This document discusses logging best practices for Java applications. It covers how to set up logging frameworks like Log4j and SLF4j, what types of information are useful to log, and how to analyze logs to debug and improve applications. The document provides examples of good logging messages and techniques for querying, aggregating, and learning from log data using tools like grep, awk and scripting languages.

1. ©2010 Improving Enterprises, Inc.
Logging For Fun and Profit
Jane Prusakova
Jane.Prusakova@ImprovingEnterprises.com
http://softwareandotherthings.blogspot.com
Improving Enterprises
Houston JUG
2014

2. ©2010 Improving Enterprises, Inc.
Jane Prusakova
Hard-core developer
Data science geek
Dancer and photographer

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Why
How-to
Using the results
So lets talk about logging…

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Peek into the soul
Real setup
Real data
Real users
Real work
Dev environment
Happy path
Minimum load
Limited data

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Inside the cloud
No programming-by-coincidence
Replaces debugging
Works on server

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Logging [vs Debugging]
Set up once, always ready to use
Minimum disruption to the flow
Works locally and in the cloud
Faster data collection
Collected data is persisted

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Logging helps with…
Integration
Multithreading
High load

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What else?
Trace what the users do
Features
Sequences
Data flow

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When to use logging?
New systems
New features
Bug fixing

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Are logs forever?
Permanent
Errors
Extraordinary situations

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Some are seasonal
Very temporary
Debugging info
Trace bug fixes

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Others last awhile
Medium term
Data flow
Resource utilization
Optimization points

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How-to
Using the results

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Yes, I want to setup logging!
Pick a framework
Configure logging
Storage and access
Getting value from logs

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Log4J SLF4J
Apache MIT license
Logging framework Logging facade
Configuration, log levels and categories,
rotate log files, thread-safe logging.
Support: online documentation, tutorials,
online forums.

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Database Files
Easy to parse,
hard to evolve
Very hard to parse,
easy to change
Uses DB
connections
Requires FS
access
Can cause app
crash, loss of logs,
slow down app
Can fill up space

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Where to write logs?
Console
File system
DB
Queue mechanisms
Combination

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Logging configuration
Java properties-style
XML
Programmatically
Hard-coded
Takes preference

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Log Message
What to log with the message
Importance level
Timestamp
Where in the code
Data being processed
System state info

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Example layout
log4j.appender.stdout.layout.ConversionPattern=
%d %5p [%t] (%F:%L) - %m%n
- Date
- Log Level
- Thread name
- File and line (slow to retrieve)

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Good log messages
Support code
Correctness
Readability
Performance

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Better log messages
Relate to generating point
Code
Data

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Best log messages
Are formatted for
Readability
Aggregation

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Logging objects
.toString()
org.apache.commons.lang.builder.ToStringBuilder
log.info(myDataObject);

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What to log
Input and output
Errors and exceptions
Computation results
Integration points
Thread rendezvous

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Input and output
public int calculateCost(String user, Flight flight)
{
log.debug(“calculateCost: “ + user + “: “ +
flight);
int cost = … ;
… // cost calculation
log.debug((“calculateCost: cost for “ + user
+ “: “ + flight + “ = “ + cost);
return cost;
}

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Errors and exceptions
// do dangerous work
result = DoDangerousWork(user, flight);
}
catch (Exception e)
{
log.error(“methodName: exception in
DoDangerousWork for “ + user + “: “ + flight, e);
// recover or re-throw
}

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More
Log destination, parameters, outcomes
DB calls
SOAP calls
Wait time
Thread joins
Available memory
Memory-intensive operations
Intermediate calculation results
For complicated calculations

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Avoid
Little information
… // step 1
logger.info(“did something - XXXX”);
… // step 2
logger.info(“did more work - YYYY”);
foreach (…) {
… // useful work
logger.info(“working hard”);
}

30. ©2010 Improving Enterprises, Inc.
Avoid
Code cluttering
if (logger.isDebugEnabled()) {
logger.debug(“Show Views");
logger.debug("Address:n" +
view.getAddress());
logger.debug("Email:n" +
view.getEmail());
}

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Watch for logging-caused bugs
Side effects
Logger.info(count++ + “: did something”);
Errors and exceptions
Logger.info(m.GetValue());

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DRY principle applies

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Growing, growing…
Naming conventions
Rotation
by size
by time

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You’ve got logs!
Real-time access
Deal with space limitations
Warning: large datasets

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Security
Absolutely
No Sensitive Data

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Using the results

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Learning from the logs
Never rely on eye-balling
WYS is not WYG
Distinguish common and
rare

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Code and logs

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Process log
Line by line
No need to load entire file
Aggregate events
Relate to time

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Tools
Utilities
grep
sort
uniq
Scripting
Perl
Awk
sed

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Learning from the logs
> grep userName myApplication-MMDDYYYY.log
> grep methodName myApplication-MMDDYYYY.log
> grep FATAL myApplication-MMDDYYYY.log
> tail -2000 myApplication-MMDDYYYY.log | grep Exception

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Querying logs using Awk
GNU Awk www.gnu.org/software/gawk
Create histograms
Wait times
Number of calls
Exceptions per time period or per user
Compare and relate events
Exception stacks traces
Outcomes by caller or thread

43. ©2010 Improving Enterprises, Inc.
AWK demo
Logs for the demo generously
provided by
http://42graphy.org/

44. ©2010 Improving Enterprises, Inc.
Jane.Prusakova at ImprovingEnterprises.com
SoftwareAndOtherThings.blogspot.com
@jprusakova
Jane Prusakova
http://www.slideshare.net/jprusakova

45. ©2010 Improving Enterprises, Inc.
Logging For Fun and Profit
Jane Prusakova
Jane.Prusakova@ImprovingEnterprises.com
http://softwareandotherthings.blogspot.com
Improving Enterprises
Houston JUG
2014