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Node.js v12.20.0-rc.2 Documentation
Table of Contents
Trace events#
Source Code: lib/trace_events.js
The trace_events
module provides a mechanism to centralize tracing information
generated by V8, Node.js core, and userspace code.
Tracing can be enabled with the --trace-event-categories
command-line flag
or by using the trace_events
module. The --trace-event-categories
flag
accepts a list of comma-separated category names.
The available categories are:
node
: An empty placeholder.node.async_hooks
: Enables capture of detailedasync_hooks
trace data. Theasync_hooks
events have a uniqueasyncId
and a specialtriggerId
triggerAsyncId
property.node.bootstrap
: Enables capture of Node.js bootstrap milestones.node.console
: Enables capture ofconsole.time()
andconsole.count()
output.node.dns.native
: Enables capture of trace data for DNS queries.node.environment
: Enables capture of Node.js Environment milestones.node.fs.sync
: Enables capture of trace data for file system sync methods.node.perf
: Enables capture of Performance API measurements.node.perf.usertiming
: Enables capture of only Performance API User Timing measures and marks.node.perf.timerify
: Enables capture of only Performance API timerify measurements.
node.promises.rejections
: Enables capture of trace data tracking the number of unhandled Promise rejections and handled-after-rejections.node.vm.script
: Enables capture of trace data for thevm
module'srunInNewContext()
,runInContext()
, andrunInThisContext()
methods.v8
: The V8 events are GC, compiling, and execution related.
By default the node
, node.async_hooks
, and v8
categories are enabled.
node --trace-event-categories v8,node,node.async_hooks server.js
Prior versions of Node.js required the use of the --trace-events-enabled
flag to enable trace events. This requirement has been removed. However, the
--trace-events-enabled
flag may still be used and will enable the
node
, node.async_hooks
, and v8
trace event categories by default.
node --trace-events-enabled
# is equivalent to
node --trace-event-categories v8,node,node.async_hooks
Alternatively, trace events may be enabled using the trace_events
module:
const trace_events = require('trace_events');
const tracing = trace_events.createTracing({ categories: ['node.perf'] });
tracing.enable(); // Enable trace event capture for the 'node.perf' category
// do work
tracing.disable(); // Disable trace event capture for the 'node.perf' category
Running Node.js with tracing enabled will produce log files that can be opened
in the chrome://tracing
tab of Chrome.
The logging file is by default called node_trace.${rotation}.log
, where
${rotation}
is an incrementing log-rotation id. The filepath pattern can
be specified with --trace-event-file-pattern
that accepts a template
string that supports ${rotation}
and ${pid}
:
node --trace-event-categories v8 --trace-event-file-pattern '${pid}-${rotation}.log' server.js
The tracing system uses the same time source
as the one used by process.hrtime()
.
However the trace-event timestamps are expressed in microseconds,
unlike process.hrtime()
which returns nanoseconds.
The features from this module are not available in Worker
threads.
The trace_events
module#
Tracing
object#
The Tracing
object is used to enable or disable tracing for sets of
categories. Instances are created using the trace_events.createTracing()
method.
When created, the Tracing
object is disabled. Calling the
tracing.enable()
method adds the categories to the set of enabled trace event
categories. Calling tracing.disable()
will remove the categories from the
set of enabled trace event categories.
tracing.categories
#
A comma-separated list of the trace event categories covered by this
Tracing
object.
tracing.disable()
#
Disables this Tracing
object.
Only trace event categories not covered by other enabled Tracing
objects
and not specified by the --trace-event-categories
flag will be disabled.
const trace_events = require('trace_events');
const t1 = trace_events.createTracing({ categories: ['node', 'v8'] });
const t2 = trace_events.createTracing({ categories: ['node.perf', 'node'] });
t1.enable();
t2.enable();
// Prints 'node,node.perf,v8'
console.log(trace_events.getEnabledCategories());
t2.disable(); // Will only disable emission of the 'node.perf' category
// Prints 'node,v8'
console.log(trace_events.getEnabledCategories());
tracing.enable()
#
Enables this Tracing
object for the set of categories covered by the
Tracing
object.
tracing.enabled
#
- <boolean>
true
only if theTracing
object has been enabled.
trace_events.createTracing(options)
#
options
<Object>categories
<string[]> An array of trace category names. Values included in the array are coerced to a string when possible. An error will be thrown if the value cannot be coerced.
- Returns: <Tracing>.
Creates and returns a Tracing
object for the given set of categories
.
const trace_events = require('trace_events');
const categories = ['node.perf', 'node.async_hooks'];
const tracing = trace_events.createTracing({ categories });
tracing.enable();
// do stuff
tracing.disable();
trace_events.getEnabledCategories()
#
- Returns: <string>
Returns a comma-separated list of all currently-enabled trace event
categories. The current set of enabled trace event categories is determined
by the union of all currently-enabled Tracing
objects and any categories
enabled using the --trace-event-categories
flag.
Given the file test.js
below, the command
node --trace-event-categories node.perf test.js
will print
'node.async_hooks,node.perf'
to the console.
const trace_events = require('trace_events');
const t1 = trace_events.createTracing({ categories: ['node.async_hooks'] });
const t2 = trace_events.createTracing({ categories: ['node.perf'] });
const t3 = trace_events.createTracing({ categories: ['v8'] });
t1.enable();
t2.enable();
console.log(trace_events.getEnabledCategories());