public DeploymentInstanceSpec requireInstance(InstanceName name) {
        Optional<DeploymentInstanceSpec> instance = instance(name);
        if (instance.isEmpty())
            throw new IllegalArgumentException("No instance '" + name + "' in deployment.xml. Instances: " +
                                               instances().stream().map(spec -> spec.name().toString()).collect(joining(",")));
        return instance.get();
    }
Returns the instance step containing the given instance name
config-model-api/src/main/java/com/yahoo/config/application/api/DeploymentSpec.java