Add polling for changes from Docker

README.md

@@ -1,3 +1,7 @@
 # docker-batch-scheduler
 
 WIP for a docker batch scheduler
+
+Uses https://godoc.org/github.com/robfig/cron for cron expressions.
+
+Sets schedules based on label `cron.schedule="* * * * *"`

main.go

@@ -10,6 +10,8 @@ import (
 	"time"
 )
 
+var WatchInterval = (5 * time.Second)
+
 type ContainerStartJob struct {
 	Client      *client.Client
 	ContainerID string
@@ -19,26 +21,20 @@ type ContainerStartJob struct {
 }
 
 func (job ContainerStartJob) Run() {
-	fmt.Println("Starting: ", job.Name)
+	fmt.Println("Starting:", job.Name)
 	err := job.Client.ContainerStart(job.Context, job.ContainerID, types.ContainerStartOptions{})
 	if err != nil {
 		panic(err)
 	}
 }
 
-func main() {
-	cli, err := client.NewEnvClient()
-	if err != nil {
-		panic(err)
-	}
-
+func QueryScheduledJobs(cli *client.Client) (jobs []ContainerStartJob) {
+	fmt.Println("Scanning containers for new schedules...")
 	containers, err := cli.ContainerList(context.Background(), types.ContainerListOptions{All: true})
 	if err != nil {
 		panic(err)
 	}
 
-	jobs := []ContainerStartJob{}
-
 	for _, container := range containers {
 		if val, ok := container.Labels["cron.schedule"]; ok {
 			jobName := strings.Join(container.Names, "/")
@@ -52,19 +48,43 @@ func main() {
 		}
 	}
 
-	c := cron.New()
+	return
+}
 
+func ScheduleJobs(c *cron.Cron, jobs []ContainerStartJob) {
 	for _, job := range jobs {
-		fmt.Println("Scheduling ", job.Name, "(", job.Schedule, ")")
+		fmt.Printf("Scheduling %s (%s) with schedule '%s'\n", job.Name, job.ContainerID[:10], job.Schedule)
 		c.AddJob(job.Schedule, job)
 	}
+}
 
-	// Start the cron job threads
-	c.Start()
+func main() {
+	cli, err := client.NewEnvClient()
+	if err != nil {
+		panic(err)
+	}
+
+	// Create a cron
+	c := cron.New()
 
 	// Start the loop
 	for {
-		time.Sleep(5 * time.Second)
 		fmt.Println("Tick...")
+
+		// HACK: This is risky as it could fall on the same interval as a task and that task would get skipped
+		// It would be best to manage a ContainerID to Job mapping and then remove entries that are missing
+		// in the new list and add new entries. However, cron does not support this yet.
+
+		// Stop and create a new cron
+		c.Stop()
+		c = cron.New()
+
+		// Schedule jobs again
+		jobs := QueryScheduledJobs(cli)
+		ScheduleJobs(c, jobs)
+		c.Start()
+
+		// Sleep until the next query time
+		time.Sleep(WatchInterval)
 	}
 }