Log files are essential for monitoring and troubleshooting applications. They provide insight into the application’s behaviour and help identify issues. In a Node.js application, you might want to create an API that allows you to view these logs easily. This can be particularly useful for centralized log management, debugging, and remote access to log data.

This article will guide you through the process of creating a simple API to view logs in a Node.js application. We will use the express framework to build our API and a logging library to manage the log files.

Introduction to Log Management

Logs are records of events that happen in an application. They help developers understand what’s going on under the hood and are invaluable for diagnosing issues. Common practices for managing logs include:

• Centralized logging: Collecting logs from different sources in one place.
• Log rotation: Managing the size and number of log files.
• Access control: Restricting who can view or manipulate logs.

Using Winston logging Framework

Steps to create API to view logs in Node.js using Winston

Step 1: Install the required dependencies, You will need to install some packages like express, morgan, cors, and winston to create the API. You can install them using npm by running the following command in your terminal

npm install express winston

Step 2: Create an Express app, Create a new file app.js and import the necessary modules

const express = require('express');
const winston = require('winston');
const app = express();

Step 3:  You can use Winston to log more detailed information.

const logger = winston.createLogger({
    level: 'info',
    format: winston.format.json(),
    defaultMeta: { service: 'your-service-name' },
    transports: [
        new winston.transports.File({ filename: 
           'error.log', level: 'error' }),
        new winston.transports.File({ 
            filename: 'combined.log' 
        })
    ]
});

if (process.env.NODE_ENV !== 'production') {
    logger.add(new winston.transports.Console({
        format: winston.format.simple()
    }));
}

Step 4: Define the API endpoint: You can define a route to retrieve the logs and return them as a JSON response.

app.get('/logs', (req, res) => {
    logger.query({ order: 'desc', limit: 100 }, 
    (err, result) => {
        if (err) {
            res.status(500).send({ 
                error: 'Error retrieving logs' 
               });
        } else {
            res.send(result);
        }
    });
});

 Step 6: Start the server: Finally, start the Express app by listening on a port:

const port = process.env.PORT || 3000;

app.listen(port, () => {
    console.log(`Server listening on port ${port}`);
});

This API will retrieve the latest 100 logs and return them as a JSON response when the /logs endpoint is accessed. You can customize the logger to log more detailed information or write the logs to different files or databases based on your requirements.

We set the logger’s log level to ‘info‘ and format the log entries in JSON format. We created a route path “/logs” that query the latest last 100 log entries using logger.query(). We pass in an object with options for the query – order is set to ‘desc’ to retrieve the latest entries first, and the limit is set to 100 to retrieve the latest 100 entries.  

Example: In this example, we create a Winston logger instance with winston.createLogger(). The logger has two types of transport – one for logging into the console and one for logging into a file named app.log. 

// index.js

const express = require('express');
const winston = require('winston');
const app = express();

// Create a Winston logger with transports
// for  logging to console and file
const logger = winston.createLogger({
    level: 'info',
    format: winston.format.json(),
    transports: [
        new winston.transports.Console(),
        new winston.transports.File({ 
            filename: 'app.log' 
        })
    ]
});

// Define an API route for viewing the logs
app.get('/logs', (req, res) => {

    // Query the logger for the latest log entries
    logger.query({ order: 'desc', limit: 100 },
        (err, results) => {
            if (err) {

                // If an error occurs, send an
                // error response
                res.status(500).send({ 
                    error: 'Error retrieving logs' 
                });
            } else {

                // If successful, send the log 
                // entries as a response
                res.send(results);
            }
        });
});

// Start the server and log a message when
// it's ready
app.listen(3000, () => {
    logger.info('Server started on port 3000');
});

Step to Run Application: Run the application using the following command from the root directory of the project

node index.js

Output:

 Routing the “/logs” Path on the browser:

 The output is in JSON format. The output is also saved in a file called app.log:

Using Log4js logging framework

In this code, we first import the necessary packages: Express and Log4js. We then configure Log4js to write logs to a file called logs.log using the file appender. We create a logger object that we can use to write logs to. We then create an endpoint for our API to view logs, which reads logs from the file and returns them as a JSON response.

Example: Implementation to create API to view logs in Node.js using above method.

// index.js

const express = require("express");
const app = express();
const log4js = require("log4js");
const fs = require("fs");

// Configure log4js to write logs to a file
log4js.configure({
    appenders: { 
        file: { 
            type: "file", 
            filename: "logs.log" 
        } 
    },
    categories: {
        default: {
            appenders:
                ["file"], level: "info"
        }
    },
});

// Create a logger object
const logger = log4js.getLogger();

// Create a route to view logs
app.get("/logs", (req, res) => {

    // Read logs from file
    const logs = fs.readFileSync("logs.log", "utf8");

    // Return logs as a JSON response
    res.json({ logs: logs });
});

// Example logging statements
logger.info("Info message");
logger.warn("Warning message");
logger.error("Error message");

// Start the server
app.listen(3000, () => {
    console.log("Server started on port 3000");
});

Run the index.js file using the below command:

node index.js 

Output:

The output is also saved in logs.log file: