Example of stractured logging with MDC using log4js for NodeJs

appWithMdc.js

import fetch from "node-fetch";
import {logger} from "./logWithMdc.js";

const fetchReceipt = (receiptId, log) => {
    log.info(`Fetching receipt for ${receiptId}`)
    // https://jsonplaceholder.typicode.com/todos/{id} can be used for testing
    return fetch(`https://jsonplaceholder.typicode.com/todos/${receiptId}`)
        .then(res => {
            if (res.ok) {
                log.info(`Successfully fetched receipt for ${receiptId}`)
                return Promise.resolve(res);
            } else {
                log.error(`Unable to fetch receipt for ${receiptId}`)
                return res.text().then(reason => Promise.reject(new Error(reason)));
            }
        })
}

const fetchReceipts = (events, mainLog) => {
    return Promise.all(events.map(event => {
        const log = logger();
        log.addContext("traceId", event.traceId);
        log.addContext("orderId", event.orderId);

        return fetchReceipt(event.receiptId, log);
    }));
};

const app = (log) => {
    const events = [
        {
            traceId: "traceId-1",
            orderId: "orderId-1",
            receiptId: "1"
        },
        {
            traceId: "traceId-2",
            orderId: "orderId-2",
            receiptId: "2"
        },
        {
            traceId: "traceId-3",
            orderId: "orderId-3",
            receiptId: "3"
        },
        {
            traceId: "traceId-4",
            orderId: "orderId-4",
            receiptId: "4"
        },
        {
            traceId: "traceId-5",
            orderId: "orderId-5",
            receiptId: "5"
        }
    ]

    return fetchReceipts(events, log);
}

const execute = () => {
    const log = logger();

    app(log)
        .then(receipts => log.info("Successfully fetched receipts"))
        .catch(reason => log.error("Unable to fetch some of the receipts", reason))
}

execute();

appWithoutMdc.js

import fetch from "node-fetch";
import {logger} from "./logWithoutMdc.js";

const fetchReceipt = (receiptId, log) => {
    log.info(`Fetching receipt for ${receiptId}`)
    // https://jsonplaceholder.typicode.com/todos/{id} can be used for testing
    return fetch(`/receipts/${receiptId}`)
        .then(res => {
            if (res.ok) {
                log.info(`Successfully fetched receipt for ${receiptId}`)
                return Promise.resolve(res);
            } else {
                log.error(`Unable to fetch receipt for ${receiptId}`)
                return res.text().then(reason => Promise.reject(new Error(reason)));
            }
        })
}

const fetchReceipts = (events, log) => Promise.all(events.map(event => fetchReceipt(event.receiptId, log)));

const app = (log) => {
    const events = [
        {
            traceId: "traceId-1",
            orderId: "orderId-1",
            receiptId: "1"
        },
        {
            traceId: "traceId-2",
            orderId: "orderId-2",
            receiptId: "2"
        },
        {
            traceId: "traceId-3",
            orderId: "orderId-3",
            receiptId: "3"
        },
        {
            traceId: "traceId-4",
            orderId: "orderId-4",
            receiptId: "4"
        },
        {
            traceId: "traceId-5",
            orderId: "orderId-5",
            receiptId: "5"
        }
    ]

    return fetchReceipts(events, log);
}

const execute = () => {
    const log = logger();

    app(log)
        .then(receipts => log.info("Successfully fetched receipts"))
        .catch(reason => log.error("Unable to fetch some of the receipts", reason))
}

execute();

exampleLogWithMdc.txt

{"timestamp":"<timestamp>","level":"INFO","message":"Fetching receipt for 1","traceId":"traceId-1","orderId":"orderId-1"}
{"timestamp":"<timestamp>","level":"INFO","message":"Fetching receipt for 2","traceId":"traceId-2","orderId":"orderId-2"}
{"timestamp":"<timestamp>","level":"INFO","message":"Fetching receipt for 3","traceId":"traceId-3","orderId":"orderId-3"}
{"timestamp":"<timestamp>","level":"INFO","message":"Fetching receipt for 4","traceId":"traceId-4","orderId":"orderId-4"}
{"timestamp":"<timestamp>","level":"INFO","message":"Fetching receipt for 5","traceId":"traceId-5","orderId":"orderId-5"}
{"timestamp":"<timestamp>","level":"INFO","message":"Successfully fetched receipt for 3","traceId":"traceId-3","orderId":"orderId-3"}
{"timestamp":"<timestamp>","level":"INFO","message":"Successfully fetched receipt for 4","traceId":"traceId-4","orderId":"orderId-4"}
{"timestamp":"<timestamp>","level":"INFO","message":"Successfully fetched receipt for 2","traceId":"traceId-2","orderId":"orderId-2"}
{"timestamp":"<timestamp>","level":"INFO","message":"Successfully fetched receipt for 1","traceId":"traceId-1","orderId":"orderId-1"}
{"timestamp":"<timestamp>","level":"INFO","message":"Successfully fetched receipt for 5","traceId":"traceId-5","orderId":"orderId-5"}
{"timestamp":"<timestamp>","level":"INFO","message":"Successfully fetched receipts"}

exampleLogWithoutMdc.txt

[<timestamp>] default - Fetching receipt for 1
[<timestamp>] default - Fetching receipt for 2
[<timestamp>] default - Fetching receipt for 3
[<timestamp>] default - Fetching receipt for 4
[<timestamp>] default - Fetching receipt for 5
[<timestamp>] default - Successfully fetched receipt for 2
[<timestamp>] default - Successfully fetched receipt for 4
[<timestamp>] default - Successfully fetched receipt for 1
[<timestamp>] default - Successfully fetched receipt for 5
[<timestamp>] default - Successfully fetched receipt for 3
[<timestamp>] default - Successfully fetched receipts

logWithMdc.js

import log4js from "log4js";

const format = (elements) =>
  elements.map((element) =>
    !(element instanceof Error) ? element.toString() : `${element.stack}`
  );

log4js.addLayout("json", (config) => (logEvent) => {
  const { startTime, data, level, context } = logEvent;
  const newLog = {
    timestamp: startTime,
    level: level.levelStr,
    message: format(data).join(config.separator),
    ...context,
  };
  return JSON.stringify(newLog);
});

log4js.configure({
  appenders: {
    stdout: { type: "stdout", layout: { type: "json", separator: "," } },
    stderr: { type: "stderr", layout: { type: "json", separator: "," } },
    error: { type: "logLevelFilter", appender: "stderr", level: "warn" },
    information: {
      type: "logLevelFilter",
      appender: "stdout",
      level: "debug",
      maxLevel: "info",
    },
  },
  categories: {
    default: {
      appenders: ["information", "error"],
      level: "debug",
      enableCallStack: true,
    },
  },
});

const logger = log4js.getLogger;

export { logger };

logWithoutMdc.js

import log4js from "log4js";

log4js.configure({
  appenders: {
    stdout: { type: "stdout", layout: { type: "basic", separator: "," } },
    stderr: { type: "stderr", layout: { type: "basic", separator: "," } },
    error: { type: "logLevelFilter", appender: "stderr", level: "warn" },
    information: {
      type: "logLevelFilter",
      appender: "stdout",
      level: "debug",
      maxLevel: "info",
    },
  },
  categories: {
    default: {
      appenders: ["information", "error"],
      level: "debug",
      enableCallStack: true,
    },
  },
});

const logger = log4js.getLogger;

export { logger };