Flag overrides

​

About flag overrides

• An override takes precedence over LaunchDarkly data for its key. This includes evaluations that use the flag as a prerequisite and evaluations that use the segment in a rule.
• The SDK serves overrides before it receives any data from LaunchDarkly. An override works even if the SDK never connects.
• Overrides that are in the files when your application starts are in effect after the SDK client finishes starting.
• The SDK does not serve overrides in offline mode.
• An override stays in effect even after LaunchDarkly becomes available again. While a flag has an override, changes that you make to the flag in LaunchDarkly do not affect its evaluations.

​

Prerequisites

• An SDK version that supports flag overrides. Each SDK section below lists the required version.
• An SDK configured for data saving mode. The override source is part of the data system configuration. If your application already configures the data system, add the override source to that configuration. If it does not, set up data saving mode first, and configure any persistent store or Relay Proxy settings through the data system configuration.

​

Create an override file

• flagValues: Flag keys mapped to values. The SDK serves the value to every context. It evaluates the entry as a flag that is on and serves the value as its fallthrough. The evaluation reason kind is FALLTHROUGH.
• flags: Full flag definitions, keyed by flag key. These can target contexts and use rules, prerequisites, and segments.
• segments: Segment definitions, keyed by segment key.

{
  "flagValues": {
    "my-boolean-flag-key": false,
    "my-string-flag-key": "value-1"
  },
  "flags": {
    "my-targeted-flag-key": {
      "key": "my-targeted-flag-key",
      "on": true,
      "variations": [ true, false ],
      "targets": [
        { "values": [ "context-key-123abc" ], "variation": 0 }
      ],
      "fallthrough": { "variation": 1 },
      "offVariation": 1
    }
  }
}

flagValues:
  my-boolean-flag-key: false
  my-string-flag-key: "value-1"
flags:
  my-targeted-flag-key:
    key: my-targeted-flag-key
    "on": true
    variations: [ true, false ]
    targets:
      - values: [ "context-key-123abc" ]
        variation: 0
    fallthrough:
      variation: 1
    offVariation: 1

​

Use multiple files

​

Change and remove overrides

• Polling: The SDK checks the files for changes every second. This is the default. Polling works on every file system, including network file systems and directories whose contents are replaced through symbolic links, such as a Kubernetes ConfigMap mounted as a volume. You can make the polling interval longer. The minimum is one second.
• Watching: The SDK reloads the files when the file system reports a change. This is faster than polling, but some file systems do not report changes reliably.

​

How overrides affect evaluations and events

• The evaluation reason has the overrideAffected property set to true. The reason kind is the kind that the definition produces. If the evaluation fails, for example because of a type mismatch, the reason is an error reason that still has overrideAffected set to true.
• The SDK does not send feature events or debug events for the evaluation, even if the flag configuration requests them. LaunchDarkly features that rely on feature events, such as Experimentation, do not receive data from these evaluations.
• The SDK counts the evaluation in the summary events it sends to LaunchDarkly, separately from evaluations that are not affected by an override.
• In the all flags output, an overridden flag is evaluated from its override, and every flag that is affected by an override has event tracking turned off. A flag that exists only in an override file is included. Overrides do not remove any other flag from the output.

​

Server-side SDKs

• Go
• Node.js (server-side)

​

Go

import (
    "time"

    ld "github.com/launchdarkly/go-server-sdk/v7"
    "github.com/launchdarkly/go-server-sdk/v7/ldcomponents"
    "github.com/launchdarkly/go-server-sdk/v7/ldoverrides"
)

var config ld.Config

config.DataSystem = ldcomponents.DataSystem().Default().
    Overrides(ldoverrides.FileSource().FilePaths("/etc/launchdarkly/overrides.json"))

client, _ := ld.MakeCustomClient("YOUR_SDK_KEY", config, 5*time.Second)

overrides := ldoverrides.FileSource().
    FilePaths("/etc/launchdarkly/overrides.json", "/etc/launchdarkly/team-overrides.json").
    DuplicateKeysHandling(ldoverrides.DuplicateKeysKeepFirst).
    ChangeDetection(ldoverrides.Watching)

config.DataSystem = ldcomponents.DataSystem().Default().Overrides(overrides)

_, detail, _ := client.BoolVariationDetail("my-boolean-flag-key", context, false)
if detail.Reason.IsOverrideAffected() {
    fmt.Println("An override affected this evaluation")
}

​

Node.js (server-side)

import { init } from '@launchdarkly/node-server-sdk';

const client = init('YOUR_SDK_KEY', {
  dataSystem: {
    overrides: {
      type: 'file',
      paths: ['/etc/launchdarkly/overrides.json'],
    },
  },
});

const client = init('YOUR_SDK_KEY', {
  dataSystem: {
    overrides: {
      type: 'file',
      paths: ['/etc/launchdarkly/overrides.json', '/etc/launchdarkly/team-overrides.json'],
      duplicateKeysHandling: 'ignore',
      changeDetection: 'watching',
    },
  },
});

import { init } from '@launchdarkly/node-server-sdk';
import { parse } from 'yaml';

const client = init('YOUR_SDK_KEY', {
  dataSystem: {
    overrides: {
      type: 'file',
      paths: ['/etc/launchdarkly/overrides.yaml'],
      yamlParser: parse,
    },
  },
});

const detail = await client.variationDetail('my-boolean-flag-key', context, false);
if (detail.reason.overrideAffected) {
  console.log('An override affected this evaluation');
}