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Erlang SDK reference

Read time: 4 minutes
Last edited: Jun 14, 2024
Recent major versions

Version 3 of the Erlang SDK requires Gun 2.x. To learn more about upgrading, read Erlang (server-side) SDK 2.x to 3.0 migration guide.

Version 2 of the Erlang SDK replaces users with contexts. A context is a generalized way of referring to the people, services, machines, or other resources that encounter feature flags in your product. To learn more about upgrading, read Erlang SDK 1.x to 2.0 migration guide and Best practices for upgrading users to contexts.

Code samples on this page are from the three most recent SDK versions where they differ.


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

SDK quick links

LaunchDarkly's SDKs are open source. In addition to this reference guide, we provide source, API reference documentation, and sample applications:

SDK API documentationSDK API docs
GitHub repositoryerlang-server-sdk
Sample applicationsErlang
Published moduleHex

Get started

Follow the steps below to get started using the LaunchDarkly SDK in your Erlang application.

First, download the dependency using Rebar:

{deps, [
{ldclient, "3.0.0", {pkg, launchdarkly_server_sdk}}

Then, add it to your app.src file:


If you use Elixir, you can download the dependency using Mix:

defp deps do
{:ldclient, "~> 3.0.0", hex: :launchdarkly_server_sdk}
The Erlang SDK uses an SDK key

The Erlang SDK uses an SDK key. Keys are specific to each project and environment. They are available from the Environments list for each project. To learn more about key types, read Keys.

Initialize the client

After you install the SDK dependency, create an instance of the client. Specify your SDK key here to authorize your application to connect to a particular environment within LaunchDarkly.

Use a single instance

The Erlang SDK supports starting multiple instances, but most use cases only need a single instance. Consider using multiple instances only if you need to simultaneously access more than one environment. Do not start an instance every time you need to make a variation or other SDK call.

Here is an example:

% This starts an instance with the default options
% You can also start a named instance
ldclient:start_instance("sdk-key-123abc", your_instance)

To learn more about the specific configuration options available for this SDK, read ldclient_config.

Evaluate a context

Next, check which flag variation a specific context should receive:

Flag = ldclient:variation(<<"flag-key-123abc">>, #{key => <<"context-key-123abc">>}, false)

Transport Layer Security (TLS)

The SDK includes configuration options that allow you to set custom TLS options. If you don't set these options, then the SDK will use the Erlang/OTP defaults. The default TLS connection settings in Erlang/OTP do not validate the identity or authenticity of certificates.

If your application has existing TLS options, then you can pass them to the SDK:

ldclient:start_instance(SdkKey, #{
http_options => #{
tls_options => YourOptions

The SDK also provides helper methods to create TLS options. We recommend ensuring that the SDK is using a Certificate Authorities (CA) store that is regularly updated for production environments.

Here is a list of helper methods:

  • ldclient_config:tls_basic_options(): This helper provides a basic TLS configuration suitable for development. It tries to use a CA store in the default location for many linux distributions (/etc/ssl/certs/ca-certificates.crt). If it can't find the store, then it will use the store from the certifi package.
  • ldclient_config:tls_basic_linux_options(): This helper provides a basic TLS configuration for linux. It uses the CA store located at /etc/ssl/certs/ca-certificates.crt.
  • ldclient_config:tls_ca_certfile_options(CaStorePath): This helper provides a basic TLS configuration with the CA store you specify.
  • ldclient_config:tls_basic_certifi_options(): This helper provides a basic TLS configuration that uses the certifi store. Because this store is from a dependency of the package, it is not maintained or updated by OS releases.
  • ldclient_config:with_tls_revocation(TlsOptions): This helper extends a TLS configuration with certificate revocation. Revocation is not included in the basic configuration because the Erlang/OTP does not cache revocation results. Enabling this feature incurs additional requests per request the SDK makes.

Shut down the client

Shut down the client when your application terminates. To learn more, read Shutting down.

Supported features

This SDK supports the following features: