**Use case**

Currently, `system.replicas` table has `total_replicas` and `active_replicas` columns which are useful to check/alert when some replicas are down. One problem, these columns are numbers. They tell when there is a problem, but you need to further check ZooKeeper or query all the nodes to detect what exact replicas (hosts) are down/inactive/have problems.

**Describe the solution you'd like**

Add a replicas column maybe as a map which would contain the replica name, and it's status. Or, add 2 array columns with `replica_names` and `active_replica_names`

**Alternative**

Something along the line

```sql
select path, value from system.zookeeper where path IN (
    select concat(path, '/', name) from system.zookeeper where path in (select concat(zookeeper_path, '/replicas') from system.replicas)
)
and name = 'is_active'
```
void queueUpdatingTask();
    void mutationsUpdatingTask();
    /** Clone data from another replica.
      * If replica can not be cloned throw Exception.
      */
    void cloneReplica(const String & source_replica, Coordination::Stat source_is_lost_stat, zkutil::ZooKeeperPtr & zookeeper);
    
    void cloneMetadataIfNeeded(const String & source_replica, const String & source_path, zkutil::ZooKeeperPtr & zookeeper);
    
    void cloneReplicaIfNeeded(zkutil::ZooKeeperPtr zookeeper);
    ReplicatedMergeTreeQueue::SelectedEntryPtr selectQueueEntry();
    bool processQueueEntry(ReplicatedMergeTreeQueue::SelectedEntryPtr entry);
    
    
    
    void enterLeaderElection();
    
    
    
    void exitLeaderElection();
    /** Selects the parts to merge and writes to the log.
      */
    void mergeSelectingTask();
    
    void mutationsFinalizingTask();
    /** Write the selected parts to merge into the log,
      * Call when merge_selecting_mutex is locked.
      * Returns false if any part is not in ZK.
      */
    enum class CreateMergeEntryResult { Ok, MissingPart, LogUpdated, Other };