OK, makes sense.
On 8/31/2016 1:15 AM, Flavio Junqueira wrote:
Ok, I think I get what you're saying. Perhaps you're missing that this is an
issue that Guy encountered in 3.4.6 and that is fixed in a later release. We
are discussing here a workaround for his 3.4.6 deployment, not a permanent
solution. Does it make sense?
-Flavio
On 31 Aug 2016, at 01:16, David Brower <[email protected]> wrote:
You'd be programming iptables to pass/accept things from a whitelist of peers
you're willing to talk with.
If you've got such a whitelist, you don't need to program iptables to look at
the peer address from a packet/socket and drop it, you can just do it in your
message processing code.
The second part deals with various hang situations. If you've got a critical
thread selecting/reading messages, then it can't wait forever for a stuck read
(or write). Every operation needs to be timed out in some fashion to prevent
things like hung election thread.
You get into this sort of thing when miscreants or PEN-testers start scanning
your open ports and sending you malformed or fuzzed packets that you don't
handle cleanly, or start some exchange that they don't complete.
-dB
Oracle RAC Database and Clusterware Architect
On 8/30/2016 4:54 PM, Flavio Junqueira wrote:
I'm not sure what you're suggesting, David. Could you be more specific, please?
-Flavio
On 30 Aug 2016, at 23:54, David Brower <[email protected]> wrote:
Anything you could do with iptables you can do in the process by having it drop
connections from things not on a whitelist, and not having a thread waiting
indefinitely for operations from any connection.
-dB
On 8/30/2016 2:46 PM, Flavio Junqueira wrote:
I was trying to write down an analysis and I haven't been able to come up with
anything that is foolproof. Basically, the two main issues are:
- A bad server is able to connect to a good server in the case it has a message
outstanding and is trying to establish a connection to the good server. This
happens if the server is LOOKING or has an outstanding message from the
previous round. The converse isn't true, though. A good server can't start a
connection to a bad server because the bad server doesn't have a listener.
- If we bounce servers sequentially, there is a chance that a bad server is
elected more than once along the process, which induces multiple leader
election rounds.
Perhaps this is overkill, but I was wondering if it makes sense to filter
election traffic to and from bad servers using, for example, iptables. The idea
is to a rule that are local to each server preventing the server to get
connections established for leader election. For each bad server, we stop it,
remove the rule, and bring it back up. We also stop a minority first before
stoping the bad leader.
-Flavio
On 29 Aug 2016, at 09:29, Guy Laden <[email protected]> wrote:
Hi Flavio, Thanks for your reply. The situation is that indeed all the
servers are in a bad state so it looks like we will have to perform a
cluster restart.
We played with attempts to optimize the downtime along the lines you
suggested. In testing it we ran into the issue where a server with no
Listener thread can initiate a leader election connection to a
newly-restarted server that does have a Listener. The result is a quorum
that may include 'bad' servers, even a 'bad' leader. So we tried to first
restart the higher-id servers, because lower-id servers will drop their
leader-election connections to higher id servers.
I'm told there are issues with this flow as well but have not yet
investigated the details.
I also worry about the leader-election retries done with exponential
backoff.
I guess we will play with things a bit more but at this point I am tending
towards a simple parallel restart of all servers..
Once the clusters are healthy again we will do a rolling upgrade to 3.4.8
sometime soon.
Thanks again,
Guy
On Sun, Aug 28, 2016 at 5:52 PM, Flavio Junqueira <[email protected]> wrote:
Hi Guy,
We don't have a way to restart the listener thread, so you really need to
bounce the server. I don't think there is a way of doing this without
forcing a leader election, assuming all your servers are in this bad state.
To minimize downtime, one thing you can do is to avoid bouncing the current
leader until it loses quorum support. Once it loses quorum support, you
have a quorum of healthy servers and they will elect a new, healthy leader.
At the point, you can bounce all your unhealthy servers.
You may also want to move to a later 3.4 release.
-Flavio
On 24 Aug 2016, at 23:15, Guy Laden <[email protected]> wrote:
Hi all,
It looks like due to a security scan sending "bad" traffic to the leader
election port, we have clusters in which
the leader election Listener thread is dead (unchecked exception was
thrown
and thread died - seen in the log).
(This seems to be fixed by fixed in
https://issues.apache.org/jira/browse/ZOOKEEPER-2186)
In this state, when a healthy server comes up and tries to connecnt to
the
quorum, it gets stuck on
the leader election. It establishes TCP connections to the other servers
but any traffic it sends seems
to get stuck in the receiver's TCP Recv queue (seen with netstat), and is
not read/processed by zk.
Not a good place to be :)
This is with 3.4.6
Is there a way to get such clusters back to a healthy state without loss
of
quorum / client impact?
Some way of re-starting the listener thread? or restarting the servers
in a
certain order?
e.g. If I restart a minority, say the ones with lower server id's - is
there a way to get the majority servers
to re-initiate leader election connections with them so as to connect
them
to the quorum? (and to do this without
the majority losing quorum).
Thanks,
Guy
