private static boolean assertEntryExists(GemFireCache cache, String regionName, int start,
      int end) {
    assertRegionExists(cache, regionName);
    Region region = getRegion(cache, regionName);
    for (int i = start; i < end; i++) {
      String key = "" + i;
      Object regionValue = region.get(key);
      if (regionValue == null) {
        fail("Entry for key:" + key + " 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