React Native

Eppo's open source React Native SDK can be used for both feature flagging and experiment assignment:

Getting Started

Installation

You can install the SDK with Yarn or NPM:

Yarn
NPM

yarn add @eppo/react-native-sdk

npm install @eppo/react-native-sdk

Usage

Begin by initializing a singleton instance of Eppo's client with an SDK key from the Eppo dashboard. Once initialized, the client can be used to make assignments anywhere in your app.

Initialize once

import { init } from "@eppo/react-native-sdk";

await init({ apiKey: "<SDK-KEY>" });

Assign anywhere

import * as EppoSdk from "@eppo/react-native-sdk";

const eppoClient = EppoSdk.getInstance();
const user = getCurrentUser();

const variation = eppoClient.getBooleanAssignment('show-new-feature', user.id, { 
  'country': user.country,
  'device': user.device,
}, false);

Flags and other features become available to the SDK after initialization.

After initialization, the SDK begins polling Eppo’s API at regular intervals to retrieve the most recent experiment configurations (variation values, traffic allocation, etc.). You can customize initialization and polling preferences by passing in additional initialization options.

The SDK stores these configurations in memory so that assignments thereafter are effectively instant. For more information, see the architecture overview page.

Connecting an event logger

Eppo is architected so that raw user data never leaves your system. As part of that, instead of pushing subject-level exposure events to Eppo's servers, Eppo's SDKs integrate with your existing logging system. This is done with a logging callback function defined at SDK initialization.

await init({
  apiKey: "<SDK_KEY>",
  assignmentLogger,
});

This logger takes an analytic event created by Eppo, assignment, and writes in to a table in the data warehouse (Snowflake, Databricks, BigQuery, or Redshift). You can read more on the Event Logging page.

The code below illustrates an example implementation of a logging callback using Segment. You could also use your own logging system, the only requirement is that the SDK receives a logAssignment function. Here we define an implementation of the Eppo `IAssignmentLogger interface containing a single function named logAssignment:

import { IAssignmentLogger } from "@eppo/react-native-sdk";
import { AnalyticsBrowser } from "@segment/analytics-next";

// Connect to Segment (or your own event-tracking system)
const analytics = AnalyticsBrowser.load({ writeKey: "<SEGMENT_WRITE_KEY>" });

const assignmentLogger: IAssignmentLogger = {
  logAssignment(assignment) {
    analytics.track({
      userId: assignment.subject,
      event: "Eppo Randomized Assignment",
      type: "track",
      properties: { ...assignment },
    });
  },
};

Deduplicating assignment logs

Eppo's SDK uses an internal cache to ensure that duplicate assignment events are not logged to the data warehouse. While Eppo's analytic engine will automatically deduplicate assignment records, this internal cache prevents firing unnecessary events and can help minimize costs associated with event logging.

Getting variations

Now that the SDK is initialized and connected to your event logger, you can check what variant a specific subject (typically user) should see by calling the get<Type>Assignment functions.

For example, for a string-valued flag, use getStringAssignment:

import * as EppoSdk from "@eppo/react-native-sdk";

const eppoClient = EppoSdk.getInstance();

// replace this with your own user object
const user = getCurrentUser();

const variation = eppoClient.getStringAssignment(
  'show-new-feature', // flagKey
  user.id, // subjectKey
  {'country': user.country, 'device': user.device}, // subjectAttributes
  'default-value' // defaultValue
)

Note that Eppo uses a unified API for feature gates, experiments, and mutually exclusive layers. This makes it easy to turn a flag into an experiment or vice versa without having to do a code release.

The getStringAssignment function takes four inputs to assign a variation:

flagKey - The key for the flag you are evaluating. This key is available on the feature flag detail page (see below).
subjectKey - A unique identifier for the subject being experimented on (e.g., user), typically represented by a UUID. This key is used to deterministically assign subjects to variants.
subjectAttributes - A map of metadata about the subject used for targeting. If targeting is not needed, pass in an empty object.
defaultValue - The value that will be returned if no allocation matches the subject, if the flag is not enabled, if getStringAssignment is invoked before the SDK has finished initializing, or if the SDK was not able to retrieve the flag configuration. Its type must match the get<Type>Assignment call.

Typed assignments

Every Eppo flag has a return type that is set once on creation in the dashboard. Once a flag is created, assignments in code should be made using the corresponding typed function:

getBooleanAssignment(...)
getNumericAssignment(...)
getIntegerAssignment(...)
getStringAssignment(...)
getJSONAssignment(...)

Each function has the same signature, but returns the type in the function name. The only exception is defaultValue, which should be the same type as the flag. For boolean flags for instance, you should use getBooleanAssignment, which has the following signature:

getBooleanAssignment: (
  flagKey: string,
  subjectKey: string,
  subjectAttributes: Record<string, any>,
  defaultValue: boolean,
) => boolean

To read more about different flag types, see the Flag Variations page.

Advanced Options

Initialization options

The init function accepts the following optional configuration arguments.

Option	Type	Description	Default
`assignmentLogger`	IAssignmentLogger	A callback that sends each assignment to your data warehouse. Required only for experiment analysis. See example below.	`null`
`requestTimeoutMs`	number	Timeout in milliseconds for HTTPS requests for the experiment configurations.	`5000`
`numInitialRequestRetries`	number	Number of additional times the initial configurations request will be attempted if it fails. This is the request typically synchronously waited (via `await`) for completion. A small wait will be done between requests.	`1`
`pollAfterSuccessfulInitialization`	boolean	Poll for new configurations (every 30 seconds) after successfully requesting the initial configurations.	`false`
`pollAfterFailedInitialization`	boolean	Poll for new configurations even if the initial configurations request failed.	`false`
`throwOnFailedInitialization`	boolean	Throw an error (reject the promise) if unable to fetch initial configurations during initialization.	`true`
`numPollRequestRetries`	number	If polling for updated configurations after initialization, the number of additional times a request will be attempted before giving up. Subsequent attempts are done using an exponential backoff.	`7`

Appendix

Usage in React

For usage in React, we recommend using the below EppoRandomizationProvider at the root of your component tree. By default, this component waits for initialization of the SDK before rendering its children. If waitForInitialization is set to false, the SDK getStringAssignment function will return null assignments while initializing and will only start assigning subjects when a new browser session is started.

import { useEffect, useState } from "react";

import { init } from "@eppo/react-native-sdk";

interface IEppoRandomizationProvider {
  waitForInitialization?: boolean;
  children: JSX.Element;
  loadingComponent?: JSX.Element;
}

export default function EppoRandomizationProvider({
  waitForInitialization = true,
  children,
  loadingComponent = <div>Loading...</div>,
}: IEppoRandomizationProvider): JSX.Element {
  const [isInitialized, setIsInitialized] = useState(false);
  useEffect(() => {
    init({
      apiKey: "<YOUR-SDK-KEY>",
      assignmentLogger: {
        logAssignment(assignment) {
          // logging implementation
        },
      },
    }).then(() => {
      return setIsInitialized(true);
    });
  }, []);

  if (!waitForInitialization || isInitialized) {
    return children;
  }
  return loadingComponent;
}

After the SDK is initialized, you may assign variations from any child component of EppoRandomizationProvider. We recommend wrapping the SDK code in a useMemo hook to avoid invoking the assignment logic on every render:

<EppoRandomizationProvider>
  <MyComponent />
</EppoRandomizationProvider>

function MyComponent(): JSX.Element {
  const assignedVariation = useMemo(() => {
    const eppoClient = getInstance();
    return eppoClient.getStringAssignment("<FLAG-KEY>", "<SUBJECT-KEY>", <SUBJECT-ATTRIBUTES>, "<DEFAULT-VALUE>");
  }, []);

  return (
    <div>
      {assignedVariation === "<VARIATION-KEY>" && <p>Assigned control</p>}
    </div>
  );
}

Local Storage

The SDK uses @react-native-async-storage to store experiment configurations downloaded from Eppo. This makes lookup by the get<Type>Assignment functions very fast. The configuration data stored contains the experiment key, experiment variation values, and allocations.

Debugging

You may encounter a situation where a flag assignment produces a value that you did not expect. There are functions detailed here to help you understand how flags are assigned, which will allow you to take corrective action on potential configuration issues.

Assignment Logger Schema

The SDK will invoke the logAssignment function with an assignment object that contains the following fields:

Field	Description	Example
`experiment` (string)	An Eppo experiment key	"recommendation-algo-allocation-17"
`subject` (string)	An identifier of the subject or user assigned to the experiment variation	UUID
`variation` (string)	The experiment variation the subject was assigned to	"control"
`timestamp` (string)	The time when the subject was assigned to the variation	2021-06-22T17:35:12.000Z
`subjectAttributes` (map)	A free-form map of metadata about the subject. These attributes are only logged if passed to the SDK assignment function	`{ "country": "US" }`
`featureFlag` (string)	An Eppo feature flag key	"recommendation-algo"
`allocation` (string)	An Eppo allocation key	"allocation-17"

note

More details about logging and examples (with Segment, Rudderstack, mParticle, and Snowplow) can be found in the event logging page.

React Native

Getting Started​

Installation​

Usage​

Initialize once​

Assign anywhere​

Connecting an event logger​

Deduplicating assignment logs​

Getting variations​

Typed assignments​

Advanced Options​

Initialization options​

Appendix​

Usage in React​

Local Storage​

Debugging​

Assignment Logger Schema​