[Serusers] SER Reports "out of memory"

Greger V. Teigre greger at teigre.com
Mon May 30 14:11:16 CEST 2005


See inline.
Jiri Kuthan wrote:
> At 09:24 AM 5/30/2005, Greger V. Teigre wrote:
>
> [...]
>>> * when ser starts up usrloc is "lazy-loaded"
>>> * if a usrloc record is looked up in cache and is __NOT__ found,
>>> then MySQL will be queried. If found in MySQL then the usrloc
>>> record will be put in to cache for future lookups
>>>
>>> By doing these two things we should not have a problem we
>>> excessively large subscriber bases.
>>>
>>> Thoughts?
>>
>> Makes sense.  This is how Berkeley DB and many other DBs work.  In
>> fact, the best would be to build an abstraction cache layer around
>> all the query functions that have data in the DB. This way you would
>> get the optimum performance/scalability.
>
> I have to admit I am not sufficiently familiarized with BDB. If I
> understand it right, they do confgurable in-memory caching and they
> also support some kind of master-slave replication. I am not sure
> though how this scales...(20 SERs with 20 BDBs, one of them master
> and replicating UsrLoc changes to 19 slaves who are all able to
> identify inconsistent cache?)
>
> I mean the structural problem here is dealing with r-w intensive
> Usrloc operations and still desiring to replicate for reliability.
> There is a variety of algorithms to deal with it and I don't know
> well what the respective DB systems actually do.

I'm not proposing to use BDB, it was just an example.  Databases are very 
good at replication, even two-way replication can be done quite efficiently 
through locking etc. I just took Paul's setup with cluster back-end as 
granted and wrote my comments based on that...

Thinking a bit wider and building on your comments, Jiri:
    The challenge, I think, is to handle the following things in any likely 
deployment scenario:
1. Usrloc writes to cache vs. DB
2. Replication of usrloc, multiple DBs vs. cluster, across LAN or WAN
3. Memory caching management (inconsistencies etc)

For the sake of the readers, here is how I understand SER's operations 
today:
1. Usrloc is always written to cache, DB write is controlled through 
write-through parameter
2. Replication is handled by t_replicate
3. Management of cache is not needed, the cache is always updated. However, 
an updated DB (and thus dirty cache) will not be detected

Here is how I understand Paul's proposal (and with my annotated suggestions 
from my last email :-):
1. Usrloc is always written to DB, cache is updated if it is already in the 
cache
2. Replication is handled by underlying database across DBs or in a cluster
3. If usrloc is not found, DB is checked. If cache is full, some mechanism 
for throwing out a usrloc is devised

I must admit I often fall for the argument: "let each system do what it is 
best at."
Following that, replication should only be done at an application level if 
the underlying database is not capable of doing it (if we agree that a DB is 
good at replication).  The only thing I see a DB is not capable of, is 
handling the NAT issues. So, if a given usrloc has to be represented by 
different location (ex. the registration server), then the DB cannot do 
replication. However, if the NAT issue is handled through some other means, 
ex. Call-Id aware LVS with one public IP, then the usrloc should be the same 
across DBs and the DB should handle the replication.

You don't need many subscribers before you'll want redundancy and as 
active-passive redundancy is a waste of resources, I believe an upgrade of 
the replication mechanism should soon be imminent. ;-)
    I think I have said this before, but this is my enterprise-level "dream" 
scenario:
1. Two geographically distributed server centers
2. DNS SRV for load distribution (and possible using segmentation of clients 
through their configurations if they don't support DNS SRV)
3. Each data center has Call-Id sensitive LVS in front, with one or more 
servers at the back  (a fair-sized LVS box can handle 8,000 UDP packets per 
second)
4. Each data center either has a DB cluster or two-ways SER-based 
replication
5. The data centers replicate between each other using either DB-based 
replication or two-ways SER-based replication
6. The SER-based replication is an enhanced version of t_replicate() were 
replication is to a set of servers and replication is ACKed and guaranteed 
(queue). I would suggest using the XMLRPC interface Jan has introduced
7. I think Paul's cache-suggestions are good regardless of decisions on 
replication

Entry level scenario where the same box is running LVS, SER, and DB (you can 
quickly add new boxes) has a very low cost.

>>   However, there is one more thing: You need to decide on an
>> algorithm for selecting a usrloc record to replace when the cache is
>> full.  Do you store extra info in memory for each usrloc to make the
>> right decision (ex. based on the number of lookups).
>
> You may also purchase more memory :)

Do you suggest that no mechanism should be devised when the cache limit is 
hit? ;-)  Then maybe I can suggest an email alert to the operator when a 
certain amount of the cache is full... :-D  I trust my people to act fast 
and appropriate, but not that fast and appropriate!

g-) 




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