Node.js v17.3.1 documentation

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 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

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.

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());

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();

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());