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Creating an RPC flagd provider

By default, flagd is a remote service that is accessed via grpc by a client application to retrieve feature flags. Depending on the environment, flagd therefore is usually deployed as a standalone service, e.g. as a Kubernetes Deployment, or injected as a sidecar container into the pod running the client application, as it is done in the OpenFeature Operator.


flagd Evaluation API

Fundamentally, RPC providers use the evaluation schema to connect to flagd, initiate evaluation RPCs, and listen for changes in the flag definitions. In order to do this, you must generate the gRPC primitives (message types and client) using the protobuf code generation mechanisms available in your language. If you are unable to use gRPC code generation, you can also use REST (via the connect protocol) to communicate with flagd, though in this case, you will not be able to open a stream to listen for changes.


Protobuf schemas define the contract between the flagd evaluation API and a client.

Code generation for gRPC sync

Leverage the buf CLI or protoc to generate a flagd-proxy client in the chosen technology:

Add the open-feature schema repository as a submodule

git submodule add --force

Create a buf.gen.{chosen language}.yaml for the chosen language in schemas/protobuf (if it doesn't already exist) using one of the other files as a template (find a plugin for the chosen language here) and create a pull request with this file.

Generate the code (this step ought to be automated in the build process for the chosen technology so that the generated code is never committed)

cd schemas/protobuf
buf generate --template buf.gen.{chosen language}.yaml

As an alternative to buf, use the .proto file directly along with whatever protoc-related tools or plugins available for your language.

Move the generated code (following convention for the chosen language) and add its location to .gitignore

Note that for the in-process provider only the schema package will be relevant, since RPC providers communicate directly to flagd.

Provider lifecycle, initialization and shutdown

With the release of the v0.6.0 spec, OpenFeature now outlines a lifecycle for in-process flagd provider initialization and shutdown.

In-process flagd providers should do the following to make use of OpenFeature v0.6.0 features:

  • start in a NOT_READY state
  • fetch the flag definition specified in the sync provider sources and set state to READY or ERROR in the initialization function
  • note that the SDK will automatically emit PROVIDER_READY/PROVIDER_ERROR according to the termination of the initialization function
  • throw an exception or terminate abnormally if a connection cannot be established during initialization
  • For gRPC based sources (i.e. flagd-proxy), attempt to restore the streaming connection to flagd-proxy (if the connection cannot be established or is broken):
  • If flag definition have been retrieved previously, go into STALE state to indicate that flag resolution responses are based on potentially outdated Flag definition.
  • reconnection should be attempted with an exponential back-off, with a max-delay of maxSyncRetryInterval (see configuration)
  • reconnection should be attempted up to maxSyncRetryDelay times (see configuration)
  • PROVIDER_READY and PROVIDER_CONFIGURATION_CHANGED should be emitted, in that order, after successful reconnection
  • For Kubernetes sync sources, retry to retrieve the FlagConfiguration resource, using an exponential back-off strategy, with a max-delay of maxSyncRetryInterval (see configuration)
  • emit PROVIDER_CONFIGURATION_CHANGED event and update ruleset when a configuration_change message is received on the streaming connection
  • close the streaming connection in theshutdown function
    [*] --> NOT_READY
    NOT_READY --> READY: initialize(), connection to flagd established, stream connected
    NOT_READY --> ERROR: initialize(), unable to connect or establish stream(retry)
    READY --> ERROR: stream or connection disconnected
    READY --> READY: configuration_change (emit changed*, invalidate cache)
    ERROR --> READY: reconnect successful (emit ready*, changed*, invalidate cache)
    ERROR --> ERROR: maxSyncRetries reached
    ERROR --> [*]: shutdown(), stream disconnected



flagd has a caching strategy implementable by RPC providers that support server-to-client streaming.

Cacheable flags

flagd sets the reason of a flag evaluation as STATIC when no targeting rules are configured for the flag. A client can safely store the result of a static evaluation in its cache indefinitely (until the configuration of the flag changes, see cache invalidation).

Put simply in pseudocode:

if reason == "STATIC" {
    isFlagCacheable = true

Cache invalidation

flagd emits events to the server-to-client stream, among these is the configuration_change event. The structure of this event is as such:

    "type": "delete", // ENUM:["delete","write","update"]
    "source": "/flag-configuration.json", // the source of the flag configuration
    "flagKey": "foo"

A client should invalidate the cache of any flag found in a configuration_change event to prevent stale data. If the connection drops all cache values must be cleared (any number of events may have been missed).


Expose means to configure the provider aligned with the following priority system (highest to lowest).

flowchart LR
    constructor-parameters -->|highest priority| environment-variables -->|lowest priority| defaults

Explicit declaration

This takes the form of parameters to the provider's constructor, it has the highest priority.

Environment variables

Read environment variables with sensible defaults (before applying the values explicitly declared to the constructor).

Option name Environment variable name Type & Values Default
host FLAGD_HOST String localhost
port FLAGD_PORT int 8013
tls FLAGD_TLS boolean false
socketPath FLAGD_SOCKET_PATH String null
certPath FLAGD_SERVER_CERT_PATH String null
deadline FLAGD_DEADLINE_MS int 500
cache FLAGD_CACHE String - lru, disabled lru
maxCacheSize FLAGD_MAX_CACHE_SIZE int 1000
maxEventStreamRetries FLAGD_MAX_EVENT_STREAM_RETRIES int 5
retryBackoffMs FLAGD_RETRY_BACKOFF_MS int 1000

Error handling

Handle flag evaluation errors by using the error constructors exported by the SDK (e.g. openfeature.NewProviderNotReadyResolutionError(ConnectionError)), thereby allowing the SDK to parse and handle the error appropriately.

Post creation

The following steps will extend the reach of the newly created provider to other developers of the chosen technology.

Open an issue to document the provider

Create an issue in here. This will ensure the provider is added to OpenFeature's website.

Serialization of the evaluation context

An RPC provider should serialize the OpenFeature context for use in the evaluation.proto. It should map the targeting-key into a top level property of the context, with the key "targetingKey"