctx context.Context) *slog.Logger {
    attrs := []slog.Attr{}

    if correlationID := ctx.Value("correlation_id"); correlationID != nil {
        attrs = append(attrs, slog.String("correlation_id", correlationID.(string)))
    }

    if userID := ctx.Value("user_id"); userID != nil {
        attrs = append(attrs, slog.String("user_id", userID.(string)))
    }

    return l.With(attrs...)
}

4. Log Level Guidelines

INFO Level

Use for business-significant events that operators should be aware of:

Successful completion of use cases/commands
Domain events being raised or processed
Important business state transitions
User authentication and authorization events

DEBUG Level

Use for detailed execution flow helpful during development:

Method entry/exit in domain services
Internal domain logic flow
Query execution details
Cache hit/miss information

WARN Level

Use for potentially problematic situations that don't stop execution:

Business rule violations that are handled gracefully
Retry attempts for external services
Performance degradation warnings
Non-critical validation failures

ERROR Level

Use for errors that prevent normal operation:

Unhandled errors in use cases
External service failures that can't be retried
Data consistency violations
Authentication/authorization failures

Implementation Patterns

1. Domain Aggregate Logging

package domain

import (
    "context"
    "fmt"
    "log/slog"
)

type OrderAggregate struct {
    ID         string
    CustomerID string
    Status     OrderStatus
    logger     *config.Logger
}

func NewOrderAggregate(id, customerID string, logger *config.Logger) *OrderAggregate {
    return &OrderAggregate{
        ID:         id,
        CustomerID: customerID,
        Status:     OrderStatusPending,
        logger:     logger,
    }
}

func (o *OrderAggregate) ChangeStatus(ctx context.Context, newStatus OrderStatus) error {
    oldStatus := o.Status

    // Validate business rules
    if !o.canChangeStatusTo(newStatus) {
        o.logger.Warn("Invalid status transition attempted",
            slog.String("order_id", o.ID),
            slog.String("from_status", string(oldStatus)),
            slog.String("to_status", string(newStatus)),
            slog.String("reason", "Business rule violation"),
            slog.String("correlation_id", getCorrelationID(ctx)))
        return fmt.Errorf("cannot change status from %s to %s", oldStatus, newStatus)
    }

    o.Status = newStatus

    o.logger.Info("Order status changed",
        slog.String("order_id", o.ID),
        slog.String("from_status", string(oldStatus)),
        slog.String("to_status", string(newStatus)),
        slog.String("user_id", getUserID(ctx)),
        slog.String("correlation_id", getCorrelationID(ctx)))

    // Raise domain event
    event := NewOrderStatusChangedEvent(o.ID, oldStatus, newStatus)
    o.raiseDomainEvent(event)

    o.logger.Debug("Domain event raised",
        slog.String("event_type", "OrderStatusChangedEvent"),
        slog.String("order_id", o.ID),
        slog.String("correlation_id", getCorrelationID(ctx)))

    return nil
}

func (o *OrderAggregate) canChangeStatusTo(newStatus OrderStatus) bool {
    // Business logic here
    return true
}

2. Application Service Logging

package application

import (
    "context"
    "fmt"
    "log/slog"
    "time"
)

type CreateOrderCommandHandler struct {
    orderService OrderService
    validator    Validator
    logger       *config.Logger  // Always inject *config.Logger, never *slog.Logger
}

func NewCreateOrderCommandHandler(
    orderService OrderService,
    validator Validator,
    logger *config.Logger,
) *CreateOrderCommandHandler {
    return &CreateOrderCommandHandler{
        orderService: orderService,
        validator:    validator,
        logger:       logger,
    }
}

func (h *CreateOrderCommandHandler) Handle(ctx context.Context, cmd CreateOrderCommand) (string, error) {
    // Always call WithContext at the top of every handler method.
    // This injects correlation_id / user_id and gives back a plain *slog.Logger to use locally.
    log := h.logger.WithContext(ctx)

    log.Info("CreateOrderCommand started",
        slog.String("command_id", cmd.ID),
        slog.String("customer_id", cmd.CustomerID),
        slog.Int("item_count", len(cmd.Items)),
        slog.String("command_type", "CreateOrder"))

    // Validation
    if err := h.validator.Validate(cmd); err != nil {
        log.Warn("CreateOrderCommand validation failed",
            slog.String("error", err.Error()),
            slog.String("command_type", "CreateOrder"))
        return "", fmt.Errorf("validation failed: %w", err)
    }

    // Business logic
    order, err := h.orderService.CreateOrder(ctx, cmd)
    if err != nil {
        log.Error("CreateOrderCommand failed",
            slog.String("error", err.Error()),
            slog.String("command_type", "CreateOrder"))
        return "", fmt.Errorf("failed to create order: %w", err)
    }

    log.Info("CreateOrderCommand completed successfully",
        slog.String("order_id", order.ID),
        slog.String("command_type", "CreateOrder"))

    return order.ID, nil
}

3. Infrastructure Repository Logging

package infrastructure

import (
    "context"
    "database/sql"
    "log/slog"
)

type OrderRepository struct {
    db     *sql.DB
    logger *config.Logger
}

func NewOrderRepository(db *sql.DB, logger *config.Logger) *OrderRepository {
    return &OrderRepository{
        db:     db,
        logger: logger,
    }
}

func (r *OrderRepository) Save(ctx context.Context, order *domain.Order) error {
    r.logger.Debug("Saving order to database",
        slog.String("order_id", order.ID),
        slog.String("correlation_id", getCorrelationID(ctx)))

    query := `INSERT INTO orders (id, customer_id, status, total_amount)
              VALUES ($1, $2, $3, $4)
              ON CONFLICT (id) DO UPDATE SET
              status = EXCLUDED.status, total_amount = EXCLUDED.total_amount`

    result, err := r.db.ExecContext(ctx, query,
        order.ID, order.CustomerID, order.Status, order.TotalAmount)

    if err != nil {
        r.logger.Error("Failed to save order",
            slog.String("order_id", order.ID),
            slog.String("error", err.Error()),
            slog.String("correlation_id", getCorrelationID(ctx)))
        return fmt.Errorf("failed to save order: %w", err)
    }

    rowsAffected, _ := result.RowsAffected()
    r.logger.Info("Order saved successfully",
        slog.String("order_id", order.ID),
        slog.Int64("rows_affected", rowsAffected),
        slog.String("correlation_id", getCorrelationID(ctx)))

    return nil
}

func (r *OrderRepository) FindByID(ctx context.Context, id string) (*domain.Order, error) {
    r.logger.Debug("Finding order by ID",
        slog.String("order_id", id),
        slog.String("correlation_id", getCorrelationID(ctx)))

    query := `SELECT id, customer_id, status, total_amount FROM orders WHERE id = $1`

    var order domain.Order
    err := r.db.QueryRowContext(ctx, query, id).Scan(
        &order.ID, &order.CustomerID, &order.Status, &order.TotalAmount)

    if err != nil {
        if err == sql.ErrNoRows {
            r.logger.Warn("Order not found",
                slog.String("order_id", id),
                slog.String("correlation_id", getCorrelationID(ctx)))
            return nil, domain.ErrOrderNotFound
        }

        r.logger.Error("Failed to find order",
            slog.String("order_id", id),
            slog.String("error", err.Error()),
            slog.String("correlation_id", getCorrelationID(ctx)))
        return nil, fmt.Errorf("failed to find order: %w", err)
    }

    r.logger.Debug("Order found successfully",
        slog.String("order_id", id),
        slog.String("correlation_id", getCorrelationID(ctx)))

    return &order, nil
}

4. Event Handler Logging

package application

import (
    "context"
    "log/slog"
)

type OrderStatusChangedEventHandler struct {
    notificationService NotificationService
    logger             *config.Logger
}

func NewOrderStatusChangedEventHandler(
    notificationService NotificationService,
    logger *config.Logger,
) *OrderStatusChangedEventHandler {
    return &OrderStatusChangedEventHandler{
        notificationService: notificationService,
        logger:             logger,
    }
}

func (h *OrderStatusChangedEventHandler) Handle(ctx context.Context, event domain.OrderStatusChangedEvent) error {
    log := h.logger.WithContext(ctx)
    log.Info("Processing domain event",
        slog.String("event_type", "OrderStatusChangedEvent"),
        slog.String("order_id", event.OrderID),
        slog.String("from_status", string(event.OldStatus)),
        slog.String("to_status", string(event.NewStatus)),
        slog.String("correlation_id", event.CorrelationID))

    if err := h.notificationService.SendOrderStatusUpdate(ctx, event); err != nil {
        log.Error("Failed to process domain event",
            slog.String("event_type", "OrderStatusChangedEvent"),
            slog.String("order_id", event.OrderID),
            slog.String("error", err.Error()),
            slog.String("correlation_id", event.CorrelationID))
        return fmt.Errorf("failed to send notification: %w", err)
    }

    log.Info("Domain event processed successfully",
        slog.String("event_type", "OrderStatusChangedEvent"),
        slog.String("order_id", event.OrderID),
        slog.String("correlation_id", event.CorrelationID))

    return nil
}

5. HTTP Middleware for Correlation ID

package middleware

import (
    "context"
    "log/slog"
    "net/http"
    "time"

    "github.com/google/uuid"
)

type LoggingMiddleware struct {
    logger *config.Logger
}

func NewLoggingMiddleware(logger *config.Logger) *LoggingMiddleware {
    return &LoggingMiddleware{logger: logger}
}

func (m *LoggingMiddleware) Handler(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        start := time.Now()

        // Get or generate correlation ID
        correlationID := r.Header.Get("X-Correlation-ID")
        if correlationID == "" {
            correlationID = uuid.New().String()
        }

        // Add correlation ID to context
        ctx := context.WithValue(r.Context(), "correlation_id", correlationID)
        r = r.WithContext(ctx)

        // Add correlation ID to response header
        w.Header().Set("X-Correlation-ID", correlationID)

        // Wrap response writer to capture status code
        wrappedWriter := &responseWriter{ResponseWriter: w, statusCode: http.StatusOK}

        m.logger.Info("HTTP request started",
            slog.String("method", r.Method),
            slog.String("path", r.URL.Path),
            slog.String("remote_addr", r.RemoteAddr),
            slog.String("user_agent", r.UserAgent()),
            slog.String("correlation_id", correlationID))

        next.ServeHTTP(wrappedWriter, r)

        duration := time.Since(start)
        m.logger.Info("HTTP request completed",
            slog.String("method", r.Method),
            slog.String("path", r.URL.Path),
            slog.Int("status_code", wrappedWriter.statusCode),
            slog.Duration("duration", duration),
            slog.String("correlation_id", correlationID))
    })
}

type responseWriter struct {
    http.ResponseWriter
    statusCode int
}

func (rw *responseWriter) WriteHeader(code int) {
    rw.statusCode = code
    rw.ResponseWriter.WriteHeader(code)
}

Security and Privacy Guidelines

1. Never Log Sensitive Data

Passwords, tokens, or API keys
Credit card numbers or payment details
Personal identification numbers
Full email addresses (use masked versions)
Full phone numbers (use masked versions)

2. Data Masking Patterns

package logging

import (
    "strings"
)

// MaskEmail masks email address for logging
func MaskEmail(email string) string {
    parts := strings.Split(email, "@")
    if len(parts) != 2 {
        return "invalid_email"
    }

    username := parts[0]
    domain := parts[1]

    if len(username) <= 2 {
        return "**@" + domain
    }

    return username[:2] + "***@" + domain
}

// MaskPhone masks phone number for logging
func MaskPhone(phone string) string {
    if len(phone) < 4 {
        return "****"
    }
    return "****" + phone[len(phone)-4:]
}

// Example usage
log.Info("User login attempt",
    slog.String("email", MaskEmail(user.Email)),
    slog.Bool("success", loginResult.IsSuccess),
    slog.String("correlation_id", correlationID))

Performance Considerations

1. Conditional Logging for Expensive Operations

// ✅ GOOD - Check log level before expensive operations
if logger.Enabled(ctx, slog.LevelDebug) {
    complexData := serializeComplexObject(calculationResult)
    logger.Debug("Complex calculation result",
        slog.String("order_id", orderID),
        slog.String("details", complexData),
        slog.String("correlation_id", correlationID))
}

2. Use Structured Logging Parameters

// ✅ GOOD - Structured parameters
logger.Info("Order processed",
    slog.String("order_id", order.ID),
    slog.Float64("amount", order.Amount))

// ❌ BAD - String formatting
logger.Info(fmt.Sprintf("Order processed - OrderId: %s, Amount: %.2f",
    order.ID, order.Amount))

Error Handling and Logging

1. Error Wrapping and Logging Pattern

func (s *OrderService) ProcessOrder(ctx context.Context, orderID string) error {
    logger := s.logger.With(
        slog.String("order_id", orderID),
        slog.String("correlation_id", getCorrelationID(ctx)))

    order, err := s.repository.FindByID(ctx, orderID)
    if err != nil {
        if errors.Is(err, domain.ErrOrderNotFound) {
            logger.Warn("Order not found for processing",
                slog.String("error", err.Error()))
            return fmt.Errorf("order not found: %w", err)
        }

        logger.Error("Failed to retrieve order for processing",
            slog.String("error", err.Error()))
        return fmt.Errorf("failed to retrieve order: %w", err)
    }

    if err := order.Process(); err != nil {
        var domainErr *domain.DomainError
        if errors.As(err, &domainErr) {
            logger.Warn("Domain rule violation during order processing",
                slog.String("rule", domainErr.Rule),
                slog.String("details", domainErr.Details))
        } else {
            logger.Error("Unexpected error during order processing",
                slog.String("error", err.Error()))
        }
        return fmt.Errorf("failed to process order: %w", err)
    }

    logger.Info("Order processed successfully")
    return nil
}

2. Context Helper Functions

package logging

import "context"

func getCorrelationID(ctx context.Context) string {
    if id, ok := ctx.Value("correlation_id").(string); ok {
        return id
    }
    return "unknown"
}

func getUserID(ctx context.Context) string {
    if id, ok := ctx.Value("user_id").(string); ok {
        return id
    }
    return "anonymous"
}

func getTraceID(ctx context.Context) string {
    if id, ok := ctx.Value("trace_id").(string); ok {
        return id
    }
    return ""
}

Monitoring and Observability

1. Health Check Logging

func (h *HealthChecker) CheckDatabase(ctx context.Context) error {
    start := time.Now()

    err := h.db.PingContext(ctx)
    duration := time.Since(start)

    if err != nil {
        h.logger.Error("Database health check failed",
            slog.String("component", "database"),
            slog.Duration("duration", duration),
            slog.String("error", err.Error()))
        return err
    }

    h.logger.Info("Database health check passed",
        slog.String("component", "database"),
        slog.Duration("duration", duration))

    return nil
}

2. Performance Metrics

func (s *Service) measureOperation(ctx context.Context, operationName string, fn func() error) error {
    start := time.Now()

    err := fn()
    duration := time.Since(start)

    s.logger.Info("Operation completed",
        slog.String("operation", operationName),
        slog.Duration("duration", duration),
        slog.Bool("success", err == nil),
        slog.String("correlation_id", getCorrelationID(ctx)))

    return err
}

Testing Considerations

1. Logger Mock for Testing

package testing

import (
    "bytes"
    "log/slog"
    "testing"
)

func NewTestLogger(t *testing.T) (*config.Logger, *bytes.Buffer) {
    var buf bytes.Buffer
    handler := slog.NewJSONHandler(&buf, &slog.HandlerOptions{
        Level: slog.LevelDebug,
    })
    return &config.Logger{Logger: slog.New(handler)}, &buf
}

// Example test
func TestOrderService_CreateOrder(t *testing.T) {
    logger, logBuffer := NewTestLogger(t)
    service := NewOrderService(mockRepo, logger)

    // Act
    err := service.CreateOrder(ctx, command)

    // Assert
    assert.NoError(t, err)

    logOutput := logBuffer.String()
    assert.Contains(t, logOutput, "Order created")
    assert.Contains(t, logOutput, command.ID)
}

2. Integration Test Logging

func TestIntegration_OrderWorkflow(t *testing.T) {
    // Setup test logger that outputs to test logs
    logger := slog.New(slog.NewTextHandler(testWriter{t}, &slog.HandlerOptions{
        Level: slog.LevelDebug,
    }))

    // Test implementation with detailed logging
}

type testWriter struct {
    t *testing.T
}

func (tw testWriter) Write(p []byte) (n int, err error) {
    tw.t.Log(string(p))
    return len(p), nil
}

Configuration and Environment Setup

1. Environment-Based Logger Configuration

package config

import (
    "log/slog"
    "os"
    "strings"
)

type LogConfig struct {
    Level   string `json:"level" env:"LOG_LEVEL" default:"info"`
    Format  string `json:"format" env:"LOG_FORMAT" default:"json"`
    Service string `json:"service" env:"SERVICE_NAME" default:"unknown"`
}

func NewLogger(config LogConfig) *slog.Logger {
    var level slog.Level
    switch strings.ToLower(config.Level) {
    case "debug":
        level = slog.LevelDebug
    case "info":
        level = slog.LevelInfo
    case "warn":
        level = slog.LevelWarn
    case "error":
        level = slog.LevelError
    default:
        level = slog.LevelInfo
    }

    opts := &slog.HandlerOptions{
        Level:     level,
        AddSource: level == slog.LevelDebug,
    }

    var handler slog.Handler
    if config.Format == "text" {
        handler = slog.NewTextHandler(os.Stdout, opts)
    } else {
        handler = slog.NewJSONHandler(os.Stdout, opts)
    }

    return slog.New(handler).With(
        slog.String("service", config.Service),
    )
}

Quick Reference Checklist

When implementing logging in Go DDD backend systems, ensure:

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