96 lines
2.8 KiB
Markdown
96 lines
2.8 KiB
Markdown
# Child loggers
|
|
|
|
Let's assume we want to have `"module":"foo"` added to every log within a
|
|
module `foo.js`.
|
|
|
|
To accomplish this, simply use a child logger:
|
|
|
|
```js
|
|
'use strict'
|
|
// imports a pino logger instance of `require('pino')()`
|
|
const parentLogger = require('./lib/logger')
|
|
const log = parentLogger.child({module: 'foo'})
|
|
|
|
function doSomething () {
|
|
log.info('doSomething invoked')
|
|
}
|
|
|
|
module.exports = {
|
|
doSomething
|
|
}
|
|
```
|
|
|
|
## Cost of child logging
|
|
|
|
Child logger creation is fast:
|
|
|
|
```
|
|
benchBunyanCreation*10000: 564.514ms
|
|
benchBoleCreation*10000: 283.276ms
|
|
benchPinoCreation*10000: 258.745ms
|
|
benchPinoExtremeCreation*10000: 150.506ms
|
|
```
|
|
|
|
Logging through a child logger has little performance penalty:
|
|
|
|
```
|
|
benchBunyanChild*10000: 556.275ms
|
|
benchBoleChild*10000: 288.124ms
|
|
benchPinoChild*10000: 231.695ms
|
|
benchPinoExtremeChild*10000: 122.117ms
|
|
```
|
|
|
|
Logging via the child logger of a child logger also has negligible overhead:
|
|
|
|
```
|
|
benchBunyanChildChild*10000: 559.082ms
|
|
benchPinoChildChild*10000: 229.264ms
|
|
benchPinoExtremeChildChild*10000: 127.753ms
|
|
```
|
|
|
|
## Duplicate keys caveat
|
|
|
|
It's possible for naming conflicts to arise between child loggers and
|
|
children of child loggers.
|
|
|
|
This isn't as bad as it sounds, even if the same keys between
|
|
parent and child loggers are used, Pino resolves the conflict in the sanest way.
|
|
|
|
For example, consider the following:
|
|
|
|
```js
|
|
const pino = require('pino')
|
|
pino(pino.destination('./my-log'))
|
|
.child({a: 'property'})
|
|
.child({a: 'prop'})
|
|
.info('howdy')
|
|
```
|
|
|
|
```sh
|
|
$ cat my-log
|
|
{"pid":95469,"hostname":"MacBook-Pro-3.home","level":30,"msg":"howdy","time":1459534114473,"a":"property","a":"prop","v":1}
|
|
```
|
|
|
|
Notice how there's two key's named `a` in the JSON output. The sub-childs properties
|
|
appear after the parent child properties.
|
|
|
|
At some point the logs will most likely be processed (for instance with a [transport](transports.md)),
|
|
and this generally involves parsing. `JSON.parse` will return an object where the conflicting
|
|
namespace holds the final value assigned to it:
|
|
|
|
```sh
|
|
$ cat my-log | node -e "process.stdin.once('data', (line) => console.log(JSON.stringify(JSON.parse(line))))"
|
|
{"pid":95469,"hostname":"MacBook-Pro-3.home","level":30,"msg":"howdy","time":"2016-04-01T18:08:34.473Z","a":"prop","v":1}
|
|
```
|
|
|
|
Ultimately the conflict is resolved by taking the last value, which aligns with Bunyans child logging
|
|
behavior.
|
|
|
|
There may be cases where this edge case becomes problematic if a JSON parser with alternative behavior
|
|
is used to process the logs. It's recommended to be conscious of namespace conflicts with child loggers,
|
|
in light of an expected log processing approach.
|
|
|
|
One of Pino's performance tricks is to avoid building objects and stringifying
|
|
them, so we're building strings instead. This is why duplicate keys between
|
|
parents and children will end up in log output.
|