Moves sort to a query-time decision and adds back the limit.

consul/prepared_query_endpoint.go

@@ -160,17 +160,6 @@ func parseService(svc *structs.ServiceQuery) error {
 	// - OnlyPassing is just a boolean so doesn't need further validation.
 	// - Tags is a free-form list of tags and doesn't need further validation.
 
-	// Sort order must be one of the allowed values, or if not given we
-	// default to "shuffle" so there's load balancing.
-	switch svc.Sort {
-	case structs.QueryOrderShuffle:
-	case structs.QueryOrderSort:
-	case "":
-		svc.Sort = structs.QueryOrderShuffle
-	default:
-		return fmt.Errorf("Bad Sort '%s'", svc.Sort)
-	}
-
 	return nil
 }
 
@@ -226,10 +215,13 @@ func (p *PreparedQuery) Execute(args *structs.PreparedQueryExecuteRequest,
 	// Shuffle the results in case coordinates are not available if they
 	// requested an RTT sort.
 	reply.Nodes.Shuffle()
-	if query.Service.Sort == structs.QueryOrderSort {
-		if err := p.srv.sortNodesByDistanceFrom(args.Source, reply.Nodes); err != nil {
-			return err
-		}
+	if err := p.srv.sortNodesByDistanceFrom(args.Source, reply.Nodes); err != nil {
+		return err
+	}
+
+	// Apply the limit if given.
+	if args.Limit > 0 && len(reply.Nodes) > args.Limit {
+		reply.Nodes = reply.Nodes[:args.Limit]
 	}
 
 	// In the happy path where we found some healthy nodes we go with that
@@ -237,7 +229,7 @@ func (p *PreparedQuery) Execute(args *structs.PreparedQueryExecuteRequest,
 	// by the query setup.
 	if len(reply.Nodes) == 0 {
 		wrapper := &queryServerWrapper{p.srv}
-		if err := queryFailover(wrapper, query, args.QueryOptions, reply); err != nil {
+		if err := queryFailover(wrapper, query, args, reply); err != nil {
 			return err
 		}
 	}
@@ -274,6 +266,11 @@ func (p *PreparedQuery) ExecuteRemote(args *structs.PreparedQueryExecuteRemoteRe
 	// balance the load across the results.
 	reply.Nodes.Shuffle()
 
+	// Apply the limit if given.
+	if args.Limit > 0 && len(reply.Nodes) > args.Limit {
+		reply.Nodes = reply.Nodes[:args.Limit]
+	}
+
 	return nil
 }
 
@@ -403,7 +400,7 @@ func (q *queryServerWrapper) GetOtherDatacentersByDistance() ([]string, error) {
 // queryFailover runs an algorithm to determine which DCs to try and then calls
 // them to try to locate alternative services.
 func queryFailover(q queryServer, query *structs.PreparedQuery,
-	options structs.QueryOptions,
+	args *structs.PreparedQueryExecuteRequest,
 	reply *structs.PreparedQueryExecuteResponse) error {
 
 	// Build a candidate list of DCs, starting with the nearest N from RTTs.
@@ -433,12 +430,16 @@ func queryFailover(q queryServer, query *structs.PreparedQuery,
 		}
 	}
 
-	// Now try the selected DCs in priority order.
+	// Now try the selected DCs in priority order. Note that we pass along
+	// the limit since it can be applied remotely to save bandwidth. We also
+	// pass along the consistency mode information we were given, so that
+	// applies to the remote query as well.
 	for _, dc := range dcs {
 		remote := &structs.PreparedQueryExecuteRemoteRequest{
 			Datacenter:   dc,
 			Query:        *query,
-			QueryOptions: options,
+			Limit:        args.Limit,
+			QueryOptions: args.QueryOptions,
 		}
 		if err := q.ForwardDC("PreparedQuery.ExecuteRemote", dc, remote, reply); err != nil {
 			return err

consul/structs/prepared_query.go

@@ -4,11 +4,6 @@ import (
 	"time"
 )
 
-const (
-	QueryOrderShuffle = "shuffle"
-	QueryOrderSort    = "near_agent"
-)
-
 const (
 	QueryTTLMax = 24 * time.Hour
 	QueryTTLMin = 10 * time.Second
@@ -52,10 +47,6 @@ type ServiceQuery struct {
 	// this list it must be present. If the tag is preceded with "~" then
 	// it is disallowed.
 	Tags []string
-
-	// Sort has one of the QueryOrder* options which control how the output
-	// is sorted. If this is left blank we default to "shuffle".
-	Sort string
 }
 
 // PreparedQuery defines a complete prepared query, and is the structure we
@@ -115,9 +106,22 @@ func (q *PreparedQueryRequest) RequestDatacenter() string {
 
 // PreparedQueryExecuteRequest is used to execute a prepared query.
 type PreparedQueryExecuteRequest struct {
-	Datacenter    string
+	// Datacenter is the target this request is intended for.
+	Datacenter string
+
+	// QueryIDOrName is the ID of a query _or_ the name of one, either can
+	// be provided.
 	QueryIDOrName string
-	Source        QuerySource
+
+	// Limit will trim the resulting list down to the given limit.
+	Limit int
+
+	// Source is used to sort the results relative to a given node using
+	// network coordinates.
+	Source QuerySource
+
+	// QueryOptions (unfortunately named here) controls the consistency
+	// settings for the query lookup itself, as well as the service lookups.
 	QueryOptions
 }
 
@@ -127,11 +131,20 @@ func (q *PreparedQueryExecuteRequest) RequestDatacenter() string {
 }
 
 // PreparedQueryExecuteRemoteRequest is used when running a local query in a
-// remote datacenter. We have to ship the entire query over since it won't be
-// present in the remote state store.
+// remote datacenter.
 type PreparedQueryExecuteRemoteRequest struct {
+	// Datacenter is the target this request is intended for.
 	Datacenter string
-	Query      PreparedQuery
+
+	// Query is a copy of the query to execute.  We have to ship the entire
+	// query over since it won't be present in the remote state store.
+	Query PreparedQuery
+
+	// Limit will trim the resulting list down to the given limit.
+	Limit int
+
+	// QueryOptions (unfortunately named here) controls the consistency
+	// settings for the the service lookups.
 	QueryOptions
 }