ARQ - Logging

ARQ uses SLF4j as the API and ships with Log4J as a deployment system.  You can use Java 1.4 logging instead.

ARQ does not output any logging messages at level INFO in normal operation. The code uses level TRACE and DEBUG.  Running with logging set to an application at INFO will cause no output in normal operation. Output below INFO can be very verbose and is intended mainly to help debug ARQ. WARN and FATAL messages are only used when something is wrong.

The root of all the loggers is org.apache.jena. org.apache.jena.query is the application API.  org.apache.jena.sparql is the implementation and extensions points.

If using in Tomcat, or other system that provides complex class loading arrangements, be careful about loading from jars in both the web application and the system directories as this can cause separate logging systems to be created (this may not matter).

The ARQ and RIOT command line utilities look for a file "log4j.properties" in the current directory to control logging during command execution.

Logger Names Name Constant Logger Use
org.apache.jena.arq.info ARQ.logInfoName ARQ.getLoggerInfo() General information
org.apache.jena.arq.exec ARQ.logExecName ARQ.getLoggerExec() Execution information

There is some code to cover simple situations. See the log4j documentation for full details of log4j.

The reading of log4j.properties from the current directory is achieved by a call to org.apache.jena.atlas.logging.Log.setlog4j().

Example log4j.properties file:

log4j.rootLogger=INFO, stdlog

log4j.appender.stdlog=org.apache.log4j.ConsoleAppender
## log4j.appender.stdlog.target=System.err
log4j.appender.stdlog.layout=org.apache.log4j.PatternLayout
log4j.appender.stdlog.layout.ConversionPattern=%d{HH:mm:ss} %-5p %-25c{1} :: %m%n

# Execution logging
log4j.logger.org.apache.jena.arq.info=INFO
log4j.logger.org.apache.jena.arq.exec=INFO

# Other
log4j.logger.org.apache.jena=WARN
log4j.logger.org.apache.jena=WARN

A Fuseki server output can include ARQ execution logging.

Note: ARQ performs some direct control of logging in the test suite and depends on log4j to compile.

Execution Logging

ARQ can log query and update execution details globally or for an individual operations. This adds another level of control on top of the logger level controls.

Explanatory messages are controlled by the Explain.InfoLevel level in the execution context.

The logger used is called org.apache.jena.arq.exec. Message are sent at level "info". So for log4j, the following can be set in the log4j.properties file:

log4j.logger.org.apache.jena.arq.exec=INFO

The context setting is for key (Java constant) ARQ.symLogExec. To set globally:

ARQ.setExecutionLogging(Explain.InfoLevel.ALL) ;

and it may also be set on an individual query execution using its local context.

 try(QueryExecutiuon qExec = QueryExecutionFactory.create(...)) {
    qExec.getContext().set(ARQ.symLogExec, Explain.InfoLevel.ALL) ;
    ...
 }

On the command line:

 arq.query --explain --data data file --query=queryfile

The command tdbquery takes the same --explain argument.

Information levels

Level Effect
INFO Log each query
FINE Log each query and its algebra form after optimization
ALL Log query, algebra and every dataset access (can be expensive)
NONE No information logged

These can be specified as string, to the command line tools, or using the constants in Explain.InfoLevel.

 qExec.getContext().set(ARQ.symLogExec, Explain.InfoLevel.FINE) ;

 arq.query --set arq:logExec=FINE --data data file --query=queryfile

ARQ documentation index