From 9699dc2a8510df913fde14b812f61e1cf77ca9a6 Mon Sep 17 00:00:00 2001
From: Alex Antonov <alex@antonov.ws>
Date: Thu, 22 Mar 2018 13:00:05 -0400
Subject: [PATCH] Added cluster/leader endpoint

---
 cluster/leadership.go     | 32 ++++++++++++++++++++++++++++++++
 docs/configuration/api.md | 20 ++++++++++++++++++++
 server/server.go          |  3 +++
 3 files changed, 55 insertions(+)

diff --git a/cluster/leadership.go b/cluster/leadership.go
index af01b5290..c6ff904b8 100644
--- a/cluster/leadership.go
+++ b/cluster/leadership.go
@@ -2,15 +2,22 @@ package cluster
 
 import (
 	"context"
+	"net/http"
 	"time"
 
 	"github.com/cenk/backoff"
+	"github.com/containous/mux"
 	"github.com/containous/traefik/log"
 	"github.com/containous/traefik/safe"
 	"github.com/containous/traefik/types"
 	"github.com/docker/leadership"
+	"github.com/unrolled/render"
 )
 
+var templatesRenderer = render.New(render.Options{
+	Directory: "nowhere",
+})
+
 // Leadership allows leadership election using a KV store
 type Leadership struct {
 	*safe.Pool
@@ -98,7 +105,32 @@ func (l *Leadership) onElection(elected bool) {
 	}
 }
 
+type leaderResponse struct {
+	Leader bool `json:"leader"`
+}
+
+func (l *Leadership) getLeaderHandler(response http.ResponseWriter, request *http.Request) {
+	leader := &leaderResponse{Leader: l.IsLeader()}
+
+	status := http.StatusOK
+	if !leader.Leader {
+		// Set status to be `429`, as this will typically cause load balancers to stop sending requests to the instance without removing them from rotation.
+		status = http.StatusTooManyRequests
+	}
+
+	err := templatesRenderer.JSON(response, status, leader)
+	if err != nil {
+		log.Error(err)
+	}
+}
+
 // IsLeader returns true if current node is leader
 func (l *Leadership) IsLeader() bool {
 	return l.leader.Get().(bool)
 }
+
+// AddRoutes add dashboard routes on a router
+func (l *Leadership) AddRoutes(router *mux.Router) {
+	// Expose cluster leader
+	router.Methods(http.MethodGet).Path("/api/cluster/leader").HandlerFunc(l.getLeaderHandler)
+}
diff --git a/docs/configuration/api.md b/docs/configuration/api.md
index cdcddf939..b2db8517a 100644
--- a/docs/configuration/api.md
+++ b/docs/configuration/api.md
@@ -43,6 +43,7 @@ For more customization, see [entry points](/configuration/entrypoints/) document
 | Path                                                            | Method           | Description                               |
 |-----------------------------------------------------------------|------------------|-------------------------------------------|
 | `/`                                                             |     `GET`        | Provides a simple HTML frontend of Træfik |
+| `/cluster/leader`                                               |     `GET`        | JSON leader true/false response           |
 | `/health`                                                       |     `GET`        | JSON health metrics                       |
 | `/api`                                                          |     `GET`        | Configuration for all providers           |
 | `/api/providers`                                                |     `GET`        | Providers                                 |
@@ -222,6 +223,25 @@ curl -s "http://localhost:8080/api" | jq .
 }
 ```
 
+### Cluster Leadership
+
+```shell
+curl -s "http://localhost:8080/cluster/leader" | jq .
+```
+```shell
+< HTTP/1.1 200 OK
+< Content-Type: application/json; charset=UTF-8
+< Date: xxx
+< Content-Length: 15
+```
+If the given node is not a cluster leader, an HTTP status of `429-Too-Many-Requests` will be returned.
+```json
+{
+  // current leadership status of the queried node
+  "leader": true
+}
+```
+
 ### Health
 
 ```shell
diff --git a/server/server.go b/server/server.go
index f02af806e..78ca7a83f 100644
--- a/server/server.go
+++ b/server/server.go
@@ -739,6 +739,9 @@ func (s *Server) addInternalRoutes(entryPointName string, router *mux.Router) {
 
 	if s.globalConfiguration.API != nil && s.globalConfiguration.API.EntryPoint == entryPointName {
 		s.globalConfiguration.API.AddRoutes(router)
+		if s.leadership != nil {
+			s.leadership.AddRoutes(router)
+		}
 	}
 }