private MockId getIdToDelete(Set<MockId> ids, boolean ttlUpdated) {
    MockId deleteCandidate = null;
    for (MockId id : ids) {
      if (liveKeys.contains(id) && ttlUpdated == ttlUpdatedKeys.contains(id)) {
        deleteCandidate = id;
        break;
      }
    }
    if (deleteCandidate == null) {
      throw new IllegalStateException("Could not find a key to delete in set: " + ids);
    }
    deletedKeys.add(deleteCandidate);
    liveKeys.remove(deleteCandidate);
    return deleteCandidate;
  }
Gets an id to delete from {@code ids} by picking the first live key encountered.
@param ids the {@link MockId}s to choose from
@param ttlUpdated {@code true} if the ID returned has to have undergone a TTL update. {@code false} if the ID
                              returned should NOT have gone through a TTL update
@return an id to delete from {@code ids}
ambry-store/src/test/java/com/github/ambry/store/BlobStoreTest.java