[GitHub] [zookeeper] symat commented on a change in pull request #1228: ZOOKEEPER-3698: fixing NoRouteToHostException when starting large cluster locally

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symat commented on a change in pull request #1228: ZOOKEEPER-3698: fixing NoRouteToHostException when starting large cluster locally
URL: https://github.com/apache/zookeeper/pull/1228#discussion_r368575308
 
 

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 File path: zookeeper-docs/src/main/resources/markdown/zookeeperAdmin.md
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 @@ -1542,6 +1555,22 @@ the variable does.
     ZAB protocol and the Fast Leader Election protocol. Default
     value is **false**.
 
+* *multiAddress.reachabilityCheckEnabled* :
+    (Java system property: **zookeeper.multiAddress.reachabilityCheckEnabled**)
+    **New in 3.6.0:**
+    Since ZooKeeper 3.6.0 you can also [specify multiple addresses](#id_multi_address) 
+    for each ZooKeeper server instance (this can increase availability when multiple physical 
+    network interfaces can be used parallel in the cluster). ZooKeeper will perform ICMP ECHO requests
+    or try to establish a TCP connection on port 7 (Echo) of the destination host in order to find 
+    the reachable addresses. This happens only if you provide multiple addresses in the configuration.
+    The reachable check can fail if you hit some ICMP rate-limitation, (e.g. on MacOS) when you try to 
+    start a large (e.g. 11+) ensemble members cluster on a single machine for testing. 
+    
+    Default value is **true**. By setting this parameter to 'false' you can disable the reachability checks. 
+    Please note, disabling the reachability check will cause the cluster not to be able to reconfigure 
+    itself properly during network problems, so the disabling is advised only during testing. 
 
 Review comment:
   Thanks for checking! :)
   
   The whole purpose of the multi-address feature is to always try to use an address which works. The current implementation is (in case of the leader election) always filters the address list using `InetAddress.isReachable()` calls to find out which is the working server address. This will cause ICMP calls (or TCP connections on port 7 (Echo) of the destination host), depending on the native implementation (see: https://docs.oracle.com/javase/7/docs/api/java/net/InetAddress.html#isReachable(int) )
   
   So if the `InetAddress.isReachable` can not reach the host, then the current multi-address feature will not able to take the given address as a working one. Basically right now it can not distinguish between the case of a broken network link (when the whole node is unreachable) and the case of a disabled ICMP (when only the ICMP port and the port 7 is disabled in the firewall of the destination host). I am not an expert in cluster / firewall operation, so I can not tell how serious is this limitation.
   
   One way to improve this could be to implement something like the `ruok` 4LW command for the server ports. Some simple request-response messages that only shows that the server is alive and listen on the given election / quorum port. Then we could use that instead of the ICMP calls. I think this would be a reasonable improvement, but maybe more like a separate task, out of the scope of 3.6.0.
   
   What do you think?
   
   (also: do you think I should extend the documentation, or you just wanted to elaborate here in the PR?)

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