Github user jasobrown commented on a diff in the pull request:

    https://github.com/apache/cassandra/pull/191#discussion_r168316916
  
    --- Diff: src/java/org/apache/cassandra/net/MessagingService.java ---
    @@ -1664,4 +1676,113 @@ public static boolean 
isEncryptedConnection(InetAddressAndPort address)
             }
             return true;
         }
    +
    +    public void blockForPeers()
    +    {
    +        // TODO make these yaml props?
    +        int alivePercent = Integer.getInteger(Config.PROPERTY_PREFIX + 
"blockForPeers.percent", 70);
    +        if (alivePercent < 0)
    +            alivePercent = 0;
    +        else if (alivePercent > 100)
    +            alivePercent = 100;
    +
    +        int aliveTimeoutSecs = Integer.getInteger(Config.PROPERTY_PREFIX + 
"blockForPeers.timeout_in_secs", 10);
    +        if (aliveTimeoutSecs < 0)
    +            aliveTimeoutSecs = 1;
    +        else if (aliveTimeoutSecs > 100)
    +            aliveTimeoutSecs = 100;
    +
    +        if (alivePercent > 0)
    +            blockForPeers(alivePercent, aliveTimeoutSecs);
    +    }
    +
    +    private void blockForPeers(int targetAlivePercent, int 
aliveTimeoutSecs)
    +    {
    +        // grab a snapshot of the current cluster from Gossiper. this is 
completely prone to race conditions, but it's
    +        // good enough for the purposes of blocking until some certain 
percentage of nodes are considered 'alive'/connected.
    +        Set<Map.Entry<InetAddressAndPort, EndpointState>> peers = new 
HashSet<>(Gossiper.instance.getEndpointStates());
    +
    +        // remove current node from the set
    +        peers = peers.stream()
    +                     .filter(entry -> 
!entry.getKey().equals(FBUtilities.getBroadcastAddressAndPort()))
    +                     .collect(Collectors.toSet());
    +
    +        final int totalSize = peers.size();
    +
    +        // don't block if there's no other nodes in the cluster (or we 
don't know about them)
    +        if (totalSize <= 1)
    +            return;
    +
    +        logger.info("choosing to block until {}% of peers are marked 
alive; max time to wait = {} seconds", targetAlivePercent, aliveTimeoutSecs);
    +
    +        // first, send out a ping message to open up the non-gossip 
connections
    +        AtomicInteger connectedCount = sendPingMessages(peers);
    --- End diff --
    
    >> Do the connections consume resources if there are no queued messages?
    
    Reviewing `OutboundMessageConnection`, no, they do not consume much in 
terms of resources. I'm fine either way creating the large message connections 
eagerly or lazily, so I'll add that in.
    
    >> do we even need to send a message or is just instructing the transport 
system to open them enough?
    
    Because connections are unidirectional, and as we'll want each "pair" of 
outbound/inbound connections for each connection type (gossip, small message, 
large message) to be established, the only way to do that is to send a 'real' 
message that the peer can respond to. That being said, perhaps I can work in 
something that lets a Ping/Pong message "select" which type of transport it 
wants to be sent on.
    



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