Fastly SDK reference

Overview

This topic documents how to get started with the Fastly SDK, and links to reference information on all of the supported features.

Get started

After you complete the Create a Fastly integration process in an existing project, follow these instructions to start using the LaunchDarkly Fastly SDK:

• Install the SDK
• Initialize the client
• Evaluate a flag

Install the SDK

First, install the Fastly SDK as a dependency in your application using your application’s dependency manager.

Here’s how:

$
yarn add @launchdarkly/fastly-server-sdk

Next, import the LaunchDarkly client in your application code:

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import { KVStore } from 'fastly:kv-store';
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import { init } from '@launchdarkly/fastly-server-sdk';

Initialize the client

After you install and import the SDK, configure a Fastly KV Store. Then, initialize an LDClient using your LaunchDarkly client-side ID, the Fastly KV store name, and the Fastly events backend name. The client must be initialized when processing requests.

Here’s how:

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const KV_STORE_NAME = 'launchdarkly';
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const EVENTS_BACKEND_NAME = 'launchdarkly';
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const store = new KVStore(KV_STORE_NAME);
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async function handleRequest(event: FetchEvent) {
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  const ldClient = init('client-side-id-123abc', store, {
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    eventsBackendName: EVENTS_BACKEND_NAME,
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  });
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  await ldClient.waitForInitialization();
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  // The rest of your handler code goes here
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}

The client must be initialized and used when processing requests, not during built-time initialization.

Events are enabled by default in the Fastly SDK, which means the Fastly SDK supports Experimentation and metrics use cases by default. To learn more, read Experimentation and metric events.

Evaluate a flag

After you initialize the client, wait for the waitForInitialization function to resolve. When waitForInitialization is resolved, the client can serve feature flags.

Using the client, you can check which variation a particular context will receive for a given feature flag. In your Fastly application, place the variation code so that it is invoked as needed.

Here is an example:

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import type { LDContext } from '@launchdarkly/js-server-sdk-common';
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const ldContext: LDContext = {
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  kind: 'org',
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  key: 'org-key-123abc',
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  someAttribute: 'example-attribute-value',
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}
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const flagValue = await ldClient.variation('flag-key-123abc', ldContext, false)

The SDK caches all KV data during initialization to reduce the number of backend requests needed to fetch KV data. This means changes to feature flags or segments will not be picked up during the lifecycle of a single request instance.

Flush events

You must flush events before your worker exits to ensure that they are sent back to LaunchDarkly. To learn more, read Flushing events.

Supported features

• Anonymous contexts and users
• Configuration
• Context configuration
• Evaluating flags
• Flag evaluation reasons
• Flushing events
• Getting all flags
• Logging configuration
• Secure mode
• Sending custom events