private static boolean assertRegionExists(GemFireCache cache, String regionName) {
    Region region = cache.getRegion(regionName);
    if (region == null) {
      throw new Error("Region: " + regionName + " does not exist");
    }
    return true;
  }
This test starts up multiple servers from the current code base and multiple servers from the
old version and executes puts and gets on a new server and old server and verifies that the
results are present. Note that the puts have overlapping region keys just to test new puts and
replaces
geode-core/src/upgradeTest/java/org/apache/geode/internal/cache/rollingupgrade/RollingUpgrade2DUnitTestBase.java