/
apdx-logging.lyx
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apdx-logging.lyx
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#LyX 1.6.7 created this file. For more info see http://www.lyx.org/
\lyxformat 345
\begin_document
\begin_header
\textclass book
\use_default_options false
\language english
\inputencoding auto
\font_roman default
\font_sans default
\font_typewriter default
\font_default_family default
\font_sc false
\font_osf false
\font_sf_scale 100
\font_tt_scale 100
\graphics default
\paperfontsize default
\spacing single
\use_hyperref false
\papersize default
\use_geometry false
\use_amsmath 1
\use_esint 1
\cite_engine basic
\use_bibtopic false
\paperorientation portrait
\secnumdepth 3
\tocdepth 3
\paragraph_separation indent
\defskip medskip
\quotes_language english
\papercolumns 1
\papersides 1
\paperpagestyle default
\tracking_changes false
\output_changes false
\author ""
\author ""
\end_header
\begin_body
\begin_layout Chapter
Logging in Lift
\begin_inset CommandInset label
LatexCommand label
name "cha:Logging-in-Lift"
\end_inset
\end_layout
\begin_layout Standard
Logging is a useful part of any application, Lift app or otherwise.
Logging can be used to audit user actions, give insight into runtime performanc
e and operation, and even to troubleshoot and debug issues.
Lift comes with a thin logging facade that sits on top of the SLF4J library
\begin_inset Foot
status open
\begin_layout Plain Layout
\begin_inset Flex URL
status collapsed
\begin_layout Plain Layout
http://www.slf4j.org/
\end_layout
\end_inset
\end_layout
\end_inset
.
This facade provides simple access to most common logging functions and
aims to be easy to use, flexible, and most important, inconspicuous.
If you do decide that Lift's logging facilities don't meet your needs,
it's possible to use any Java logging framework you desire, but it's still
useful to understand Lift's framework since Lift uses it internally for
logging.
\end_layout
\begin_layout Section
Logging Backend
\begin_inset CommandInset label
LatexCommand label
name "sec:Logging-Backend"
\end_inset
\end_layout
\begin_layout Standard
The detailed configuration of SLF4J is outside the scope of this book, but
we would like to highlight a few key notes concerning SLF4J.
First, SLF4J splits the API from the implementation into separate JAR files.
This allows you to choose the backing implementation that best suits your
deployment.
For example, SLF4J contains implementations for JDK logging, log4j, and
more.
For example, the JBoss application server version 5 and newer comes bundled
with its own implementation of the SLF4J API, so you can simply deploy
a Lift WAR on JBoss and use the server's configuration and logging.
Similarly, Jetty comes bundled with the SLF4J API.
\end_layout
\begin_layout Standard
Outside of container-provided implementations, the logback logging framework
\begin_inset Foot
status open
\begin_layout Plain Layout
\begin_inset Flex URL
status collapsed
\begin_layout Plain Layout
http://logback.qos.ch/
\end_layout
\end_inset
\end_layout
\end_inset
is a very nice implementation of the SLF4J API.
It's written by the same people who wrote log4j and has much improved performan
ce and functionality over log4j
\begin_inset Foot
status open
\begin_layout Plain Layout
\begin_inset Flex URL
status collapsed
\begin_layout Plain Layout
http://logback.qos.ch/reasonsToSwitch.html
\end_layout
\end_inset
\end_layout
\end_inset
.
Because logback-classic implements the SLF4J API, you simply need to drop
the logback-classic jar into your classpath, or add it as a dependency
in your WAR to use it.
One particularly nice feature of logback is that if it can't locate a configura
tion file (which can be written in XML, Groovy, or soon, Scala), it will
default to logging everything to the console.
\end_layout
\begin_layout Section
Basic Logging
\end_layout
\begin_layout Standard
Logging in Lift is performed via the
\family typewriter
net.liftweb.common.Logger
\family default
object and trait.
The
\family typewriter
Logger
\family default
object provides a set of utility methods for configuration and instantiation
of logger instances.
The
\family typewriter
Logger
\family default
trait can be mixed into your snippet, comet, or other classes to provide
direct methods for logging (we'll cover these in Section
\begin_inset CommandInset ref
LatexCommand ref
reference "sub:Logging-Methods"
\end_inset
).
\end_layout
\begin_layout Subsection
Logging Setup
\end_layout
\begin_layout Standard
The first step in utilizing Lift's logging is to configure the logger.
As we mentioned in Section
\begin_inset CommandInset ref
LatexCommand ref
reference "sec:Logging-Backend"
\end_inset
, Lift uses the SLF4J framework.
The configuration of SLF4J depends on which backing implementation you
use, but Lift comes with helpers for both log4j (
\family typewriter
net.liftweb.common.Log4j
\family default
) and for logback (
\family typewriter
net.liftweb.common.Logback
\family default
).
These helpers are utilized with the
\family typewriter
Logger
\family default
object's
\family typewriter
setup
\family default
var.
The
\family typewriter
Log4j
\family default
helpers provides methods that can be used to load a configuration from
a
\family typewriter
String
\family default
, a file (either XML or properties), or with Lift's default (console) logging.
The
\family typewriter
Logback
\family default
helper provides a single method to load a configuration from an XML file.
Listing
\begin_inset CommandInset ref
LatexCommand ref
reference "lst:Configuring-Logback-via"
\end_inset
shows how you could use the
\family typewriter
Logback
\family default
helper in the
\family typewriter
Boot.boot
\family default
method to configure logging from the
\emph on
logconfig.xml
\emph default
file.
\end_layout
\begin_layout Standard
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
\begin_inset Caption
\begin_layout Plain Layout
Configuring Logback via Logger.setup
\begin_inset CommandInset label
LatexCommand label
name "lst:Configuring-Logback-via"
\end_inset
\end_layout
\end_inset
\end_layout
\begin_layout Plain Layout
def boot {
\end_layout
\begin_layout Plain Layout
// Get a reference to the config from our classpath
\end_layout
\begin_layout Plain Layout
val logUrl = LiftRules.getResource("logconfig.xml")
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
// Apply the reference, if found
\end_layout
\begin_layout Plain Layout
logUrl.foreach { Logger.setup = Logback.withFile(_) }
\end_layout
\begin_layout Plain Layout
...
\end_layout
\begin_layout Plain Layout
}
\end_layout
\end_inset
\end_layout
\begin_layout Subsection
Obtaining a Logger
\end_layout
\begin_layout Standard
There are two basic means of obtaining a logger instance.
The first is to mix the
\family typewriter
Logger
\family default
trait into your class.
The second is to instantiate a logger using the
\family typewriter
Logger
\family default
object's
\family typewriter
apply
\family default
methods.
\end_layout
\begin_layout Standard
Mixing the
\family typewriter
Logger
\family default
trait into your class is a very simple way to add logging methods in your
code, as shown in Listing.
When you do this, the underlying logger will have a name derived from your
class's dynamic type.
For example, Listing
\begin_inset CommandInset ref
LatexCommand ref
reference "lst:Mixing-Logger-into"
\end_inset
shows the definition of our
\family typewriter
Accounts
\family default
snippet object with a
\family typewriter
Logger
\family default
trait mixed in.
When we log in the
\family typewriter
Accounts
\family default
object the name will be the fully-qualified classname, or
\begin_inset Quotes eld
\end_inset
com.pocketchangeapp.snippet.Accounts
\begin_inset Quotes erd
\end_inset
in our case.
When you mix the Logger trait into your class, you can access the logging
methods (Section
\begin_inset CommandInset ref
LatexCommand ref
reference "sub:Logging-Methods"
\end_inset
) directly.
\end_layout
\begin_layout Standard
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
\begin_inset Caption
\begin_layout Plain Layout
Mixing Logger into a Class
\begin_inset CommandInset label
LatexCommand label
name "lst:Mixing-Logger-into"
\end_inset
\end_layout
\end_inset
\end_layout
\begin_layout Plain Layout
object Accounts extends DispatchSnippet with Logger {
\end_layout
\begin_layout Plain Layout
...
\end_layout
\begin_layout Plain Layout
warn("This is a warning")
\end_layout
\end_inset
\end_layout
\begin_layout Standard
The second basic way to obtain a logger instance is to construct one directly
via the
\family typewriter
Logger
\family default
object's
\family typewriter
apply
\family default
method, as shown in Listing
\begin_inset CommandInset ref
LatexCommand ref
reference "lst:Constructing-a-Logger"
\end_inset
.
The
\family typewriter
apply
\family default
method takes either a
\family typewriter
String
\family default
or
\family typewriter
Class
\family default
that will be used to determine the constructed logger's name.
In our example, we use the
\family typewriter
classOf[Boot]
\family default
, so our logger will be named
\begin_inset Quotes eld
\end_inset
bootstrap.liftweb.Boot
\begin_inset Quotes erd
\end_inset
.
When you obtain a logger via construction you need to access the logging
methods via the logger instance.
\end_layout
\begin_layout Standard
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
\begin_inset Caption
\begin_layout Plain Layout
Constructing a Logger instance
\begin_inset CommandInset label
LatexCommand label
name "lst:Constructing-a-Logger"
\end_inset
\end_layout
\end_inset
\end_layout
\begin_layout Plain Layout
class Boot {
\end_layout
\begin_layout Plain Layout
// Set up a logger to use for startup messages
\end_layout
\begin_layout Plain Layout
val logger = Logger(classOf[Boot])
\end_layout
\begin_layout Plain Layout
...
\end_layout
\begin_layout Plain Layout
logger.warn("This is a warning")
\end_layout
\end_inset
\end_layout
\begin_layout Standard
There is a third, hybrid, approach to obtaining a
\family typewriter
Logger
\family default
that allows you to mix in the
\family typewriter
Logger
\family default
trait while controlling the logger name.
Listing
\begin_inset CommandInset ref
LatexCommand ref
reference "lst:Mixing-in-a-named-logger"
\end_inset
shows how we can mix in th trait and then override the underlying SLF4J
logger with our own named instance.
\end_layout
\begin_layout Standard
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
\begin_inset Caption
\begin_layout Plain Layout
Mixing in a named Logger
\begin_inset CommandInset label
LatexCommand label
name "lst:Mixing-in-a-named-logger"
\end_inset
\end_layout
\end_inset
\end_layout
\begin_layout Plain Layout
class AddEntry extends StatefulSnippet with Logger {
\end_layout
\begin_layout Plain Layout
// Use a different name for our logger
\end_layout
\begin_layout Plain Layout
import org.slf4j.LoggerFactory
\end_layout
\begin_layout Plain Layout
override val _logger = LoggerFactory.getLogger("EntryEdit")
\end_layout
\end_inset
\end_layout
\begin_layout Subsection
Logging Methods
\begin_inset CommandInset label
LatexCommand label
name "sub:Logging-Methods"
\end_inset
\end_layout
\begin_layout Standard
The
\family typewriter
Logger
\family default
trait provides some basic log methods which we'll summarize here.
Each log method comes in three forms: one with just a
\begin_inset Formula $\Rightarrow Object$
\end_inset
argument, one with a
\begin_inset Formula $\Rightarrow Object$
\end_inset
and
\family typewriter
Throwable
\family default
argument, and one with a
\begin_inset Formula $\Rightarrow Object$
\end_inset
,
\family typewriter
Throwable
\family default
, and
\family typewriter
Marker
\family default
argument.
These correspond roughly to the SLF4J log methods, although the order of
the parameters is different and the parameters are passed by-name.
Pass-by-name arguments are used so that computation of the log message
can be deferred.
This is useful to avoid processing messages for log statements below the
current logging threshold, a topic we'll cover more in Section
\begin_inset CommandInset ref
LatexCommand ref
reference "sec:Log-Levels"
\end_inset
.
\end_layout
\begin_layout Description
trace This logs a message at trace level.
Trace level is generally intended for very detailed
\begin_inset Quotes eld
\end_inset
tracing
\begin_inset Quotes erd
\end_inset
of processing, even more detailed than debug level.
\end_layout
\begin_layout Description
debug Logs a message at debug level.
This level is usually used to output internal variable values or other
information that is useful in debugging and troubleshooting an app.
\end_layout
\begin_layout Description
info Logs a message at info level.
This level is appropriate for general information about the app.
\end_layout
\begin_layout Description
warn Logs a message at warning level.
This level should be used for reporting issues that are in error but can
be handled cleanly, such as someone trying to submit a character string
for a numeric field value.
\end_layout
\begin_layout Description
error Logs a message at error level.
This level should be used for messages relating to errors that can't be
handled cleanly, such as a failure to connect to a backing database.
\end_layout
\begin_layout Description
assertLog This allows you to test an assertion condition and if true, logs
the assertion as well as a given message.
\end_layout
\begin_layout Standard
Listing
\begin_inset CommandInset ref
LatexCommand ref
reference "lst:Some-example-logging"
\end_inset
shows our REST API authentication hook, which uses a few different Logging
methods within the handler method.
\end_layout
\begin_layout Standard
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
\begin_inset Caption
\begin_layout Plain Layout
Some example logging
\begin_inset CommandInset label
LatexCommand label
name "lst:Some-example-logging"
\end_inset
\end_layout
\end_inset
\end_layout
\begin_layout Plain Layout
LiftRules.authentication = HttpBasicAuthentication("PocketChange") {
\end_layout
\begin_layout Plain Layout
case (userEmail, userPass, _) => {
\end_layout
\begin_layout Plain Layout
logger.debug("Authenticating: " + userEmail)
\end_layout
\begin_layout Plain Layout
User.find(By(User.email, userEmail)).map { user =>
\end_layout
\begin_layout Plain Layout
if (user.password.match_?(userPass)) {
\end_layout
\begin_layout Plain Layout
logger.info("Auth succeeded for " + userEmail)
\end_layout
\begin_layout Plain Layout
User.logUserIn(user)
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
// Set an MDC for logging purposes
\end_layout
\begin_layout Plain Layout
MDC.put("user", user.shortName)
\end_layout
\begin_layout Plain Layout
\end_layout
\begin_layout Plain Layout
// Compute all of the user roles
\end_layout
\begin_layout Plain Layout
userRoles(user.editable.map(acct => AuthRole("editAcct:" + acct.id))
++
\end_layout
\begin_layout Plain Layout
user.allAccounts.map(acct => AuthRole("viewAcct:" + acct.id)))
\end_layout
\begin_layout Plain Layout
true
\end_layout
\begin_layout Plain Layout
} else {
\end_layout
\begin_layout Plain Layout
logger.warn("Auth failed for " + userEmail)
\end_layout
\begin_layout Plain Layout
false
\end_layout
\begin_layout Plain Layout
}
\end_layout
\begin_layout Plain Layout
} openOr false
\end_layout
\begin_layout Plain Layout
}
\end_layout
\begin_layout Plain Layout
}
\end_layout
\end_inset
\end_layout
\begin_layout Section
Log Level Guards
\begin_inset CommandInset label
LatexCommand label
name "sec:Log-Levels"
\end_inset
\end_layout
\begin_layout Standard
We want to provide a brief discussion on the use of log guards and why they're
usually not needed with Lift's log framework.
A log guard is a simple test to see if a given log statement will actually
be processed.
The Log object provides a test method (returning a boolean) for each log
level:
\end_layout
\begin_layout Itemize
isDebugEnabled
\end_layout
\begin_layout Itemize
isErrorEnabled
\end_layout
\begin_layout Itemize
isInfoEnabled
\end_layout
\begin_layout Itemize
isTraceEnabled
\end_layout
\begin_layout Itemize
isWarnEnabled
\end_layout
\begin_layout Standard
Log guards are fairly common in logging frameworks to avoid expensive computatio
n of log message that won't actually be used.
This is particularly relevant with debug logging, since they often cover
a large section of code and usually aren't enabled in production.
The Log object can implicitly do log guards for you because of the pass-by-name
message parameters.
As we showed in listing
\begin_inset CommandInset ref
LatexCommand ref
reference "lst:Some-example-logging"
\end_inset
, simply converting your log message into a closure allows the Log object
decide whether to execute the closure based on the current log level.
You get the flexibility and simplicity of adding log statements anywhere
you want without explicit log guards, without losing the performance benefit
of the guards.
To explain it a bit more, let's assume for instace that the
\family typewriter
debug
\family default
method would have been declared as
\family typewriter
def debug(msg:AnyRef): Unit.
\family default
When
\family typewriter
debug
\family default
would be called the parameter will be first evaluated and then passed to
the method.
Inside the method we have the test to see if the debug level is enabled
to know if we actaully need to trace that message or not.
Well in this case even if the debugging level is turned off we still have
the evaluation of the parameters and that leads to unnecessary computing
and in an application that uses logging heavily that would likely lead
to a performance impact.
\end_layout
\begin_layout Section
Logging Mapper Queries
\begin_inset CommandInset label
LatexCommand label
name "sec:Logging-Mapper-Queries"
\end_inset
\end_layout
\begin_layout Standard
If you want to log Mapper query activity, there are two main approaches.
The first is to utilize the
\family typewriter
net.liftweb.mapper.DB.addLogFunc
\family default
method to add your own logging function.
A logging function is of the type
\begin_inset Formula $(DBLog,Long)\Rightarrow Any$
\end_inset
.
The
\family typewriter
DBLog
\family default
trait contains two separate lists of log entries, one for meta operations
(such as
\family typewriter
getFetchSize
\family default
) and one for actual work statements (such as
\family typewriter
executeQuery
\family default
).
You can access these two log lists via either the
\family typewriter
metaEntries