# Analyzing Webserver Errors
In modern web development, monitoring and logging are critical aspects of maintaining robust and reliable applications. Telemetry allows developers to track various events and system metrics, which can be crucial for debugging and improving user experience. In this post, we’ll explore how to use the Telemetry API to catch and log all errors in an Express.js web server. Although this guide only covers Express, the ideas are applicable to all web servers, from Rails to Elixir, etc.
### Setting Up Telemetry
Before we can log any data, we need to set up Telemetry. For this example, we'll use the Telemetry JavaScript SDK.
First, install the Telemetry SDK in your project:
```bash
npm install telemetry-sh
```
Then, initialize the Telemetry client in your application with your API key:
```javascript
import telemetry from "telemetry-sh";
telemetry.init("YOUR_API_KEY");
```
### Creating an Express Application
Next, let's set up a basic Express application. If you don't already have Express installed, you can add it to your project with the following command:
```bash
npm install express
```
Now, create a simple Express server:
```javascript
const express = require('express');
const app = express();
const port = 3000;
app.get('/', (req, res) => {
res.send('Hello World!');
});
app.listen(port, () => {
console.log(`Example app listening at http://localhost:${port}`);
});
```
### Middleware for Error Handling
To catch and log all errors, we need to create a middleware function that will handle errors. This middleware should be added after all other middleware and route handlers.
```javascript
// Error handling middleware
app.use((err, req, res, next) => {
// Log the error using Telemetry
telemetry.log('express_errors', {
message: err.message,
stack: err.stack,
path: req.path,
method: req.method,
timestamp: new Date().toISOString()
});
// Respond with a generic error message
res.status(500).send('Something went wrong!');
});
```
### Example Error Route
For demonstration purposes, let's create a route that will throw an error:
```javascript
app.get('/error', (req, res) => {
throw new Error('This is a test error');
});
```
### Complete Example
Here’s the complete code for an Express server with error logging using Telemetry:
```javascript
const express = require('express');
const telemetry = require('telemetry-sh');
telemetry.init("YOUR_API_KEY");
const app = express();
const port = 3000;
app.get('/', (req, res) => {
res.send('Hello World!');
});
app.get('/error', (req, res) => {
throw new Error('This is a test error');
});
// Error handling middleware
app.use((err, req, res, next) => {
// Log the error using Telemetry
telemetry.log('express_errors', {
message: err.message,
stack: err.stack,
path: req.path,
method: req.method,
timestamp: new Date().toISOString()
});
// Respond with a generic error message
res.status(500).send('Something went wrong!');
});
app.listen(port, () => {
console.log(`Example app listening at http://localhost:${port}`);
});
```
### Querying and Displaying Error Data
Now, let's query the logged errors and use the data in a simple dashboard. We'll query the error logs and display them in an HTML table.
First let's create the JSON endpoint to get the data from Telemetry:
```js
app.get('/api/errors', async (req, res) => {
const results = await telemetry.query(`
SELECT
message,
stack,
path,
method,
timestamp
FROM
express_errors
ORDER BY
timestamp DESC
LIMIT 10
`);
res.json(results);
});
```
Lastly we create a simple HTML dashboard to display the error data:
```html
Error Dashboard
Error Dashboard
| Timestamp |
Message |
Path |
Method |
Stack |
```
### Conclusion
By following these steps, you can easily integrate Telemetry into your Express application to catch and log all errors. This setup will help you monitor and debug your application more effectively, ensuring a better experience for your users. The Telemetry API provides a powerful tool for tracking various events and metrics, and with the JavaScript SDK, integrating it into your projects is straightforward and efficient.
---
# Agent Instructions: Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.
Perform an HTTP GET request on the current page URL with the `ask` query parameter:
```
GET https://docs.telemetry.sh/guides/analyzing-webserver-errors.md?ask=
```
The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.
Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.