public String toString() {
        StringBuilder sb = new StringBuilder(poolName + ": ");
        synchronized (connections) {
            for (JRTConnection conn : connections.values()) {
                sb.append(conn.toString());
                sb.append("\n");
            }
        }
        return sb.toString();
    }
Preconditions:
1. the current connection is unhealthy and should not be selected when switching
2. There is more than 1 source.
config/src/main/java/com/yahoo/vespa/config/JRTConnectionPool.java