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From Christopher Schultz <ch...@christopherschultz.net>
Subject Re: Most strict / conservative setting for SimpleTcpCluster - channelSendOptions
Date Tue, 25 Jul 2017 18:48:29 GMT
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Clemens,

On 7/25/17 9:54 AM, Clemens Eisserer wrote:
> Hi there,
> 
> What is the strictest / most conservative setting for 
> channelSendOptions when using SimpleTcpCluster for session
> replication (synchronous + ack + ??) ? I have a web-app where each
> request dependes on the session-state of the previous one and
> unfortunatelyI have to deploy in an environment where a round-robin
> load-balancer is used.

What happens if the user makes two requests at once?

> I had a look at the documentation (the examples online were not
> that clear - in the docs it is only mentioned 8 is async, whereas 6
> is later used without any explanation), however to be honest I am
> not sure which combination I actually require.

The channelSendOptions are a bitwise-OR'd combination of flags. Each
flag is a certain power of two (a single 1 bit) so if you see a value
that isn't a power of two (like 6), then you need to decompose that
number into its parts:

6 = 0110b
     ^^
     |+------ 0010 = 2 = "Use Ack"
     +------- 0100 = 4 = "Synchronized Ack"

So channelSendOptions="6" means "use ack, and use synchronized ack".

I'm not expert, but it seems you really only have two options:

1. Use ACK (yes/no)
and
2. If you use ACK, do you want them to be synchronous or asynchronous

I'm not sure why there are 3 options, here, since 2 of them make no
sense when the "Use ACK" is not set.

For a "conservative" configuration (where, I believe you mean that you
want to guarantee delivery and as much consistency as possible, at the
cost of performance), I believe you want "Use ACK" and "Sync ACK"
which would be that value of "6" you mentioned earlier.

- -chris

References:
https://tomcat.apache.org/tomcat-8.0-doc/config/cluster.html#Attributes

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