nomad: adding special 'system' scheduler

nomad/config.go

@@ -8,6 +8,7 @@ import (
 	"time"
 
 	"github.com/hashicorp/memberlist"
+	"github.com/hashicorp/nomad/nomad/structs"
 	"github.com/hashicorp/nomad/scheduler"
 	"github.com/hashicorp/raft"
 	"github.com/hashicorp/serf/serf"
@@ -164,6 +165,7 @@ func DefaultConfig() *Config {
 	for name := range scheduler.BuiltinSchedulers {
 		c.EnabledSchedulers = append(c.EnabledSchedulers, name)
 	}
+	c.EnabledSchedulers = append(c.EnabledSchedulers, structs.JobTypeSystem)
 
 	// Increase our reap interval to 3 days instead of 24h.
 	c.SerfConfig.ReconnectTimeout = 3 * 24 * time.Hour

nomad/job_endpoint.go

@@ -33,6 +33,9 @@ func (j *Job) Register(args *structs.JobRegisterRequest, reply *structs.JobRegis
 	if args.Job.Type == "" {
 		return fmt.Errorf("missing job type for registration")
 	}
+	if args.Job.Type == structs.JobTypeSystem {
+		return fmt.Errorf("job type cannot be system")
+	}
 
 	// Ensure priorities are bounded
 	if args.Job.Priority < structs.JobMinPriority {

nomad/server.go

@@ -99,6 +99,10 @@ type Server struct {
 	// plans that are waiting to be assessed by the leader
 	planQueue *PlanQueue
 
+	// systemSched is the special system scheduler used for
+	// various maintanence and book keeping.
+	systemSched *SystemScheduler
+
 	left         bool
 	shutdown     bool
 	shutdownCh   chan struct{}
@@ -157,6 +161,9 @@ func NewServer(config *Config) (*Server, error) {
 		shutdownCh:  make(chan struct{}),
 	}
 
+	// Create the system scheduler
+	s.systemSched = &SystemScheduler{srv: s}
+
 	// Initialize the RPC layer
 	// TODO: TLS...
 	if err := s.setupRPC(nil); err != nil {

nomad/structs/structs.go

@@ -440,6 +440,9 @@ func (n *NetworkResource) Add(delta *NetworkResource) {
 }
 
 const (
+	// JobTypeSystem is reserved for internal system tasks and is
+	// always handled by the SystemScheduler.
+	JobTypeSystem  = "system"
 	JobTypeService = "service"
 	JobTypeBatch   = "batch"
 )

nomad/worker.go

@@ -190,10 +190,15 @@ func (w *Worker) invokeScheduler(eval *structs.Evaluation) error {
 		return fmt.Errorf("failed to snapshot state: %v", err)
 	}
 
-	// Create the scheduler
-	sched, err := scheduler.NewScheduler(eval.Type, snap, w)
-	if err != nil {
-		return fmt.Errorf("failed to instantiate scheduler: %v", err)
+	// Create the scheduler, or use the special system scheduler
+	var sched scheduler.Scheduler
+	if eval.Type == structs.JobTypeSystem {
+		sched = w.srv.systemSched
+	} else {
+		sched, err = scheduler.NewScheduler(eval.Type, snap, w)
+		if err != nil {
+			return fmt.Errorf("failed to instantiate scheduler: %v", err)
+		}
 	}
 
 	// Process the evaluation