Open Host Service & Published Language

Also Known As

TL;DR

Explanation

Open Host Service (OHS) defines a clear API contract (the Published Language) that downstream contexts use directly — typically an OpenAPI spec, event schema, or protocol buffer definition. All downstream contexts integrate using this one stable protocol rather than each negotiating a custom translation. Distinguished from Customer-Supplier: OHS publishes a general interface rather than tailoring it to each downstream. Distinguished from Shared Kernel: OHS is one-directional — upstream publishes, downstream consumes without tight coupling.

Common Misconception

Why It Matters

Common Mistakes

• Published Language tightly coupled to the internal domain model — decouple the public API representation from internal implementation
• No versioning strategy for the Published Language — breaking changes break all consumers simultaneously
• OHS shaped by one consumer's needs — the OHS should be designed for all consumers
• Treating every outbound API as OHS — reserve for contexts with multiple consumers needing a stable general interface

Code Examples

// No Published Language — N custom integrations:
// Orders → Billing: custom internal format
// Orders → Shipping: different custom format
// Orders → Analytics: yet another format
// Adding new consumer: negotiate a new custom integration
// N consumers = N integrations to maintain forever

// Open Host Service — one stable Published Language:
// Orders publishes OpenAPI spec v1:
// GET /orders/{id} → { order_id, status, line_items, total }

// All consumers use the same protocol:
class BillingService {
    public function processOrder(int $orderId): void {
        $order = $this->ordersApi->getOrder($orderId); // Published Language
        // No custom translation layer needed
    }
}
// Adding new consumer: read the spec, integrate directly
// 1 protocol maintains instead of N

References

• ↗ https://www.domainlanguage.com/ddd/reference/

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