README

Go FlagZ

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Dynamic, thread-safe flag variables that can be modified at runtime through etcd or Kubernetes.

For a similar project for JVM languages (Java, scala) see java-flagz

This sounds crazy. Why?

File-based or command-line configuration can only be changed when a service restarts. Dynamic flags provide flexibility in normal operations and emergencies. Two examples:

  • A new feature launches that you want to A/B test. You want to gradually enable it for a certain fraction of user requests (1%, 5%, 20%, 50%, 100%) without the need to restart servers.
  • Your service is getting overloaded and you want to disable certain costly features. You can't afford restarting because you'd lose important capacity.

All of this can be done simultaneously across a whole shard of your services.

Features

  • compatible with popular flag replacement spf13/pflag (e.g. ones using spf13/cobra)
  • dynamic flag that are thread-safe and efficient:
    • DynInt64
    • DynFloat64
    • DynString
    • DynDuration
    • DynStringSlice
    • DynJSON - a flag that takes an arbitrary JSON struct
    • DynProto3 - a flag that takes a proto3 struct in JSONpb or binary form
  • validator functions for each flag, allows the user to provide checks for newly set values
  • notifier functions allow user code to be subscribed to flag changes
  • Kubernetes ConfigMap watcher, see configmap/README.md.
  • etcd based watcher that syncs values from a distributed Key-Value store into the program's memory
  • Prometheus metric for checksums of the current flag configuration
  • a /debug/flagz HandlerFunc endpoint that allows for easy inspection of the service's runtime configuration

Here's a teaser of the debug endpoint:

Status Endpoint

Examples

Declare a single pflag.FlagSet in some public package (e.g. common.SharedFlagSet) that you'll use throughout your server.

Dynamic JSON flag with a validator and notifier
var (
  limitsConfigFlag = flagz.DynJSON(
    common.SharedFlagSet, 
    "rate_limiting_config", 
    &rateLimitConfig{ DefaultRate: 10, Policy: "allow"},
    "Config for service's rate limit",
  ).WithValidator(rateLimitConfigValidator).WithNotifier(onRateLimitChange)
)

This declares a JSON flag of type rateLimitConfig with a default value. Whenever the config changes (statically or dynamically) the rateLimitConfigValidator will be called. If it returns no errors, the flag will be updated and onRateLimitChange will be called with both old and new, allowing the rate-limit mechanism to re-tune.

Dynamic feature flags

var (
  featuresFlag = flagz.DynStringSlice(common.SharedFlagSet, "enabled_features", []string{"fast_index"}, "list of enabled feature markers")
)
...
func MyHandler(resp http.ResponseWriter, req *http.Request) {
   ...
   if existsInStringSlice("fast_index", featuresFlag.Get()) {
     doFastIndex(req)
   }
   ...
}

All access to featuresFlag, which is a []string flag, is synchronised across go-routines using atomic pointer swaps.

Watching for changes from etcd

// First parse the flags from the command line, as normal.
common.SharedFlagSet.Parse(os.Args[1:])
w, err := watcher.New(common.SharedFlagSet, etcdClient, "/my_service/flagz", logger)
if err != nil {
  logger.Fatalf("failed setting up %v", err)
}
// Read flagz from etcd and update their values in common.SharedFlagSet
if err := w.Initialize(); err != nil {
	log.Fatalf("failed setting up %v", err)
}
// Start listening of dynamic flags from etcd.
w.Start()

The watcher's go-routine will watch for etcd value changes and synchronise them with values in memory. In case a value fails parsing or the user-specified validator, the key in etcd will be atomically rolled back.

More examples:

Status

This code is production quality. It's been running happily in production at Improbable for a few months.

Features planned:

  • - #11 monitoring of FlagSet checksus using a Prometheus handler
  • - #12 support for standard flag (requires changes in spf13/pflag interfaces)
License

go-flagz is released under the Apache 2.0 license. See the LICENSE file for details.

Documentation

Index

Constants

This section is empty.

Variables

This section is empty.

Functions

func ChecksumFlagSet

func ChecksumFlagSet(flagSet *pflag.FlagSet, flagFilter func(flag *pflag.Flag) bool) []byte

    ChecksumFlagSet will generate a FNV of the *set* values in a FlagSet.

    func IsFlagDynamic

    func IsFlagDynamic(f *flag.Flag) bool

      IsFlagDynamic returns whether the given Flag has been created in a Dynamic mode.

      func MarkFlagDynamic

      func MarkFlagDynamic(f *flag.Flag)

        MarkFlagDynamic marks the flag as Dynamic and changeable at runtime.

        func ReadFileFlags

        func ReadFileFlags(flagSet *flag.FlagSet) error

          ReadFileFlags parses the flagset to discover all "fileread" flags and evaluates them.

          By reading and evaluating it means: attempts to read the file and set the value.

          func ValidateDynFloat64Range

          func ValidateDynFloat64Range(fromInclusive float64, toInclusive float64) func(float64) error

            ValidateDynFloat64Range returns a validator that checks if the float value is in range.

            func ValidateDynInt64Range

            func ValidateDynInt64Range(fromInclusive int64, toInclusive int64) func(int64) error

              ValidateDynInt64Range returns a validator function that checks if the integer value is in range.

              func ValidateDynStringMatchesRegex

              func ValidateDynStringMatchesRegex(matcher *regexp.Regexp) func(string) error

                ValidateDynStringMatchesRegex returns a validator function that checks all flag's values against regex.

                func ValidateDynStringSetMinElements

                func ValidateDynStringSetMinElements(count int) func(map[string]struct{}) error

                  ValidateDynStringSetMinElements validates that the given string slice has at least x elements.

                  func ValidateDynStringSliceMinElements

                  func ValidateDynStringSliceMinElements(count int) func([]string) error

                    ValidateDynStringSliceMinElements validates that the given string slice has at least x elements.

                    Types

                    type DynDurationValue

                    type DynDurationValue struct {
                    	// contains filtered or unexported fields
                    }

                      DynDurationValue is a flag-related `time.Duration` value wrapper.

                      func DynDuration

                      func DynDuration(flagSet *flag.FlagSet, name string, value time.Duration, usage string) *DynDurationValue

                        DynDuration creates a `Flag` that represents `time.Duration` which is safe to change dynamically at runtime.

                        func (*DynDurationValue) Get

                        func (d *DynDurationValue) Get() time.Duration

                          Get retrieves the value in a thread-safe manner.

                          func (*DynDurationValue) Set

                          func (d *DynDurationValue) Set(input string) error

                            Set updates the value from a string representation in a thread-safe manner. This operation may return an error if the provided `input` doesn't parse, or the resulting value doesn't pass an optional validator. If a notifier is set on the value, it will be invoked in a separate go-routine.

                            func (*DynDurationValue) String

                            func (d *DynDurationValue) String() string

                              String represents the canonical representation of the type.

                              func (*DynDurationValue) Type

                              func (d *DynDurationValue) Type() string

                                Type is an indicator of what this flag represents.

                                func (*DynDurationValue) WithNotifier

                                func (d *DynDurationValue) WithNotifier(notifier func(oldValue time.Duration, newValue time.Duration)) *DynDurationValue

                                  WithNotifier adds a function is called every time a new value is successfully set. Each notifier is executed in a new go-routine.

                                  func (*DynDurationValue) WithValidator

                                  func (d *DynDurationValue) WithValidator(validator func(time.Duration) error) *DynDurationValue

                                    WithValidator adds a function that checks values before they're set. Any error returned by the validator will lead to the value being rejected. Validators are executed on the same go-routine as the call to `Set`.

                                    type DynFloat64Value

                                    type DynFloat64Value struct {
                                    	// contains filtered or unexported fields
                                    }

                                      DynFloat64Value is a flag-related `float64` value wrapper.

                                      func DynFloat64

                                      func DynFloat64(flagSet *flag.FlagSet, name string, value float64, usage string) *DynFloat64Value

                                        DynFloat64 creates a `Flag` that represents `float64` which is safe to change dynamically at runtime.

                                        func (*DynFloat64Value) Get

                                        func (d *DynFloat64Value) Get() float64

                                          Get retrieves the value in a thread-safe manner.

                                          func (*DynFloat64Value) Set

                                          func (d *DynFloat64Value) Set(input string) error

                                            Set updates the value from a string representation in a thread-safe manner. This operation may return an error if the provided `input` doesn't parse, or the resulting value doesn't pass an optional validator. If a notifier is set on the value, it will be invoked in a separate go-routine.

                                            func (*DynFloat64Value) String

                                            func (d *DynFloat64Value) String() string

                                              String returns the canonical string representation of the type.

                                              func (*DynFloat64Value) Type

                                              func (d *DynFloat64Value) Type() string

                                                Type is an indicator of what this flag represents.

                                                func (*DynFloat64Value) WithNotifier

                                                func (d *DynFloat64Value) WithNotifier(notifier func(oldValue float64, newValue float64)) *DynFloat64Value

                                                  WithNotifier adds a function is called every time a new value is successfully set. Each notifier is executed in a new go-routine.

                                                  func (*DynFloat64Value) WithValidator

                                                  func (d *DynFloat64Value) WithValidator(validator func(float64) error) *DynFloat64Value

                                                    WithValidator adds a function that checks values before they're set. Any error returned by the validator will lead to the value being rejected. Validators are executed on the same go-routine as the call to `Set`.

                                                    type DynInt64Value

                                                    type DynInt64Value struct {
                                                    	// contains filtered or unexported fields
                                                    }

                                                      DynInt64Value is a flag-related `int64` value wrapper.

                                                      func DynInt64

                                                      func DynInt64(flagSet *flag.FlagSet, name string, value int64, usage string) *DynInt64Value

                                                        DynInt64 creates a `Flag` that represents `int64` which is safe to change dynamically at runtime.

                                                        func (*DynInt64Value) Get

                                                        func (d *DynInt64Value) Get() int64

                                                          Get retrieves the value in a thread-safe manner.

                                                          func (*DynInt64Value) Set

                                                          func (d *DynInt64Value) Set(input string) error

                                                            Set updates the value from a string representation in a thread-safe manner. This operation may return an error if the provided `input` doesn't parse, or the resulting value doesn't pass an optional validator. If a notifier is set on the value, it will be invoked in a separate go-routine.

                                                            func (*DynInt64Value) String

                                                            func (d *DynInt64Value) String() string

                                                              String returns the canonical string representation of the type.

                                                              func (*DynInt64Value) Type

                                                              func (d *DynInt64Value) Type() string

                                                                Type is an indicator of what this flag represents.

                                                                func (*DynInt64Value) WithNotifier

                                                                func (d *DynInt64Value) WithNotifier(notifier func(oldValue int64, newValue int64)) *DynInt64Value

                                                                  WithNotifier adds a function is called every time a new value is successfully set. Each notifier is executed in a new go-routine.

                                                                  func (*DynInt64Value) WithValidator

                                                                  func (d *DynInt64Value) WithValidator(validator func(int64) error) *DynInt64Value

                                                                    WithValidator adds a function that checks values before they're set. Any error returned by the validator will lead to the value being rejected. Validators are executed on the same go-routine as the call to `Set`.

                                                                    type DynJSONValue

                                                                    type DynJSONValue struct {
                                                                    	// contains filtered or unexported fields
                                                                    }

                                                                      DynJSONValue is a flag-related JSON struct value wrapper.

                                                                      func DynJSON

                                                                      func DynJSON(flagSet *flag.FlagSet, name string, value interface{}, usage string) *DynJSONValue

                                                                        DynJSON creates a `Flag` that is backed by an arbitrary JSON which is safe to change dynamically at runtime. The `value` must be a pointer to a struct that is JSON (un)marshallable. New values based on the default constructor of `value` type will be created on each update.

                                                                        func (*DynJSONValue) Get

                                                                        func (d *DynJSONValue) Get() interface{}

                                                                          Get retrieves the value in its original JSON struct type in a thread-safe manner.

                                                                          func (*DynJSONValue) PrettyString

                                                                          func (d *DynJSONValue) PrettyString() string

                                                                            PrettyString returns a nicely structured representation of the type. In this case it returns a pretty-printed JSON.

                                                                            func (*DynJSONValue) Set

                                                                            func (d *DynJSONValue) Set(input string) error

                                                                              Set updates the value from a string representation in a thread-safe manner. This operation may return an error if the provided `input` doesn't parse, or the resulting value doesn't pass an optional validator. If a notifier is set on the value, it will be invoked in a separate go-routine.

                                                                              func (*DynJSONValue) String

                                                                              func (d *DynJSONValue) String() string

                                                                                String returns the canonical string representation of the type.

                                                                                func (*DynJSONValue) Type

                                                                                func (d *DynJSONValue) Type() string

                                                                                  Type is an indicator of what this flag represents.

                                                                                  func (*DynJSONValue) WithFileFlag

                                                                                  func (d *DynJSONValue) WithFileFlag(defaultPath string) *DynJSONValue

                                                                                    WithFileFlag adds an companion <name>_path flag that allows this value to be read from a file with flagz.ReadFileFlags.

                                                                                    This is useful for reading large JSON files as flags. If the companion flag's value (whether default or overwritten) is set to empty string, nothing is read.

                                                                                    Flag value reads are subject to notifiers and validators.

                                                                                    func (*DynJSONValue) WithNotifier

                                                                                    func (d *DynJSONValue) WithNotifier(notifier func(oldValue interface{}, newValue interface{})) *DynJSONValue

                                                                                      WithNotifier adds a function is called every time a new value is successfully set. Each notifier is executed in a new go-routine.

                                                                                      func (*DynJSONValue) WithValidator

                                                                                      func (d *DynJSONValue) WithValidator(validator func(interface{}) error) *DynJSONValue

                                                                                        WithValidator adds a function that checks values before they're set. Any error returned by the validator will lead to the value being rejected. Validators are executed on the same go-routine as the call to `Set`.

                                                                                        type DynStringSetValue

                                                                                        type DynStringSetValue struct {
                                                                                        	// contains filtered or unexported fields
                                                                                        }

                                                                                          DynStringSetValue is a flag-related `map[string]struct{}` value wrapper.

                                                                                          func DynStringSet

                                                                                          func DynStringSet(flagSet *flag.FlagSet, name string, value []string, usage string) *DynStringSetValue

                                                                                            DynStringSet creates a `Flag` that represents `map[string]struct{}` which is safe to change dynamically at runtime. Unlike `pflag.StringSlice`, consecutive sets don't append to the slice, but override it.

                                                                                            func (*DynStringSetValue) Contains

                                                                                            func (d *DynStringSetValue) Contains(val string) bool

                                                                                              Contains returns whether the specified string is in the flag.

                                                                                              func (*DynStringSetValue) Get

                                                                                              func (d *DynStringSetValue) Get() map[string]struct{}

                                                                                                Get retrieves the value in a thread-safe manner.

                                                                                                func (*DynStringSetValue) Set

                                                                                                func (d *DynStringSetValue) Set(val string) error

                                                                                                  Set updates the value from a string representation in a thread-safe manner. This operation may return an error if the provided `input` doesn't parse, or the resulting value doesn't pass an optional validator. If a notifier is set on the value, it will be invoked in a separate go-routine.

                                                                                                  func (*DynStringSetValue) String

                                                                                                  func (d *DynStringSetValue) String() string

                                                                                                    String represents the canonical representation of the type.

                                                                                                    func (*DynStringSetValue) Type

                                                                                                    func (d *DynStringSetValue) Type() string

                                                                                                      Type is an indicator of what this flag represents.

                                                                                                      func (*DynStringSetValue) WithNotifier

                                                                                                      func (d *DynStringSetValue) WithNotifier(notifier func(oldValue map[string]struct{}, newValue map[string]struct{})) *DynStringSetValue

                                                                                                        WithNotifier adds a function that is called every time a new value is successfully set. Each notifier is executed asynchronously in a new go-routine.

                                                                                                        func (*DynStringSetValue) WithValidator

                                                                                                        func (d *DynStringSetValue) WithValidator(validator func(map[string]struct{}) error) *DynStringSetValue

                                                                                                          WithValidator adds a function that checks values before they're set. Any error returned by the validator will lead to the value being rejected. Validators are executed on the same go-routine as the call to `Set`.

                                                                                                          type DynStringSliceValue

                                                                                                          type DynStringSliceValue struct {
                                                                                                          	// contains filtered or unexported fields
                                                                                                          }

                                                                                                            DynStringSliceValue is a flag-related `time.Duration` value wrapper.

                                                                                                            func DynStringSlice

                                                                                                            func DynStringSlice(flagSet *flag.FlagSet, name string, value []string, usage string) *DynStringSliceValue

                                                                                                              DynStringSlice creates a `Flag` that represents `[]string` which is safe to change dynamically at runtime. Unlike `pflag.StringSlice`, consecutive sets don't append to the slice, but override it.

                                                                                                              func (*DynStringSliceValue) Get

                                                                                                              func (d *DynStringSliceValue) Get() []string

                                                                                                                Get retrieves the value in a thread-safe manner.

                                                                                                                func (*DynStringSliceValue) Set

                                                                                                                func (d *DynStringSliceValue) Set(val string) error

                                                                                                                  Set updates the value from a string representation in a thread-safe manner. This operation may return an error if the provided `input` doesn't parse, or the resulting value doesn't pass an optional validator. If a notifier is set on the value, it will be invoked in a separate go-routine.

                                                                                                                  func (*DynStringSliceValue) String

                                                                                                                  func (d *DynStringSliceValue) String() string

                                                                                                                    String represents the canonical representation of the type.

                                                                                                                    func (*DynStringSliceValue) Type

                                                                                                                    func (d *DynStringSliceValue) Type() string

                                                                                                                      Type is an indicator of what this flag represents.

                                                                                                                      func (*DynStringSliceValue) WithNotifier

                                                                                                                      func (d *DynStringSliceValue) WithNotifier(notifier func(oldValue []string, newValue []string)) *DynStringSliceValue

                                                                                                                        WithNotifier adds a function that is called every time a new value is successfully set. Each notifier is executed asynchronously in a new go-routine.

                                                                                                                        func (*DynStringSliceValue) WithValidator

                                                                                                                        func (d *DynStringSliceValue) WithValidator(validator func([]string) error) *DynStringSliceValue

                                                                                                                          WithValidator adds a function that checks values before they're set. Any error returned by the validator will lead to the value being rejected. Validators are executed on the same go-routine as the call to `Set`.

                                                                                                                          type DynStringValue

                                                                                                                          type DynStringValue struct {
                                                                                                                          	// contains filtered or unexported fields
                                                                                                                          }

                                                                                                                            DynStringValue is a flag-related `time.Duration` value wrapper.

                                                                                                                            func DynString

                                                                                                                            func DynString(flagSet *flag.FlagSet, name string, value string, usage string) *DynStringValue

                                                                                                                              DynString creates a `Flag` that represents `string` which is safe to change dynamically at runtime.

                                                                                                                              func (*DynStringValue) Get

                                                                                                                              func (d *DynStringValue) Get() string

                                                                                                                                Get retrieves the value in a thread-safe manner.

                                                                                                                                func (*DynStringValue) Set

                                                                                                                                func (d *DynStringValue) Set(val string) error

                                                                                                                                  Set updates the value from a string representation in a thread-safe manner. This operation may return an error if the provided `input` doesn't parse, or the resulting value doesn't pass an optional validator. If a notifier is set on the value, it will be invoked in a separate go-routine.

                                                                                                                                  func (*DynStringValue) String

                                                                                                                                  func (d *DynStringValue) String() string

                                                                                                                                    String represents the canonical representation of the type.

                                                                                                                                    func (*DynStringValue) Type

                                                                                                                                    func (d *DynStringValue) Type() string

                                                                                                                                      Type is an indicator of what this flag represents.

                                                                                                                                      func (*DynStringValue) WithNotifier

                                                                                                                                      func (d *DynStringValue) WithNotifier(notifier func(oldValue string, newValue string)) *DynStringValue

                                                                                                                                        WithNotifier adds a function is called every time a new value is successfully set. Each notifier is executed in a new go-routine.

                                                                                                                                        func (*DynStringValue) WithValidator

                                                                                                                                        func (d *DynStringValue) WithValidator(validator func(string) error) *DynStringValue

                                                                                                                                          WithValidator adds a function that checks values before they're set. Any error returned by the validator will lead to the value being rejected. Validators are executed on the same go-routine as the call to `Set`.

                                                                                                                                          type FileReadValue

                                                                                                                                          type FileReadValue struct {
                                                                                                                                          	// contains filtered or unexported fields
                                                                                                                                          }

                                                                                                                                            FileReadValue is a flag that wraps another flag and makes it readable from a local file in the filesystem.

                                                                                                                                            func FileReadFlag

                                                                                                                                            func FileReadFlag(flagSet *flag.FlagSet, parentFlagName string, defaultFilePath string) *FileReadValue

                                                                                                                                              FileReadFlag creates a `Flag` that allows you to pass a flag.

                                                                                                                                              If defaultFilePath is non empty, the flagz.ReadFileFlags will expect the file to be there.

                                                                                                                                              func (*FileReadValue) Set

                                                                                                                                              func (f *FileReadValue) Set(path string) error

                                                                                                                                              func (*FileReadValue) String

                                                                                                                                              func (f *FileReadValue) String() string

                                                                                                                                              func (*FileReadValue) Type

                                                                                                                                              func (f *FileReadValue) Type() string

                                                                                                                                              type StatusEndpoint

                                                                                                                                              type StatusEndpoint struct {
                                                                                                                                              	// contains filtered or unexported fields
                                                                                                                                              }

                                                                                                                                                StatusEndpoint is a collection of `http.HandlerFunc` that serve debug pages about a given `FlagSet.

                                                                                                                                                func NewStatusEndpoint

                                                                                                                                                func NewStatusEndpoint(flagSet *flag.FlagSet) *StatusEndpoint

                                                                                                                                                  NewStatusEndpoint creates a new debug `http.HandlerFunc` collection for a given `FlagSet`

                                                                                                                                                  func (*StatusEndpoint) ListFlags

                                                                                                                                                  func (e *StatusEndpoint) ListFlags(resp http.ResponseWriter, req *http.Request)

                                                                                                                                                    ListFlags provides an HTML and JSON `http.HandlerFunc` that lists all Flags of a `FlagSet`. Additional URL query parameters can be used such as `type=[dynamic,static]` or `only_changed=true`.

                                                                                                                                                    Directories

                                                                                                                                                    Path Synopsis
                                                                                                                                                    examples