Table of Contents

• Tracing

Tracing#

Trace Event provides a mechanism to centralize tracing information generated by V8, Node.js core, and userspace code.

Tracing can be enabled by passing the --trace-events-enabled flag when starting a Node.js application.

The set of categories for which traces are recorded can be specified using the --trace-event-categories flag followed by a list of comma separated category names.

The available categories are:

• node
• node.async_hooks - Enables capture of detailed async_hooks trace data.
• node.bootstrap - Enables capture of Node.js bootstrap milestones.
• 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.
• v8

By default the node, node.async_hooks, and v8 categories are enabled.

node --trace-events-enabled --trace-event-categories v8,node,node.async_hooks server.js

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}. For example:

node --trace-events-enabled --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.