Improve health endpoint. Add average response middleware and hook to metrics endpoint. (#6)

* WiP. Add average response middleware and hook to health endpoint.

* Add standard health formatting, average response time for last 50 requests.

* Replace homebrew metrics with prometheus to placate the Vrachnis/Panagiotou duo

* Add unit suffix on prometheus metric

* Remove dangling sprintf-js dependency and require statement.

package.json

@@ -41,7 +41,8 @@
     "proj4": "2.4.4",
     "request": "2.83.0",
     "sharp": "^0.20.0",
-    "tileserver-gl-styles": "1.2.0"
+    "tileserver-gl-styles": "1.2.0",
+    "prom-client": "11.0.0"
   },
   "devDependencies": {
     "should": "^11.2.0",

src/avgresp.js

@@ -0,0 +1,42 @@
+#!/usr/bin/env node
+'use strict';
+
+var prometheus = require('prom-client');
+
+module.exports = {
+    avgresp: avgresp
+};
+
+
+var history;
+var samples;
+var fullhistory = false;
+
+/**
+ * Create middleware to record and output average response times
+ * @param  {Object} options
+ * @return {function}
+ */
+function avgresp(options) {
+
+    var opts = options || {};
+
+    history = 50;
+    samples = new Array(history);
+    var currentIndex = 0;
+
+    const respSummary = new prometheus.Summary({
+        name: "tileserver_static_latency_seconds",
+        help: "The tileserver response time in seconds"
+    });
+
+    return function avgresp(req, res, next) {
+        var end = respSummary.startTimer();
+
+        res.on('finish', function() {
+            end();
+        })
+
+        next();
+    }
+}

src/server.js

@@ -21,7 +21,11 @@ var packageJson = require('../package'),
     serve_rendered = null,
     serve_style = require('./serve_style'),
     serve_data = require('./serve_data'),
-    utils = require('./utils');
+    utils = require('./utils'),
+    avgresp = require('./avgresp'),
+    prometheus = require('prom-client');
+
+prometheus.collectDefaultMetrics({ timeout: 5000 });
 
 var isLight = packageJson.name.slice(-6) == '-light';
 if (!isLight) {
@@ -52,6 +56,8 @@ function start(opts) {
     }));
   }
 
+  app.use(/\/styles\/.*\/static\/.*$/, avgresp.avgresp());
+
   var config = opts.config || null;
   var configPath = null;
   if (opts.configPath) {
@@ -391,12 +397,30 @@ function start(opts) {
     console.log('Startup complete');
     startupComplete = true;
   });
+
   app.get('/health', function(req, res, next) {
+
+    var healthTemplate = {
+    "service": "tileserver",
+    "status": "200",
+    "message": "OK"
+    };
+
+    var statusCode = 200;
+
     if (startupComplete) {
-      return res.status(200).send('OK');
+        statusCode = 200;
+        healthTemplate.message = "OK";
     } else {
-      return res.status(503).send('Starting');
+        statusCode = 503;
+        healthTemplate.message = "Starting";
     }
+    healthTemplate.status = statusCode;
+    return res.status(statusCode).send(healthTemplate);
+  });
+
+  app.get('/metrics', function(req, res, next){
+     res.end(prometheus.register.metrics());
   });
 
   var server = app.listen(process.env.PORT || opts.port, process.env.BIND || opts.bind, function() {