Trace Events

Trace Event 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 detailed async_hooks trace data. The async_hooks events have a unique asyncId and a special triggerId triggerAsyncId property.
• node.bootstrap - Enables capture of Node.js bootstrap milestones.
• node.console - Enables capture of console.time() and console.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 the vm module's runInNewContext(), runInContext(), and runInThisContext() 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

Starting with Node.js 10.0.0, 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

• <string>

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 the Tracing 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());