15.5 Logging
A logger accepts events that contain information to be logged for interested parties. A log receiver represents an interested party that receives logged events asynchronously. Each event has a topic and level of detail, and a log receiver subscribes to logging events at a certain level of detail (and higher) for a specific topic or for all topics. The levels, in increasing order of detail, are 'fatal, 'error, 'warning, 'info, and 'debug.
To help organize logged events, a logger can have a default topic and/or a parent logger. Every event reported to a logger is also propagated to its parent (if any), while the event message is prefixed with the logger’s topic (if any) if the message doesn’t already have a topic. Furthermore, events that are propagated from a logger to its parent can be filtered by level and topic.
On start-up, Racket creates an initial logger that is used to record events from the core run-time system. For example, an 'debug event is reported for each garbage collection (see Garbage Collection). For this initial logger, two log receivers are also created: one that writes events to the process’s original error output port, and one that writes events to the system log. The level of written events in each case is system-specific, and the default can be changed through command-line flags (see Command Line) or through environment variables:
If the PLTSTDERR environment variable is defined and is not overridden by a command-line flag, it determines the level of the log receiver that propagates events to the original error port.
The environment variable’s value can be a ‹level›: none, fatal, error, warning, info, or debug; all events the corresponding level of higher are printed. After an initial ‹level›, the value can contain whitespace-separated specifications of the form ‹level›@‹topic›, which prints events whose topics match ‹topic› only at the given ‹level› or higher (where a ‹topic› contains any character other than whitespace or @). Leading and trailing whitespace is ignored. For example, the value "error debug@GC" prints all events at the 'error level and higher, but prints events for the topic 'GC at the 'debug level and higher (which includes all levels).
The default is "error".
If the PLTSYSLOG environment variable is defined and is not overridden by a command-line flag, it determines the level of the log receiver that propagates events to the system log. The possible values are the same as for PLTSTDERR.
The default is "none" for Unix or "error" for Windows and Mac OS X.
The current-logger parameter determines the current logger that is used by forms such as log-warning. On start-up, the initial value of this parameter is the initial logger. The run-time system sometimes uses the current logger to report events. For example, the bytecode compiler sometimes reports 'warning events when it detects an expression that would produce a run-time error if evaluated.
Changed in version 6.6.0.2 of package base: Prior to version 6.6.0.2, parsing of PLTSTDERR and PLTSYSLOG was very strict. Leading and trailing whitespace was forbidden, and anything other than exactly one space character separating two specifications was rejected.
15.5.1 Creating Loggers
procedure
(make-logger [ topic parent propagate-level propagate-topic ...] ...) → logger? topic : (or/c symbol? #f) = #f parent : (or/c logger? #f) = #f propagate-level : log-level/c = 'debug propagate-topic : (or/c #f symbol?) = #f
The optional propagate-level and propagate-topic arguments constrain the events that are propagated from the new logger to parent (when parent is not #f) in the same way that events are described for a log receiver in make-log-receiver. By default, all events are propagated to parent.
Changed in version 6.1.1.3 of package base: Removed an optional argument to specify a notification callback, and added propagate-level and propagate-topic constraints for events to propagate.
procedure
(logger-name logger) → (or/c symbol? #f)
logger : logger?
parameter
(current-logger logger) → void? logger : logger?
syntax
(define-logger id)
15.5.2 Logging Events
procedure
(log-message logger level [ topic] message data [ prefix-message?]) → void? logger : logger? level : log-level/c topic : (or/c symbol? #f) = (logger-name logger) message : string? data : any/c prefix-message? : any/c = #t
Log receivers can filter events based on topic. In addition, if topic and prefix-message? are not #f, then message is prefixed with the topic followed by ": " before it is sent to receivers.
Changed in version 6.0.1.10 of package base: Added the prefix-message? argument.
procedure
(log-level? logger level [topic]) → boolean?
logger : logger? level : log-level/c topic : (or/c symbol? #f) = #f
Use this function to avoid work generating an event for log-message if no receiver is interested in the information; this shortcut is built into log-fatal, log-error, log-warning, log-info, log-debug, and forms bound by define-logger, however, so it should not be used with those forms.
The result of this function can change if a garbage collection determines that a log receiver is no longer accessible (and therefore that any event information it receives will never become accessible).
Changed in version 6.1.1.3 of package base: Added the topic argument.
procedure
(log-max-level logger [topic]) → (or/c log-level/c #f)
logger : logger? topic : (or/c symbol? #f) = #f
Changed in version 6.1.1.3 of package base: Added the topic argument.
procedure
(log-all-levels logger) →
(list/c (or/c #f log-level/c) (or/c #f symbol?) ... ...) logger : logger?
The result is suitable as a sequence of arguments to make-log-receiver (after a logger argument) to create a new receiver for events that currently have receivers in logger.
Added in version 6.1.1.4 of package base.
procedure
(log-level-evt logger) → evt?
logger : logger?
The condition reported by the event is a conservative approximation: the event can become ready for synchronization even if the results of log-level?, log-max-level, and log-all-levels are unchanged. Nevertheless, the expectation is that events produced by log-level-evt become ready infrequently, because they are triggered by the creation of a log receiver.
Added in version 6.1.1.4 of package base.
syntax
(log-fatal string-expr)
(log-fatal format-string-expr v ...)
syntax
(log-error string-expr)
(log-error format-string-expr v ...)
syntax
(log-warning string-expr)
(log-warning format-string-expr v ...)
syntax
(log-info string-expr)
(log-info format-string-expr v ...)
syntax
(log-debug string-expr)
(log-debug format-string-expr v ...)
These form are convenient for using the current logger, but libraries
should generally use a logger for a specific topic—
For each log-level,
(log-level string-expr)
is equivalent to
(let ([l (current-logger)]) (when (log-level? l 'level) (log-message l 'level string-expr (current-continuation-marks))))
while
(log-level format-string-expr v ...)
is equivalent to
15.5.3 Receiving Logged Events
procedure
(log-receiver? v) → boolean?
v : any/c
procedure
(make-log-receiver logger level [topic ...] ...) → log-receiver?
logger : logger? level : log-level/c topic : (or/c #f symbol?) = #f
A log receiver is a synchronizable event. It becomes ready for synchronization when a logging event is received, so use sync to receive a logged event. The log receiver’s synchronization result is an immutable vector containing four values: the level of the event as a symbol, an immutable string for the event message, an arbitrary value that was supplied as the last argument to log-message when the event was logged, and a symbol or #f for the event topic.
Multiple pairs of level and topic can be provided to indicate different specific levels for different topics (where topic defaults to #f only for the last given level). A level for a #f topic applies only to events whose topic does not match any other provided topic. If the same topic is provided multiple times, the level provided with the last instance in the argument list takes precedence.
15.5.4 Additional Logging Functions
| (require racket/logging) | package: base |
procedure
(log-level/c v) → boolean?
v : any/c
Added in version 6.3 of package base.
procedure
(with-intercepted-logging interceptor proc level [ topic ...] ...) → any
interceptor :
(-> (vector/c log-level/c string? any/c (or/c symbol? #f)) any) proc : (-> any) level : log-level/c topic : (or/c #f symbol?) = #f
> (let ([warning-counter 0]) (with-intercepted-logging (lambda (l) (when (eq? (vector-ref l 0) 'warning) (set! warning-counter (add1 warning-counter)))) (lambda () (log-warning "Warning!") (log-warning "Warning again!") (+ 2 2)) 'warning) warning-counter) 2
Added in version 6.3 of package base.
procedure
(with-logging-to-port port proc level [ topic ...] ...) → any port : output-port? proc : (-> any) level : log-level/c topic : (or/c #f symbol?) = #f
> (let ([my-log (open-output-string)]) (with-logging-to-port my-log (lambda () (log-warning "Warning World!") (+ 2 2)) 'warning) (get-output-string my-log)) "Warning World!\n"
Added in version 6.3 of package base.