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From Paul Gale <paul.n.g...@gmail.com>
Subject Re: Message Group Limitations (how many simulataneous groups are supported?)
Date Tue, 26 Jan 2016 14:56:23 GMT
*>Using a key such as social security number for message groups is going to
be challenging simply due to the number of groups involved*
This can be overcome by using the hashing technique I described earlier
which has worked out nicely for us YMMV. In practice we've found that more
often than not that the item we wish to group on greatly exceeds 1024 hence
the effectiveness of the hashing method.

*>The default Message Group Map implementation was recently changed to use
an LRU Cache of message groups.*
When did this change? Perhaps I'm missing something but I've looked at the
source for 5.11, 5.12 and 5.13 and they all
​ appear to be​
using CachedMessageGroupMapFactory - and its implementation doesn't appear
to have changed either. Just wondering.

Thanks,
Paul

On Mon, Jan 25, 2016 at 5:49 PM, artnaseef <art@artnaseef.com> wrote:

> The default Message Group Map implementation was recently changed to use an
> LRU Cache of message groups.
>
> Here's the issue with message groups - the broker does not know the set of
> message group IDs ahead of time and must allow for any number of group IDs
> to be used.  If the total set of possible message group IDs is small, this
> is not a major concern, but if it is large, then tracking message group
> owners over time acts like a memory leak (consider the memory needed to
> maintain a mapping of 1 million message groups).
>
> The cache implementation attempts to address the concern by limiting the
> map
> to only retain 1024 group ID mappings  (by default - the number appears to
> be configurable from looking at the code).  This means that once 1025 group
> IDs exist at once within the broker, assignments will get lost (and new
> consumers attached to the dropped assignments as-needed, leading to more
> dropped assignments, and so on).
>
> On the other hand, the previous default implementation used a Hash Map so
> that the hash value of each group ID determined a "bucket" to which the
> group was assigned; that bucket can then be assigned to any number of
> groups.  The bucket is assigned to a single consumer.  Like the LRU cache,
> the number of buckets is limited, thereby eliminating the possibility of a
> "pseudo-leak".  However, this leads to the issue that assignments may not
> be
> fair and a single consumer may be assigned any combination of groups
> entirely based on the hash of the group IDs.  If selectors are added to the
> mix, this easily leads to messages assigned to consumers that cannot
> consume
> the messages.  Yuck.  Add in the max page size limitation and messages
> start
> getting stuck all over the place - double yuck.
>
> The best practice in general is to look for ways to avoid order
> dependencies
> (e.g. attaching sequence numbers to messages so that the processor can
> determine when messages are received out-of-order and then suspend
> processing until the late messages are received).  Camel's aggregator
> and/or
> resequencer processors can help here.
>
> Using a key such as social security number for message groups is going to
> be
> challenging simply due to the number of groups involved, and the memory
> leak
> concern mentioned above.  If guarantees can be met, such as "no more than
> 1000 SS numbers will ever have pending messages at a time," then the
> concerns can be eliminated.  Probably the hash map solution will be the
> best
> bet here - at the expense of reduced fairness of mappings (one consumer can
> easily carry more than its share) and eliminating the feasibility of
> selectors (although I usually recommend against using selectors with
> message
> groups anyway).
>
>
>
>
> --
> View this message in context:
> http://activemq.2283324.n4.nabble.com/Message-Group-Limitations-how-many-simulataneous-groups-are-supported-tp4706412p4706419.html
> Sent from the ActiveMQ - User mailing list archive at Nabble.com.
>

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