commit fba1a94836f131945d5da6b92ef1e775c85cf700
Author: Richard Hillmann <richie@project0.de>
Date:   Tue Mar 14 00:23:31 2017 +0100

    init project

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..9493596
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+.idea/
+exporter
\ No newline at end of file
diff --git a/Dockerfile b/Dockerfile
new file mode 100644
index 0000000..d548fc6
--- /dev/null
+++ b/Dockerfile
@@ -0,0 +1,7 @@
+FROM busybox:latest
+
+COPY exporter /bin/exporter
+
+EXPOSE 8081
+
+ENTRYPOINT ["/bin/exporter"]
\ No newline at end of file
diff --git a/INSTRUCTIONS.md b/INSTRUCTIONS.md
new file mode 100644
index 0000000..1de3192
--- /dev/null
+++ b/INSTRUCTIONS.md
@@ -0,0 +1,91 @@
+# SysEng coding challenge: Prometheus exporter
+
+Thanks for trying our coding challenge for systems engineers.
+
+## Prerequisites
+
+You need to be able to run Docker on your development computer to work on this
+challenge. Please run
+```
+docker run -it -p 8080:8080 beorn7/syseng-challenge
+```
+
+Then run `curl http://localhost:8080/stats`. The output should look similar to
+the following:
+
+```json
+{
+  "requestCounters": {
+    "200": 65221,
+    "404": 14066,
+    "500": 12618
+  },
+  "requestRates": {
+    "200": 100,
+    "404": 1
+  },
+  "duration": {
+    "count": 91905,
+    "sum": 4484.3037570333245,
+    "average": 0.024613801985478054
+  }
+}
+```
+
+If you aren't already, you should make yourself familiar with the
+[Prometheus monitoring and alerting system](https://prometheus.io). A good
+starting point is Brian Brazil's very concise
+[talk](https://www.youtube.com/watch?v=cwRmXqXKGtk) at FOSDEM 2016.
+
+## The Challenge
+
+Imagine the little binary you have started above is an instance of a
+microservice that is running replicated with hundreds of instances on a
+computing cluster. For the sake of the challenge, we are not interested in what
+the service is actually doing. We are only interested in its metrics, and we
+want to simulate monitoring it with Prometheus. Thankfully, the service is
+providing metrics in JSON format via its `/stats` endpoint, as you have seen
+above. The `requestCounters` tell you how often each HTTP status code has been
+served during the lifetime of the binary. The `requestRates` tell you the same
+but for the last second, i.e. they give you the current QPS. The `duration`
+tells you how many requests have been served in total during the lifetime of
+the binary (`count`) and how much total time those requests have taken in
+seconds (`sum`). The `average` is the time in seconds a request has taken,
+averaged over the last second.
+
+Unfortunately, Prometheus cannot ingest JSON directly but requires a custom
+format. Usually, you would write your microservice in a way that it would
+expose metrics in a format suitable for Prometheus directly. Let's imagine that
+this direct instrumentation is, for some reason, not feasible in this case. (In
+reality, this situation often arises when monitoring 3rd party software that
+does not happen to be instrumented for Prometheus specifically.) The usual
+solution is to write a so-called
+[exporter](https://prometheus.io/docs/instrumenting/exporters/), a little glue
+program that retrieves metrics from a 3rd party system and exposes them in the
+Prometheus way.
+
+Your task is to write, in a language of your choice, such an exporter for the
+simulated microservice running in your Docker right now.
+
+It might be helpful to also start a Prometheus server and scrape your exporter
+and explore the possibilities enabled by your metrics. The simulated
+microservice instance is looping through a number of scenarios over the course
+of about 15 minutes.
+
+## Bonus questions
+
+Optionally, you may answer the following questions. Thinking about them might
+also help you solve the coding challenge in a meaningful way. Keep answers
+short. It's really just about sketching out a few ideas. If we invite you for
+on-site interviews, we will have plenty of time to discuss them in detail.
+
+1. What are good ways of deploying hundreds of instances of our simulated
+   service? How would you deploy your exporter? And how would you configure
+   Prometheus to monitor them all?
+2. What graphs about the service would you plot in a dashboard builder like
+   Grafana? Ideally, you can come up with PromQL expressions for them.
+3. What would you alert on? What would be the urgency of the various alerts?
+   Again, it would be great if you could formulate alerting conditions with
+   PromQL.
+4. If you were in control of the microservice, which exported metrics would you
+   add or modify next?
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..4b6e64b
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,34 @@
+GO    := GO15VENDOREXPERIMENT=1 go
+pkgs   = $(shell $(GO) list ./... | grep -v /vendor/)
+
+DOCKER_IMAGE_NAME       ?= exporter
+DOCKER_IMAGE_TAG        ?= latest
+
+
+all: format build test
+
+style:
+	@echo ">> checking code style"
+	@! gofmt -d $(shell find . -path ./vendor -prune -o -name '*.go' -print) | grep '^'
+
+test:
+	@echo ">> running tests"
+	@$(GO) test -short $(pkgs)
+
+format:
+	@echo ">> formatting code"
+	@$(GO) fmt $(pkgs)
+
+vet:
+	@echo ">> vetting code"
+	@$(GO) vet $(pkgs)
+
+build:
+	@echo ">> building binaries"
+	@$(GO) build -o exporter
+
+docker:
+	@echo ">> building docker image"
+	@docker build -t "$(DOCKER_IMAGE_NAME):$(DOCKER_IMAGE_TAG)" .
+
+.PHONY: all style format build test vet docker
\ No newline at end of file
diff --git a/exporter.go b/exporter.go
new file mode 100644
index 0000000..f1d6189
--- /dev/null
+++ b/exporter.go
@@ -0,0 +1,263 @@
+package main
+
+import (
+	"encoding/json"
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"syscall"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+	"github.com/prometheus/common/log"
+	"github.com/prometheus/common/version"
+	"net/url"
+	"strings"
+)
+
+const (
+	metricsPath = "/metrics"
+	namespace   = "app"
+)
+
+var (
+	listenAddress        = flag.String("listen", ":8080", "The address to listen on for HTTP requests.")
+	endpointApp          = flag.String("endpoint", "http://localhost:8050/stats", "HTTP API address of the application")
+	hostname             = flag.String("hostname", "", "Optional hostname which will be added to the exported metrics (defaults to $HOSTNAME)")
+	prometheusConstLabel = parseConstLabel()
+)
+
+// run before creating the descriptor
+func parseConstLabel() prometheus.Labels {
+	// parse flags in an early state, so we can retrieve the instance id
+	flag.Parse()
+
+	if *hostname != "" {
+		// Try to set hostname from env var, so we can see if an pod does not work as expected
+		if value, found := syscall.Getenv("HOSTNAME"); found {
+			hostname = &value
+		}
+	}
+
+	// generate constant label if hostname is present
+	if *hostname != "" {
+		return prometheus.Labels{"hostname": *hostname}
+	}
+	return prometheus.Labels{}
+}
+
+var (
+	// Create the prometheus descriptors
+	descUp = prometheus.NewDesc(
+		prometheus.BuildFQName(namespace, "", "up"),
+		"Was the last query successful.",
+		nil, prometheusConstLabel,
+	)
+	descRequestCount = prometheus.NewDesc(
+		prometheus.BuildFQName(namespace, "", "request_count"),
+		"How many requests processed, partitioned by status code.",
+		[]string{"code"}, prometheusConstLabel,
+	)
+	descRequestCountTotal = prometheus.NewDesc(
+		prometheus.BuildFQName(namespace, "", "request_count_total"),
+		"How many requests processed of all status codes.",
+		nil, prometheusConstLabel,
+	)
+	descRequestRates = prometheus.NewDesc(
+		prometheus.BuildFQName(namespace, "", "request_rates"),
+		"How many requests processed in the last second, partitioned by status code.",
+		[]string{"code"}, prometheusConstLabel,
+	)
+	descRequestRatesTotal = prometheus.NewDesc(
+		prometheus.BuildFQName(namespace, "", "request_rates_total"),
+		"How many requests processed in the last second.",
+		nil, prometheusConstLabel,
+	)
+	descDurationSum = prometheus.NewDesc(
+		prometheus.BuildFQName(namespace, "", "duration_sum"),
+		"How much time consumed the requests in summary.",
+		nil, prometheusConstLabel,
+	)
+	descDurationAvg = prometheus.NewDesc(
+		prometheus.BuildFQName(namespace, "", "duration_avg"),
+		"How much time consumed the requests in average.",
+		nil, prometheusConstLabel,
+	)
+)
+
+// AppStats represent the schema of the returned json
+type AppStats struct {
+	// RequestCounters are the served status codes during app lifetime
+	RequestCounters map[string]int `json:"requestCounters"`
+	// RequestRates are the served status codes for the last second (QPS)
+	RequestRates map[string]int `json:"requestRates"`
+	// Duration represent some request stats during the lifetime
+	Duration *AppDuration `json:"duration"`
+}
+
+// AppDuration the schema of the returned duration part of the json
+type AppDuration struct {
+	// Count is the total served request in the lifetime
+	Count int `json:"count"`
+	// Sum is the total time of taken time the requests have taken in seconds
+	Sum float64 `json:"sum"`
+	// Average time of usage a request has taken.
+	Average float64 `json:"average"`
+}
+
+// appScraper is a helper to retrieve stats in a generic way
+type appScraper struct {
+	endpoint string
+	client   *http.Client
+}
+
+// stats returns the fetched and parsed json
+func (s *appScraper) stats() (*AppStats, error) {
+	var stats AppStats
+	response, err := s.client.Get(s.endpoint)
+	if err != nil {
+		return nil, err
+	}
+
+	buf, err := ioutil.ReadAll(response.Body)
+	if err != nil {
+		return nil, err
+	}
+
+	err = json.Unmarshal(buf, &stats)
+	if err != nil {
+		return nil, err
+	}
+
+	// validate returned json is complete
+	// requestCounter and requestRates should be empty maps, so we are fine here
+	if stats.Duration == nil {
+		return nil, fmt.Errorf("Invalid JSON returned, could not retreive duration.*")
+	}
+
+
+
+	return &stats, err
+}
+
+// Exporter implements prometheus.Collector
+type Exporter struct {
+	scraper *appScraper
+}
+
+// Describe implements prometheus.Describe
+func (e *Exporter) Describe(ch chan<- *prometheus.Desc) {
+	ch <- descUp
+	ch <- descRequestCount
+	ch <- descRequestCountTotal
+	ch <- descRequestRates
+	ch <- descDurationSum
+	ch <- descDurationAvg
+
+}
+
+func (e *Exporter) Collect(ch chan<- prometheus.Metric) {
+
+	stats, err := e.scraper.stats()
+	if err != nil {
+		ch <- prometheus.MustNewConstMetric(
+			descUp, prometheus.GaugeValue, 0,
+		)
+		log.Error("Failed to scrape app stats: ", err)
+		return
+	}
+	ch <- prometheus.MustNewConstMetric(
+		descUp, prometheus.GaugeValue, 1,
+	)
+
+	// Add counter per code on the fly (no need to update code if there are additional codes)
+	for code, count := range stats.RequestCounters {
+		ch <- prometheus.MustNewConstMetric(
+			descRequestCount, prometheus.CounterValue, float64(count), code,
+		)
+	}
+
+	// Add total of all requests
+	ch <- prometheus.MustNewConstMetric(
+		descRequestCountTotal, prometheus.CounterValue, float64(stats.Duration.Count),
+	)
+
+	// Add rates per code on the fly (no need to update code if there are additional codes)
+	ratesSum := 0
+	for code, count := range stats.RequestRates {
+		ch <- prometheus.MustNewConstMetric(
+			descRequestRates, prometheus.GaugeValue, float64(count), code,
+		)
+		ratesSum += count
+	}
+	// Additional sum of the rates, such like the requestCounter
+	ch <- prometheus.MustNewConstMetric(
+		descRequestRatesTotal, prometheus.CounterValue, float64(ratesSum),
+	)
+
+	ch <- prometheus.MustNewConstMetric(
+		descDurationSum, prometheus.CounterValue, stats.Duration.Sum,
+	)
+	ch <- prometheus.MustNewConstMetric(
+		descDurationAvg, prometheus.GaugeValue, stats.Duration.Average,
+	)
+
+}
+
+func NewExporter(endpoint string) (*Exporter, error) {
+
+	if !strings.Contains(endpoint, "://") {
+		endpoint = "http://" + endpoint
+	}
+	u, err := url.Parse(endpoint)
+	if err != nil {
+		return nil, fmt.Errorf("invalid endpoint URL: %s", err)
+	}
+	if u.Host == "" || (u.Scheme != "http" && u.Scheme != "https") {
+		return nil, fmt.Errorf("invalid endpoint URL: %s", endpoint)
+	}
+
+	// use custom http client with specific timeout
+	client := &http.Client{
+		Timeout: time.Duration(100*time.Millisecond),
+	}
+
+	// create api client
+	appScraper := &appScraper{
+		client:   client,
+		endpoint: endpoint,
+	}
+
+	return &Exporter{
+		scraper: appScraper,
+	}, nil
+}
+
+func init() {
+	prometheus.MustRegister(version.NewCollector(fmt.Sprintf("%s_exporter", namespace)))
+}
+
+func main() {
+
+	exporter, err := NewExporter(*endpointApp)
+	if err != nil {
+		log.Fatal(err)
+	}
+	prometheus.MustRegister(exporter)
+
+	http.Handle(metricsPath, promhttp.Handler())
+	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		w.Write([]byte(`<html>
+             <head><title>App Exporter</title></head>
+             <body>
+             <h1>App Exporter</h1>
+             <p><a href='` + metricsPath + `'>Metrics</a></p>
+             </body>
+             </html>`))
+	})
+
+	log.Infoln("Listening on", *listenAddress)
+	log.Fatal(http.ListenAndServe(*listenAddress, nil))
+}
diff --git a/exporter_test.go b/exporter_test.go
new file mode 100644
index 0000000..f07553c
--- /dev/null
+++ b/exporter_test.go
@@ -0,0 +1,116 @@
+package main
+
+import (
+	"testing"
+	"net/http/httptest"
+	"fmt"
+	"net/http"
+	"reflect"
+)
+
+func TestScraper(t *testing.T) {
+
+	tests := []struct {
+		json     string
+		expected *AppStats
+		ok bool
+	}{
+		{
+			json: `
+{
+  "requestCounters": {
+    "200": 65221,
+    "404": 14066,
+    "500": 12618
+  },
+  "requestRates": {
+    "200": 100,
+    "404": 1
+  },
+  "duration": {
+    "count": 91905,
+    "sum": 4484.3037570333245,
+    "average": 0.024613801985478054
+  }
+}
+`,
+			expected: &AppStats{
+				RequestCounters: map[string]int{
+					"200": 65221,
+					"404": 14066,
+					"500": 12618,
+				},
+				RequestRates: map[string]int{
+					"200": 100,
+					"404": 1,
+				},
+				Duration: &AppDuration{
+					Count:   91905,
+					Sum:     4484.3037570333245,
+					Average: 0.024613801985478054,
+
+				},
+			},
+			ok: true,
+		},
+		{
+			json: "invalid",
+			ok: false,
+		},
+	}
+
+	for i, test := range tests {
+		server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.Header().Set("Content-Type", "application/json")
+			fmt.Fprintln(w, test.json)
+		}))
+		defer server.Close()
+
+		scraper := appScraper{
+			endpoint: server.URL,
+			client:   http.DefaultClient,
+		}
+		stats, err := scraper.stats()
+
+		if err != nil{
+			if !test.ok{
+				continue
+			}
+			t.Fatalf("Test %v: http.Get(%q) unexpected error: %v", i, server.URL, err)
+		}
+
+		if !reflect.DeepEqual(*test.expected.Duration, *stats.Duration) {
+			t.Fatalf("Test %v: Duration expected %v, got %v", i, *test.expected.Duration, *stats.Duration)
+		}
+		if !reflect.DeepEqual(test.expected.RequestCounters, stats.RequestCounters) {
+			t.Fatalf("Test %v: RequestCounters expected %v, got %v", i, test.expected.RequestCounters, stats.RequestCounters)
+		}
+		if !reflect.DeepEqual(test.expected.RequestRates, stats.RequestRates) {
+			t.Fatalf("Test %v: RequestRates expected %v, got %v", i, test.expected.RequestRates, stats.RequestRates)
+		}
+	}
+}
+
+// Yes, this is stolen from the consul_exporter, but why reinvent the wheel? ;-)
+func TestNewExporter(t *testing.T) {
+	cases := []struct {
+		uri string
+		ok  bool
+	}{
+		{uri: "", ok: false},
+		{uri: "localhost:8500", ok: true},
+		{uri: "https://localhost:8500", ok: true},
+		{uri: "http://some.where:8500", ok: true},
+		{uri: "fuuuu://localhost:8500", ok: false},
+	}
+
+	for _, test := range cases {
+		_, err := NewExporter(test.uri)
+		if test.ok && err != nil {
+			t.Errorf("expected no error w/ %q, but got %q", test.uri, err)
+		}
+		if !test.ok && err == nil {
+			t.Errorf("expected error w/ %q, but got %q", test.uri, err)
+		}
+	}
+}