// Endpoint is an example API endpoint. In a real application, the backend of // your service would be contacting several external and internal API endpoints // which may be the bottleneck of your application. // // For example purposes we just sleep for 200ms before responding to simulate a // slow API endpoint as the bottleneck of your application. func Endpoint(w http.ResponseWriter, r *http.Request) { // Extract the trace from the headers and join it with a new child span. carrier := opentracing.HTTPHeaderTextMapCarrier(r.Header) span, err := opentracing.GlobalTracer().Join(r.URL.Path, opentracing.TextMap, carrier) if err != nil { return } defer span.Finish() span.SetTag("Request.Host", r.Host) span.SetTag("Request.Method", r.Method) addHeaderTags(span, r.Header) time.Sleep(200 * time.Millisecond) fmt.Fprintf(w, "Slept for 200ms!") }
func main() { var ( httpAddr = flag.String("http.addr", ":8000", "Address for HTTP (JSON) server") consulAddr = flag.String("consul.addr", "", "Consul agent address") retryMax = flag.Int("retry.max", 3, "per-request retries to different instances") retryTimeout = flag.Duration("retry.timeout", 500*time.Millisecond, "per-request timeout, including retries") ) flag.Parse() // Logging domain. var logger log.Logger { logger = log.NewLogfmtLogger(os.Stderr) logger = log.NewContext(logger).With("ts", log.DefaultTimestampUTC) logger = log.NewContext(logger).With("caller", log.DefaultCaller) } // Service discovery domain. In this example we use Consul. var client consulsd.Client { consulConfig := api.DefaultConfig() if len(*consulAddr) > 0 { consulConfig.Address = *consulAddr } consulClient, err := api.NewClient(consulConfig) if err != nil { logger.Log("err", err) os.Exit(1) } client = consulsd.NewClient(consulClient) } // Transport domain. tracer := stdopentracing.GlobalTracer() // no-op ctx := context.Background() r := mux.NewRouter() // Now we begin installing the routes. Each route corresponds to a single // method: sum, concat, uppercase, and count. // addsvc routes. { // Each method gets constructed with a factory. Factories take an // instance string, and return a specific endpoint. In the factory we // dial the instance string we get from Consul, and then leverage an // addsvc client package to construct a complete service. We can then // leverage the addsvc.Make{Sum,Concat}Endpoint constructors to convert // the complete service to specific endpoint. var ( tags = []string{} passingOnly = true endpoints = addsvc.Endpoints{} ) { factory := addsvcFactory(addsvc.MakeSumEndpoint, tracer, logger) subscriber := consulsd.NewSubscriber(client, factory, logger, "addsvc", tags, passingOnly) balancer := lb.NewRoundRobin(subscriber) retry := lb.Retry(*retryMax, *retryTimeout, balancer) endpoints.SumEndpoint = retry } { factory := addsvcFactory(addsvc.MakeConcatEndpoint, tracer, logger) subscriber := consulsd.NewSubscriber(client, factory, logger, "addsvc", tags, passingOnly) balancer := lb.NewRoundRobin(subscriber) retry := lb.Retry(*retryMax, *retryTimeout, balancer) endpoints.ConcatEndpoint = retry } // Here we leverage the fact that addsvc comes with a constructor for an // HTTP handler, and just install it under a particular path prefix in // our router. r.PathPrefix("addsvc/").Handler(addsvc.MakeHTTPHandler(ctx, endpoints, tracer, logger)) } // stringsvc routes. { // addsvc had lots of nice importable Go packages we could leverage. // With stringsvc we are not so fortunate, it just has some endpoints // that we assume will exist. So we have to write that logic here. This // is by design, so you can see two totally different methods of // proxying to a remote service. var ( tags = []string{} passingOnly = true uppercase endpoint.Endpoint count endpoint.Endpoint ) { factory := stringsvcFactory(ctx, "GET", "/uppercase") subscriber := consulsd.NewSubscriber(client, factory, logger, "stringsvc", tags, passingOnly) balancer := lb.NewRoundRobin(subscriber) retry := lb.Retry(*retryMax, *retryTimeout, balancer) uppercase = retry } { factory := stringsvcFactory(ctx, "GET", "/count") subscriber := consulsd.NewSubscriber(client, factory, logger, "stringsvc", tags, passingOnly) balancer := lb.NewRoundRobin(subscriber) retry := lb.Retry(*retryMax, *retryTimeout, balancer) count = retry } // We can use the transport/http.Server to act as our handler, all we // have to do provide it with the encode and decode functions for our // stringsvc methods. r.Handle("/stringsvc/uppercase", httptransport.NewServer(ctx, uppercase, decodeUppercaseRequest, encodeJSONResponse)) r.Handle("/stringsvc/count", httptransport.NewServer(ctx, count, decodeCountRequest, encodeJSONResponse)) } // Interrupt handler. errc := make(chan error) go func() { c := make(chan os.Signal) signal.Notify(c, syscall.SIGINT, syscall.SIGTERM) errc <- fmt.Errorf("%s", <-c) }() // HTTP transport. go func() { logger.Log("transport", "HTTP", "addr", *httpAddr) errc <- http.ListenAndServe(*httpAddr, r) }() // Run! logger.Log("exit", <-errc) }
func main() { var ( debugAddr = flag.String("debug.addr", ":8080", "Debug and metrics listen address") grpcAddr = flag.String("grpc.addr", ":8082", "gRPC (HTTP) listen address") appdashAddr = flag.String("appdash.addr", "", "Enable Appdash tracing via an Appdash server host:port") ircNick = flag.String("irc.nick", "gogrpctest", "IRC nickname") ircSecret = flag.String("irc.secret", "", "IRC password") ircHost = flag.String("irc.addr", "irc.freenode.net:6667", "IRC host system") ) flag.Parse() // Logging domain. var logger log.Logger { logger = log.NewLogfmtLogger(os.Stdout) logger = log.NewContext(logger).With("ts", log.DefaultTimestampUTC) logger = log.NewContext(logger).With("caller", log.DefaultCaller) } logger.Log("msg", "hello") defer logger.Log("msg", "goodbye") // Metrics domain. var cx, cxerr, joins metrics.Counter { // Business level metrics. cx = prometheus.NewCounterFrom(stdprometheus.CounterOpts{ Namespace: "relay", Name: "connect_succeeded", Help: "Total count of connects via the Connect method.", }, []string{}) cxerr = prometheus.NewCounterFrom(stdprometheus.CounterOpts{ Namespace: "relay", Name: "connect_failed", Help: "Total count of connect errors via the Connect method.", }, []string{}) joins = prometheus.NewCounterFrom(stdprometheus.CounterOpts{ Namespace: "relay", Name: "channels_joined", Help: "Total count of channels joined via the Join method.", }, []string{}) } var duration metrics.Histogram { // Transport level metrics. duration = prometheus.NewSummaryFrom(stdprometheus.SummaryOpts{ Namespace: "relay", Name: "request_duration_ns", Help: "Request duration in nanoseconds.", }, []string{"method", "success"}) } // Tracing domain. var tracer stdopentracing.Tracer { if *appdashAddr != "" { logger := log.NewContext(logger).With("tracer", "Appdash") logger.Log("addr", *appdashAddr) tracer = appdashot.NewTracer(appdash.NewRemoteCollector(*appdashAddr)) } else { logger := log.NewContext(logger).With("tracer", "none") logger.Log() tracer = stdopentracing.GlobalTracer() // no-op } } // Business domain. var service relay.Service { service = relay.NewBasicService(*ircNick, *ircHost, *ircSecret) service = relay.ServiceLoggingMiddleware(logger)(service) service = relay.ServiceInstrumentingMiddleware(cx, cxerr, joins)(service) } // Endpoint domain. var connectEndpoint endpoint.Endpoint { connectDuration := duration.With("method", "Connect") connectLogger := log.NewContext(logger).With("method", "Connect") connectEndpoint = relay.MakeConnectEndpoint(service) connectEndpoint = opentracing.TraceServer(tracer, "Connect")(connectEndpoint) connectEndpoint = relay.EndpointInstrumentingMiddleware(connectDuration)(connectEndpoint) connectEndpoint = relay.EndpointLoggingMiddleware(connectLogger)(connectEndpoint) } var joinEndpoint endpoint.Endpoint { joinDuration := duration.With("method", "Join") joinLogger := log.NewContext(logger).With("method", "Join") joinEndpoint = relay.MakeJoinEndpoint(service) joinEndpoint = opentracing.TraceServer(tracer, "Join")(joinEndpoint) joinEndpoint = relay.EndpointInstrumentingMiddleware(joinDuration)(joinEndpoint) joinEndpoint = relay.EndpointLoggingMiddleware(joinLogger)(joinEndpoint) } endpoints := relay.Endpoints{ ConnectEndpoint: connectEndpoint, JoinEndpoint: joinEndpoint, } // Mechanical domain. errc := make(chan error) ctx := context.Background() // Interrupt handler. go func() { c := make(chan os.Signal, 1) signal.Notify(c, syscall.SIGINT, syscall.SIGTERM) errc <- fmt.Errorf("%s", <-c) }() // Debug listener. go func() { logger := log.NewContext(logger).With("transport", "debug") m := http.NewServeMux() m.Handle("/debug/pprof/", http.HandlerFunc(pprof.Index)) m.Handle("/debug/pprof/cmdline", http.HandlerFunc(pprof.Cmdline)) m.Handle("/debug/pprof/profile", http.HandlerFunc(pprof.Profile)) m.Handle("/debug/pprof/symbol", http.HandlerFunc(pprof.Symbol)) m.Handle("/debug/pprof/trace", http.HandlerFunc(pprof.Trace)) m.Handle("/metrics", stdprometheus.Handler()) logger.Log("addr", *debugAddr) errc <- http.ListenAndServe(*debugAddr, m) }() // gRPC transport. go func() { logger := log.NewContext(logger).With("transport", "gRPC") ln, err := net.Listen("tcp", *grpcAddr) if err != nil { errc <- err return } srv := relay.MakeGRPCServer(ctx, endpoints, tracer, logger) s := grpc.NewServer() pb.RegisterRelayServer(s, srv) logger.Log("addr", *grpcAddr) errc <- s.Serve(ln) }() // Run! logger.Log("exit", <-errc) }
func main() { // The relaycli presumes no service discovery system, and expects users to // provide the direct address of a relaysvc. This presumption is reflected in // the relaycli binary and the the client packages: the -transport.addr flags // and various client constructors both expect host:port strings. var ( grpcAddr = flag.String("grpc.addr", "", "gRPC (HTTP) address of relaysvc") appdashAddr = flag.String("appdash.addr", "", "Enable Appdash tracing via an Appdash server host:port") method = flag.String("method", "connect", "connect, join") ) flag.Parse() if len(flag.Args()) != 1 { fmt.Fprintf(os.Stderr, "usage: relaycli [flags] <s>\n") os.Exit(1) } var tracer stdopentracing.Tracer { if *appdashAddr != "" { tracer = appdashot.NewTracer(appdash.NewRemoteCollector(*appdashAddr)) } else { tracer = stdopentracing.GlobalTracer() // no-op } } var ( service relay.Service err error ) if *grpcAddr != "" { conn, err := grpc.Dial(*grpcAddr, grpc.WithInsecure(), grpc.WithTimeout(time.Second)) if err != nil { fmt.Fprintf(os.Stderr, "error: %v", err) os.Exit(1) } defer conn.Close() service = grpcclient.New(conn, tracer, log.NewNopLogger()) } else { fmt.Fprintf(os.Stderr, "error: no remote address specified\n") os.Exit(1) } if err != nil { fmt.Fprintf(os.Stderr, "error: %v\n", err) os.Exit(1) } switch *method { case "connect": s := flag.Args()[0] v, err := service.Connect(context.Background(), s) if err != nil { fmt.Fprintf(os.Stderr, "error: %v\n", err) os.Exit(1) } fmt.Fprintf(os.Stdout, "Connect to %s: %s\n", s, v) case "join": c := flag.Args()[0] v, err := service.Join(context.Background(), c) if err != nil { fmt.Fprintf(os.Stderr, "error: %v\n", err) os.Exit(1) } fmt.Fprintf(os.Stdout, "Join channel %s: %s\n", c, v) default: fmt.Fprintf(os.Stderr, "error: invalid method %q\n", method) os.Exit(1) } }
func main() { var ( debugAddr = flag.String("debug.addr", ":8080", "Debug and metrics listen address") httpAddr = flag.String("http.addr", ":8081", "HTTP listen address") grpcAddr = flag.String("grpc.addr", ":8082", "gRPC (HTTP) listen address") thriftAddr = flag.String("thrift.addr", ":8083", "Thrift listen address") thriftProtocol = flag.String("thrift.protocol", "binary", "binary, compact, json, simplejson") thriftBufferSize = flag.Int("thrift.buffer.size", 0, "0 for unbuffered") thriftFramed = flag.Bool("thrift.framed", false, "true to enable framing") zipkinAddr = flag.String("zipkin.addr", "", "Enable Zipkin tracing via a Kafka server host:port") appdashAddr = flag.String("appdash.addr", "", "Enable Appdash tracing via an Appdash server host:port") lightstepToken = flag.String("lightstep.token", "", "Enable LightStep tracing via a LightStep access token") ) flag.Parse() // Logging domain. var logger log.Logger { logger = log.NewLogfmtLogger(os.Stdout) logger = log.NewContext(logger).With("ts", log.DefaultTimestampUTC) logger = log.NewContext(logger).With("caller", log.DefaultCaller) } logger.Log("msg", "hello") defer logger.Log("msg", "goodbye") // Metrics domain. var ints, chars metrics.Counter { // Business level metrics. ints = prometheus.NewCounter(stdprometheus.CounterOpts{ Namespace: "addsvc", Name: "integers_summed", Help: "Total count of integers summed via the Sum method.", }, []string{}) chars = prometheus.NewCounter(stdprometheus.CounterOpts{ Namespace: "addsvc", Name: "characters_concatenated", Help: "Total count of characters concatenated via the Concat method.", }, []string{}) } var duration metrics.TimeHistogram { // Transport level metrics. duration = metrics.NewTimeHistogram(time.Nanosecond, prometheus.NewSummary(stdprometheus.SummaryOpts{ Namespace: "addsvc", Name: "request_duration_ns", Help: "Request duration in nanoseconds.", }, []string{"method", "success"})) } // Tracing domain. var tracer stdopentracing.Tracer { if *zipkinAddr != "" { logger := log.NewContext(logger).With("tracer", "Zipkin") logger.Log("addr", *zipkinAddr) collector, err := zipkin.NewKafkaCollector( strings.Split(*zipkinAddr, ","), zipkin.KafkaLogger(logger), ) if err != nil { logger.Log("err", err) os.Exit(1) } tracer, err = zipkin.NewTracer( zipkin.NewRecorder(collector, false, "localhost:80", "addsvc"), ) if err != nil { logger.Log("err", err) os.Exit(1) } } else if *appdashAddr != "" { logger := log.NewContext(logger).With("tracer", "Appdash") logger.Log("addr", *appdashAddr) tracer = appdashot.NewTracer(appdash.NewRemoteCollector(*appdashAddr)) } else if *lightstepToken != "" { logger := log.NewContext(logger).With("tracer", "LightStep") logger.Log() // probably don't want to print out the token :) tracer = lightstep.NewTracer(lightstep.Options{ AccessToken: *lightstepToken, }) defer lightstep.FlushLightStepTracer(tracer) } else { logger := log.NewContext(logger).With("tracer", "none") logger.Log() tracer = stdopentracing.GlobalTracer() // no-op } } // Business domain. var service addsvc.Service { service = addsvc.NewBasicService() service = addsvc.ServiceLoggingMiddleware(logger)(service) service = addsvc.ServiceInstrumentingMiddleware(ints, chars)(service) } // Endpoint domain. var sumEndpoint endpoint.Endpoint { sumDuration := duration.With(metrics.Field{Key: "method", Value: "Sum"}) sumLogger := log.NewContext(logger).With("method", "Sum") sumEndpoint = addsvc.MakeSumEndpoint(service) sumEndpoint = opentracing.TraceServer(tracer, "Sum")(sumEndpoint) sumEndpoint = addsvc.EndpointInstrumentingMiddleware(sumDuration)(sumEndpoint) sumEndpoint = addsvc.EndpointLoggingMiddleware(sumLogger)(sumEndpoint) } var concatEndpoint endpoint.Endpoint { concatDuration := duration.With(metrics.Field{Key: "method", Value: "Concat"}) concatLogger := log.NewContext(logger).With("method", "Concat") concatEndpoint = addsvc.MakeConcatEndpoint(service) concatEndpoint = opentracing.TraceServer(tracer, "Concat")(concatEndpoint) concatEndpoint = addsvc.EndpointInstrumentingMiddleware(concatDuration)(concatEndpoint) concatEndpoint = addsvc.EndpointLoggingMiddleware(concatLogger)(concatEndpoint) } endpoints := addsvc.Endpoints{ SumEndpoint: sumEndpoint, ConcatEndpoint: concatEndpoint, } // Mechanical domain. errc := make(chan error) ctx := context.Background() // Interrupt handler. go func() { c := make(chan os.Signal, 1) signal.Notify(c, syscall.SIGINT, syscall.SIGTERM) errc <- fmt.Errorf("%s", <-c) }() // Debug listener. go func() { logger := log.NewContext(logger).With("transport", "debug") m := http.NewServeMux() m.Handle("/debug/pprof/", http.HandlerFunc(pprof.Index)) m.Handle("/debug/pprof/cmdline", http.HandlerFunc(pprof.Cmdline)) m.Handle("/debug/pprof/profile", http.HandlerFunc(pprof.Profile)) m.Handle("/debug/pprof/symbol", http.HandlerFunc(pprof.Symbol)) m.Handle("/debug/pprof/trace", http.HandlerFunc(pprof.Trace)) m.Handle("/metrics", stdprometheus.Handler()) logger.Log("addr", *debugAddr) errc <- http.ListenAndServe(*debugAddr, m) }() // HTTP transport. go func() { logger := log.NewContext(logger).With("transport", "HTTP") h := addsvc.MakeHTTPHandler(ctx, endpoints, tracer, logger) logger.Log("addr", *httpAddr) errc <- http.ListenAndServe(*httpAddr, h) }() // gRPC transport. go func() { logger := log.NewContext(logger).With("transport", "gRPC") ln, err := net.Listen("tcp", *grpcAddr) if err != nil { errc <- err return } srv := addsvc.MakeGRPCServer(ctx, endpoints, tracer, logger) s := grpc.NewServer() pb.RegisterAddServer(s, srv) logger.Log("addr", *grpcAddr) errc <- s.Serve(ln) }() // Thrift transport. go func() { logger := log.NewContext(logger).With("transport", "Thrift") var protocolFactory thrift.TProtocolFactory switch *thriftProtocol { case "binary": protocolFactory = thrift.NewTBinaryProtocolFactoryDefault() case "compact": protocolFactory = thrift.NewTCompactProtocolFactory() case "json": protocolFactory = thrift.NewTJSONProtocolFactory() case "simplejson": protocolFactory = thrift.NewTSimpleJSONProtocolFactory() default: errc <- fmt.Errorf("invalid Thrift protocol %q", *thriftProtocol) return } var transportFactory thrift.TTransportFactory if *thriftBufferSize > 0 { transportFactory = thrift.NewTBufferedTransportFactory(*thriftBufferSize) } else { transportFactory = thrift.NewTTransportFactory() } if *thriftFramed { transportFactory = thrift.NewTFramedTransportFactory(transportFactory) } transport, err := thrift.NewTServerSocket(*thriftAddr) if err != nil { errc <- err return } logger.Log("addr", *thriftAddr) errc <- thrift.NewTSimpleServer4( thriftadd.NewAddServiceProcessor(addsvc.MakeThriftHandler(ctx, endpoints)), transport, transportFactory, protocolFactory, ).Serve() }() // Run! logger.Log("exit", <-errc) }
func main() { // The addcli presumes no service discovery system, and expects users to // provide the direct address of an addsvc. This presumption is reflected in // the addcli binary and the the client packages: the -transport.addr flags // and various client constructors both expect host:port strings. For an // example service with a client built on top of a service discovery system, // see profilesvc. var ( httpAddr = flag.String("http.addr", "", "HTTP address of addsvc") grpcAddr = flag.String("grpc.addr", "", "gRPC (HTTP) address of addsvc") thriftAddr = flag.String("thrift.addr", "", "Thrift address of addsvc") thriftProtocol = flag.String("thrift.protocol", "binary", "binary, compact, json, simplejson") thriftBufferSize = flag.Int("thrift.buffer.size", 0, "0 for unbuffered") thriftFramed = flag.Bool("thrift.framed", false, "true to enable framing") zipkinAddr = flag.String("zipkin.addr", "", "Enable Zipkin tracing via a Kafka Collector host:port") appdashAddr = flag.String("appdash.addr", "", "Enable Appdash tracing via an Appdash server host:port") lightstepToken = flag.String("lightstep.token", "", "Enable LightStep tracing via a LightStep access token") method = flag.String("method", "sum", "sum, concat") ) flag.Parse() if len(flag.Args()) != 2 { fmt.Fprintf(os.Stderr, "usage: addcli [flags] <a> <b>\n") os.Exit(1) } // This is a demonstration client, which supports multiple tracers. // Your clients will probably just use one tracer. var tracer stdopentracing.Tracer { if *zipkinAddr != "" { collector, err := zipkin.NewKafkaCollector( strings.Split(*zipkinAddr, ","), zipkin.KafkaLogger(log.NewNopLogger()), ) if err != nil { fmt.Fprintf(os.Stderr, "%v\n", err) os.Exit(1) } tracer, err = zipkin.NewTracer( zipkin.NewRecorder(collector, false, "localhost:8000", "addcli"), ) if err != nil { fmt.Fprintf(os.Stderr, "%v\n", err) os.Exit(1) } } else if *appdashAddr != "" { tracer = appdashot.NewTracer(appdash.NewRemoteCollector(*appdashAddr)) } else if *lightstepToken != "" { tracer = lightstep.NewTracer(lightstep.Options{ AccessToken: *lightstepToken, }) defer lightstep.FlushLightStepTracer(tracer) } else { tracer = stdopentracing.GlobalTracer() // no-op } } // This is a demonstration client, which supports multiple transports. // Your clients will probably just define and stick with 1 transport. var ( service addsvc.Service err error ) if *httpAddr != "" { service, err = httpclient.New(*httpAddr, tracer, log.NewNopLogger()) } else if *grpcAddr != "" { conn, err := grpc.Dial(*grpcAddr, grpc.WithInsecure(), grpc.WithTimeout(time.Second)) if err != nil { fmt.Fprintf(os.Stderr, "error: %v", err) os.Exit(1) } defer conn.Close() service = grpcclient.New(conn, tracer, log.NewNopLogger()) } else if *thriftAddr != "" { // It's necessary to do all of this construction in the func main, // because (among other reasons) we need to control the lifecycle of the // Thrift transport, i.e. close it eventually. var protocolFactory thrift.TProtocolFactory switch *thriftProtocol { case "compact": protocolFactory = thrift.NewTCompactProtocolFactory() case "simplejson": protocolFactory = thrift.NewTSimpleJSONProtocolFactory() case "json": protocolFactory = thrift.NewTJSONProtocolFactory() case "binary", "": protocolFactory = thrift.NewTBinaryProtocolFactoryDefault() default: fmt.Fprintf(os.Stderr, "error: invalid protocol %q\n", *thriftProtocol) os.Exit(1) } var transportFactory thrift.TTransportFactory if *thriftBufferSize > 0 { transportFactory = thrift.NewTBufferedTransportFactory(*thriftBufferSize) } else { transportFactory = thrift.NewTTransportFactory() } if *thriftFramed { transportFactory = thrift.NewTFramedTransportFactory(transportFactory) } transportSocket, err := thrift.NewTSocket(*thriftAddr) if err != nil { fmt.Fprintf(os.Stderr, "error: %v\n", err) os.Exit(1) } transport := transportFactory.GetTransport(transportSocket) if err := transport.Open(); err != nil { fmt.Fprintf(os.Stderr, "error: %v\n", err) os.Exit(1) } defer transport.Close() client := thriftadd.NewAddServiceClientFactory(transport, protocolFactory) service = thriftclient.New(client) } else { fmt.Fprintf(os.Stderr, "error: no remote address specified\n") os.Exit(1) } if err != nil { fmt.Fprintf(os.Stderr, "error: %v\n", err) os.Exit(1) } switch *method { case "sum": a, _ := strconv.ParseInt(flag.Args()[0], 10, 64) b, _ := strconv.ParseInt(flag.Args()[1], 10, 64) v, err := service.Sum(context.Background(), int(a), int(b)) if err != nil { fmt.Fprintf(os.Stderr, "error: %v\n", err) os.Exit(1) } fmt.Fprintf(os.Stdout, "%d + %d = %d\n", a, b, v) case "concat": a := flag.Args()[0] b := flag.Args()[1] v, err := service.Concat(context.Background(), a, b) if err != nil { fmt.Fprintf(os.Stderr, "error: %v\n", err) os.Exit(1) } fmt.Fprintf(os.Stdout, "%q + %q = %q\n", a, b, v) default: fmt.Fprintf(os.Stderr, "error: invalid method %q\n", method) os.Exit(1) } }