private static ChainSpecification getChainSpec(ComponentRegistry<DocprocChain> allChains, ComponentSpecification componentSpec) {
        DocprocChain docprocChain = allChains.getComponent(componentSpec);
        if (docprocChain == null) throw new IllegalArgumentException("Chain '" + componentSpec + "' not found.");

        return docprocChain.getChainSpecification();
    }
Returns indexing cluster implicitly created by this,
or empty if an explicit cluster is used (or if called before the build phase)
config-model/src/main/java/com/yahoo/vespa/model/content/Content.java