/**
 * Increment and get the number of request
 * per defined interval (currently 1 seconds)
 *
 * @param   key to use for request rate counting
 *
 * @return  promise object returning the request rate
 */
function incrementRequestRate( key ) {
	// Redis request interval in milliseconds
	// (currently set as 1 second)
	let reqInterval = 1000;

	// Lets increment and get the request rate
	// Inside a promise
	let reqPromise = redisClient.incr( key );

	//
	// asynchronously set the expiry
	// in the background.
	//
	// In practise due to the "delay" between
	// the increment, and the ttl get+expire set,
	// the request interval is slightly larger than
	// the value configured for "reqInterval"
	//
	reqPromise.then(function() {
		// Get the current value, ttl in milliseconds
		// which is -1, if its not configured
		redisClient.pttl( key ).then(function(ttlValue) {
			if( ttlValue == -1 ) {
				// ttl expiry is not configure,
				// configure it to be reqInterval
				redisClient.pexpire( key, reqInterval );
			}
		});
	});

	// Lets return the promise with the result
	return reqPromise;
}

/**
 * Increment and get the number of request
 * per defined interval (currently 1 seconds).
 *
 * Throws an exception inside the promise if the rate limit is reached.
 *
 * @param  key to use for request rate counting
 * @param  limit to the number of request per interval (seconds) supported.
 *         (optional, defaults to 10)
 *
 * @return  promise object, returning either the request rate
 *          (if below the limit), or an error
 */
function applyRequestRateLimit( key, limit ) {
	// Normalize the limit to default value if not set
	limit = limit || 10;

	// Increment the request rate,
	// and respond accordingly
	return incrementRequestRate(key).then(function(rate) {
		if( rate > limit ) {
			throw "request rate limit reached";
		}
		return rate;
	});
}