djfdev / lambda-logger-node

A middleware logger that implements the MDC logging pattern for use in AWS NodeJS Lambdas.

lambda-logger-node

A middleware logger that implements the MDC logging pattern for use in AWS NodeJS Lambdas

This logger does two things.

1. Replaces console.log with a curried function that prepends any configured key/value pairs (or the default ones) onto the logged message. Both the format and values can be customized. The default format is

traceId={{traceId}} {{date}} appname={{appname}} version={{version}} severity={{severity}} | {{originalMessage}}

2. Replaces the lambda callback (as well as context.[fail|done|succeed] for legacy lambdas) with a function will log an access log before passing the result onto AWS. It will look something like This

requestURL=/some/route requestMethod=GET elapsedTime=-11 accessToken=Bearer 12 restApiId=request-id apigTraceId=098sgs7d-fi2n465-12msr

Installation

npm install lambda-logger-node

Usage

Lambda handler

var logger = require('lambda-logger-node')

exports.handler = logger(handler)

function handler (event, context, callback) {
  // your lambda handler
}

Or, with more middleware

var logger = require('lambda-logger-node')
var moreMiddleware = require('more-middleware')

exports.handler = logger(moreMiddleware(handler))

function handler (event, context, callback) {
  // your lambda handler
}

Or, with functional composition

var compose = require('compose-func')
var logger = require('lambda-logger-node')
var moreMiddleware = require('more-middleware')

exports.handler = compose(logger, moreMiddleware)(handler)

function handler (event, context, callback) {
  // your lambda handler
}

Restoring console.log

If you do not like the replacement of console.log you can restore the original one and use the log function on the module export to get the prepended logs.

var logger = require('lambda-logger-node')

exports.handler = logger(handler)

function handler (event, context, callback) {
  logger.restoreConsoleLog()
  logger.log('This log will have the access log data prepended on it')
  console.log('This will be a normal log')
  // your lambda handler
}

Customizing logs

The console.log replacement prepends on any configured key/value pairs using a custom string interpolation function. It can be controlled in two ways

• logger.format is the format string that will be interpolated and prepended to all console.log calls.

The format string is on [[module.export]].format. The default value is

'traceId={{traceId}} {{date}} appname={{appname}} version={{version}}'

You can set this to any value, at any time, to change the log-prepend output. The replacement values inside of {{traceId}} can be set with [[module.export]].setKey(key, value). Values can be either literals (string, integer, object) or functions. If a function is used, it will be called with no arguments at log-time to get the value (this is actually how the {{date}} token is implemented).

var logger = require('lambda-logger-node')
// Add more values
logger.format += ' someCustomValue={{custom1}} anotherCustomValue={{custom2}}'

// OR change the format string entirely
logger.format = 'traceId={{traceId}} someCustomValue={{custom1}} {{date}} appname={{appname}}'

// Set token values
logger.setKey('appname', 'custom-app-name')
logger.setKey('custom1', 'some-constant-value')
logger.setKey('custom2', () => Math.random())

// customize date format
logger.setKey('date', () => customDateFormattter(Date.now()))

// Change the log delimiter
logger.delimiter = ':'

The Final Log

After you return from your handler the logger will make one final log entry with either the error or the result from your handler. This log has a special format controlled by the logModule.successFormat and logModule.errorFormat. By default they are both the same:

Final log format

var finalFormat = logFormat + 'requestURL={{requestURL}} requestMethod={{requestMethod}} elapsedTime={{elapsedTime}} accessToken={{accessToken}} apiKey={{apiKey}} restApiId={{restApiId}} apigTraceId={{apigTraceId}} restResourceId={{restResourceId}} result={{result}}'

You can customize either format by setting logModule.successFormat or logModule.errorFormat properties. If the logModule.successFormat or logModule.errorFormat are falsy when the handler results in a success or failure, respectively, no final log will be made.

Warning: If you clear the logModule.successFormat or logModule.errorFormat at the end of your handler to supress the final log all future success or failure logs will be supressed. This is because the logger does not reset these properties on each lambda invocation. If you only want to supress a single final log you can call logModule.supressCurrentFinalLog, or make sure to reset the logModule.successFormat or logModule.errorFormat at the beginning of your handler.

Supressing the final log

You can stop the logger from logging all success or error logs by setting logModule.successFormat or logModule.errorFormat to a falsy value. However, these are global properties and persist beyond a single lambda invocation. If you want to supress only the current invocation's final log you can call logModule.supressCurrentFinalLog(). The flag this call sets is reset every time the handler runs, so you cannot call it before your handler. It is intended to be called at the end of the handler to cancel just the current invocation.

Log Levels

The logger provides two methods to log with different severity.

• console.(warn|info|error|log|debug)
• logger.(debug|info|warn|error)

These will include a severity=${loglevel} entry in the log.

Setting Minimum Log Level

Use logger.setMinimumLogLevel('debug|info|warn|error') to ignore log levels below the selected one. For example, calling logger.setMinimumLogLevel('info') will cause calls to logger.debug() to be ignored. This allows the same code to be deployed to development and production, while keeping the production logs quieter.