@Override
  public List<? extends PartitionId> getFullyWritablePartitionIds(String partitionClass) {
    throw new IllegalStateException();
  }
Implementation of {@link ClusterMap} that has well known structure and can provide peer lists via the
{@link #getPeers(String)} method for verification.
<p/>
Can also throw exceptions on demand.
ambry-frontend/src/test/java/com/github/ambry/frontend/GetPeersHandlerTest.java