feat: updating context using headers (#1641)

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Able to update context map using headers present in 
- OFREP requests
- Connect Requests (via Flag Evaluator V2 service)

### Related Issues
Fixes #1583 

### Notes
Context values passed via headers is high priority

If same context key is updated via
- Headers
- Request Body
- Static Config

_Context via Headers will be considered_

### Usage 
```
flagd start --port 8013 --uri file:./samples/example_flags.flagd.json -H Header=contextKey
```
or
```
flagd start --port 8013 --uri file:./samples/example_flags.flagd.json --context-from-header Header=contextKey
```

---------

Signed-off-by: Rahul Baradol <rahul.baradol.14@gmail.com>

Author: Rahul-Baradol

core/pkg/service/iservice.go

@@ -23,16 +23,17 @@ type Notification struct {
 type ReadinessProbe func() bool
 
 type Configuration struct {
-	ReadinessProbe ReadinessProbe
-	Port           uint16
-	ManagementPort uint16
-	ServiceName    string
-	CertPath       string
-	KeyPath        string
-	SocketPath     string
-	CORS           []string
-	Options        []connect.HandlerOption
-	ContextValues  map[string]any
+	ReadinessProbe             ReadinessProbe
+	Port                       uint16
+	ManagementPort             uint16
+	ServiceName                string
+	CertPath                   string
+	KeyPath                    string
+	SocketPath                 string
+	CORS                       []string
+	Options                    []connect.HandlerOption
+	ContextValues              map[string]any
+	HeaderToContextKeyMappings map[string]string
 }
 
 /*

docs/reference/flagd-cli/flagd_start.md

@@ -11,26 +11,27 @@ flagd start [flags]
 ### Options
 
 ```
-  -X, --context-value stringToString    add arbitrary key value pairs to the flag evaluation context (default [])
-  -C, --cors-origin strings             CORS allowed origins, * will allow all origins
-  -h, --help                            help for start
-  -z, --log-format string               Set the logging format, e.g. console or json (default "console")
-  -m, --management-port int32           Port for management operations (default 8014)
-  -t, --metrics-exporter string         Set the metrics exporter. Default(if unset) is Prometheus. Can be override to otel - OpenTelemetry metric exporter. Overriding to otel require otelCollectorURI to be present
-  -r, --ofrep-port int32                ofrep service port (default 8016)
-  -A, --otel-ca-path string             tls certificate authority path to use with OpenTelemetry collector
-  -D, --otel-cert-path string           tls certificate path to use with OpenTelemetry collector
-  -o, --otel-collector-uri string       Set the grpc URI of the OpenTelemetry collector for flagd runtime. If unset, the collector setup will be ignored and traces will not be exported.
-  -K, --otel-key-path string            tls key path to use with OpenTelemetry collector
-  -I, --otel-reload-interval duration   how long between reloading the otel tls certificate from disk (default 1h0m0s)
-  -p, --port int32                      Port to listen on (default 8013)
-  -c, --server-cert-path string         Server side tls certificate path
-  -k, --server-key-path string          Server side tls key path
-  -d, --socket-path string              Flagd unix socket path. With grpc the evaluations service will become available on this address. With http(s) the grpc-gateway proxy will use this address internally.
-  -s, --sources string                  JSON representation of an array of SourceConfig objects. This object contains 2 required fields, uri (string) and provider (string). Documentation for this object: https://flagd.dev/reference/sync-configuration/#source-configuration
-  -g, --sync-port int32                 gRPC Sync port (default 8015)
-  -e, --sync-socket-path string         Flagd sync service socket path. With grpc the sync service will be available on this address.
-  -f, --uri .yaml/.yml/.json            Set a sync provider uri to read data from, this can be a filepath, URL (HTTP and gRPC), FeatureFlag custom resource, or GCS or Azure Blob. When flag keys are duplicated across multiple providers the merge priority follows the index of the flag arguments, as such flags from the uri at index 0 take the lowest precedence, with duplicated keys being overwritten by those from the uri at index 1. Please note that if you are using filepath, flagd only supports files with .yaml/.yml/.json extension.
+  -H, --context-from-header stringToString   add key-value pairs to map header values to context values, where key is Header name, value is context key (default [])
+  -X, --context-value stringToString         add arbitrary key value pairs to the flag evaluation context (default [])
+  -C, --cors-origin strings                  CORS allowed origins, * will allow all origins
+  -h, --help                                 help for start
+  -z, --log-format string                    Set the logging format, e.g. console or json (default "console")
+  -m, --management-port int32                Port for management operations (default 8014)
+  -t, --metrics-exporter string              Set the metrics exporter. Default(if unset) is Prometheus. Can be override to otel - OpenTelemetry metric exporter. Overriding to otel require otelCollectorURI to be present
+  -r, --ofrep-port int32                     ofrep service port (default 8016)
+  -A, --otel-ca-path string                  tls certificate authority path to use with OpenTelemetry collector
+  -D, --otel-cert-path string                tls certificate path to use with OpenTelemetry collector
+  -o, --otel-collector-uri string            Set the grpc URI of the OpenTelemetry collector for flagd runtime. If unset, the collector setup will be ignored and traces will not be exported.
+  -K, --otel-key-path string                 tls key path to use with OpenTelemetry collector
+  -I, --otel-reload-interval duration        how long between reloading the otel tls certificate from disk (default 1h0m0s)
+  -p, --port int32                           Port to listen on (default 8013)
+  -c, --server-cert-path string              Server side tls certificate path
+  -k, --server-key-path string               Server side tls key path
+  -d, --socket-path string                   Flagd unix socket path. With grpc the evaluations service will become available on this address. With http(s) the grpc-gateway proxy will use this address internally.
+  -s, --sources string                       JSON representation of an array of SourceConfig objects. This object contains 2 required fields, uri (string) and provider (string). Documentation for this object: https://flagd.dev/reference/sync-configuration/#source-configuration
+  -g, --sync-port int32                      gRPC Sync port (default 8015)
+  -e, --sync-socket-path string              Flagd sync service socket path. With grpc the sync service will be available on this address.
+  -f, --uri .yaml/.yml/.json                 Set a sync provider uri to read data from, this can be a filepath, URL (HTTP and gRPC), FeatureFlag custom resource, or GCS or Azure Blob. When flag keys are duplicated across multiple providers the merge priority follows the index of the flag arguments, as such flags from the uri at index 0 take the lowest precedence, with duplicated keys being overwritten by those from the uri at index 1. Please note that if you are using filepath, flagd only supports files with .yaml/.yml/.json extension.
 ```
 
 ### Options inherited from parent commands

flagd/cmd/start.go

@@ -37,6 +37,7 @@ const (
 	syncSocketPathFlagName     = "sync-socket-path"
 	uriFlagName                = "uri"
 	contextValueFlagName       = "context-value"
+	headerToContextKeyFlagName = "context-from-header"
 )
 
 func init() {
@@ -84,6 +85,8 @@ func init() {
 		"from disk")
 	flags.StringToStringP(contextValueFlagName, "X", map[string]string{}, "add arbitrary key value pairs "+
 		"to the flag evaluation context")
+	flags.StringToStringP(headerToContextKeyFlagName, "H", map[string]string{}, "add key-value pairs to map " +
+		"header values to context values, where key is Header name, value is context key")
 
 	bindFlags(flags)
 }
@@ -107,6 +110,7 @@ func bindFlags(flags *pflag.FlagSet) {
 	_ = viper.BindPFlag(syncSocketPathFlagName, flags.Lookup(syncSocketPathFlagName))
 	_ = viper.BindPFlag(ofrepPortFlagName, flags.Lookup(ofrepPortFlagName))
 	_ = viper.BindPFlag(contextValueFlagName, flags.Lookup(contextValueFlagName))
+	_ = viper.BindPFlag(headerToContextKeyFlagName, flags.Lookup(headerToContextKeyFlagName))
 }
 
 // startCmd represents the start command
@@ -156,25 +160,31 @@ var startCmd = &cobra.Command{
 			contextValuesToMap[k] = v
 		}
 
+		headerToContextKeyMappings := make(map[string]string)
+		for k, v := range viper.GetStringMapString(headerToContextKeyFlagName) {
+			headerToContextKeyMappings[k] = v
+		}
+
 		// Build Runtime -----------------------------------------------------------
 		rt, err := runtime.FromConfig(logger, Version, runtime.Config{
-			CORS:                  viper.GetStringSlice(corsFlagName),
-			MetricExporter:        viper.GetString(metricsExporter),
-			ManagementPort:        viper.GetUint16(managementPortFlagName),
-			OfrepServicePort:      viper.GetUint16(ofrepPortFlagName),
-			OtelCollectorURI:      viper.GetString(otelCollectorURI),
-			OtelCertPath:          viper.GetString(otelCertPathFlagName),
-			OtelKeyPath:           viper.GetString(otelKeyPathFlagName),
-			OtelReloadInterval:    viper.GetDuration(otelReloadIntervalFlagName),
-			OtelCAPath:            viper.GetString(otelCAPathFlagName),
-			ServiceCertPath:       viper.GetString(serverCertPathFlagName),
-			ServiceKeyPath:        viper.GetString(serverKeyPathFlagName),
-			ServicePort:           viper.GetUint16(portFlagName),
-			ServiceSocketPath:     viper.GetString(socketPathFlagName),
-			SyncServicePort:       viper.GetUint16(syncPortFlagName),
-			SyncServiceSocketPath: viper.GetString(syncSocketPathFlagName),
-			SyncProviders:         syncProviders,
-			ContextValues:         contextValuesToMap,
+			CORS:                       viper.GetStringSlice(corsFlagName),
+			MetricExporter:             viper.GetString(metricsExporter),
+			ManagementPort:             viper.GetUint16(managementPortFlagName),
+			OfrepServicePort:           viper.GetUint16(ofrepPortFlagName),
+			OtelCollectorURI:           viper.GetString(otelCollectorURI),
+			OtelCertPath:               viper.GetString(otelCertPathFlagName),
+			OtelKeyPath:                viper.GetString(otelKeyPathFlagName),
+			OtelReloadInterval:         viper.GetDuration(otelReloadIntervalFlagName),
+			OtelCAPath:                 viper.GetString(otelCAPathFlagName),
+			ServiceCertPath:            viper.GetString(serverCertPathFlagName),
+			ServiceKeyPath:             viper.GetString(serverKeyPathFlagName),
+			ServicePort:                viper.GetUint16(portFlagName),
+			ServiceSocketPath:          viper.GetString(socketPathFlagName),
+			SyncServicePort:            viper.GetUint16(syncPortFlagName),
+			SyncServiceSocketPath:      viper.GetString(syncSocketPathFlagName),
+			SyncProviders:              syncProviders,
+			ContextValues:              contextValuesToMap,
+			HeaderToContextKeyMappings: headerToContextKeyMappings,
 		})
 		if err != nil {
 			rtLogger.Fatal(err.Error())

flagd/pkg/runtime/from_config.go

@@ -42,7 +42,8 @@ type Config struct {
 	SyncProviders []sync.SourceConfig
 	CORS          []string
 
-	ContextValues map[string]any
+	ContextValues              map[string]any
+	HeaderToContextKeyMappings map[string]string
 }
 
 // FromConfig builds a runtime from startup configurations
@@ -106,6 +107,7 @@ func FromConfig(logger *logger.Logger, version string, config Config) (*Runtime,
 		Port:   config.OfrepServicePort,
 	},
 		config.ContextValues,
+		config.HeaderToContextKeyMappings,
 	)
 	if err != nil {
 		return nil, fmt.Errorf("error creating ofrep service")
@@ -146,15 +148,16 @@ func FromConfig(logger *logger.Logger, version string, config Config) (*Runtime,
 		OfrepService: ofrepService,
 		Service:      connectService,
 		ServiceConfig: service.Configuration{
-			Port:           config.ServicePort,
-			ManagementPort: config.ManagementPort,
-			ServiceName:    svcName,
-			KeyPath:        config.ServiceKeyPath,
-			CertPath:       config.ServiceCertPath,
-			SocketPath:     config.ServiceSocketPath,
-			CORS:           config.CORS,
-			Options:        options,
-			ContextValues:  config.ContextValues,
+			Port:                       config.ServicePort,
+			ManagementPort:             config.ManagementPort,
+			ServiceName:                svcName,
+			KeyPath:                    config.ServiceKeyPath,
+			CertPath:                   config.ServiceCertPath,
+			SocketPath:                 config.ServiceSocketPath,
+			CORS:                       config.CORS,
+			Options:                    options,
+			ContextValues:              config.ContextValues,
+			HeaderToContextKeyMappings: config.HeaderToContextKeyMappings,
 		},
 		SyncImpl: iSyncs,
 	}, nil

flagd/pkg/service/flag-evaluation/connect_service.go

@@ -172,6 +172,7 @@ func (s *ConnectService) setupServer(svcConf service.Configuration) (net.Listene
 		s.eventingConfiguration,
 		s.metrics,
 		svcConf.ContextValues,
+		svcConf.HeaderToContextKeyMappings,
 	)
 
 	_, newHandler := evaluationV1.NewServiceHandler(newFes, append(svcConf.Options, marshalOpts)...)

flagd/pkg/service/flag-evaluation/flag_evaluator.go

@@ -3,6 +3,7 @@ package service
 import (
 	"context"
 	"fmt"
+	"net/http"
 	"time"
 
 	schemaV1 "buf.build/gen/go/open-feature/flagd/protocolbuffers/go/schema/v1"
@@ -72,7 +73,7 @@ func (s *OldFlagEvaluationService) ResolveAll(
 		Flags: make(map[string]*schemaV1.AnyFlag),
 	}
 
-	values, _, err := s.eval.ResolveAllValues(sCtx, reqID, mergeContexts(req.Msg.GetContext().AsMap(), s.contextValues))
+	values, _, err := s.eval.ResolveAllValues(sCtx, reqID, mergeContexts(req.Msg.GetContext().AsMap(), s.contextValues, req.Header(), make(map[string]string)))
 	if err != nil {
 		s.logger.WarnWithID(reqID, fmt.Sprintf("error resolving all flags: %v", err))
 		return nil, fmt.Errorf("error resolving flags. Tracking ID: %s", reqID)
@@ -179,11 +180,13 @@ func (s *OldFlagEvaluationService) ResolveBoolean(
 		sCtx,
 		s.logger,
 		s.eval.ResolveBooleanValue,
+		req.Header(),
 		req.Msg.GetFlagKey(),
 		req.Msg.GetContext(),
 		&booleanResponse{schemaV1Resp: res},
 		s.metrics,
 		s.contextValues,
+		make(map[string]string),
 	)
 	if err != nil {
 		span.RecordError(err)
@@ -206,11 +209,13 @@ func (s *OldFlagEvaluationService) ResolveString(
 		sCtx,
 		s.logger,
 		s.eval.ResolveStringValue,
+		req.Header(),
 		req.Msg.GetFlagKey(),
 		req.Msg.GetContext(),
 		&stringResponse{schemaV1Resp: res},
 		s.metrics,
 		s.contextValues,
+		make(map[string]string),
 	)
 	if err != nil {
 		span.RecordError(err)
@@ -233,11 +238,13 @@ func (s *OldFlagEvaluationService) ResolveInt(
 		sCtx,
 		s.logger,
 		s.eval.ResolveIntValue,
+		req.Header(),
 		req.Msg.GetFlagKey(),
 		req.Msg.GetContext(),
 		&intResponse{schemaV1Resp: res},
 		s.metrics,
 		s.contextValues,
+		make(map[string]string),
 	)
 	if err != nil {
 		span.RecordError(err)
@@ -260,11 +267,13 @@ func (s *OldFlagEvaluationService) ResolveFloat(
 		sCtx,
 		s.logger,
 		s.eval.ResolveFloatValue,
+		req.Header(),
 		req.Msg.GetFlagKey(),
 		req.Msg.GetContext(),
 		&floatResponse{schemaV1Resp: res},
 		s.metrics,
 		s.contextValues,
+		make(map[string]string),
 	)
 	if err != nil {
 		span.RecordError(err)
@@ -287,11 +296,13 @@ func (s *OldFlagEvaluationService) ResolveObject(
 		sCtx,
 		s.logger,
 		s.eval.ResolveObjectValue,
+		req.Header(),
 		req.Msg.GetFlagKey(),
 		req.Msg.GetContext(),
 		&objectResponse{schemaV1Resp: res},
 		s.metrics,
 		s.contextValues,
+		make(map[string]string),
 	)
 	if err != nil {
 		span.RecordError(err)
@@ -301,28 +312,34 @@ func (s *OldFlagEvaluationService) ResolveObject(
 	return res, err
 }
 
-// mergeContexts combines values from the request context with the values from the config --context-values flag.
-// Request context values have a higher priority.
-func mergeContexts(reqCtx, configFlagsCtx map[string]any) map[string]any {
+// mergeContexts combines context values from headers, static context (from cli) and request context. 
+// highest priority > header-context-from-cli > static-context-from-cli > request-context > lowest priority
+func mergeContexts(reqCtx, configFlagsCtx map[string]any, headers http.Header, headerToContextKeyMappings map[string]string) map[string]any {
 	merged := make(map[string]any)
 	for k, v := range reqCtx {
 		merged[k] = v
 	}
 	for k, v := range configFlagsCtx {
 		merged[k] = v
 	}
+	for header, contextKey := range headerToContextKeyMappings {
+		if values, ok := headers[header]; ok {
+			merged[contextKey] = values[0]
+		}
+	}
 	return merged
 }
 
 // resolve is a generic flag resolver
-func resolve[T constraints](ctx context.Context, logger *logger.Logger, resolver resolverSignature[T], flagKey string,
+func resolve[T constraints](ctx context.Context, logger *logger.Logger, resolver resolverSignature[T], header http.Header, flagKey string,
 	evaluationContext *structpb.Struct, resp response[T], metrics telemetry.IMetricsRecorder,
-	configContextValues map[string]any,
+	configContextValues map[string]any, configHeaderToContextKeyMappings map[string]string,
 ) error {
 	reqID := xid.New().String()
 	defer logger.ClearFields(reqID)
 
-	mergedContext := mergeContexts(evaluationContext.AsMap(), configContextValues)
+	mergedContext := mergeContexts(evaluationContext.AsMap(), configContextValues, header, configHeaderToContextKeyMappings)	
+
 	logger.WriteFields(
 		reqID,
 		zap.String("flag-key", flagKey),