feature: implement <forward>

For migration purposes it is useful that the operator can override the backend target.
Users of routing tools like stackset https://github.com/zalando-incubator/stackset-controller can set the backend to something that the operators decide.

Signed-off-by: Sandor Szücs <[email protected]>

Author: szuecs

config/config.go

@@ -128,6 +128,7 @@ type Config struct {
 	RoutesFile         string               `yaml:"routes-file"`
 	RoutesURLs         *listFlag            `yaml:"routes-urls"`
 	InlineRoutes       string               `yaml:"inline-routes"`
+	ForwardURL         string               `yaml:"forward-url"`
 	AppendFilters      *defaultFiltersFlags `yaml:"default-filters-append"`
 	PrependFilters     *defaultFiltersFlags `yaml:"default-filters-prepend"`
 	DisabledFilters    *listFlag            `yaml:"disabled-filters"`
@@ -445,6 +446,7 @@ func NewConfig() *Config {
 	flag.StringVar(&cfg.RoutesFile, "routes-file", "", "file containing route definitions")
 	flag.Var(cfg.RoutesURLs, "routes-urls", "comma separated URLs to route definitions in eskip format")
 	flag.StringVar(&cfg.InlineRoutes, "inline-routes", "", "inline routes in eskip format")
+	flag.StringVar(&cfg.ForwardURL, "forward-url", "", "target url of the <forward> backend")
 	flag.Int64Var(&cfg.SourcePollTimeout, "source-poll-timeout", int64(3000), "polling timeout of the routing data sources, in milliseconds")
 	flag.Var(cfg.AppendFilters, "default-filters-append", "set of default filters to apply to append to all filters of all routes")
 	flag.Var(cfg.PrependFilters, "default-filters-prepend", "set of default filters to apply to prepend to all filters of all routes")
@@ -861,6 +863,7 @@ func (c *Config) ToOptions() skipper.Options {
 		WatchRoutesFile: c.RoutesFile,
 		RoutesURLs:      c.RoutesURLs.values,
 		InlineRoutes:    c.InlineRoutes,
+		ForwardURL:      c.ForwardURL,
 		DefaultFilters: &eskip.DefaultFilters{
 			Prepend: c.PrependFilters.filters,
 			Append:  c.AppendFilters.filters,

eskip/eskip.go

@@ -5,6 +5,7 @@ package eskip
 import (
 	"errors"
 	"fmt"
+	"maps"
 	"math/rand"
 	"net/url"
 	"regexp"
@@ -209,8 +210,31 @@ const (
 	LoopBackend
 	DynamicBackend
 	LBBackend
+	ForwardBackend
 )
 
+type fwdBackend struct {
+	target string
+}
+
+func ForwardPreProcessor(s string) *fwdBackend {
+	return &fwdBackend{
+		target: s,
+	}
+}
+
+func (fb *fwdBackend) Do(routes []*Route) []*Route {
+	for i := range routes {
+		switch routes[i].BackendType {
+		case ForwardBackend:
+			routes[i].Backend = fb.target
+			routes[i].BackendType = NetworkBackend
+		}
+	}
+
+	return routes
+}
+
 var errMixedProtocols = errors.New("loadbalancer endpoints cannot have mixed protocols")
 
 // Route definition used during the parser processes the raw routing
@@ -223,6 +247,7 @@ type parsedRoute struct {
 	loopback    bool
 	dynamic     bool
 	lbBackend   bool
+	forward     bool
 	backend     string
 	lbAlgorithm string
 	lbEndpoints []string
@@ -372,9 +397,7 @@ func (r *Route) Copy() *Route {
 
 	if len(r.Headers) > 0 {
 		c.Headers = make(map[string]string)
-		for k, v := range r.Headers {
-			c.Headers[k] = v
-		}
+		maps.Copy(c.Headers, r.Headers)
 	}
 
 	if len(r.HeaderRegexps) > 0 {
@@ -420,6 +443,8 @@ func BackendTypeFromString(s string) (BackendType, error) {
 		return DynamicBackend, nil
 	case "lb":
 		return LBBackend, nil
+	case "forward":
+		return ForwardBackend, nil
 	default:
 		return -1, fmt.Errorf("unsupported backend type: %s", s)
 	}
@@ -438,6 +463,8 @@ func (t BackendType) String() string {
 		return "dynamic"
 	case LBBackend:
 		return "lb"
+	case ForwardBackend:
+		return "forward"
 	default:
 		return "unknown"
 	}
@@ -578,6 +605,8 @@ func newRouteDefinition(r *parsedRoute) (*Route, error) {
 		rd.BackendType = DynamicBackend
 	case r.lbBackend:
 		rd.BackendType = LBBackend
+	case r.forward:
+		rd.BackendType = ForwardBackend
 	default:
 		rd.BackendType = NetworkBackend
 	}
@@ -630,7 +659,7 @@ func parse(start int, code string) ([]*parsedRoute, []*Predicate, []*Filter, err
 	return lexer.routes, lexer.predicates, lexer.filters, lexer.err
 }
 
-// Parses a route expression or a routing document to a set of route definitions.
+// Parse a route expression or a routing document to a set of route definitions.
 func Parse(code string) ([]*Route, error) {
 	parsedRoutes, err := parseDocument(code)
 	if err != nil {
@@ -680,7 +709,7 @@ func MustParseFilters(s string) []*Filter {
 	return p
 }
 
-// Parses a filter chain into a list of parsed filter definitions.
+// ParseFilters parses a filter chain into a list of parsed filter definitions.
 func ParseFilters(f string) ([]*Filter, error) {
 	f = strings.TrimSpace(f)
 	if f == "" {
@@ -726,7 +755,7 @@ const (
 	alphabet = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
 )
 
-// generate weak random id for a route if
+// GenerateIfNeeded generates a weak random id for a route if
 // it doesn't have one.
 //
 // Deprecated: do not use, generate valid route id that matches [a-zA-Z_] yourself.

eskip/eskip_test.go

@@ -166,6 +166,16 @@ func TestParse(t *testing.T) {
 			BackendType: DynamicBackend,
 		}},
 		"",
+	}, {
+		"forward",
+		`* -> setRequestHeader("X-Foo", "bar") -> <forward>`,
+		[]*Route{{
+			Filters: []*Filter{
+				{Name: "setRequestHeader", Args: []interface{}{"X-Foo", "bar"}},
+			},
+			BackendType: ForwardBackend,
+		}},
+		"",
 	}, {
 		"multiple routes",
 		`r1: Path("/foo") -> <shunt>; r2: Path("/bar") -> "https://www.example.org"`,

eskip/example_test.go

@@ -17,8 +17,15 @@ package eskip_test
 import (
 	"fmt"
 	"log"
+	"net/http"
+	"net/http/httptest"
 
 	"github.com/zalando/skipper/eskip"
+	"github.com/zalando/skipper/filters"
+	"github.com/zalando/skipper/filters/builtin"
+	"github.com/zalando/skipper/proxy/proxytest"
+	"github.com/zalando/skipper/routing"
+	"github.com/zalando/skipper/routing/testdataclient"
 )
 
 func Example() {
@@ -220,3 +227,43 @@ func ExampleParseFilters() {
 	// second filter, first arg: 3.14
 	// second filter, second arg: Hello, world!
 }
+
+func ExampleForwardBackend() {
+	backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer backend.Close()
+
+	doc := `r: * -> <forward>;`
+	routes := eskip.MustParse(doc)
+
+	spec := builtin.NewStatus()
+	fr := make(filters.Registry)
+	fr.Register(spec)
+
+	dc := testdataclient.New(routes)
+	defer dc.Close()
+
+	proxy := proxytest.WithRoutingOptions(fr, routing.Options{
+		DataClients:   []routing.DataClient{dc},
+		PreProcessors: []routing.PreProcessor{eskip.ForwardPreProcessor(backend.URL)},
+	})
+	defer proxy.Close()
+
+	client := proxy.Client()
+
+	rsp, err := client.Get("http://hello.world")
+	if err != nil {
+		fmt.Printf("Failed to GET hello.world: %v\n", err)
+		return
+	}
+	defer rsp.Body.Close()
+	if rsp.StatusCode != http.StatusOK {
+		fmt.Printf("Failed to GET OK from http://hello.world, got: %v\n", rsp.StatusCode)
+		return
+	}
+	fmt.Println("hello skipper")
+
+	// output:
+	// hello skipper
+}

eskip/lexer.go

@@ -59,6 +59,7 @@ var openarrowPrefixedTokens = []*fixedScanner{
 	{shunt, "<shunt>"},
 	{loopback, "<loopback>"},
 	{dynamic, "<dynamic>"},
+	{forward, "<forward>"},
 }
 
 func (fs *fixedScanner) scan(code string) (t token, rest string, err error) {