This version is still in development and is not considered stable yet.

Endpoint URL Property Placeholders

The pipelite-spring-starter module resolves ${property.key} placeholders embedded in the URL of a flow’s source (fromSource(…)) and sink (toSink(…)), using the Spring Environment as the value source at context startup time.

Overview

Endpoint URLs in Pipelite flows are plain strings such as kafka://orders or queue://orders. In production deployments the target resource names often vary between environments (dev, staging, production) and must be externalised to configuration files or environment variables.

By embedding standard Spring property placeholders directly in the URL string, you can keep flow definitions deployment-agnostic:

@DefineFlow
public FlowDefinition orderFlow() {
    return Pipelite.defineFlow("order-flow")
        .fromSource("kafka://${app.kafka.topic.orders}")    (1)
        .toSink("slf4j://order-log")
        .build();
}

1	`${app.kafka.topic.orders}` is resolved against the Spring `Environment` before the URL is parsed.

application.properties:

app.kafka.topic.orders=orders-v2

At startup the context sees kafka://orders-v2. The same works in toSink(…).

All standard Spring Environment sources are supported: application.properties, application.yml, OS environment variables, JVM system properties, and any custom PropertySource.

Resolution at startup

Placeholder resolution happens once, eagerly, during PipeliteContext.start(), just before each endpoint URL is parsed. The resolved value becomes the permanent key under which the flow is registered in the internal registry — not the raw placeholder string. This means that routing and lookups (e.g. PipeliteContext.supplyExchange("queue://<name>", …)) must use the resolved name, not the original ${…} literal.

Fail-fast behaviour

If a placeholder cannot be resolved — because the property is absent from every Environment source — Pipelite propagates the failure immediately during startup with an IllegalArgumentException. The application context will not start with an unresolved placeholder. There is no fallback to a literal ${key} value.

Where placeholders are not resolved

Only the URLs of fromSource(…) and toSink(…) are resolved. A ${…} placeholder written anywhere else in a flow definition is left as it is, and the first time a message is delivered there it fails with Malformed endpoint url '${…}':

the URL of a wireTap(…);
the destinations of toRoute(…) and toRecipientList(…);
the target of an error channel, err.toChannel(…).

For these, write the URL literally. A routing destination that depends on the message can use a #{…} expression instead (see Routing).

Plain-Java usage

Without pipelite-spring-starter, the default resolver is a no-op: placeholders in URL strings are not substituted and reach the URL parser unchanged, causing a startup failure (see Fail-fast behaviour).

To use dynamic URLs outside Spring, supply a custom EndpointURLPropertyResolver when creating the context:

EndpointURLPropertyResolver resolver =
    rawUrl -> rawUrl.replace("${key}", System.getenv("MY_SOURCE_NAME")); (1)

PipeliteContext context = Pipelite.createContext(resolver);

1	Any substitution logic can be used here; the resolved URL is passed directly to the endpoint parser.