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