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From "Joseph Lynch (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-14358) OutboundTcpConnection can hang for many minutes when nodes restart
Date Tue, 03 Apr 2018 18:01:00 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-14358?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16424369#comment-16424369
] 

Joseph Lynch commented on CASSANDRA-14358:
------------------------------------------

[~djoshi3] We send RSTs on listen overflows via the {{tcp_abort_on_overflow}} sysctl, so
I don't think that this is a simple listen overflow. You're absolutely right that nodes coming
up can get overwhelmed in 2.1/3.0/3.11, and I think it's really great that trunk fixes that;
but I don't think this is the bug I've identified here. In this case a Cassandra node that
_didn't_ restart continues trying to send on a dead connection forever until the kernel steps
in and cuts off the connection.

 [~jasobrown]
{quote}For trunk, we already use IdleStateHandler in the outbound pipeline for the "not making
progress sending data" case. Take a look at MessageOutHandler#userEventTriggered. iirc, WriteTimeoutHandler
/ ReadTimeoutHandler does not fit our use case for messaging; I'd have to look again to page
it back in.
{quote}
I am definitely not a netty expert, but I think that {{IdleStateHandler}} needs to be paired
with a {{WriteTimeoutHandler}} or {{ReadTimeoutHandler}} in order to effectively identify
such blackhole partitions. The app is sending data on the socket, it's just never going anywhere.
My understanding is that {{IdleStateHandler}} just sends data if there is none.
{quote}Further, Joseph Lynch, I thought your problem was in pre-4.0? There is a problem (or,
at least, non-optimization) with MessagingService.SocketThread where the single-threaded accept
thread handles receiving the first message pf the c* internode messaging protocol, which may
require executing the TLS handshake before that can be received (if using internode TLS).
Thus a single new establishing connection may block the accept thread for 500-1000 ms for
a cross-datacenter connection. Even worse, if you are using the Ec2MultiregionSnitch, all
connections for the local region will be torn down and built anew as we switch from the public
IP to the region-local private IP - more traffic on the accept thread. So one slow connection
interaction screws the whole accept thread.
{quote}
Yes, our problem is with pre-4.0, but I'm 99% sure we can mitigate this by setting {{tcp_retries2}}
to 3 or 5 to tell the OS to cover over the app bug. If Cassandra trunk is resilient to the
(rare) bug I think the sysctl is a reasonable workaround for earlier versions, what do you
think? It's also worth noting that this kind of partition probably only occurs frequently
in stateful networks that swallow RSTs to unknown flows such as AWS VPC (although as [~aweisberg]
points out, power failure could manifest similarly).
{quote}I have an internal patch for 3.0 which, when a new socket is accepted, moves all processing
(internode protocol handshake, TLS handshake) to a separate thread (via an executor group).
So, if one connection runs into a problem, the other incoming connections are not held up.
I've been reticent about OSS'ing this late in the 3.0/3.11 branches, even though it's not
that clever or invasive. Let me know if you'd be interested in giving it shot and I can post
it. If it's useful/helpful we can consider committing it, as well.
{quote}
I don't think that would fix this particular bug, but it is awesome and fixes other issues.
I volunteer to test it and review it if that helps get it merged to 3.0/3.11.

> OutboundTcpConnection can hang for many minutes when nodes restart
> ------------------------------------------------------------------
>
>                 Key: CASSANDRA-14358
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-14358
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Streaming and Messaging
>         Environment: Cassandra 2.1.19 (also reproduced on 3.0.15), running with {{internode_encryption:
all}} and the EC2 multi region snitch on Linux 4.13 within the same AWS region. Smallest
cluster I've seen the problem on is 12 nodes, reproduces more reliably on 40+ and 300 node
clusters consistently reproduce on at least one node in the cluster.
> So all the connections are SSL and we're connecting on the internal ip addresses (not
the public endpoint ones).
> Potentially relevant sysctls:
> {noformat}
> /proc/sys/net/ipv4/tcp_syn_retries = 2
> /proc/sys/net/ipv4/tcp_synack_retries = 5
> /proc/sys/net/ipv4/tcp_keepalive_time = 7200
> /proc/sys/net/ipv4/tcp_keepalive_probes = 9
> /proc/sys/net/ipv4/tcp_keepalive_intvl = 75
> /proc/sys/net/ipv4/tcp_retries2 = 15
> {noformat}
>            Reporter: Joseph Lynch
>            Priority: Major
>         Attachments: 10 Minute Partition.pdf
>
>
> I've been trying to debug nodes not being able to see each other during longer (~5 minute+)
Cassandra restarts in 3.0.x and 2.1.x which can contribute to {{UnavailableExceptions}} during
rolling restarts of 3.0.x and 2.1.x clusters for us. I think I finally have a lead. It appears
that prior to trunk (with the awesome Netty refactor) we do not set socket connect timeouts
on SSL connections (in 2.1.x, 3.0.x, or 3.11.x) nor do we set {{SO_TIMEOUT}} as far as I can
tell on outbound connections either. I believe that this means that we could potentially block
forever on {{connect}} or {{recv}} syscalls, and we could block forever on the SSL Handshake
as well. I think that the OS will protect us somewhat (and that may be what's causing the
eventual timeout) but I think that given the right network conditions our {{OutboundTCPConnection}}
threads can just be stuck never making any progress until the OS intervenes.
> I have attached some logs of such a network partition during a rolling restart where
an old node in the cluster has a completely foobarred {{OutboundTcpConnection}} for ~10 minutes
before finally getting a {{java.net.SocketException: Connection timed out (Write failed)}}
and immediately successfully reconnecting. I conclude that the old node is the problem because
the new node (the one that restarted) is sending ECHOs to the old node, and the old node is
sending ECHOs and REQUEST_RESPONSES to the new node's ECHOs, but the new node is never getting
the ECHO's. This appears, to me, to indicate that the old node's {{OutboundTcpConnection}} thread
is just stuck and can't make any forward progress. By the time we could notice this and slap
TRACE logging on, the only thing we see is ~10 minutes later a {{SocketException}} inside
{{writeConnected}}'s flush and an immediate recovery. It is interesting to me that the exception
happens in {{writeConnected}} and it's a _connection timeout_ (and since we see {{Write failure}} I
believe that this can't be a connection reset), because my understanding is that we should
have a fully handshaked SSL connection at that point in the code.
> Current theory:
>  # "New" node restarts,  "Old" node calls [newSocket|https://github.com/apache/cassandra/blob/6f30677b28dcbf82bcd0a291f3294ddf87dafaac/src/java/org/apache/cassandra/net/OutboundTcpConnection.java#L433]
>  # Old node starts [creating a new|https://github.com/apache/cassandra/blob/6f30677b28dcbf82bcd0a291f3294ddf87dafaac/src/java/org/apache/cassandra/net/OutboundTcpConnectionPool.java#L141] SSL
socket 
>  # SSLSocket calls [createSocket|https://github.com/apache/cassandra/blob/cassandra-3.11/src/java/org/apache/cassandra/security/SSLFactory.java#L98],
which conveniently calls connect with a default timeout of "forever". We could hang here forever
until the OS kills us.
>  # If we continue, we get to [writeConnected|https://github.com/apache/cassandra/blob/6f30677b28dcbf82bcd0a291f3294ddf87dafaac/src/java/org/apache/cassandra/net/OutboundTcpConnection.java#L263]
which eventually calls [flush|https://github.com/apache/cassandra/blob/6f30677b28dcbf82bcd0a291f3294ddf87dafaac/src/java/org/apache/cassandra/net/OutboundTcpConnection.java#L341]
on the output stream and also can hang forever. I think the probability is especially high
when a node is restarting and is overwhelmed with SSL handshakes and such.
> I don't fully understand the attached traceback as it appears we are getting a {{Connection
Timeout}} from a {{send}} failure (my understanding is you can only get a connection timeout
prior to a send), but I think it's reasonable that we have a timeout configuration issue. I'd
like to try to make Cassandra robust to networking issues like this via maybe:
>  # Change the {{SSLSocket}} {{getSocket}} methods to provide connection timeouts of 2s
(equivalent to trunk's [timeout|https://github.com/apache/cassandra/blob/11496039fb18bb45407246602e31740c56d28157/src/java/org/apache/cassandra/net/async/NettyFactory.java#L329])
>  # Appropriately set recv timeouts via {{SO_TIMEOUT}}, maybe something like 2 minutes
(in old versions via [setSoTimeout|https://docs.oracle.com/javase/8/docs/api/java/net/Socket.html#setSoTimeout-int-],
in trunk via [SO_TIMEOUT|http://netty.io/4.0/api/io/netty/channel/ChannelOption.html#SO_TIMEOUT]
>  # Since we can't set send timeouts afaik (thanks java) maybe we can have some kind of
watchdog that ensures OutboundTcpConnection is making progress in its queue and if it doesn't
make any progress for ~30s-1m, forces a disconnect.
> If anyone has insight or suggestions, I'd be grateful. I am going to rule out if this
is keepalive duration by setting tcp_keepalive_probes to like 1 and maybe tcp_retries2 to
like 8 get more information about the state of the tcp connections the next time this happens.
It's a very rare bug and when it does happen I only have 10 minutes to jump on the nodes and
fix it before it fixes itself so I'll do my best.



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