Repository: allegro/marathon-consul (go)

Issue:
Support Marathon SSE
Since there are rumors about deprecating http callbacks and their limited throughput and performance footprint we need to consider implementing [SSE support](http://mesosphere.github.io/marathon/docs/rest-api.html#get-v2-events).

As @kamilchm [proposed](https://github.com/allegro/marathon-consul/pull/140#issuecomment-261197732) when [marathon#4637](https://github.com/mesosphere/marathon/issues/4637) ([marathon#4671](https://github.com/mesosphere/marathon/pull/4671)) will be solved we can significantly reduce network traffic and network connections in comparision to callbacks.

Pull request:
Fixes #143 | Support Marathon SSE

Patch:
diff --git a/README.md b/README.md
index 68adc50..087d181 100644
--- a/README.md
+++ b/README.md
@@ -7,6 +7,11 @@ Register [Marathon](https://mesosphere.github.io/marathon/) Tasks as [Consul](ht
 forwards it to Consul agents. It also re-syncs all the information from Marathon
 to Consul on startup and repeats it with given interval.
 
+Note: In the future release Event Bus (callbacks) will be considered deprecated and eventually
+removed in favor of Event Stream (SSE).
+Right now marathon-consul is supporting both solutions.
+SSE is provided as experimental feature, disabled by default ([more](#sse-support)).
+
 ## Code
 
 This project is based on
@@ -101,7 +106,6 @@ curl -X POST 'http://marathon.service.consul:8080/v2/eventSubscriptions?callback
 ```
 The event subscription should be set to `localhost` to reduce network traffic.
 
-
 ## Usage
 
 ### Marathon masters
@@ -165,11 +169,12 @@ for more details.
 ### Sync
 
 - The scheduled Marathon-consul sync may run in two modes:
-    - Only on node that is the current [Marathon-leader](https://mesosphere.github.io/marathon/docs/rest-api.html#get-v2-leader), `sync-leader` parameter should be set to `hostname:port` the current node appears in the Marathon cluster. 
+    - Only on node that is the current [Marathon-leader](https://mesosphere.github.io/marathon/docs/rest-api.html#get-v2-leader), `sync-leader` parameter should be set to `hostname:port` the current node appears in the Marathon cluster.
       This mode is **enabled by default** and the `sync-leader` property is set to the hostname resolved by OS.
       Note that there is a difference between `sync-leader` and `marathon-location`: `sync-leader` is used for node leadership detection (should be set to cluster-wide node name), while `marathon-location` is used for connection purpose (may be set to `localhost`)
     - On every node, `sync-force` parameter should be set to `true`
 
+
 ### Options
 
 Argument                    | Default         | Description
@@ -215,6 +220,8 @@ sync-force                  | `false`         | Force leadership-independent Mar
 sync-interval               | `15m0s`         | Marathon-consul sync interval
 sync-leader                 |                 | Marathon cluster-wide node name (defaults to <hostname>:8080), the sync will run only if the specified node is the current Marathon-leader
 workers-pool-size           | `10`            | Number of concurrent workers processing events
+sse-enabled                 | `false`         | Enable marathon-consul SSE on this node
+web-enabled                 | `true`          | Enable marathon-consul Web callbacks on this node
 
 ### Endpoints
 
@@ -227,7 +234,7 @@ Endpoint  | Description
 
 ### Register under multiple ports
 
-If you need to map your Marathon task into multiple service registrations in Consul, you can configure marathon-consul 
+If you need to map your Marathon task into multiple service registrations in Consul, you can configure marathon-consul
 via Marathon's `portDefinitions`:
 
 ```
@@ -276,15 +283,15 @@ curl -X GET http://localhost:8500/v1/catalog/service/my-app-other-name
 ],
 "ServicePort": 31293,
 ...
-``` 
+```
 
-If any port definition contains the `consul` label, then advanced configuration mode is enabled. As a result, only the ports 
+If any port definition contains the `consul` label, then advanced configuration mode is enabled. As a result, only the ports
 containing this label are registered, under the name specified as the label's value – with empty value resolved to default name.
-Names don't have to be unique – you can have multiple registrations under the same name, but on different ports, 
+Names don't have to be unique – you can have multiple registrations under the same name, but on different ports,
 perhaps with different tags. Note that the `consul` label still needs to be present in the top-level application labels, even
 though its value won't have any effect.
 
-Tags configured in the top-level application labels will be added to all registrations. Tags configured in the port definition 
+Tags configured in the top-level application labels will be added to all registrations. Tags configured in the port definition
 labels will be added only to corresponding registrations.
 
 All registrations share the same `marathon-task` tag.
@@ -294,9 +301,19 @@ All registrations share the same `marathon-task` tag.
 Until 1.x.x marathon-consul would register services in Consul with registration id equal to related Marathon task id. Since 1.x.x registration ids are different and
 an additional tag, `marathon-task`, is added to each registration.
 
-If you update marathon-consul from version 0.x.x to 1.x.x, expect the synchronization phase during the first startup to 
+If you update marathon-consul from version 0.x.x to 1.x.x, expect the synchronization phase during the first startup to
 reregister all healthy services managed by marathon-consul to the new format. Unhealthy services will get deregistered in the process.
 
+## SSE Support
+
+In future callback interface between marathon and marathon-consul will be replaced by SSE.
+While using SSE please consider:
+- SSE is using Web module config for queues, event sizes, in the future will be moved to sse module,
+- SSE is using sync-leader config for determining current leader, when this value match leader returned by marathon (/v2/leader endpoint)
+then SSE is started on this instance,
+- when enabled SSE is spawning its own own set of workers and separated dispatcher,
+- be advised to disable marathon callback subscription when enabling SSE, otherwise it might result in doubling registers and deregisers.
+
 ## Known limitations
 
 The following section describes known limitations in `marathon-consul`.
diff --git a/apps/task.go b/apps/task.go
index d999c8c..7f04d7b 100644
--- a/apps/task.go
+++ b/apps/task.go
@@ -3,10 +3,16 @@ package apps
 import (
 	"encoding/json"
 	"strings"
+
+	"github.com/allegro/marathon-consul/time"
 )
 
 type Task struct {
-	ID                 TaskID              `json:"id"`
+	ID TaskID `json:"id"`
+	// Timestamp field is not a part of a Marathon task object.
+	// It's only present in StatusUpdateEventType and we are using this struct for decoding it.
+	// As well as for Marathon Task.
+	Timestamp          time.Timestamp      `json:"timestamp"`
 	TaskStatus         string              `json:"taskStatus"`
 	AppID              AppID               `json:"appId"`
 	Host               string              `json:"host"`
diff --git a/config/config.go b/config/config.go
index ce593f5..7ace9ba 100644
--- a/config/config.go
+++ b/config/config.go
@@ -12,6 +12,7 @@ import (
 	"github.com/allegro/marathon-consul/marathon"
 	"github.com/allegro/marathon-consul/metrics"
 	"github.com/allegro/marathon-consul/sentry"
+	"github.com/allegro/marathon-consul/sse"
 	"github.com/allegro/marathon-consul/sync"
 	"github.com/allegro/marathon-consul/web"
 	flag "github.com/ogier/pflag"
@@ -20,6 +21,7 @@ import (
 type Config struct {
 	Consul   consul.Config
 	Web      web.Config
+	SSE      sse.Config
 	Sync     sync.Config
 	Marathon marathon.Config
 	Metrics  metrics.Config
@@ -81,6 +83,10 @@ func (config *Config) parseFlags() {
 	flag.IntVar(&config.Web.QueueSize, "events-queue-size", 1000, "Size of events queue")
 	flag.IntVar(&config.Web.WorkersCount, "workers-pool-size", 10, "Number of concurrent workers processing events")
 	flag.Int64Var(&config.Web.MaxEventSize, "event-max-size", 4096, "Maximum size of event to process (bytes)")
+	flag.BoolVar(&config.Web.Enabled, "web-enabled", true, "Enable web events (callbacks).")
+
+	// SSE
+	flag.BoolVar(&config.SSE.Enabled, "sse-enabled", false, "Enable sse event stream.")
 
 	// Sync
 	flag.BoolVar(&config.Sync.Enabled, "sync-enabled", true, "Enable Marathon-consul scheduled sync")
diff --git a/debian/config.json b/debian/config.json
index 04e4dc2..47485f3 100644
--- a/debian/config.json
+++ b/debian/config.json
@@ -22,7 +22,11 @@
     "Listen": ":4000",
     "QueueSize": 1000,
     "WorkersCount": 10,
-    "MaxEventSize": 4096
+    "MaxEventSize": 4096,
+    "Enabled": true
+  },
+  "SSE": {
+    "Enabled": false
   },
   "Sync": {
     "Enabled": true,
@@ -55,4 +59,4 @@
       "Level": "error"
     }
   }
-}
\ No newline at end of file
+}
diff --git a/events/event_handler.go b/events/event_handler.go
index bfbd4a5..30569f2 100644
--- a/events/event_handler.go
+++ b/events/event_handler.go
@@ -102,6 +102,8 @@ func (fh *eventHandler) handleHealthyTask(body []byte) error {
 		log.WithError(err).Error("Body generated error")
 		return err
 	}
+	delay := taskHealthChange.Timestamp.Delay()
+	metrics.UpdateGauge("events.read.delay.current", int64(delay))
 
 	appID := taskHealthChange.AppID
 	taskID := taskHealthChange.TaskID()
@@ -146,11 +148,12 @@ func (fh *eventHandler) handleHealthyTask(body []byte) error {
 
 func (fh *eventHandler) handleStatusEvent(body []byte) error {
 	task, err := apps.ParseTask(body)
-
 	if err != nil {
 		log.WithError(err).WithField("Body", body).Error("Could not parse event body")
 		return err
 	}
+	delay := task.Timestamp.Delay()
+	metrics.UpdateGauge("events.read.delay.current", int64(delay))
 
 	log.WithFields(log.Fields{
 		"Id":         task.ID,
diff --git a/events/sse_events.go b/events/sse_events.go
new file mode 100644
index 0000000..eba2589
--- /dev/null
+++ b/events/sse_events.go
@@ -0,0 +1,149 @@
+package events
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"io"
+)
+
+// Event holds state of parsed fields from marathon EventStream
+type SSEEvent struct {
+	Type        string
+	Body        []byte
+	ID          string
+	Delay       string
+	maxLineSize int64
+}
+
+var (
+	lineFeed = []byte("\n")
+	colon    = []byte{':'}
+	space    = []byte{' '}
+)
+
+func (e *SSEEvent) parseLine(line []byte) bool {
+	// https://www.w3.org/TR/2011/WD-eventsource-20110208/
+	// Quote: Lines must be separated by either a U+000D CARRIAGE RETURN U+000A
+	// LINE FEED (CRLF) character pair, a single U+000A LINE FEED (LF) character,
+	// or a single U+000D CARRIAGE RETURN (CR) character.
+
+	//If the line is empty (a blank line)
+	if len(line) == 0 || bytes.Compare(line, lineFeed) == 0 {
+		//Dispatch the event, as defined below.
+		return !e.isEmpty()
+	}
+
+	//If the line starts with a U+003A COLON character (:)
+	if bytes.HasPrefix(line, colon) {
+		//Ignore the line.
+		return false
+	}
+
+	var field string
+	var value []byte
+	//If the line contains a U+003A COLON character (:)
+	//Collect the characters on the line before the first U+003A COLON character (:), and let field be that string.
+	split := bytes.SplitN(line, colon, 2)
+	if len(split) == 2 {
+		field = string(split[0])
+		//Collect the characters on the line after the first U+003A COLON character (:), and let value be that string.
+		//If value starts with a U+0020 SPACE character, remove it from value.
+		value = bytes.TrimPrefix(split[1], space)
+	} else {
+		//Otherwise, the string is not empty but does not contain a U+003A COLON character (:)
+		//Process the field using the steps described below, using the whole line as the field name,
+		//and the empty string as the field value.
+		field = string(line)
+		value = []byte{}
+
+	}
+	stringValue := string(value)
+	//If the field name is
+	switch field {
+	case "event":
+		//Set the event name buffer to field value.
+		e.Type = stringValue
+	case "data":
+		//If the data buffer is not the empty string,
+		if len(value) != 0 {
+			//Append the field value to the data buffer,
+			//then append a single U+000A LINE FEED (LF) character to the data buffer.
+			e.Body = append(e.Body, value...)
+			e.Body = append(e.Body, '\n')
+		}
+	case "id":
+		//Set the last event ID buffer to the field value.
+		e.ID = stringValue
+	case "retry":
+		e.Delay = stringValue
+		// TODO consider reconnection delay
+	}
+
+	return false
+}
+
+func (e *SSEEvent) isEmpty() bool {
+	return e.Type == "" && e.Body == nil && e.ID == ""
+}
+
+func (e *SSEEvent) String() string {
+	return fmt.Sprintf("Type: %s, Body: %s", e.Type, string(e.Body))
+}
+
+func ParseSSEEvent(scanner *bufio.Scanner) (SSEEvent, error) {
+	e := SSEEvent{}
+
+	for dispatch := false; !dispatch; {
+		if !scanner.Scan() {
+			return e, io.EOF
+		}
+		line := scanner.Bytes()
+		dispatch = e.parseLine(line)
+		if err := scanner.Err(); err != nil {
+			return e, scanner.Err()
+		}
+	}
+	return e, nil
+}
+
+// ScanLines is higtly inspired by the function of the same name from bufio package,
+// but is sensitive to CR as line separator
+func ScanLines(data []byte, atEOF bool) (advance int, token []byte, err error) {
+	if atEOF && len(data) == 0 {
+		return 0, nil, nil
+	}
+	pos := lineTerminatorPosition(data)
+	if pos != 0 {
+		return pos + 1, dropCR(data[0:pos]), nil
+	}
+	// If we're at EOF, we have a final, non-terminated line. Return it.
+	if atEOF {
+		return len(data), dropCR(data), nil
+	}
+	// Request more data.
+	return 0, nil, nil
+}
+
+func lineTerminatorPosition(data []byte) int {
+	// https://www.w3.org/TR/2011/WD-eventsource-20110208/
+	// Quote: Lines must be separated by either a U+000D CARRIAGE RETURN U+000A
+	// LINE FEED (CRLF) character pair, a single U+000A LINE FEED (LF) character,
+	// or a single U+000D CARRIAGE RETURN (CR) character.
+	if i := bytes.IndexByte(data, '\n'); i >= 0 {
+		// We have a full newline-terminated line
+		return i
+	} else if i := bytes.IndexByte(data, '\r'); i >= 0 {
+		// We have a full CR terminated line
+		return i
+	}
+	return 0
+}
+
+// dropCR drops a terminal \r from the data.
+func dropCR(data []byte) []byte {
+	if len(data) > 0 && data[len(data)-1] == '\r' {
+		return data[0 : len(data)-1]
+	}
+	return data
+}
diff --git a/events/task_health_change.go b/events/task_health_change.go
index 6f79243..496c444 100644
--- a/events/task_health_change.go
+++ b/events/task_health_change.go
@@ -6,10 +6,11 @@ import (
 	"regexp"
 
 	"github.com/allegro/marathon-consul/apps"
+	"github.com/allegro/marathon-consul/time"
 )
 
 type TaskHealthChange struct {
-	Timestamp string `json:"timestamp"`
+	Timestamp time.Timestamp `json:"timestamp"`
 	// Prefer TaskID() instead of ID
 	ID         apps.TaskID `json:"id"`
 	InstanceID string      `json:"instanceId"`
diff --git a/events/web_events.go b/events/web_events.go
index a2ddb30..5ab3ce3 100644
--- a/events/web_events.go
+++ b/events/web_events.go
@@ -3,31 +3,13 @@ package events
 import (
 	"encoding/json"
 	"errors"
-	"strings"
-	"time"
-)
-
-type Timestamp struct {
-	time.Time
-}
-
-func (t *Timestamp) UnmarshalJSON(b []byte) (err error) {
-	s := strings.Trim(string(b), "\"")
-	if s == "null" {
-		t.Time = time.Time{}
-		return
-	}
-	t.Time, err = time.Parse(time.RFC3339Nano, s)
-	return
-}
 
-func (t *Timestamp) String() string {
-	return t.Format(time.RFC3339Nano)
-}
+	"github.com/allegro/marathon-consul/time"
+)
 
 type WebEvent struct {
-	Type      string    `json:"eventType"`
-	Timestamp Timestamp `json:"timestamp"`
+	Type      string         `json:"eventType"`
+	Timestamp time.Timestamp `json:"timestamp"`
 }
 
 func ParseEvent(jsonBlob []byte) (WebEvent, error) {
@@ -37,7 +19,7 @@ func ParseEvent(jsonBlob []byte) (WebEvent, error) {
 		return WebEvent{}, err
 	} else if webEvent.Type == "" {
 		return WebEvent{}, errors.New("Missing event type")
-	} else if webEvent.Timestamp.Unix() == (time.Time{}).Unix() {
+	} else if webEvent.Timestamp.Missing() {
 		return WebEvent{}, errors.New("Missing timestamp")
 	}
 	return webEvent, nil
diff --git a/main.go b/main.go
index 9691718..3e25410 100644
--- a/main.go
+++ b/main.go
@@ -9,6 +9,7 @@ import (
 	"github.com/allegro/marathon-consul/marathon"
 	"github.com/allegro/marathon-consul/metrics"
 	"github.com/allegro/marathon-consul/sentry"
+	"github.com/allegro/marathon-consul/sse"
 	"github.com/allegro/marathon-consul/sync"
 	"github.com/allegro/marathon-consul/web"
 )
@@ -34,19 +35,26 @@ func main() {
 	}
 
 	consulInstance := consul.New(config.Consul)
-	remote, err := marathon.New(config.Marathon)
+	// TODO(tz) - move Leader from sync module to highest level config, access like config.Leader
+	remote, err := marathon.New(config.Marathon, config.Sync.Leader)
 	if err != nil {
 		log.Fatal(err.Error())
 	}
 
 	sync.New(config.Sync, remote, consulInstance, consulInstance.AddAgentsFromApps).StartSyncServicesJob()
 
-	handler, stop := web.NewHandler(config.Web, remote, consulInstance)
-	defer stop()
+	if config.SSE.Enabled {
+		stopSSE := sse.NewHandler(config.SSE, config.Web, remote, consulInstance)
+		defer stopSSE()
+	}
+
+	if config.Web.Enabled {
+		handler, stop := web.NewHandler(config.Web, remote, consulInstance)
+		defer stop()
+		http.HandleFunc("/events", handler)
+	}
 
-	// set up routes
 	http.HandleFunc("/health", web.HealthHandler)
-	http.HandleFunc("/events", handler)
 
 	log.WithField("Port", config.Web.Listen).Info("Listening")
 	log.Fatal(http.ListenAndServe(config.Web.Listen, nil))
diff --git a/marathon/marathon.go b/marathon/marathon.go
index 5730baf..7bf545e 100644
--- a/marathon/marathon.go
+++ b/marathon/marathon.go
@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"net/url"
 	"strings"
+	"time"
 
 	log "github.com/Sirupsen/logrus"
 	"github.com/allegro/marathon-consul/apps"
@@ -19,11 +20,14 @@ type Marathoner interface {
 	App(apps.AppID) (*apps.App, error)
 	Tasks(apps.AppID) ([]apps.Task, error)
 	Leader() (string, error)
+	EventStream([]string, int, int) (*Streamer, error)
+	AmILeader() (bool, error)
 }
 
 type Marathon struct {
 	Location string
 	Protocol string
+	MyLeader string
 	Auth     *url.Userinfo
 	client   *http.Client
 }
@@ -32,7 +36,7 @@ type LeaderResponse struct {
 	Leader string `json:"leader"`
 }
 
-func New(config Config) (*Marathon, error) {
+func New(config Config, leader string) (*Marathon, error) {
 	var auth *url.Userinfo
 	if len(config.Username) == 0 && len(config.Password) == 0 {
 		auth = nil
@@ -45,10 +49,12 @@ func New(config Config) (*Marathon, error) {
 			InsecureSkipVerify: !config.VerifySsl,
 		},
 	}
+	// TODO(tz) - consider passing desiredEvents as config
 	return &Marathon{
 		Location: config.Location,
 		Protocol: config.Protocol,
 		Auth:     auth,
+		MyLeader: leader,
 		client: &http.Client{
 			Transport: transport,
 			Timeout:   config.Timeout.Duration,
@@ -59,7 +65,7 @@ func New(config Config) (*Marathon, error) {
 func (m Marathon) App(appID apps.AppID) (*apps.App, error) {
 	log.WithField("Location", m.Location).Debug("Asking Marathon for " + appID)
 
-	body, err := m.get(m.urlWithQuery(fmt.Sprintf("/v2/apps/%s", appID), params{"embed": "apps.tasks"}))
+	body, err := m.get(m.urlWithQuery(fmt.Sprintf("/v2/apps/%s", appID), params{"embed": []string{"apps.tasks"}}))
 	if err != nil {
 		return nil, err
 	}
@@ -69,7 +75,7 @@ func (m Marathon) App(appID apps.AppID) (*apps.App, error) {
 
 func (m Marathon) ConsulApps() ([]*apps.App, error) {
 	log.WithField("Location", m.Location).Debug("Asking Marathon for apps")
-	body, err := m.get(m.urlWithQuery("/v2/apps", params{"embed": "apps.tasks", "label": apps.MarathonConsulLabel}))
+	body, err := m.get(m.urlWithQuery("/v2/apps", params{"embed": []string{"apps.tasks"}, "label": []string{apps.MarathonConsulLabel}}))
 	if err != nil {
 		return nil, err
 	}
@@ -106,6 +112,53 @@ func (m Marathon) Leader() (string, error) {
 	return leaderResponse.Leader, err
 }
 
+// EventStream method creates Streamer handler which is configured based on marathon
+// client and credentials.
+func (m Marathon) EventStream(desiredEvents []string, retries, retryBackoff int) (*Streamer, error) {
+	subURL := m.urlWithQuery("/v2/events", params{"event_type": desiredEvents})
+
+	// Before creating actual streamer, this function blocks until configured leader for this (m) reciever is elected.
+	// When leaderPoll function successfully exit this instance of marathon-consul,
+	// consider itself as a new leader and initializes Streamer.
+	err := m.leaderPoll()
+	if err != nil {
+		log.WithError(err).Fatal("Leader poll failed. Check marathon and previous errors. Exiting")
+	}
+
+	return &Streamer{
+		subURL: subURL,
+		client: &http.Client{
+			Transport: m.client.Transport,
+		},
+		retries:      retries,
+		retryBackoff: retryBackoff,
+	}, nil
+}
+
+// leaderPoll just blocks until configured myleader is equal to
+// leader returned from marathon (/v2/leader endpoint)
+func (m Marathon) leaderPoll() error {
+	pollTicker := time.Tick(1 * time.Second)
+	retries := 5
+	i := 0
+	for range pollTicker {
+		leading, err := m.AmILeader()
+		if err != nil {
+			if i >= retries {
+				return fmt.Errorf("Failed to get a leader after %d retries", i)
+			}
+			i++
+			log.WithError(err).Error("Error while getting leader")
+			continue
+		}
+		if leading {
+			return nil
+		}
+		log.Debug("I am not leader")
+	}
+	return nil
+}
+
 func (m Marathon) get(url string) ([]byte, error) {
 	request, err := http.NewRequest("GET", url, nil)
 	if err != nil {
@@ -157,7 +210,7 @@ func (m Marathon) url(path string) string {
 	return m.urlWithQuery(path, nil)
 }
 
-type params map[string]string
+type params map[string][]string
 
 func (m Marathon) urlWithQuery(path string, params params) string {
 	marathon := url.URL{
@@ -167,9 +220,16 @@ func (m Marathon) urlWithQuery(path string, params params) string {
 		Path:   path,
 	}
 	query := marathon.Query()
-	for key, value := range params {
-		query.Add(key, value)
+	for key, values := range params {
+		for _, value := range values {
+			query.Add(key, value)
+		}
 	}
 	marathon.RawQuery = query.Encode()
 	return marathon.String()
 }
+
+func (m Marathon) AmILeader() (bool, error) {
+	leader, err := m.Leader()
+	return m.MyLeader == leader, err
+}
diff --git a/marathon/marathon_stub.go b/marathon/marathon_stub.go
index bfbf594..97f272b 100644
--- a/marathon/marathon_stub.go
+++ b/marathon/marathon_stub.go
@@ -42,6 +42,14 @@ func (m *MarathonerStub) Leader() (string, error) {
 	return m.leader, nil
 }
 
+func (m *MarathonerStub) EventStream([]string, int, int) (*Streamer, error) {
+	return &Streamer{}, nil
+}
+
+func (m *MarathonerStub) AmILeader() (bool, error) {
+	return false, nil
+}
+
 func (m *MarathonerStub) Interactions() bool {
 	m.interactionsMu.RLock()
 	defer m.interactionsMu.RUnlock()
diff --git a/marathon/streamer.go b/marathon/streamer.go
new file mode 100644
index 0000000..2d1e5c1
--- /dev/null
+++ b/marathon/streamer.go
@@ -0,0 +1,75 @@
+package marathon
+
+import (
+	"bufio"
+	"context"
+	"fmt"
+	"net/http"
+	"time"
+
+	log "github.com/Sirupsen/logrus"
+)
+
+type Streamer struct {
+	Scanner      *bufio.Scanner
+	cancel       context.CancelFunc
+	client       *http.Client
+	subURL       string
+	retries      int
+	retryBackoff int
+	noRecover    bool
+}
+
+func (s *Streamer) stop() {
+	s.cancel()
+}
+func (s *Streamer) Stop() {
+	s.noRecover = true
+	s.stop()
+}
+
+func (s *Streamer) Start() error {
+	req, err := http.NewRequest("GET", s.subURL, nil)
+	if err != nil {
+		log.Fatal("Unable to create Streamer request")
+		return nil
+	}
+	req.Header.Set("Accept", "text/event-stream")
+	ctx, cancel := context.WithCancel(context.Background())
+	s.cancel = cancel
+	req = req.WithContext(ctx)
+	res, err := s.client.Do(req)
+	if err != nil {
+		s.cancel()
+		return err
+	}
+	if res.StatusCode != http.StatusOK {
+		log.WithFields(log.Fields{
+			"Location": s.subURL,
+			"Method":   "GET",
+		}).Errorf("Got status code : %d", res.StatusCode)
+	}
+	log.WithFields(log.Fields{
+		"SubUrl": s.subURL,
+		"Method": "GET",
+	}).Debug("Subsciption success")
+	s.Scanner = bufio.NewScanner(res.Body)
+
+	return nil
+}
+
+func (s *Streamer) Recover() error {
+	if s.noRecover {
+		return fmt.Errorf("Streamer is not recoverable")
+	}
+	s.stop()
+
+	err := s.Start()
+	i := 0
+	for ; err != nil && i <= s.retries; err = s.Start() {
+		seconds := time.Duration(i * s.retryBackoff)
+		time.Sleep(seconds * time.Second)
+		i++
+	}
+	return err
+}
diff --git a/sse/config.go b/sse/config.go
new file mode 100644
index 0000000..3de3ab2
--- /dev/null
+++ b/sse/config.go
@@ -0,0 +1,7 @@
+package sse
+
+type Config struct {
+	Enabled      bool
+	Retries      int
+	RetryBackoff int
+}
diff --git a/sse/sse.go b/sse/sse.go
new file mode 100644
index 0000000..94f3909
--- /dev/null
+++ b/sse/sse.go
@@ -0,0 +1,72 @@
+package sse
+
+import (
+	"net/http"
+	"time"
+
+	log "github.com/Sirupsen/logrus"
+	"github.com/allegro/marathon-consul/events"
+	"github.com/allegro/marathon-consul/marathon"
+	"github.com/allegro/marathon-consul/service"
+	"github.com/allegro/marathon-consul/web"
+)
+
+type Stop func()
+type Handler func(w http.ResponseWriter, r *http.Request)
+
+func NewHandler(config Config, webConfig web.Config, marathon marathon.Marathoner, serviceOperations service.ServiceRegistry) Stop {
+	stopChannels := make([]chan<- events.StopEvent, webConfig.WorkersCount, webConfig.WorkersCount)
+	eventQueue := make(chan events.Event, webConfig.QueueSize)
+	for i := 0; i < webConfig.WorkersCount; i++ {
+		handler := events.NewEventHandler(i, serviceOperations, marathon, eventQueue)
+		stopChannels[i] = handler.Start()
+	}
+
+	sse := newSSEHandler(eventQueue, marathon, webConfig.MaxEventSize, config)
+	dispatcherStop := sse.start()
+
+	guardQuit := leaderGuard(sse.Streamer, marathon)
+	stopChannels = append(stopChannels, dispatcherStop, guardQuit)
+
+	return stop(stopChannels)
+}
+
+func stop(channels []chan<- events.StopEvent) Stop {
+	return func() {
+		for _, channel := range channels {
+			channel <- events.StopEvent{}
+		}
+	}
+}
+
+// leaderGuard is a watchdog goroutine,
+// periodically checks if leader has changed
+// if change is detected, passed streamer is stopped - unable to recover
+// if this goroutine is quited, agent is stopped - unable to recover
+func leaderGuard(s *marathon.Streamer, m marathon.Marathoner) chan<- events.StopEvent {
+	// TODO(tz) - consider launching this goroutine from marathon,
+	// no need to pass marathon reciever then ??
+	quit := make(chan events.StopEvent)
+
+	go func() {
+		ticker := time.Tick(5 * time.Second)
+		for {
+			select {
+			case <-ticker:
+				if iAMLeader, err := m.AmILeader(); !iAMLeader && err != nil {
+					// Leader changed, not revocerable.
+					s.Stop()
+					log.Error("Tearing down SSE stream, marathon leader changed.")
+					return
+				} else if err != nil {
+					log.WithError(err).Error("Leader Guard error while checking leader.")
+				}
+			case <-quit:
+				log.Info("Recieved quit notification. Quit checker")
+				s.Stop()
+				return
+			}
+		}
+	}()
+	return quit
+}
diff --git a/sse/sse_handler.go b/sse/sse_handler.go
new file mode 100644
index 0000000..659a3f0
--- /dev/null
+++ b/sse/sse_handler.go
@@ -0,0 +1,111 @@
+package sse
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"time"
+
+	"github.com/allegro/marathon-consul/events"
+	"github.com/allegro/marathon-consul/marathon"
+	"github.com/allegro/marathon-consul/metrics"
+
+	log "github.com/Sirupsen/logrus"
+)
+
+// SSEHandler defines handler for marathon event stream, opening and closing
+// subscription
+type SSEHandler struct {
+	config      Config
+	eventQueue  chan events.Event
+	loc         string
+	client      *http.Client
+	close       context.CancelFunc
+	req         *http.Request
+	Streamer    *marathon.Streamer
+	maxLineSize int64
+}
+
+func newSSEHandler(eventQueue chan events.Event, service marathon.Marathoner, maxLineSize int64, config Config) *SSEHandler {
+
+	streamer, err := service.EventStream(
+		[]string{events.StatusUpdateEventType, events.HealthStatusChangedEventType},
+		config.Retries,
+		config.RetryBackoff,
+	)
+	if err != nil {
+		log.WithError(err).Fatal("Unable to start Streamer")
+	}
+
+	return &SSEHandler{
+		config:      config,
+		eventQueue:  eventQueue,
+		Streamer:    streamer,
+		maxLineSize: maxLineSize,
+	}
+}
+
+// Open connection to marathon v2/events
+func (h *SSEHandler) start() chan<- events.StopEvent {
+	stopChan := make(chan events.StopEvent)
+	go func() {
+		<-stopChan
+		h.stop()
+	}()
+
+	go func() {
+		defer h.stop()
+
+		err := h.Streamer.Start()
+		if err != nil {
+			log.WithError(err).Error("Unable to start streamer")
+		}
+		// buffer used for token storage,
+		// if token is greater than buffer, empty token is stored
+		buffer := make([]byte, h.maxLineSize)
+		// configure streamer scanner :)
+		h.Streamer.Scanner.Buffer(buffer, cap(buffer))
+		h.Streamer.Scanner.Split(events.ScanLines)
+		for {
+			metrics.Time("events.read", func() { h.handle() })
+		}
+	}()
+	return stopChan
+}
+
+func (h *SSEHandler) handle() {
+	e, err := events.ParseSSEEvent(h.Streamer.Scanner)
+	if err != nil {
+		if err == io.EOF {
+			// Event could be partial at this point
+			h.enqueueEvent(e)
+		}
+		log.WithError(err).Error("Error when parsing the event")
+		err = h.Streamer.Recover()
+		if err != nil {
+			log.WithError(err).Fatalf("Unable to recover streamer")
+		}
+	}
+	metrics.Mark("events.read." + e.Type)
+	if e.Type != events.StatusUpdateEventType && e.Type != events.HealthStatusChangedEventType {
+		log.Debugf("%s is not supported", e.Type)
+		metrics.Mark("events.read.drop")
+		return
+	}
+	h.enqueueEvent(e)
+}
+
+func (h *SSEHandler) enqueueEvent(e events.SSEEvent) {
+	select {
+	case h.eventQueue <- events.Event{Timestamp: time.Now(), EventType: e.Type, Body: e.Body}:
+		metrics.Mark("events.read.accept")
+	default:
+		log.Error("Events queue full. Dropping the event")
+		metrics.Mark("events.read.drop")
+	}
+}
+
+// Close connections managed by context
+func (h *SSEHandler) stop() {
+	h.Streamer.Stop()
+}
diff --git a/sync/sync.go b/sync/sync.go
index e4fa180..899855a 100644
--- a/sync/sync.go
+++ b/sync/sync.go
@@ -223,4 +223,4 @@ func marathonTaskIdsSet(marathonApps []*apps.App) map[apps.TaskID]struct{} {
 		}
 	}
 	return tasksSet
-}
\ No newline at end of file
+}
diff --git a/time/time.go b/time/time.go
index 4c4009b..c81fede 100644
--- a/time/time.go
+++ b/time/time.go
@@ -25,10 +25,18 @@ func (t Timestamp) MarshalJSON() ([]byte, error) {
 	return json.Marshal(t.String())
 }
 
+func (t *Timestamp) Delay() time.Duration {
+	return time.Now().Sub(t.Time)
+}
+
 func (t *Timestamp) String() string {
 	return t.Format(time.RFC3339Nano)
 }
 
+func (t *Timestamp) Missing() bool {
+	return t.Unix() == (time.Time{}).Unix()
+}
+
 type Interval struct {
 	time.Duration
 }
diff --git a/web/config.go b/web/config.go
index acdcb38..ac7cadb 100644
--- a/web/config.go
+++ b/web/config.go
@@ -5,4 +5,5 @@ type Config struct {
 	QueueSize    int
 	WorkersCount int
 	MaxEventSize int64
+	Enabled      bool
 }
diff --git a/web/web_handler.go b/web/web_handler.go
index 88febfb..07ff4f2 100644
--- a/web/web_handler.go
+++ b/web/web_handler.go
@@ -50,8 +50,6 @@ func (h *EventHandler) Handle(w http.ResponseWriter, r *http.Request) {
 		}
 
 		metrics.Mark("events.requests." + e.Type)
-		delay := time.Now().Unix() - e.Timestamp.Unix()
-		metrics.UpdateGauge("events.requests.delay.current", delay)
 		log.WithFields(log.Fields{"EventType": e.Type, "OriginalTimestamp": e.Timestamp.String()}).Debug("Received event")
 
 		if e.Type != events.StatusUpdateEventType && e.Type != events.HealthStatusChangedEventType {


Test patch:
diff --git a/apps/task_test.go b/apps/task_test.go
index 5e8136a..0bba314 100644
--- a/apps/task_test.go
+++ b/apps/task_test.go
@@ -5,6 +5,7 @@ import (
 	"io/ioutil"
 	"testing"
 
+	"github.com/allegro/marathon-consul/time"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -13,6 +14,7 @@ func TestParseTask(t *testing.T) {
 
 	testTask := &Task{
 		ID:                 "my-app_0-1396592784349",
+		Timestamp:          time.Timestamp{},
 		AppID:              "/my-app",
 		Host:               "slave-1234.acme.org",
 		Ports:              []int{31372},
@@ -40,6 +42,7 @@ func TestParseTasks(t *testing.T) {
 	expectedTasks := []Task{
 		{
 			ID:                 "test.47de43bd-1a81-11e5-bdb6-e6cb6734eaf8",
+			Timestamp:          time.Timestamp{},
 			AppID:              "/test",
 			Host:               "192.168.2.114",
 			Ports:              []int{31315},
diff --git a/config/config_test.go b/config/config_test.go
index a121cce..4733c2f 100644
--- a/config/config_test.go
+++ b/config/config_test.go
@@ -10,6 +10,7 @@ import (
 	"github.com/allegro/marathon-consul/marathon"
 	"github.com/allegro/marathon-consul/metrics"
 	"github.com/allegro/marathon-consul/sentry"
+	"github.com/allegro/marathon-consul/sse"
 	"github.com/allegro/marathon-consul/sync"
 	timeutil "github.com/allegro/marathon-consul/time"
 	"github.com/allegro/marathon-consul/web"
@@ -108,6 +109,10 @@ func TestConfig_ShouldBeMergedWithFileDefaultsAndFlags(t *testing.T) {
 			QueueSize:    1000,
 			WorkersCount: 10,
 			MaxEventSize: 4096,
+			Enabled:      true,
+		},
+		SSE: sse.Config{
+			Enabled: false,
 		},
 		Sync: sync.Config{
 			Interval: timeutil.Interval{Duration: 15 * time.Minute},
diff --git a/events/sse_events_test.go b/events/sse_events_test.go
new file mode 100644
index 0000000..3cbdf3a
--- /dev/null
+++ b/events/sse_events_test.go
@@ -0,0 +1,269 @@
+package events
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestEvent_IfEventIsEmptyReturnsFalse(t *testing.T) {
+	t.Parallel()
+	// given
+	event := &SSEEvent{
+		Type: "status_update_event",
+		Body: []byte(`{"id": "simpleId"}`),
+		ID:   "id",
+	}
+	// when
+	expected := false
+	actual := event.isEmpty()
+	// then
+	assert.Equal(t, expected, actual)
+}
+
+func TestEvent_IfEventIsEmptyReturnsTrue(t *testing.T) {
+	t.Parallel()
+	// given
+	event := &SSEEvent{}
+	// when
+	expected := true
+	actual := event.isEmpty()
+	// then
+	assert.Equal(t, expected, actual)
+}
+
+func TestParseLine_WhenStautsUpdateEventPassed(t *testing.T) {
+	t.Parallel()
+	// given
+	event := &SSEEvent{}
+	line0 := []byte("id: 0")
+	line1 := []byte("event: status_update_event")
+	line2 := []byte("data: testData")
+	expected0 := "0"
+	expected1 := "status_update_event"
+	expected2 := []byte("testData\n")
+	// when
+	event.parseLine(line0)
+	event.parseLine(line1)
+	event.parseLine(line2)
+	// then
+	assert.Equal(t, expected0, event.ID)
+	assert.Equal(t, expected1, event.Type)
+	assert.Equal(t, string(expected2), string(event.Body))
+}
+
+func TestParseLine_WhenGarbageIsProvidedBodyShouldBeNil(t *testing.T) {
+	t.Parallel()
+	// given
+	event := &SSEEvent{}
+	line := []byte("garbage data")
+	expectedBody := []byte(nil)
+	// when
+	_ = event.parseLine(line)
+	// then
+	assert.Equal(t, expectedBody, event.Body)
+}
+
+func BenchmarkParseLine(b *testing.B) {
+	// given
+	longTestData := bytes.Repeat([]byte("testData"), 1)
+	longLine := append([]byte("data: "), longTestData...)
+	expectedEvent := &SSEEvent{Body: append(longTestData, []byte("\n")...)}
+
+	var event *SSEEvent
+	for i := 0; i <= b.N; i++ {
+		event = &SSEEvent{}
+		event.parseLine(longLine)
+	}
+
+	// then
+	assert.Equal(b, string(expectedEvent.Body), string(event.Body))
+
+}
+
+var parseEventCases = []struct {
+	in            string
+	expectedEvent SSEEvent
+}{
+	{": No Event", SSEEvent{}},
+	{"event: status_update_event\ndata: testData\n",
+		SSEEvent{Type: "status_update_event", Body: []byte("testData\n")},
+	},
+	{"event: status_update_event\ndata: testData\ndummydata",
+		SSEEvent{Type: "status_update_event", Body: []byte("testData\n")},
+	},
+	{"event: status_update_event\ndata",
+		SSEEvent{Type: "status_update_event"},
+	},
+	{"event: status_update_event\ndata:\n",
+		SSEEvent{Type: "status_update_event"},
+	},
+	{"event: some_event\ndata: abc\ndata: def",
+		SSEEvent{Type: "some_event", Body: []byte("abc\ndef\n")},
+	},
+	{"event: some_event\ndata: aaa\ndata: ccc\ndata: 10",
+		SSEEvent{Type: "some_event", Body: []byte("aaa\nccc\n10\n")},
+	},
+	{"event: some_event\ndata: abc\nid: 12",
+		SSEEvent{Type: "some_event", Body: []byte("abc\n"), ID: "12"},
+	},
+	{"data: abc\n",
+		SSEEvent{Body: []byte("abc\n")},
+	},
+}
+
+func TestParseSSEEvent_TestCases(t *testing.T) {
+	t.Parallel()
+
+	for _, testCase := range parseEventCases {
+		reader := strings.NewReader(testCase.in)
+		sscanner := bufio.NewScanner(reader)
+		// when
+		actualEvent, _ := ParseSSEEvent(sscanner)
+		// then
+		assert.Equal(t, testCase.expectedEvent, actualEvent)
+
+	}
+}
+
+var parseEventMultipleDataCases = []struct {
+	in             string
+	expectedEvents []SSEEvent
+}{
+	{"\n\n\n\n\n\n\n",
+		[]SSEEvent{
+			SSEEvent{},
+		},
+	},
+	{"event: status_update_event\ndata: testData\n\nevent: some_event\ndata: someData",
+		[]SSEEvent{
+			SSEEvent{Type: "status_update_event", Body: []byte("testData\n")},
+			SSEEvent{Type: "some_event", Body: []byte("someData\n")},
+		},
+	},
+	{"event: status_update_event\ndata: testData\n\nid: 13\ndata: someData\n\nid: 14\ndata: abc\n\nid: 15\ndata: def\n",
+		[]SSEEvent{
+			SSEEvent{Type: "status_update_event", Body: []byte("testData\n")},
+			SSEEvent{ID: "13", Body: []byte("someData\n")},
+			SSEEvent{ID: "14", Body: []byte("abc\n")},
+			SSEEvent{ID: "15", Body: []byte("def\n")},
+		},
+	},
+	{"data: testData\n\ndata: someData\n\ndata: abc\n\ndata: def\n",
+		[]SSEEvent{
+			SSEEvent{Body: []byte("testData\n")},
+			SSEEvent{Body: []byte("someData\n")},
+			SSEEvent{Body: []byte("abc\n")},
+			SSEEvent{Body: []byte("def\n")},
+		},
+	},
+	{"data: testData\nretry: 10\ndummy: dummy field\n\ndata: someData\n\ndata: abc\n\ndata: def\n",
+		[]SSEEvent{
+			SSEEvent{Body: []byte("testData\n"), Delay: "10"},
+			SSEEvent{Body: []byte("someData\n")},
+			SSEEvent{Body: []byte("abc\n")},
+			SSEEvent{Body: []byte("def\n")},
+		},
+	},
+}
+
+func TestParseSSEEvent_MultipleDataTestCases(t *testing.T) {
+	t.Parallel()
+
+	for _, testCase := range parseEventMultipleDataCases {
+
+		reader := strings.NewReader(testCase.in)
+		sscanner := bufio.NewScanner(reader)
+
+		var err error
+		var actualEvent SSEEvent
+		for i := 0; len(testCase.expectedEvents) > i && err != fmt.Errorf("EOF"); i++ {
+			// when
+			actualEvent, err = ParseSSEEvent(sscanner)
+			// then
+			assert.Equal(t, testCase.expectedEvents[i], actualEvent)
+		}
+
+	}
+}
+
+func BenchmarkParseEvent(b *testing.B) {
+	var event SSEEvent
+	expectedEvent := SSEEvent{
+		Type:  "some event type",
+		ID:    "1",
+		Body:  []byte("some data\nnext data\n"),
+		Delay: "10",
+	}
+
+	for i := 0; i <= b.N; i++ {
+		reader := strings.NewReader("event: some event type\nid: 1\ndata: some data\ndata: next data\nretry: 10\n")
+		sscanner := bufio.NewScanner(reader)
+		event, _ = ParseSSEEvent(sscanner)
+	}
+
+	assert.Equal(b, expectedEvent, event)
+
+}
+
+func TestParseSSEEvent_WhenVeryLongLineIsOnStream(t *testing.T) {
+	t.Parallel()
+	// given
+	veryLongEventName := strings.Repeat("a", 1016)
+	veryLongLine := fmt.Sprintf("event: %s\n", veryLongEventName)
+
+	sreader := strings.NewReader(veryLongLine)
+	sscanner := bufio.NewScanner(sreader)
+	expectedEventType := veryLongEventName
+	// when
+	event, _ := ParseSSEEvent(sscanner)
+	// then
+	assert.Equal(t, expectedEventType, event.Type)
+}
+
+func TestParseSSEEvent_WhenVeryLongLineIsLongerThanMaxLineSize(t *testing.T) {
+	t.Parallel()
+	// given
+	veryLongEventName := strings.Repeat("a", 10240)
+	veryLongLine := fmt.Sprintf("event: %s\n\n", veryLongEventName)
+	sreader := strings.NewReader(veryLongLine)
+	sscanner := bufio.NewScanner(sreader)
+	buffer := make([]byte, 1024)
+	sscanner.Buffer(buffer, cap(buffer))
+	expectedEventType := ""
+	// when
+	event, _ := ParseSSEEvent(sscanner)
+	// then
+	assert.Equal(t, expectedEventType, event.Type)
+}
+
+var scanLineCases = []struct {
+	in              []byte
+	atEOL           bool
+	expectedAdvance int
+	expectedToken   []byte
+}{
+	{[]byte("abcd\n"), false, 5, []byte("abcd")},
+	{[]byte("abcd\r"), false, 5, []byte("abcd")},
+	{[]byte("abcd\r\n"), false, 6, []byte("abcd")},
+	{[]byte("abcd"), false, 0, []byte(nil)},
+	{[]byte("abcd"), true, 4, []byte("abcd")},
+	{[]byte("abcd\n"), true, 5, []byte("abcd")},
+}
+
+func TestScanLine_TestCases(t *testing.T) {
+	t.Parallel()
+	for _, testCase := range scanLineCases {
+		// when
+		advance, token, err := ScanLines(testCase.in, testCase.atEOL)
+		// then
+		require.NoError(t, err)
+		assert.Equal(t, testCase.expectedAdvance, advance)
+		assert.Equal(t, testCase.expectedToken, token)
+	}
+}
diff --git a/events/task_health_change_test.go b/events/task_health_change_test.go
index 284c23d..ae00475 100644
--- a/events/task_health_change_test.go
+++ b/events/task_health_change_test.go
@@ -3,12 +3,22 @@ package events
 import (
 	"encoding/json"
 	"testing"
+	gotime "time"
 
+	"github.com/allegro/marathon-consul/time"
 	"github.com/stretchr/testify/assert"
 )
 
+func StringToTimestamp(date string) time.Timestamp {
+	t, err := gotime.Parse(gotime.RFC3339Nano, date)
+	if err != nil {
+		return time.Timestamp{Time: gotime.Time{}}
+	}
+	return time.Timestamp{Time: t}
+}
+
 var testHealthChange = &TaskHealthChange{
-	Timestamp: "2014-03-01T23:29:30.158Z",
+	Timestamp: StringToTimestamp("2014-03-01T23:29:30.158Z"),
 	ID:        "my-app_0-1396592784349",
 	Alive:     true,
 	AppID:     "/my-app",
diff --git a/marathon/marathon_test.go b/marathon/marathon_test.go
index 4ccf03a..8d7ae74 100644
--- a/marathon/marathon_test.go
+++ b/marathon/marathon_test.go
@@ -17,7 +17,7 @@ func TestMarathon_AppsWhenMarathonReturnEmptyList(t *testing.T) {
 	defer server.Close()
 
 	url, _ := url.Parse(server.URL)
-	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"})
+	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"}, "")
 	m.client.Transport = transport
 	// when
 	apps, err := m.ConsulApps()
@@ -29,7 +29,7 @@ func TestMarathon_AppsWhenMarathonReturnEmptyList(t *testing.T) {
 func TestMarathon_AppsWhenConfigIsWrong(t *testing.T) {
 	t.Parallel()
 	// given
-	m, _ := New(Config{Location: "not::valid/location", Protocol: "HTTP"})
+	m, _ := New(Config{Location: "not::valid/location", Protocol: "HTTP"}, "")
 	// when
 	apps, err := m.ConsulApps()
 	//then
@@ -43,7 +43,7 @@ func TestMarathon_AppsWhenServerIsNotResponding(t *testing.T) {
 	}
 	t.Parallel()
 	// given
-	m, _ := New(Config{Location: "unknown:22", Protocol: "HTTP"})
+	m, _ := New(Config{Location: "unknown:22", Protocol: "HTTP"}, "")
 	// when
 	apps, err := m.ConsulApps()
 	//then
@@ -62,7 +62,7 @@ func TestMarathon_AppsWhenMarathonConnectionFailedShouldNotRetry(t *testing.T) {
 	defer server.Close()
 
 	url, _ := url.Parse(server.URL)
-	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"})
+	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"}, "")
 	m.client.Transport = transport
 	// when
 	apps, err := m.ConsulApps()
@@ -83,7 +83,7 @@ func TestMarathon_TasksWhenMarathonConnectionFailedShouldNotRetry(t *testing.T)
 	defer server.Close()
 
 	url, _ := url.Parse(server.URL)
-	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"})
+	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"}, "")
 	m.client.Transport = transport
 	// when
 	tasks, err := m.Tasks("/app/id")
@@ -104,7 +104,7 @@ func TestMarathon_AppWhenMarathonConnectionFailedShouldNotRetry(t *testing.T) {
 	defer server.Close()
 
 	url, _ := url.Parse(server.URL)
-	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"})
+	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"}, "")
 	m.client.Transport = transport
 	// when
 	app, err := m.App("/app/id")
@@ -121,7 +121,7 @@ func TestMarathon_AppsWhenMarathonReturnEmptyResponse(t *testing.T) {
 	defer server.Close()
 
 	url, _ := url.Parse(server.URL)
-	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"})
+	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"}, "")
 	m.client.Transport = transport
 	// when
 	apps, err := m.ConsulApps()
@@ -137,7 +137,7 @@ func TestMarathon_AppsWhenMarathonReturnMalformedJsonResponse(t *testing.T) {
 	defer server.Close()
 
 	url, _ := url.Parse(server.URL)
-	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"})
+	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"}, "")
 	m.client.Transport = transport
 	// when
 	app, err := m.App("/test/app")
@@ -153,7 +153,7 @@ func TestMarathon_AppWhenMarathonReturnEmptyApp(t *testing.T) {
 	defer server.Close()
 
 	url, _ := url.Parse(server.URL)
-	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"})
+	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"}, "")
 	m.client.Transport = transport
 	// when
 	app, err := m.App("/test/app")
@@ -169,7 +169,7 @@ func TestMarathon_AppWhenMarathonReturnEmptyResponse(t *testing.T) {
 	defer server.Close()
 
 	url, _ := url.Parse(server.URL)
-	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"})
+	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"}, "")
 	m.client.Transport = transport
 	// when
 	app, err := m.App("/test/app")
@@ -185,7 +185,7 @@ func TestMarathon_AppWhenMarathonReturnMalformedJsonResponse(t *testing.T) {
 	defer server.Close()
 
 	url, _ := url.Parse(server.URL)
-	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"})
+	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"}, "")
 	m.client.Transport = transport
 	// when
 	apps, err := m.ConsulApps()
@@ -207,7 +207,7 @@ func TestMarathon_TasksWhenMarathonReturnEmptyList(t *testing.T) {
 	}]}`)
 	defer server.Close()
 	url, _ := url.Parse(server.URL)
-	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"})
+	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"}, "")
 	m.client.Transport = transport
 	// when
 	tasks, err := m.Tasks("//test/app")
@@ -222,7 +222,7 @@ func TestMarathon_TasksWhenMarathonReturnEmptyResponse(t *testing.T) {
 	server, transport := stubServer("/v2/apps/test/app/tasks", ``)
 	defer server.Close()
 	url, _ := url.Parse(server.URL)
-	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"})
+	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"}, "")
 	m.client.Transport = transport
 	// when
 	tasks, err := m.Tasks("/test/app")
@@ -237,7 +237,7 @@ func TestMarathon_TasksWhenMarathonReturnMalformedJsonResponse(t *testing.T) {
 	server, transport := stubServer("/v2/apps/test/app/tasks", ``)
 	defer server.Close()
 	url, _ := url.Parse(server.URL)
-	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"})
+	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"}, "")
 	m.client.Transport = transport
 	// when
 	tasks, err := m.Tasks("/test/app")
@@ -251,7 +251,7 @@ func TestConfig_transport(t *testing.T) {
 	// given
 	config := Config{VerifySsl: false}
 	// when
-	marathon, _ := New(config)
+	marathon, _ := New(config, "")
 	// then
 	transport, ok := marathon.client.Transport.(*http.Transport)
 	assert.True(t, ok)
@@ -263,7 +263,7 @@ func TestUrl_WithoutAuth(t *testing.T) {
 	// given
 	config := Config{Location: "localhost:8080", Protocol: "http"}
 	// when
-	m, _ := New(config)
+	m, _ := New(config, "")
 	// then
 	assert.Equal(t, "http://localhost:8080/v2/apps", m.url("/v2/apps"))
 }
@@ -273,7 +273,7 @@ func TestUrl_WithAuth(t *testing.T) {
 	// given
 	config := Config{Location: "localhost:8080", Protocol: "http", Username: "peter", Password: "parker"}
 	// when
-	m, _ := New(config)
+	m, _ := New(config, "")
 	// then
 	assert.Equal(t, "http://peter:parker@localhost:8080/v2/apps", m.url("/v2/apps"))
 }
@@ -285,7 +285,7 @@ func TestLeader_SuccessfulResponse(t *testing.T) {
 	server, transport := stubServer("/v2/leader", `{"leader": "some.leader.host:8081"}`)
 	defer server.Close()
 	url, _ := url.Parse(server.URL)
-	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"})
+	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"}, "")
 	m.client.Transport = transport
 
 	// when
@@ -303,7 +303,7 @@ func TestLeader_ErrorOnMalformedJsonResponse(t *testing.T) {
 	server, transport := stubServer("/v2/leader", "{")
 	defer server.Close()
 	url, _ := url.Parse(server.URL)
-	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"})
+	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"}, "")
 	m.client.Transport = transport
 
 	// when
@@ -325,7 +325,7 @@ func TestLeader_NotRetryOnFailingResponse(t *testing.T) {
 	})
 	defer server.Close()
 	url, _ := url.Parse(server.URL)
-	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"})
+	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"}, "")
 	m.client.Transport = transport
 
 	// when
@@ -337,6 +337,77 @@ func TestLeader_NotRetryOnFailingResponse(t *testing.T) {
 	assert.Empty(t, leader)
 }
 
+func TestLeaderPoll_PassingRunningOnLeader(t *testing.T) {
+	t.Parallel()
+
+	// given
+	server, transport := stubServer("/v2/leader", `{"leader": "this.leader:8080"}`)
+	defer server.Close()
+	url, _ := url.Parse(server.URL)
+	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"}, "this.leader:8080")
+	m.client.Transport = transport
+
+	// when
+	err := m.leaderPoll()
+
+	//then
+	assert.NoError(t, err)
+}
+
+func TestAMILeader_PassingRunningOnLeader(t *testing.T) {
+	t.Parallel()
+
+	// given
+	server, transport := stubServer("/v2/leader", `{"leader": "this.leader:8080"}`)
+	defer server.Close()
+	url, _ := url.Parse(server.URL)
+	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"}, "this.leader:8080")
+	m.client.Transport = transport
+
+	// when
+	leading, err := m.AmILeader()
+
+	//then
+	assert.True(t, leading)
+	assert.NoError(t, err)
+}
+
+func TestAMILeader_NotPassingNotRunningOnLeader(t *testing.T) {
+	t.Parallel()
+
+	// given
+	server, transport := stubServer("/v2/leader", `{"leader": "other.leader:8080"}`)
+	defer server.Close()
+	url, _ := url.Parse(server.URL)
+	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"}, "this.leader:8080")
+	m.client.Transport = transport
+
+	// when
+	leading, err := m.AmILeader()
+
+	//then
+	assert.False(t, leading)
+	assert.NoError(t, err)
+}
+
+func TestEventStream_PassingStreamerCreated(t *testing.T) {
+	t.Parallel()
+
+	// given
+	server, transport := stubServer("/v2/leader", `{"leader": "this.leader:8080"}`)
+	defer server.Close()
+	url, _ := url.Parse(server.URL)
+	m, _ := New(Config{Location: url.Host, Protocol: "HTTP"}, "this.leader:8080")
+	m.client.Transport = transport
+
+	// when
+	streamer, err := m.EventStream([]string{}, 1, 1)
+
+	//then
+	assert.NoError(t, err)
+	assert.IsType(t, &Streamer{}, streamer)
+}
+
 // http://keighl.com/post/mocking-http-responses-in-golang/
 func stubServer(uri string, body string) (*httptest.Server, *http.Transport) {
 	return mockServer(func(w http.ResponseWriter, r *http.Request) {
diff --git a/sync/error_marathon_stub_test.go b/sync/error_marathon_stub_test.go
index 9be9cb2..ed06fec 100644
--- a/sync/error_marathon_stub_test.go
+++ b/sync/error_marathon_stub_test.go
@@ -4,6 +4,7 @@ import (
 	"errors"
 
 	"github.com/allegro/marathon-consul/apps"
+	"github.com/allegro/marathon-consul/marathon"
 )
 
 type errorMarathon struct {
@@ -24,3 +25,11 @@ func (m errorMarathon) Tasks(appID apps.AppID) ([]apps.Task, error) {
 func (m errorMarathon) Leader() (string, error) {
 	return "", errors.New("Error")
 }
+
+func (m errorMarathon) EventStream([]string, int, int) (*marathon.Streamer, error) {
+	return &marathon.Streamer{}, errors.New("Error")
+}
+
+func (m errorMarathon) AmILeader() (bool, error) {
+	return false, errors.New("Error")
+}