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From Rainer Jung <>
Subject Re: mod_jk error detection
Date Wed, 25 Jul 2007 20:00:14 GMT
Hi Scott,

> I thoroughly enjoyed the updated docs.  It is just what I needed.  I
> just want to mention a few inferences I have now from reading it.


> In a load balanced setup using connect_timeout and prepost_timeout, this
> will protect me from sending either newly established connections (rare
> event due to persistence) as well as each and every individual request
> from being sent to a failed tomcat node based on CPING/CPONG messages.
> These messages only detect whether or not the container (I'm using
> tomcat) is healthy enough to respond to such a message but not
> necessarily anything more, correct?  Basically, its ajp listener is
> responsive.  Plus, if I need more high speed error detection I can use

That's correct.

> reply_timeout.  Sound correct?

That one, reply_timeout, is not really meant for high speed detection. 
Usually you've got an ap, that every now and then needs 10 or 20 seconds 
for an answer and you don't like to disable a worker automatically 
because of those rare events. So normally one sets reply_timeout to 1, 2 
or 3 minutes.

Now with the new max_reply_timeouts one can experiment with lower 
values. It's new, so not enough experience for good suggestions.

> I get confused on the recovery_options section.  How does it work in a
> load balanced environment?  If tomcat receives a request and processes
> some of it followed by a catastrophic failure before completing the
> response, what exactly does a repeated request from the client do?
> Assuming recovery_options is set to 0.

Value "0" means, if you don't get any part of the answer and an error 
occurs (network, reply_timeout, ...) then send the same request again to 
another member of the load balancer (if a working member is remaining).

That's why you usualy really want to not use value "0" in case your app 
has data changing use cases. Most apps have.

If you use REST principles and HEAD and GET is always idempotent for 
your app, the new (version 1.2.24) bits 8 and 16 are your friend!

> Also, I get confused with the section describing the retries directive.
> In a load balanced environment, would the connector retry no matter the
> state (tcp state here) of the connection whether it be established
> already?  Would it retry against the same backend tomcat server?  The
> reason I ask is because the docs say "If the load balancer can not get a
> free connection for a member worker from the pool, it will try again a
> number of times given by retries." I highlighted the words that confuse
> me.

We have to strongly make a difference between retries of a non-lb worker 
and of a load balancer worker. A normal worker has a simple retry 
procedure, independant of the fact, if it is used directly or as part of 
an lb. If it detects an error it uses another pool connection and by 
default tries once more.

An lb has another idea of retries. It uses retries if all connections to 
a backend are busy. For Apache with default config, this should never 
happen, because we allow as many connections as threads per process. So 
any request should be able to get a connection without waiting (maybe it 
needs to start a new one). For the other web servers we don't have a 
good way to detect the "correct" pool size. In some cases even for 
Apache it might be interesting to use a smaler pool size, in case the 
backend is only used occasionally and/or you want to prevent it from 
getting flodded in case of congestion. Then you might run out of 
available connections and requests will have to wait. LB retries 
configure this waiting.

> Every 60 seconds would we expect the connector to attempt to send a
> valid request to a backend tomcat and fail or once a worker goes into
> error state do we only check with CPING/CPONG requests during the
> maintenance cycle?

The maintenance uses a real request and handles it as if the backend 
wouldn't have failed. If you enabled CPing/CPong this means, that it 
would detect a still broken backend early and transparently send the 
request to another member. Because no part of the request (the CPing 
doesn't count) already has been send, the failover to another member 
happens independently of recovery_options (i.e. even with 
recovery_options 3).

If you like to improve the page about load balancing or the timeouts 
page, or you want to add some parts about retries and recovery: 
contributions are welcome.



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