> For the complete documentation index, see [llms.txt](https://developer.harness.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://developer.harness.io/feature-flags/use-feature-flags/use-ff/ff-sdks/server-sdks/feature-flag-sdks-go-application.md). # Go SDK Reference {% hint style="info" %} **NOTE** This SDK works with Harness Self-Managed Enterprise Edition (on premises). {% endhint %} This topic describes how to use the Harness Feature Flags Go SDK for your Go application. For getting started quickly, you can use our [sample code from the SDK README](https://github.com/harness/ff-golang-server-sdk). You can also [clone](https://docs.github.com/en/repositories/creating-and-managing-repositories/cloning-a-repository) and run a sample application from the [Go SDK GitHub Repository.](https://github.com/harness/ff-golang-server-sdk) ### Before you begin Make sure you read and understand: * [Feature Flags Overview](/feature-flags/new-to-feature-flags/get-started/overview.md) * [Getting Started with Feature Flags](/feature-flags/new-to-feature-flags/get-started/onboarding-guide.md) * [Client-Side and Server-Side SDKs](/feature-flags/use-feature-flags/use-ff/ff-sdks/sdk-overview/client-side-and-server-side-sdks.md) * [Communication Strategy Between SDKs and Harness Feature Flags](/feature-flags/use-feature-flags/use-ff/ff-sdks/sdk-overview/communication-sdks-harness-feature-flags.md) ### Version Latest SDK version can be found on [GitHub Release Page](https://github.com/harness/ff-golang-server-sdk/releases) ### Requirements To use this SDK, make sure you: * Starting with SDK version v0.1.21, Golang version 1.20 or later is required. * Earlier versions of the SDK require Golang versions newer than 1.6 but older than 1.19. * For installation details, please refer to [Golang's official installation guide](https://go.dev/doc/install). * [Download the SDK from our GitHub repository](https://github.com/harness/ff-golang-server-sdk) * Create a Go application, or [clone](https://docs.github.com/en/repositories/creating-and-managing-repositories/cloning-a-repository) our [sample application](https://github.com/harness/ff-golang-server-sdk/blob/main/examples/getting_started.go). * [Create a Feature Flag on the Harness Platform](/feature-flags/use-feature-flags/use-ff/ff-creating-flag/create-a-feature-flag.md). If you are following along with the SDK README sample code, make sure your flag is called `harnessappdemodarkmode`. * [Create an SDK key and make a copy of it](/feature-flags/use-feature-flags/use-ff/ff-creating-flag/create-a-project.md#create-an-sdk-key). ### Install the SDK Install the SDK using the following Go command: ``` go get github.com/harness/ff-golang-server-sdk ``` ### Import the SDK Import the SDK using the following Go command: ``` import harness "github.com/harness/ff-golang-server-sdk/client" ``` ### Initialize the SDK To initialize the Go SDK, you need to: 1. Add your Server SDK Key to connect to your Harness Environment. 2. (Optional) Configure the SDK options. For more details on what features you can configure for this SDK, go to [Configure the SDK](#configure-the-sdk). 3. Complete the initialization by creating an instance of the Feature Flag client and passing in the Server SDK Key and Configuration. 4. Add a Target that you want to Evaluate against a Feature Flag. #### Add the Server SDK Key To connect to the correct Environment that you set up on the Harness Platform, you need to add the Server SDK Key from that Environment. Input the Server SDK Key into the `sdkKey` parameter. For example: ``` client, err := harness.NewCfClient(sdkKey) ``` #### Configure the SDK You can configure the following features of the SDK: | | | | | | ---------------- | --------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------- | | **Name** | **Example** | **Description** | **Default Value** | | baseUrl | `harness.WithURL("https://config.ff.harness.io/api/1.0")` | The URL used to fetch Feature Flag Evaluations. When using the Relay Proxy, change this to: `http://localhost:7000` | `https://config.ff.harness.io/api/1.0` | | eventUrl | `harness.WithEventsURL("https://events.ff.harness.io/api/1.0")` | The URL for posting metrics data to the Feature Flag service. When using the Relay Proxy, change this to: `http://localhost:7000` | `https://events.ff.harness.io/api/1.0` | | pollInterval | `harness.WithPullInterval(60))` | The interval **in seconds** that we poll for changes when you are using stream mode. | `60 (seconds)` | | streamEnabled | `harness.WithStreamEnabled(false)` | Set to `true` to enable streaming mode.Set to `false` to disable streaming mode. | `true` | | analyticsEnabled | `harness.WithAnalyticsEnabled(false)` | Set to `true` to enable analytics.Set to `false` to disable analytics.**Note**: Analytics are not cached. | `true` | For further configuration options and samples, such as configuring your logger or using the SDK with the Relay Proxy, go to [Additional Options](#additional-options). #### Complete the initialization Complete the initialization by creating an instance of the Feature Flag client and passing in the `sdkKey`, and any configuration options. For example: ``` // Create Options client, err := harness.NewCfClient(myApiKey,  harness.WithURL("https://config.ff.harness.io/api/1.0"),  harness.WithEventsURL("https://events.ff.harness.io/api/1.0"),  harness.WithPullInterval(1), harness.WithStreamEnabled(false)) ``` #### Block initialization By default, when initializing the Harness Feature Flags client, the initialization process is non-blocking. This means that the client creation call returns immediately, allowing your application to continue its startup process without waiting for the client to be fully initialized. If you evaluate a flag before the client has finished initializing, the default variation you provided can be returned as the evaluation result, because the SDK has not finished caching your remote Flag configuration stored in Harness. You can choose to wait for the client to finish initializing before continuing. To achieve this, you can use the `WithWaitForInitialized` option, which blocks until the client is fully initialized. Example usage: ```go client, err := harness.NewCfClient(sdkKey, harness.WithWaitForInitialized(true)) if err != nil { log.ErrorF("could not connect to FF servers %s", err) } result, err := client.BoolVariation("identifier_of_your_boolean_flag", &target, false) ``` In this example, WaitForInitialized blocks for up to 5 authentication attempts. If the client is not initialized within 5 authentication attempts, it returns an error. This can be useful if you need to unblock after a certain time. ```go // Try to authenticate only 5 times before returning a result client, err := harness.NewCfClient(sdkKey, harness.WithWaitForInitialized(true), harness.WithMaxAuthRetries(5)) if err != nil { log.Fatalf("client did not initialize in time: %s", err) } ``` #### Add a target
What is a Target? Targets are used to control which users see which Variation of a Feature Flag, for example, if you want to do internal testing, you can enable the Flag for some users and not others. When creating a Target, you give it a name and a unique identifier. Often Targets are users but you can create a Target from anything that can be uniquely identified, such as an app or a machine.
For more information about Targets, go to [Targeting Users With Flags](/feature-flags/use-feature-flags/use-ff/ff-target-management/targeting-users-with-flags.md). To add a Target, build it and pass in arguments for the following: | | | | | | ------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------- | ----------------------------------------------------------------- | | **Parameter** | **Description** | **Required?** | **Example** | | `Identifier` | Unique ID for the Target.Read **Regex requirements for Target names and identifiers** below for accepted characters. | Required | `Identifier: "HT_1"` | | `Name` | Name for this Target. This does not have to be unique. **Note**: If you don’t provide a value, the name will be the same as the identifier.Read **Regex requirements for Target names and identifiers** below for accepted characters. | Optional**Note**: If you don't want to send a name, don't send the parameter. Sending an empty argument will cause an error. | `Name: "Harness_Target_1"` | | `Attributes` | Additional data you can store for a Target, such as email addresses or location. | Optional | `Attributes: &map[string]interface{}{"email":"demo@harness.io"},` |
Regex requirements for Target names and identifiers **Identifier** Regex: `^[A-Za-z0-9.@_-]*$`\ Must consist of only alphabetical characters, numbers, and the following symbols:\ . (period)\ @ (at sign)\ -(dash)\ \_ (underscore) The characters can be lowercase or uppercase but cannot include accented letters, for example `Cafe_789`. **Name** Regex: `^[\\p{L}\\d .@_-]*$` Must consist of only alphabetical characters, numbers, and the following symbols:\ . (period)\ @ (at sign)\ -(dash)\ \_ (underscore)\ (space) The characters can be lowercase or uppercase and can include accented letters, for example `Café_123`.
For example: **Create a Target** ``` target2 := evaluation.Target{ Identifier: "HT_1", Name: "Harness_Target_1", Attributes: &map[string]interface{}{"email":"demo@harness.io"}, } ``` **Create a Target with the builder** If you create a Target with the builder, use `Custom` instead of `Attributes`. ``` target := dto.NewTargetBuilder("HT_1"). Name("Harness_Target_1"). Custom("email", "demo@harness.io"). Build() ``` ### Evaluate a Flag Evaluating a Flag is when the SDK processes all Flag rules and returns the correct Variation of that Flag for the Target you provide. If a matching Flag can’t be found, or the SDK can’t remotely fetch flags, the default value is returned. This will be indicated by: 1. The SDK will log an `info` level log that the default variation was returned. 2. Evaluation calls will return an `error` as well as the default variation that you can handle. There are different methods for the different Variation types and for each method you need to pass in: * Identifier of the Flag you want to evaluate * The Target object you want to evaluate against * The default Variation For example: #### Evaluate a string Variation ``` client.StringVariation(flagName, &target, "default_string") ``` #### Evaluate a boolean Variation ``` showFeature, err := client.BoolVariation(featureFlagKey, &target, false) ``` #### Evaluate a number Variation ``` client.NumberVariation(flagName, &target, -1) ``` #### Evaluate a JSON Variation ``` client.JSONVariation(flagName, &target, types.JSON{"darkmode": false}) ``` {% hint style="info" %} If you evaluate a feature flag when initialization fails, the default variation you provided is returned as the evaluation result. {% endhint %} ### Test Your App is Connected to Harness When you receive a response showing the current status of your Feature Flag, go to the Harness Platform and toggle the Flag on and off. Then, check your app to verify if the Flag Variation displayed is updated with the Variation you toggled. {% hint style="info" %} **NOTE** The SDK must run for at least 60 seconds before it sends metrics. Please ensure metrics have not been disabled in the SDK. {% endhint %} ### Close the SDK client In most applications, you won't need to close the SDK client. However, you should close the SDK client if: * Your application is about to terminate. Closing the client ensures that all associated resources are released. * You have determined that you do not need to evaluate flags again in your application lifecycle. {% hint style="info" %} **IMPORTANT** The SDK does not evaluate flags after the client is closed. {% endhint %} To close the SDK client: * Assuming you have initialized an SDK client instance named `client`, call the following function: ``` client.close() ``` ### Additional options #### Configure your logger The SDK has a default logger, however, you can provide your own logger to the SDK by passing it in as a configuration option. For example, the following creates an instance of the logrus logger and passes it in as a configuration option: ``` logger := logrus.New() logger.SetLevel(logrus.ErrorLevel)  // Create a feature flag client client, err := harness.NewCfClient(sdkKey, harness.WithLogger(logger)) ``` #### Use the Relay Proxy When using your Feature Flag SDKs with a [Harness Relay Proxy](/feature-flags/use-feature-flags/use-ff/relay-proxy/relay-proxy.md) you need to change the default URL and events URL to `http://localhost:7000` when initializing the SDK. For example: ``` client, err := harness.NewCfClient(apiKey, harness.WithURL("http://localhost:7000"), harness.WithEventsURL("http://localhost:7000")) ``` #### Configure your HTTP Client The SDK has a default HTTP client, however, you can provide your own HTTP client to the SDK by passing it in as a configuration option. For example, the following creates an HTTP client using custom CAs for Harness Self-Managed Enterprise Edition (on premises). ``` // Create a custom TLS configuration tlsConfig := &tls.Config{ RootCAs: certPool, } transport := &http.Transport{ TLSClientConfig: tlsConfig, } httpClient := http.Client{Transport: transport} client, err := harness.NewCfClient(apiKey, harness.WithEventsURL("https://ffserver:8003/api/1.0"), harness.WithURL("https://ffserver:8003/api/1.0"), harness.WithHTTPClient(&httpClient)) ``` For a full example of providing custom CAs for Harness Self-Managed Enterprise Edition, see our [TLS Example](https://github.com/harness/ff-golang-server-sdk/blob/main/examples/tls/example.go) ### Sample code for a Go application Here is a sample code for integrating with the Go SDK: ``` package main  import ( "context" "fmt" "log" "time"   harness "github.com/harness/ff-golang-server-sdk/client" "github.com/harness/ff-golang-server-sdk/dto" "github.com/sirupsen/logrus" )  const sdkKey = "your SDK key"  const featureFlag = "harnessappdemodarkmode"  func main() { logger := logrus.New() logger.SetLevel(logrus.ErrorLevel) client, err := harness.NewCfClient(myApiKey,  harness.WithURL("https://config.ff.harness.io/api/1.0"),  harness.WithEventsURL("https://events.ff.harness.io/api/1.0”),  harness.WithPullInterval(1), harness.WithLogger(logger), harness.WithStreamEnabled(false) ) defer func() { if err := client.Close(); err != nil { log.Printf("error while closing client err: %v", err) } }() if err != nil { log.Printf("could not connect to CF servers %v", err) } target := dto.NewTargetBuilder("HT_1"). Name("Harness_Target_1"). Custom("email", "demo@harness.io"). Build() ctx, cancel := context.WithCancel(context.Background())   go func() { for { select { case <-ctx.Done(): return default: showFeature, err := client.BoolVariation(featureFlag, &target, false)   if err != nil { fmt.Printf("Error getting value: %v", err) }   fmt.Printf("KeyFeature flag '%s' is %t for this user\n", featureFlag, showFeature) time.Sleep(10 * time.Second) } } }()   time.Sleep(5 * time.Minute) cancel() } ``` ### Troubleshooting The SDK logs the following codes for certain lifecycle events, for example authentication, which can aid troubleshooting. | **Code** | **Description** | **Log Level** | | -------- | ------------------------------------------------------------------------------------------------------------- | ------------- | | **1000** | Successfully initialized | Info | | **1001** | Failed to initialize due to authentication error | Error | | **1002** | Failed to initialize due to a missing or empty API key | Error | | **1003** | `WaitForInitialization` configuration option was provided and the SDK is waiting for initialization to finish | Info | | **2000** | Successfully authenticated | Info | | **2001** | Authentication failed with a non-recoverable error | Error | | **2002** | Authentication failed and is retrying | Warn | | **2003** | Authentication failed and max retries have been exceeded | Error | | **3000** | SDK closing | Info | | **3001** | SDK closed successfully | Info | | **4000** | Polling service started | Info | | **4001** | Polling service stopped | Info | | **5000** | Streaming connected | Info | | **5001** | Streaming disconnected | Warn | | **5002** | Streaming event received | Debug | | **5003** | Streaming disconnected and is retrying to connect | Info | | **5004** | Streaming service stopped | Info | | **6000** | Evaluation was successful | Debug | | **6001** | Evaluation failed and the default value was returned | Info | | **7000** | Metrics service has started | Info | | **7001** | Metrics service has stopped | Info | | **7002** | Metrics posting failed | Warn | | **7003** | Metrics posting success | Debug | | **7004** | Metrics max target size exceeded | Warn | | **7007** | Metrics max evaluation size reached | Warn |