Federated queries

One of FeatureQL's core strengths is its ability to federate data from multiple heterogeneous sources within a single query. This enables building features that span across databases, caches, APIs, and other data systems seamlessly.

Cross-source data federation

This powerful example demonstrates federated querying across multiple data sources:

Data sources involved:

1. Redis: Contains order details (order:* keys with JSON data)
2. PostgreSQL: Contains customer information with last order references
3. FeatureQL: Orchestrates the join between these sources

Federation workflow:

1. Customer lookup: Query PostgreSQL for customer details and last_order_id
2. Order enrichment: Use the last_order_id to fetch order details from Redis
3. Cross-source join: ADD_FIELDS() links Redis order data to PostgreSQL customer data
4. Type-safe binding: Entity annotations (BIGINT#ORDERS, BIGINT#CUSTOMERS) ensure referential integrity

Key federation features:

• Multi-source joins: Combine data from different storage systems
• Type safety: Entity relationships are preserved across sources
• Performance: Each source is queried optimally using its native access patterns