Wait for the initial endpoints of a CentralDogma client for Spring integrations

[ikhoon]

A CentralDogma client asynchronously fetches the initial endpoints using DynamicEndpointGroup.

centraldogma/client/java-armeria/src/main/java/com/linecorp/centraldogma/client/armeria/AbstractArmeriaCentralDogmaBuilder.java

Line 140 in a74df46

HealthCheckedEndpointGroup.builder(group, HttpApiV1Constants.HEALTH_CHECK_PATH)

centraldogma/client/java-armeria/src/main/java/com/linecorp/centraldogma/client/armeria/AbstractArmeriaCentralDogmaBuilder.java

Line 163 in a74df46

final DnsAddressEndpointGroupBuilder dnsAddressEndpointGroup = DnsAddressEndpointGroup

The created CentralDogma client can't get/watch stored documentation from Central Dogma replicas if the endpoints are not resolved.
If CentralDogma.whenReadyEndpoint().get(..) is not called, CentralDogma waits 3.2 seconds for endpoints to be completed.
https://github.com/line/armeria/blob/790a2db01a8514b591bde90932ab18f63693829e/core/src/main/java/com/linecorp/armeria/client/DefaultClientRequestContext.java#L320
3.2 seconds is the default connection timeout for an Armeria client.
While a server is starting, it could take longer than 3.2 seconds because the server is busy initializing the application context.

A CentralDogma bean is a managed instance provided by centraldogma-client-spring-boot-autoconfigure.
So the CentralDogma bean is created in the main thread when a Spring application context is initialized.
We can safely block the current thread with CentralDogma.whenReadyEnpoint().get(...).
For example:

class CentralDogmaSettings {
    private long initializationTimeoutMillis = 15000;
}

class CentralDogmaClientAutoConfiguration {
    @Bean
    public CentralDogma dogmaClient(...) {
        ...

        if (initializationTimeoutMillis > 0) {
            centralDogma.whenReadyEndpoint().get(initializationTimeoutMillis, ...);
        }
        return centralDogma;
    }
}