Add synchronous jobs

We needed a possibility to execute some jobs / queries synchronously every time
/metrics is requested. So I added the infrastructure to execute jobs with an
interval <= 0 synchronously when the http-handler is called.

interval: '0s' # an interval <= 0 will make the queries synchronous

Some typos fixed by Felix Hillingshaeuser, thanks!

README.md

@@ -119,6 +119,7 @@ jobs:
   # each job needs a unique name, it's used for logging and as a default label
 - name: "example"
   # interval defined the pause between the runs of this job
+  # set to 0 to make the queries synchronous
   interval: '5m'
   # cron_schedule when to execute the job in the standard CRON syntax
   # if specified, the interval is ignored

config.go

@@ -145,6 +145,8 @@ func (c *cronConfig) UnmarshalYAML(unmarshal func(interface{}) error) error {
 type Job struct {
 	log          log.Logger
 	conns        []*connection
+	Trigger      chan bool     // used to trigger execution
+	Done         chan bool     // used to tell state
 	Name         string        `yaml:"name"`          // name of this job
 	KeepAlive    bool          `yaml:"keepalive"`     // keep connection between runs?
 	Interval     time.Duration `yaml:"interval"`      // interval at which this job is run

handler.go

@@ -0,0 +1,35 @@
+package main
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+	"net/http"
+)
+
+// handlerFunc can be used as handler for http.HandleFunc()
+// all synchronous jobs will be triggered and waited for,
+// then the promhttp handler is executed
+func (ex *Exporter) handlerFunc(w http.ResponseWriter, req *http.Request) {
+	// pull all triggers on jobs with interval 0
+	for _, job := range ex.jobs {
+		// if job is nil or is async then continue to next job
+		if job == nil || job.Interval > 0 {
+			continue
+		}
+		job.Trigger <- true
+	}
+
+	// wait for all sync jobs to finish
+	for _, job := range ex.jobs {
+		if job == nil || job.Interval > 0 {
+			continue
+		}
+		<-job.Done
+	}
+
+	// get the prometheus handler
+	handler := promhttp.HandlerFor(prometheus.DefaultGatherer, promhttp.HandlerOpts{})
+
+	// execute the ServeHTTP function
+	handler.ServeHTTP(w, req)
+}

job.go

@@ -84,6 +84,16 @@ func (j *Job) Init(logger log.Logger, queries map[string]string) error {
 }
 
 func (j *Job) updateConnections() {
+	// if the interval is not set > 0, create needed channels
+	if j.Interval <= 0 {
+		if j.Trigger == nil {
+			j.Trigger = make(chan bool)
+		}
+
+		if j.Done == nil {
+			j.Done = make(chan bool)
+		}
+	}
 	// if there are no connection URLs for this job it can't be run
 	if j.Connections == nil {
 		level.Error(j.log).Log("msg", "no connections for job", "job_name", j.Name)
@@ -405,13 +415,26 @@ func (j *Job) markFailed(conn *connection) {
 
 // Run the job queries with exponential backoff, implements the cron.Job interface
 func (j *Job) Run() {
-	bo := backoff.NewExponentialBackOff()
-	bo.MaxElapsedTime = j.Interval
-	if bo.MaxElapsedTime == 0 {
-		bo.MaxElapsedTime = time.Minute
-	}
-	if err := backoff.Retry(j.runOnce, bo); err != nil {
-		level.Error(j.log).Log("msg", "Failed to run", "err", err)
+        // if the interval is 0 or lower, wait to be triggered
+        if j.Interval <= 0 {
+                // wait for trigger
+                <-j.Trigger
+                if err := j.runOnce(); err != nil {
+                        level.Error(j.log).Log("msg", "Failed to run", "err", err)
+                }
+
+                // send true into done channel
+                j.Done <- true
+        } else {
+		bo := backoff.NewExponentialBackOff()
+		bo.MaxElapsedTime = j.Interval
+		level.Info(j.log).Log("msg", "In Run()")
+		if bo.MaxElapsedTime == 0 {
+			bo.MaxElapsedTime = time.Minute
+		}
+		if err := backoff.Retry(j.runOnce, bo); err != nil {
+			level.Error(j.log).Log("msg", "Failed to run", "err", err)
+		}
 	}
 }
 

main.go

@@ -10,7 +10,6 @@ import (
 	"github.com/go-kit/log"
 	"github.com/go-kit/log/level"
 	"github.com/prometheus/client_golang/prometheus"
-	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"github.com/prometheus/common/version"
 )
 
@@ -62,8 +61,8 @@ func main() {
 	}
 	prometheus.MustRegister(exporter)
 
-	// setup and start webserver
-	http.Handle(*metricsPath, promhttp.Handler())
+	// setup and start webserver with custom function
+	http.HandleFunc(*metricsPath, exporter.handlerFunc)
 	http.HandleFunc("/healthz", func(w http.ResponseWriter, r *http.Request) { http.Error(w, "OK", http.StatusOK) })
 	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
 		w.Write([]byte(`<html>