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Eppo's Rust SDK can be used for feature flagging and experiment assignment in Rust applications.

Getting started

Install the SDK

First, add the SDK as a dependency using Cargo:

cargo add eppo

Initialize the SDK

To initialize the SDK, you will need an SDK key. You can generate one in the Eppo interface.

let mut client = eppo::ClientConfig::from_api_key("<YOUR_API_KEY>")


This creates a client instance that can be reused throughout the application lifecycle, and starts a poller thread to periodically fetch latest feature flag configuration from Eppo server.

Assign variations

Assign users to flags or experiments using the appropriate get_*_assignment function, depending on the type of the flag. For example, to get a string value from a flag, use get_string_assignment:

let variation = client
.inspect_err(|err| eprintln!("error assigning a flag: {:?}", err))

if variation == "fast_checkout" {
// Handle fast checkout
} else {
// Handle other variations
  • <FLAG_KEY> is the key that you chose when creating a flag. You can find it in the Eppo interface.
  • <SUBJECT_KEY> is a value that identifies each entity in your experiment, typically a user ID.
  • <SUBJECT_ATTRIBUTES> is a HashMap of metadata about the subject used for targeting. If targeting rules are based on attributes, they must be passed in on every assignment call. If no attributes are needed, pass an empty HashMap.

That’s it: You can start changing the feature flag on the page and see how it controls your code!

However, if you want to run experiments, there’s a little extra work to configure it properly.

Assignment Logging for Experiments

If you are using the Eppo SDK for experiment assignment (i.e., randomization), we will need to know which subjects were exposed to experiment. For that, you will need to log the assignment information.

Eppo encourages centralizing application event logging as much as possible. Accordingly, instead of implementing a new event logging framework, Eppo's SDK integrates with your existing logging system via a logging callback function defined at SDK initialization.

The code below illustrates an example implementation of a logging callback to the console and other event platforms. You could also use your own logging system, the only requirement is that you should implement AssignmentLogger trait and pass the logger when initializing the SDK.

let mut client = ClientConfig::from_api_key("<YOUR_API_KEY>")

use eppo::AssignmentLogger;

struct LocalAssignmentLogger;

impl AssignmentLogger for LocalAssignmentLogger {
fn log_assignment(&self, assignment: AssignmentEvent) {
println!("{:?}", assignment);

Typed Assignment Calls

The SDK provides typed assignment functions for different types of values:

  • get_boolean_assignment
  • get_integer_assignment
  • get_numeric_assignment
  • get_string_assignment
  • get_json_assignment

All of these functions have the same input parameters as mentioned in the "Assign variations" section.

Example usage for a boolean assignment:

if client.get_boolean_assignment("<FLAG_KEY>", "<SUBJECT_KEY>", SUBJECT_ATTRIBUTES).unwrap_or_default().unwrap_or(false) {
// Handle true assignment
} else {
// Handle false assignment