Allow HTTP GET for metrics.

You can now issue a simple HTTP GET to retrieve metrics from Cube synchronously,
rather than fetching metrics asynchronously via WebSockets. The parameters are
identical to the WebSockets API. So,

    {
      "expression": "sum(random)",
      "start": "2012-01-09T01:23:00Z",
      "stop": "2012-01-10T04:56:00Z",
      "step": 300000
    }

Is equivalent to:

   http://localhost:1081/1.0/metric
     ?expression=sum(random)
     &start=2012-01-09T01:23:00Z
     &stop=2012-01-10T04:56:00Z
     &step=300000

The results are returned as a JSON array in chronological order. If an error
occurs, an empty array of results are returned. Fixes #24.

lib/cube/server/evaluator.js

@@ -1,8 +1,38 @@
-var endpoint = require("./endpoint");
+var endpoint = require("./endpoint"),
+    url = require("url");
+
+var headers = {
+  "Content-Type": "application/json",
+  "Access-Control-Allow-Origin": "*"
+};
 
 exports.register = function(db, endpoints) {
+  var event = require("./event").getter(db),
+      metric = require("./metric").getter(db);
+
   endpoints.ws.push(
-    endpoint.exact("/1.0/event/get", require("./event").getter(db)),
-    endpoint.exact("/1.0/metric/get", require("./metric").getter(db))
+    endpoint.exact("/1.0/event/get", event),
+    endpoint.exact("/1.0/metric/get", metric)
+  );
+  endpoints.http.push(
+    endpoint.exact("GET", "/1.0/metric", metricGet)
   );
+
+  function metricGet(request, response) {
+    var values = [],
+        expected = metric(url.parse(request.url, true).query, callback);
+
+    response.writeHead(expected < 0 ? 400 : 200, headers);
+    if (expected <= 0) response.end(JSON.stringify([]));
+
+    function callback(value) {
+      if (values.push(value) === expected) {
+        response.end(JSON.stringify(values.sort(chronological)));
+      }
+    }
+  }
 };
+
+function chronological(a, b) {
+  return a.time - b.time;
+}

lib/cube/server/metric.js

@@ -261,21 +261,21 @@ exports.getter = function(db) {
     var start = new Date(request.start),
         stop = new Date(request.stop),
         id = request.id;
-    if (isNaN(start)) return util.log("invalid start: " + request.start);
-    if (isNaN(stop)) return util.log("invalid stop: " + request.stop);
+    if (isNaN(start)) return util.log("invalid start: " + request.start), -1;
+    if (isNaN(stop)) return util.log("invalid stop: " + request.stop), -1;
 
     // Parse the expression.
     // TODO store expression as JSON object, or compute canonical form
     var expression;
     try {
       expression = parser.parse(request.expression);
-    } catch (error) {
-      return util.log("invalid expression: " + error);
+    } catch (e) {
+      return util.log("invalid expression: " + e), -1;
     }
 
     // Round start and stop to the appropriate time step.
     var tier = tiers[request.step];
-    if (!tier) return util.log("invalid step: " + request.step);
+    if (!tier) return util.log("invalid step: " + request.step), -1;
     start = tier.floor(start);
     stop = tier.ceil(stop);
 
@@ -299,5 +299,10 @@ exports.getter = function(db) {
     measure(expression, start, stop, tier, expression.group
         ? ("id" in request ? callbackGroupId : callbackGroup)
         : ("id" in request ? callbackValueId : callbackValue));
+
+    // Return the expected number of values.
+    var expected = 0;
+    while ((start = tier.step(start)) < stop) ++expected;
+    return expected;
   };
 };