OSGi Log Service Tutorial using Twitter as back-end

1. 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]
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2. 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]
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3. Architecture big picture
cmp Big Picture
Client side Back-end
MyBundle LogReade rService
LogSe rvice
Conso leLog
LogSe rvice Logger LogListener
LogListener
HttpSe rv ice
LogSe rvice
Log2Tw itter
LogReade rService
ConfigAdmin
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4. Logging API
class log
Client side Back-end
«interface»
LogService
+ LOG_DEBUG: int = 4 {readOnly} «interface»
LogReaderService
+ LOG_ERROR: int = 1 {readOnly}
+ LOG_INFO: int = 3 {readOnly} + addLogListener(LogListener) : void
+ LOG_WARNING: int = 2 {readOnly} + getLog() : Enumeration
+ removeLogListener(LogListener) : void
+ log(int, String) : void
+ log(int, String, Throwable) : void
+ log(ServiceReference, int, String) : void
+ log(ServiceReference, int, String, Throwable) : void
EventListener
«interface»
«interface» LogListener
LogEntry + logged(LogEntry) : void
+ getBundle() : Bundle
+ getException() : Throwable
+ getLevel() : int
+ getMessage() : String
+ getServiceReference() : ServiceReference
+ getTime() : long
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5. LogService interaction
sd Logging
«service» «service» «interface»
LogServiceImpl LogReaderServiceImpl MyListener
Client :LogService :LogReaderService :LogListener
log(int, String)
fireEvent(LogEntry)
addLogListener(LogListener)
log(int, String)
fireEvent(LogEntry)
logged(LogEntry)
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6. 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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7. Pax bridge
sd JCL
Commons-API Pax Bridge LogService
:LogFactory «interface» «interface» Log4jLogger :PaxLoggingServiceImpl :LogReaderServiceImpl
JCLLogger :Log PaxLoggerImpl :Logger
Client :PaxLogger
getLog(Cl ass) :Log
info(Object)
inform(String, Throwable)
info(Object)
handleEvents(Bundle, ServiceReference, int, String, Throwable)
fireEvent(LogEntry)
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8. 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 Basic back-end
logger Defines a Log command
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9. Back-end configuration
 UseConfig Admin and FileInstall to manage
dynamically log listeners
cmp Logging
Configura tionAdmin
File Install ConfigAdmin
ManagedService [service.pi d = org.ops4j.pax.logging]
watching
LoggingServ ice
etc/org.ops4j.p ax.logging.cfg
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10. 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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11. 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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12. 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
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