Velossity@Training 2012-Services/Log/11

OSGi Logging: The basics

Two parts in a logger: the client bundle who wants to log and the back-end to manage logs– A very simple API for the client side– The logging back-end, or how the log entries are

displayed, stored, or processed is currently not defined by the OSGI specifications [1]

Velossity@Training 2012-Services/Log/12

Separation of concerns

Application developers just choose when and what to log, with which log level

Platform administrators choose how the log entries are displayed, stored, or processed– logging strategy can be defined not only in terms

of packages and classes, but also in terms of services and bundles [1]

Velossity@Training 2012-Services/Log/13

Architecture big picturecmp Big Picture

Back-endClient side

Logger

MyBundle

HttpSe rv ice

ConfigAdmin

ConsoleLog

Log2Tw itterLogReade rService

LogReade rService

LogSe rvice

LogSe rvice

LogSe rvice

LogListener

LogListener

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Logging APIclass log

Back-endClient side

«interface»LogEntry

+ getBundle() : Bundle+ getException() : Throwable+ getLevel() : int+ getMessage() : String+ getServiceReference() : ServiceReference+ getTime() : long

EventListener

«interface»LogListener

+ logged(LogEntry) : void

«interface»LogReaderService

+ addLogListener(LogListener) : void+ getLog() : Enumeration+ removeLogListener(LogListener) : void

«interface»LogService

+ LOG_DEBUG: int = 4 {readOnly}+ LOG_ERROR: int = 1 {readOnly}+ LOG_INFO: int = 3 {readOnly}+ LOG_WARNING: int = 2 {readOnly}

+ log(int, String) : void+ log(int, String, Throwable) : void+ log(ServiceReference, int, String) : void+ log(ServiceReference, int, String, Throwable) : void

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LogService interaction

sd Logging

Client

«service»

LogServiceImpl:LogService

«service»

LogReaderServiceImpl:LogReaderService

«interface»

MyListener:LogListener

log(int, String)

fireEvent(LogEntry)

addLogListener(LogListener)

log(int, String)

fireEvent(LogEntry)

logged(LogEntry)

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Alternative: The logging bridge

The easy way to log with OSGi is to use an “old java logging API” acting like a façade

Such solution uses a LogService implementation as the back-end for the Log implementation– Pros: compliant with Java world– Cons: not easily swappable as your code don’t

rely on the standard OSGi service

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Pax bridge

sd JCL

Pax BridgeCommons-API LogService

:LogFactory

Client

«interface»

JCLLogger :Log

«interface»

PaxLoggerImpl:PaxLogger

Log4jLogger:Logger

:PaxLoggingServiceImpl :LogReaderServiceImpl

getLog(Cl ass) :Log

info(Object)

inform(String, Throwable)

info(Object)

handleEvents(Bundle, ServiceReference, int, String, Throwable)

fireEvent(LogEntry)

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JCL open implementations

Pax Logging Compliant with most popular logger APIs (JCL, sl4j,JDK logging, …)

Uses Log4j as back-end

Easily and dynamically configurable

Knopflerfish logger

Basic back-end

Defines a Log command

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Back-end configuration

Use Config Admin and FileInstall to manage dynamically log listeners

cmp Logging

LoggingServ ice

ConfigAdmin

etc/org.ops4j.p ax.logging.cfg

File Install

ManagedService [service.pi d = org.ops4j.pax.logging]

watching

Configura tionAdmin

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Recommendations

Choose one API for all your clients Define clearly the log levels Bind to the Logging Service only if using

Dynamic frameworks Ensure that log4j.jar, commons-logging.jar,

and any other legacy API that you want to use, are NOT included in your bundle jars

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References

[1] OSGi logging: http://code.google.com/p/osgi-logging/

[2] Pax-Logging: http://www.ops4j.org/projects/pax/logging/

[3] FileInstall: http://felix.apache.org/site/apache-felix-file-install.html

[4] Knopflerfish: http://www.knopflerfish.org/releases/3.1.0/docs/bundledoc/log/index.html

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Maven multi-modules organization

| aggregator -- Run multi-modules build

| parent -- Parent of all modules

| JCL -- Logging with JCL API

| ServiceBinding -- Logging with LogService API

| Log2Twitter -- Logging back-end sending direct messages to Twitter

| TwitterService -- OSGi wrapper relying on Twitter4J*

*http://twitter4j.org/en/index.html

Software Configuration Management:svn co https://osgipatterns.svn.sourceforge.net/svnroot/osgipatterns/services/logging/tags/logging.aggregator-1.0.1

Continuous Integration:http://velossity.zapto.org:8080/job/Logging/modules