Add distributor endpoint for validating PromQL expressions

Returns a JSON object of the form:

    {
      "valid": true|false
    }

Fixes #176

Author: juliusv

cmd/cortex/main.go

@@ -310,6 +310,7 @@ func setupQuerier(
 	}).WithPrefix("/api/prom/api/v1")
 	api.Register(promRouter)
 	router.PathPrefix("/api/v1").Handler(promRouter)
+	router.Path("/validate_expr").Handler(http.HandlerFunc(distributor.ValidateExprHandler))
 	router.Path("/user_stats").Handler(http.HandlerFunc(distributor.UserStatsHandler))
 	router.Path("/graph").Handler(ui.GraphHandler())
 	router.PathPrefix("/static/").Handler(ui.StaticAssetsHandler("/api/prom/static/"))

distributor/http_server.go

@@ -4,6 +4,7 @@ import (
 	"net/http"
 
 	"github.com/prometheus/common/log"
+	"github.com/prometheus/prometheus/promql"
 	"github.com/prometheus/prometheus/storage/remote"
 
 	"github.com/weaveworks/cortex/util"
@@ -54,3 +55,10 @@ func (d *Distributor) UserStatsHandler(w http.ResponseWriter, r *http.Request) {
 
 	util.WriteJSONResponse(w, stats)
 }
+
+// ValidateExprHandler validates a PromQL expression.
+func (d *Distributor) ValidateExprHandler(w http.ResponseWriter, r *http.Request) {
+	_, err := promql.ParseExpr(r.FormValue("expr"))
+	valid := (err == nil)
+	util.WriteJSONResponse(w, map[string]bool{"valid": valid})
+}