void preprocess() {
            try {
                validateXmlFeatures(applicationPackage, logger);
                this.preprocessedApplicationPackage = applicationPackage.preprocess(zone, logger);
            } catch (IOException | RuntimeException e) {
                throw new IllegalArgumentException("Error preprocessing application package for " + applicationId +
                                                   ", session " + sessionZooKeeperClient.sessionId(), e);
            }
            checkTimeout("preprocess");
        }
The version of Vespa to build first when there are several config models, empty if latest version should be built first
configserver/src/main/java/com/yahoo/vespa/config/server/session/SessionPreparer.java