Allow the query-frontend code to proxy queries to 'vanilla' Prometheus.

docs/prometheus-frontend.yml

@@ -0,0 +1,43 @@
+# You can use the Cortex query frontend with any Prometheus-API compatible
+# service, including Prometheus and Thanos.  Use this config file to get
+# the benefits of query parallelisation and caching.
+
+# Disable the requirement that every request to Cortex has a
+# X-Scope-OrgID header. `fake` will be substituted in instead.
+auth_enabled: false
+
+# We only want to run the query-frontend module.
+target: query-frontend
+
+# We don't want the usual /api/prom prefix.
+http_prefix:
+
+server:
+  http_listen_port: 9091
+
+frontend:
+  split_queries_by_day: true
+  align_queries_with_step: true
+  cache_results: true
+  compress_responses: true
+
+  results_cache:
+    max_freshness: 1m
+    cache:
+
+      # We're going to use the in-process "FIFO" cache, but you can enable
+      # memcached below.
+      enable_fifocache: true
+      fifocache:
+        size: 1024
+        validity: 24h
+
+      # If you want to use a memcached cluster, configure a headless service
+      # in Kubernetes and Cortex will discover the individual instances using
+      # a SRV DNS query.  Cortex will then do client-side hashing to spread
+      # the load evenly.
+      # memcached:
+      #   memcached_client:
+      #     host: memcached.default.svc.cluster.local
+      #     service: memcached
+      #     consistent_hash: true

pkg/cortex/cortex.go

@@ -53,6 +53,7 @@ type Config struct {
 	Target      moduleName `yaml:"target,omitempty"`
 	AuthEnabled bool       `yaml:"auth_enabled,omitempty"`
 	PrintConfig bool       `yaml:"-"`
+	HTTPPrefix  string     `yaml:"http_prefix"`
 
 	Server         server.Config            `yaml:"server,omitempty"`
 	Distributor    distributor.Config       `yaml:"distributor,omitempty"`
@@ -82,6 +83,7 @@ func (c *Config) RegisterFlags(f *flag.FlagSet) {
 	f.Var(&c.Target, "target", "target module (default All)")
 	f.BoolVar(&c.AuthEnabled, "auth.enabled", true, "Set to false to disable auth.")
 	f.BoolVar(&c.PrintConfig, "print.config", false, "Print the config and exit.")
+	f.StringVar(&c.HTTPPrefix, "http.prefix", "/api/prom", "HTTP path prefix for Cortex API.")
 
 	c.Server.RegisterFlags(f)
 	c.Distributor.RegisterFlags(f)

pkg/cortex/modules.go

@@ -132,6 +132,14 @@ func (m *moduleName) Set(s string) error {
 	}
 }
 
+func (m *moduleName) UnmarshalYAML(unmarshal func(interface{}) error) error {
+	var s string
+	if err := unmarshal(&s); err != nil {
+		return err
+	}
+	return m.Set(s)
+}
+
 func (t *Cortex) initServer(cfg *Config) (err error) {
 	t.server, err = server.New(cfg.Server)
 	return
@@ -259,7 +267,7 @@ func (t *Cortex) initQueryFrontend(cfg *Config) (err error) {
 	}
 
 	frontend.RegisterFrontendServer(t.server.GRPC, t.frontend)
-	t.server.HTTP.PathPrefix("/api/prom").Handler(
+	t.server.HTTP.PathPrefix(cfg.HTTPPrefix).Handler(
 		t.httpAuthMiddleware.Wrap(
 			t.frontend.Handler(),
 		),

pkg/querier/frontend/frontend.go

@@ -8,6 +8,8 @@ import (
 	"io/ioutil"
 	"math/rand"
 	"net/http"
+	"net/url"
+	"path"
 	"sync"
 	"time"
 
@@ -57,6 +59,7 @@ type Config struct {
 	CacheResults                  bool `yaml:"cache_results"`
 	CompressResponses             bool `yaml:"compress_responses"`
 	queryrange.ResultsCacheConfig `yaml:"results_cache"`
+	DownstreamURL                 string `yaml:"downstream"`
 }
 
 // RegisterFlags adds the flags required to config this to the given FlagSet.
@@ -68,6 +71,7 @@ func (cfg *Config) RegisterFlags(f *flag.FlagSet) {
 	f.BoolVar(&cfg.CacheResults, "querier.cache-results", false, "Cache query results.")
 	f.BoolVar(&cfg.CompressResponses, "querier.compress-http-responses", false, "Compress HTTP responses.")
 	cfg.ResultsCacheConfig.RegisterFlags(f)
+	f.StringVar(&cfg.DownstreamURL, "frontend.downstream-url", "", "URL of downstream Prometheus.")
 }
 
 // Frontend queues HTTP requests, dispatches them to backends, and handles retries
@@ -99,6 +103,7 @@ func New(cfg Config, log log.Logger, limits *validation.Overrides) (*Frontend, e
 		log:    log,
 		queues: map[string]chan *request{},
 	}
+	f.cond = sync.NewCond(&f.mtx)
 
 	// Stack up the pipeline of various query range middlewares.
 	var queryRangeMiddleware []queryrange.Middleware
@@ -119,20 +124,44 @@ func New(cfg Config, log log.Logger, limits *validation.Overrides) (*Frontend, e
 		queryRangeMiddleware = append(queryRangeMiddleware, queryrange.InstrumentMiddleware("retry", queryRangeDuration), queryrange.NewRetryMiddleware(log, cfg.MaxRetries))
 	}
 
-	// Finally, if the user selected any query range middleware, stitch it in.
+	// If the user has specified a downstream Prometheus, then we should
+	// forward requests to that.  Otherwise we will wait for queries to
+	// contact us.
 	var roundTripper http.RoundTripper = f
+	if cfg.DownstreamURL != "" {
+		u, err := url.Parse(cfg.DownstreamURL)
+		if err != nil {
+			return nil, err
+		}
+
+		roundTripper = RoundTripFunc(func(r *http.Request) (*http.Response, error) {
+			r.URL.Scheme = u.Scheme
+			r.URL.Host = u.Host
+			r.URL.Path = path.Join(u.Path, r.URL.Path)
+			return http.DefaultTransport.RoundTrip(r)
+		})
+	}
+
+	// Finally, if the user selected any query range middleware, stitch it in.
 	if len(queryRangeMiddleware) > 0 {
 		roundTripper = queryrange.NewRoundTripper(
-			f,
-			queryrange.MergeMiddlewares(queryRangeMiddleware...).Wrap(&queryrange.ToRoundTripperMiddleware{Next: f}),
+			roundTripper,
+			queryrange.MergeMiddlewares(queryRangeMiddleware...).Wrap(&queryrange.ToRoundTripperMiddleware{Next: roundTripper}),
 			limits,
 		)
 	}
 	f.roundTripper = roundTripper
-	f.cond = sync.NewCond(&f.mtx)
 	return f, nil
 }
 
+// RoundTripFunc is to http.RoundTripper what http.HandlerFunc is to http.Handler.
+type RoundTripFunc func(*http.Request) (*http.Response, error)
+
+// RoundTrip implements http.RoundTripper.
+func (f RoundTripFunc) RoundTrip(r *http.Request) (*http.Response, error) {
+	return f(r)
+}
+
 // Close stops new requests and errors out any pending requests.
 func (f *Frontend) Close() {
 	f.mtx.Lock()