private static long requirePositive(long generation) {
        if (generation <= 0)
            throw new IllegalArgumentException("Generation must be positive, but was " + generation);

        return generation;
    }
Reindexing status common to an application, one of its clusters, or a single document type in a cluster.
configserver/src/main/java/com/yahoo/vespa/config/server/application/ApplicationReindexing.java