Hi Martin,

Am 04.08.2017 um 10:53 schrieb Martin Knoblauch:
Hi,

 just need some clarification on the mod_jk load blanacing method "Next".
The documentation states:

"If method is set to N[ext] the balancer will again use the number of
sessions to find the best worker. All remarks concerning the Session method
apply as well. The difference to the Session method is how the session
count is handled in the sliding time window. The Next method does not
divide by 2, instead it subtracts the current minimum number. This should
effectively result in a round-robin session balancing, thus the name Next.
Under high load, the two session balancing methods will result in a similar
distribution, but Next will be better if you need to distribute small
numbers of sessions. "

 What exactly is the "current minimum number"? How is the minimum taken?
From all workers in the balancer set, or only the ACTive ones? I know, I
should look it up in the code :-)

I looked up the code I wrote 6 years ago.

First: when using the session base lb methods, mod_jk needs to estimate session counts. No lb method of mod_jk contacts the backends to get real data, instead mod_jk uses the request info it sees to estimate the backend situation.

For session based methods, mod_jk counts requests, that do not include a session id assuming that those are exactly the ones that create new sessions. Of course:

a) a session id can be outdated, meaning mod_jk would not count the request as session creating but in fact it would create a new one. One can at least configure mod_jk to be aware of login pages which will always create a new session (see http://tomcat.apache.org/connectors-doc/reference/uriworkermap.html and http://tomcat.apache.org/connectors-doc/reference/apache.html and there look for "sticky_ignore").

b) a request without a session ID might not actually create a session, depending on app details. There are additional config options to teach mod_jk which URIs do not create sessions (see http://tomcat.apache.org/connectors-doc/reference/uriworkermap.html and http://tomcat.apache.org/connectors-doc/reference/apache.html and there look for "stateless").

c) sessions time out in backends and users can log out. mod_jk does not track that. One can remove the session cookie during the logout, so that the "new" requests from that user will be counted by the mod_jk session counter.

Because of these problems I typically recommend to stick to the default lb method (request counting, not session counting). But sometimes apps have resource usage dominated by sessions and then a "session" based lb method can help, especially if you find a configuration which keeps the effect of a)-c) above small.

Since all counting methods, not only session based ones, would count stuff since the last restart of mod_jk, but the current backend load situation depends much more on stuff that happened recently, we try to get rid of past counts by reducing the counters regularly. By default this happens once per minute and is done in a way that the counters are divided by 2 once per minute. That way old counter increases contribute less and less to the current counter value. For the session based method this would mean we assume half of the counted sessions die after one minute, 50% of the rest during the next minute etc. Note that the counters are integers, so e.g. a counter value of 1 will after division by 2 result in a new value 0. Most often that is no problem, because on a loaded system numbers are big and rounding down doesn't change a lot.

The next request without session id will be send to the worker with the smallest such "session" counter.

The "Next" message varies that procedure by not dividing by 2 every minute, but instead subtracting the minimum value of the backend counters. Assume after the first minute, your 4 backends have "session" counters 2, 3, 3 and 2. Then the minimum is 2, so after the minute we correct the values to 0, 1, 1 and 0. Then we add for the next minute new sessions to that counter and again subtract the new minimum etc.

When would that be helpful? It was for an application with really huge sessions but small session numbers. There was a risk that if for a minute only 0 or one sessions were created on the backends, after dividing by 2 all workers were again 0.

You can actually track the counters via the status worker, were they are exposed as column "V" (load balancer value).

Regards,

Rainer

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