winston-express-sse

Installation

$ npm install --save winston-express-sse

Usage

On the server-side

This module provides a Winston's Transport class that you must create and add to your winston configuration:

var winston = require('winston');
var ExpressSSETransportClass = require('winston-express-sse');

const logger = winston.createLogger({
  level: 'info',
  format: winston.format.json(),
  transports: [
    //
    // - Write to all logs with level `info` and below to `combined.log` 
    // - Write all logs error (and below) to `error.log`.
    //
    new ExpressSSETransportClass({level: 'error', timestamp: function(){return (new Date()).toISOString();})
  ]
});

The module also provides the Express's middleware as a static function called 'middleware'. Add these line in your express configuration code:

var winstonSse = require('winston-express-sse').middleware;

// Add the middleware function to the chain 
app.use(winstonSse);

// Be aware not to call the middleware. This module doesn't work in this way. Don't do this:
// /* wrong */ app.use(winstonSse());

The middleware adds a new method called 'enableLogEventStream' to the request object. So you can enable any route to receive SSE events from winston logs:

var router = express.Router();

/* SSE: Log */
router.get('/logstream', function (req, res, next) {
  res.enableLogEventStream();
});

On the browser-side

On the browser-side you need to build an EventSource pointing to the route on the server where you set up the SSE Stream:

  // Close previous connections
  if (logEventSource) {
    logEventSource.close();
  }
      
  // Connect to SSE with minimumLevel
  logEventSource = new EventSource('http://localhost:4000/logstream');

And subscribe for each log level event you want to receive:

  logEventSource.addEventListener('error', logEventHandler, false);
  logEventSource.addEventListener('warning', logEventHandler, false);
  logEventSource.addEventListener('info', logEventHandler, false);
  logEventSource.addEventListener('trace', logEventHandler, false);
  
  logEventHandler = function(e) {
    // e parameter contains the log level event in the property 'type' and the contents in the property 'data'    
    var message = (e.type || 'Unknown type') + ' - ' + (e.data || 'Unknown log');
    
    console.log(message);
  }

Transport configuration

The transport constructor takes these options:

• level: Minimum level of messages that this transport will log (default 'info').
• timestamp: Boolean flag indicating if we should prepend output with timestamps (default false). If function is specified, its return value will be used instead of timestamps.

Remarks

• You can add your authentication logic in your Express route before calling to 'res.enableLogStream()';

• The server will emit only SSE events with the minimum level specified in the 'level' property of the transport options

• The client must subscribe to specific levels adding listeners to event names matching the winston level name. logEventSource.addEventListener('error', ...);

• This module was tested with winston v.2.x.x and it doesn't use the node.js streams.

License

MIT © Raul