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Logger System LLD (Java)

#architecture#interview#java#systemdesign

Problem Statement

Design a Logger Framework that supports:

  • Multiple Log Levels

    • DEBUG
    • INFO
    • WARN
    • ERROR
    • FATAL

  • Multiple Output Destinations

    • Console
    • File
    • Database
    • Kafka (future)
    • ElasticSearch (future)

  • Configurable Minimum Log Level

  • Thread Safe

  • Easy to extend

Functional Requirements

  • Log message
  • Timestamp
  • Log Level
  • Destination
  • Format message

Non Functional Requirements

  • Extensible
  • Low Coupling
  • High Cohesion
  • Open Closed Principle
  • Thread Safe
  • Singleton Logger

Interview Expectation 

Application
│
▼
Logger (Singleton)
│
▼
LogManager
│
▼
Formatter
│
▼
Appender
├── ConsoleAppender
├── FileAppender
├── DatabaseAppender
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Design Patterns Used

PatternWhy
SingletonOne Logger instance
StrategyDifferent Appenders
FactoryCreate appenders
BuilderBuild LogMessage
Chain of Responsibility (Optional)Multiple appenders
Dependency InjectionLoose coupling

Step 1: Log Level 

publicenumLogLevel{DEBUG(1),INFO(2),WARN(3),ERROR(4),FATAL(5);privatefinalintpriority;LogLevel(intpriority){this.priority=priority;}publicintgetPriority(){returnpriority;}}
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Step 2: Log Message 

importjava.time.LocalDateTime;publicclassLogMessage{privatefinalLogLevellevel;privatefinalStringmessage;privatefinalLocalDateTimetimestamp;publicLogMessage(LogLevellevel,Stringmessage){this.level=level;this.message=message;this.timestamp=LocalDateTime.now();}publicLogLevelgetLevel(){returnlevel;}publicStringgetMessage(){returnmessage;}publicLocalDateTimegetTimestamp(){returntimestamp;}}
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Step 3: Formatter 

publicinterfaceLogFormatter{Stringformat(LogMessagemessage);}
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Default Formatter 

publicclassDefaultFormatterimplementsLogFormatter{@OverridepublicStringformat(LogMessagemessage){returnString.format("[%s] [%s] %s",message.getTimestamp(),message.getLevel(),message.getMessage());}}
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Step 4: Appender 

publicinterfaceAppender{voidappend(LogMessagemessage);}
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Console Appender 

publicclassConsoleAppenderimplementsAppender{privatefinalLogFormatterformatter;publicConsoleAppender(LogFormatterformatter){this.formatter=formatter;}@Overridepublicvoidappend(LogMessagemessage){System.out.println(formatter.format(message));}}
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File Appender 

importjava.io.FileWriter;importjava.io.IOException;publicclassFileAppenderimplementsAppender{privatefinalLogFormatterformatter;publicFileAppender(LogFormatterformatter){this.formatter=formatter;}@Overridepublicvoidappend(LogMessagemessage){try(FileWriterwriter=newFileWriter("app.log",true)){writer.write(formatter.format(message));writer.write("\n");}catch(IOExceptione){e.printStackTrace();}}}
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Step 5: Logger 

importjava.util.List;publicclassLogger{privatestaticvolatileLoggerinstance;privatefinalList<Appender>appenders;privateLogLevelminimumLevel;privateLogger(List<Appender>appenders,LogLevelminimumLevel){this.appenders=appenders;this.minimumLevel=minimumLevel;}publicstaticLoggergetInstance(List<Appender>appenders,LogLevellevel){if(instance==null){synchronized(Logger.class){if(instance==null){instance=newLogger(appenders,level);}}}returninstance;}publicvoidlog(LogLevellevel,Stringmessage){if(level.getPriority()<minimumLevel.getPriority())return;LogMessagelogMessage=newLogMessage(level,message);for(Appenderappender:appenders)appender.append(logMessage);}}
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Client 

importjava.util.List;publicclassMain{publicstaticvoidmain(String[]args){LogFormatterformatter=newDefaultFormatter();Appenderconsole=newConsoleAppender(formatter);Appenderfile=newFileAppender(formatter);Loggerlogger=Logger.getInstance(List.of(console,file),LogLevel.INFO);logger.log(LogLevel.INFO,"Application Started");logger.log(LogLevel.DEBUG,"Debug Message");logger.log(LogLevel.ERROR,"Database Down");}}
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Output 

[2026-07-09T10:30] [INFO] Application Started
[2026-07-09T10:31] [ERROR] Database Down
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(DEBUG ignored)

UML 

 +------------------+
| Logger |
+------------------+
| minimumLevel |
| appenders |
+------------------+
|
-------------------------------
| |
▼ ▼
+----------------+ +----------------+
| Appender | | LogFormatter |
+----------------+ +----------------+
| append() | | format() |
+----------------+ +----------------+
▲
│
-----------------------------
| | |
▼ ▼ ▼
Console File Database
Appender Appender Appender
▲
|
+--------------+
| LogMessage |
+--------------+
| level |
| timestamp |
| message |
+--------------+
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SOLID Principles

PrincipleApplication
SRPLogMessage, Appender, Formatter, Logger each have a single responsibility.
OCPAdd new appenders or formatters without modifying existing classes.
LSPAny Appender implementation can replace another.
ISPSmall focused interfaces (Appender, LogFormatter).
DIPLogger depends on abstractions rather than concrete implementations.

Common Interview Follow-up Questions

  1. How would you support asynchronous logging?
  • Introduce a BlockingQueue<LogMessage> and a dedicated worker thread or thread pool that consumes log events and dispatches them to appenders.
  1. How would you prevent logging from blocking application threads?
  • Use non-blocking queues, batching, and asynchronous appenders with configurable backpressure policies.
  1. How would you support log rotation?
  • Create a RollingFileAppender that rotates files based on size or time (daily/hourly) and manages retention.
  1. How would you support multiple log formats?
  • Implement additional LogFormatter strategies such as JsonFormatter, XmlFormatter, or custom templates.
  1. How would you enable configuration from a properties or YAML file?
  • Build a LoggerConfiguration component and an AppenderFactory to instantiate appenders based on external configuration.
  1. How would you make the logger thread-safe?
  • Keep shared state immutable where possible, use thread-safe collections if appenders are modified at runtime, and ensure appenders handle concurrent writes correctly.
  1. How would you add contextual information (e.g., request ID, user ID)?
  • Introduce a Mapped Diagnostic Context (MDC) using ThreadLocal to automatically enrich each LogMessage.
  1. How would you support filtering?
  • Add a LogFilter abstraction (Strategy pattern) to filter messages by package, marker, or custom predicates before dispatching.

Possible Extensions

  • AsyncLogger
  • RollingFileAppender
  • DatabaseAppender
  • KafkaAppender
  • ElasticSearchAppender
  • JsonFormatter
  • XMLFormatter
  • LoggerFactory
  • LoggerConfiguration
  • LogFilter
  • MDC (Mapped Diagnostic Context)
  • Rate limiting and sampling
  • Dynamic log level updates
  • Metrics for dropped/processed log events

This design demonstrates strong OOP fundamentals, SOLID principles, and extensibility while remaining concise enough to explain in a typical 30–45 minute LLD interview.

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