@emartech/json-logger

A tiny and fast logging library that outputs logs in JSON format. It has the same namespace based enabling/disabling mechanism as debug.

Installation

npm install @emartech/json-logger

Usage

Since 8.0.0, by default ECS fields will be used when logging.

If for reason you still need the old format, you need to override the outputFormat config. configure will apply this setting globally, for all instances of the logger.

const { createLogger } = require('@emartech/json-logger');

createLogger.configure({
  outputFormat: 'legacy'
});

Script

process.env.DEBUG = 'redis';
const { createLogger } = require('@emartech/json-logger');
const mongoLogger = createLogger('mongo');
const redisLogger = createLogger('redis');

redisLogger.info('connected', { domain: 'yahoo' });
// ECS format: {"event":{"action":"connected","created":"2016-08-15T08:50:23.566Z"},"log":{"logger":"redis","level":30},"domain":"yahoo"}
// Legacy format: {"name":"redis","action":"connected","level":30,"time":"2016-08-15T08:50:23.566Z","domain":"yahoo"}

mongoLogger.info('connected', { domain: 'google' });
// no output, because 'mongo' is not within namespaces (process.env.DEBUG)

redisLogger.fromError('query', new Error('Unauthorized'), { problem: 'missmatch' });
// ECS format: {"event":{"action":"query","created":"2016-08-15T08:50:23.569Z"},"log":{"logger":"redis","level":50},"error":{"type":"Error","message":"Unauthorized","stack_trace":"..."},"problem":"mismatch"}
// Legacy format: {"name":"redis","action":"query","level":50,"time":"2016-08-15T08:50:23.569Z","error_name":"Error","error_stack":"Error: Unauthorized\n    at Object.<anonymous> (/home/blacksonic/workspace/bunyan-debug/example.js:15:32)\n    at Module._compile (module.js:541:32)\n    at Object.Module._extensions..js (module.js:550:10)\n    at Module.load (module.js:458:32)\n    at tryModuleLoad (module.js:417:12)\n    at Function.Module._load (module.js:409:3)\n    at Module.runMain (module.js:575:10)\n    at run (bootstrap_node.js:352:7)\n    at startup (bootstrap_node.js:144:9)\n    at bootstrap_node.js:467:3","error_message":"Unauthorized","problem":"missmatch"}

Class

const { createLogger } = require('@emartech/json-logger');
const logger = createLogger('Exporter');

class Exporter {
  export() {
    mongoLogger.info('export', { customer_id: 123 });
  }
}

import { createLogger } from '@emartech/json-logger';
const logger = createLogger('Exporter');

class Exporter {
  export() {
    mongoLogger.info('export', { customer_id: 123 });
  }
}

Tests

import { Logger } from '@emartech/json-logger';

describe('Exporter', () => {
  it('should log', () => {
    jest.spyOn(Logger.prototype, 'info').mockReturnValue();
    const exporter = new Exporter();
    
    exporter.export();

    expect(Logger.prototype.info).toHaveBeenCalledWith('export', { customer_id: 123 });
  })
});

More examples can be found in the examples directory.

API

JsonLogger(namespace)

The default export of the library acts as a factory method. Returns a logging instance with the given namespace. The DEBUG environment variable is then used to enable these instances based on comma-delimited names. Disabled instances output no logs.

process.env.DEBUG = 'redis,mysql';
const { createLogger } = require('@emartech/json-logger');

const mongoLogger = createLogger('mongo');
// mongo instance will be disabled

const redisLogger = createLogger('redis');
// redis instance will be enabled

JsonLogger.prototype.info(action, data)

Prints the provided data to the console in JSON format.

const { createLogger } = require('@emartech/json-logger');
const redisLogger = createLogger('redis');

redisLogger.info('connected', { domain: 'yahoo' });
// ECS format: {"event":{"action":"connected","created":"2016-08-15T08:50:23.566Z"},"log":{"logger":"redis","level":30},"domain":"yahoo"}
// Legacy format: {"name":"redis","action":"connected","level":30,"time":"2016-08-15T08:50:23.566Z","domain":"yahoo"}

redisLogger.info('connected');
// ECS format: {"event":{"action":"connected","created":"2016-08-15T08:50:23.566Z"},"log":{"logger":"redis","level":30}}
// Legacy format: {"name":"redis","action":"connected","level":30,"time":"2016-08-15T08:50:23.566Z"}

By default displays the namespace of the instance (name), the current time in ISO8601 format (time), the action passed to the log method and the log level associated with the method. The second argument is assigned to these basic fields and is displayed along with them.

JsonLogger.prototype.trace(action, data)

Same as info with trace log level.

JsonLogger.prototype.debug(action, data)

Same as info with debug log level.

JsonLogger.prototype.warn(action, data)

Same as info with warn log level.

JsonLogger.prototype.error(action, data)

Same as info with error log level.

JsonLogger.prototype.fatal(action, data)

Same as info with fatal log level.

JsonLogger.prototype.fromError(action, data)

Displays an error object which formatted to fit into one line. The displayed line contains the stack trace, the name and the message of the error (for Axios errors, also request and response details). The log level defaults to error.

const { createLogger } = require('@emartech/json-logger');
const redisLogger = createLogger('redis');

redisLogger.fromError('query', new Error('Unauthorized'), { problem: 'missmatch' });
// ECS format: {"event":{"action":"query","created":"2016-08-15T08:50:23.569Z"},"log":{"logger":"redis","level":50},"error":{"type":"Error","message":"Unauthorized","stack_trace":"..."},"problem":"mismatch"}
// Legacy format: {"name":"redis","action":"query","level":50,"time":"2016-08-15T08:50:23.569Z","error_name":"Error","error_stack":"Error: Unauthorized\n    at Object.<anonymous> (/home/blacksonic/workspace/bunyan-debug/example.js:15:32)\n    at Module._compile (module.js:541:32)\n    at Object.Module._extensions..js (module.js:550:10)\n    at Module.load (module.js:458:32)\n    at tryModuleLoad (module.js:417:12)\n    at Function.Module._load (module.js:409:3)\n    at Module.runMain (module.js:575:10)\n    at run (bootstrap_node.js:352:7)\n    at startup (bootstrap_node.js:144:9)\n    at bootstrap_node.js:467:3","error_message":"Unauthorized","problem":"missmatch"}

JsonLogger.prototype.warnFromError(action, data)

Same as fromError, but with warn log level.

JsonLogger.prototype.timer()

Creates a new instance of timer that has the same methods as the logging instance (info, warn, error, fromError etc.) but also logs the elapsed time in milliseconds from the creation of the instance. The elapsed time will be logged into the duration field.

const { createLogger } = require('@emartech/json-logger');
const redisLogger = createLogger('redis');

const timer = redisLogger.timer();

// heavy task

timer.info('completed');
// Legacy format: {"name":"redis","action":"completed","level":30,"time":"2016-08-15T08:50:23.566Z","duration": 1500}
// ECS format: {"event":{"action":"completed","duration":"1500","created":"2016-08-15T08:50:23.566Z"},"log":{"logger":"redis","level":30}}

JsonLogger.configure(options)

The separate steps of the logging process can be configured here. These modifications affect all the instances of the library. With transformers we can alter the data to be logged before passing to the formatter and then to the output. It is a perfect place to add the name of the machine is running on or the request id associated with the current thread stored on a continuation local storage.

const { createLogger } = require('@emartech/json-logger');

createLogger.configure({
  formatter: JSON.stringify,
  output: console.log,
  transformers: [],
  outputFormat: 'ecs'
});

Log levels

• "fatal" (60): The service/app is going to stop or become unusable now. An operator should definitely look into this soon.
• "error" (50): Fatal for a particular request, but the service/app continues servicing other requests. An operator should look at this soon(ish).
• "warn" (40): A note on something that should probably be looked at by an operator eventually.
• "info" (30): Detail on regular operation.
• "debug" (20): Anything else, i.e. too verbose to be included in "info" level.
• "trace" (10): Logging from external libraries used by your app or very detailed application logging.

Logging request identifier automatically

You need to use the middlewares of @emartech/cls-adapter and add its transformer to the logger's configure method. This way it will log the request identifier coming from the header field (X-Request-Id) to every log line where the called function is originating from the route handler.

For automating

const Koa = require('koa');
const { createLogger } = require('@emartech/json-logger');
const clsAdapter = require('@emartech/cls-adapter');
const logger = createLogger('redis');

createLogger.configure({
  transformers: [
    clsAdapter.addContextStorageToInput()
  ]
});

const app = new Koa();
app.use(clsAdapter.getKoaMiddleware());

app.use(async () => {
  logger.info('connected');
  // Legacy format: {"name":"redis","action":"connected","level":30,"time":"2016-08-15T08:50:23.566Z","request_id":"d5caaa0e-b04e-4d94-bc88-3ed3b62dc94a"}
})