protected void checkCacheVersion() throws IOException {
        // Be very aggressive in invalidating the cache.
        boolean invalidate = true;
        GetResponse result = getClient().get(new GetRequest(getIndexPrefixWithJoiner() + EsConstants.INDEX_DATA_VERSION, "instance"), RequestOptions.DEFAULT);
        if (result.isExists()) {
            String latestDV = Long.toString(result.getVersion()) ; //$NON-NLS-1$
            if (latestDV != null && dataVersion != null && latestDV.equals(dataVersion)) {
                invalidate = false;
            } else {
                dataVersion = latestDV;
            }
        }
        if (invalidate) {
            invalidateCache();
        }
    }
Checks the ES store to see if the 'dataVersion' entry has been updated with a newer
version #.  If it has, then we need to invalidate our cache.
gateway/engine/es/src/main/java/io/apiman/gateway/engine/es/PollCachingEsRegistry.java