Designing a Logging Framework

Design a flexible logging framework that supports multiple log levels, different output destinations, and various formatting strategies.

What is the Logging Framework Problem?

Design a flexible logging framework that supports multiple log levels, different output destinations, and various formatting strategies. It must work safely when multiple threads log at the same time, without mixing up or losing log messages.

In this problem, you’ll build a framework that processes log messages through a pipeline of filters and handlers, ensuring high performance even under high concurrency.

Problem Overview

Design a logging utility that allows developers to record application events with different severity levels and route them to various destinations.

Core Requirements

Functional Requirements:

Log Levels: Support DEBUG, INFO, WARNING, ERROR, and FATAL severities.
Rich Metadata: Include timestamp, log level, message, thread name, and logger name in every record.
Multi-Destination: Route logs to Console, File, or Database simultaneously.
Configurability: Enable/disable log levels per logger; configure output destinations dynamically.
Custom Formatting: Support different strategies (e.g., Simple Text, JSON) for different destinations.
Thread Safety: Ensure logs aren’t garbled or lost when multiple threads log concurrently.

Non-Functional Requirements:

Clean OOD: Well-defined roles for Loggers, Handlers, Formatters, and Filters.
Open-Closed Principle: Easily add new log levels or output sinks without modifying core code.
Performance: Low latency—the logging process should not block the main application.
Chain of Responsibility: Use this pattern to allow logs to be processed by multiple handlers in sequence.
Maintainability: The core logging logic should be easy to test, understand, and maintain.

What’s Expected?

1. System Architecture

The framework follows a modular pipeline: Logger → Filter → Handler → Formatter.

2. Key Classes to Design

classDiagram
    class Logger {
        <<Singleton>>
        -LogLevel level
        -List~Handler~ handlers
        +log(level, msg)
        +addHandler(handler)
    }
    
    class Handler {
        <<abstract>>
        -LogLevel level
        -Formatter formatter
        +publish(logRecord)*
    }
    
    class ConsoleHandler {
        +publish()
    }
    
    class FileHandler {
        +publish()
    }

    class Formatter {
        <<interface>>
        +format(logRecord) String
    }

    Logger o-- Handler
    Handler --> Formatter
    Handler <|-- ConsoleHandler
    Handler <|-- FileHandler

System Flow

Log Processing Flow

Key Design Challenges

1. Handling Multiple Destinations

A user might want to log to the console AND a file at the same time. Hardcoding this in the Logger class violates the Single Responsibility Principle.

Solution: Use the Chain of Responsibility (or an Observer-like list of Handlers). The Logger simply broadcasts the message to a list of Appenders. Each Appender knows how to handle its specific destination.

2. Efficient Formatting

Different destinations need different formats (e.g., a simple string for the console but JSON for an ELK stack).

Solution: Use the Strategy Pattern. Each Handler is injected with a Formatter strategy. This allows you to mix and match handlers and formats dynamically (e.g., a FileHandler with a JSONFormatter).

3. Concurrency and Blocking

If multiple threads write to the same file at once, the logs will get garbled. If the file system is slow, it could hang the entire application.

Solution: Use Asynchronous Logging. The Logger puts the log message into a thread-safe BlockingQueue, and a background thread consumes the queue and writes to the destinations. This keeps the main application thread fast.

What You’ll Learn

By solving this problem, you’ll master:

✅ Pipeline Architecture - Building extensible processing stages.
✅ Decoupling - Separating log generation from log destination.
✅ Concurrency Patterns - Safe resource access across threads.
✅ SOLID Principles - Designing for long-term maintainability.

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