Principle:DataExpert io Data engineer handbook AB Test SDK Initialization
Overview
A/B Test SDK Initialization is the foundational step in setting up a server-side experimentation framework. Before any experiment variant can be served or any user can be bucketed into a test group, the experimentation SDK must be initialized with proper credentials and configuration.
Theory of A/B Testing SDK Initialization
In server-side A/B testing, the experimentation platform's SDK acts as the bridge between your application and the remote experimentation service. Initialization is the process by which the SDK:
- Authenticates with the experimentation platform using a server-side API key
- Downloads the current experiment configurations (feature flags, experiment definitions, rollout rules)
- Caches these configurations locally for fast, low-latency lookups during request handling
- Establishes the connection for event logging and data synchronization
This initialization is a one-time setup that occurs at application startup, before the server begins handling requests.
Server-Side SDK Initialization with API Key
The SDK is initialized by providing a secret API key, typically loaded from an environment variable. This key identifies your project within the experimentation platform and grants the server permission to:
- Fetch experiment configurations
- Evaluate user bucketing rules
- Log events back to the platform
Application Startup
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Load API Key from Environment
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SDK.initialize(api_key)
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SDK fetches configs from remote service
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SDK ready — server can begin handling requests
One-Time Setup at Application Startup
The initialization call is made once when the application starts, not per-request. This is critical for performance:
- The SDK operates as a module-level singleton — once initialized, it maintains state for the lifetime of the process
- Experiment configurations are cached in memory, so individual experiment lookups are fast
- The SDK periodically refreshes configurations in the background, ensuring experiments stay up to date without requiring re-initialization
SDK Manages Experiment Configurations and User Bucketing Rules
After initialization, the SDK holds all the information needed to assign users to experiment groups:
- Experiment definitions: which experiments are active, their variant names, and parameter values
- Bucketing rules: the logic that determines which users see which variant (e.g., hash-based assignment, targeting rules, percentage rollouts)
- Feature flags: on/off switches that may gate experiment eligibility
When to Apply
This principle applies when:
- Setting up a web server that serves experiment variants to users
- Building a backend service that needs to make experiment-informed decisions
- Initializing any server-side application that participates in A/B testing
Theoretical Basis
The design follows two key patterns:
- Client-Server SDK Pattern: The server-side SDK acts as a client to the experimentation platform's API. It abstracts away the complexity of configuration fetching, caching, and synchronization, exposing a simple interface for experiment evaluation.
- Lazy Evaluation of Experiment Assignments: While the SDK eagerly fetches configurations at startup, actual user assignment (bucketing) is lazy — it only happens when a specific experiment is queried for a specific user. This means initialization is lightweight and does not pre-compute assignments for all possible users.