[openstack-dev] [Heat] event table is a ticking time bomb

Sandy Walsh sandy.walsh at rackspace.com
Fri Aug 9 13:16:55 UTC 2013

On 08/08/2013 11:36 PM, Angus Salkeld wrote:
> On 08/08/13 13:16 -0700, Clint Byrum wrote:
>> Last night while reviewing a feature which would add more events to the
>> event table, it dawned on me that the event table really must be removed.
>> https://bugs.launchpad.net/heat/+bug/1209492
>> tl;dr: users can write an infinite number of rows to the event table at
>> a fairly alarming rate just by creating and updating a very large stack
>> that has no resources that cost any time or are even billable (like an
>> autoscaling launch configuration).
>> The table has no purge function, so the only way to clear out old events
>> is to delete the stack, or manually remove them directly in the database.
>> We've all been through this before, logging to a database seems great
>> until you actually do it.
>> I have some ideas for how to solve it, but I wanted to get a wider
>> audience:
>> 1) Make the event list a ring buffer. Have rows 0 - $MAX_BUFFER_SIZE in
>> each stack, and simply write each new event to the next open position,
>> wrapping at $MAX_BUFFER_SIZE. Pros: little change to current code,
>> just need an offset column added and code that will properly wrap to 0
>> at $MAX_BUFFER_SIZE. Cons: still can incur heavy transactional load on
>> the database server.A
>> 1.b) Same, but instead of rows, just maintain a blob and append the rows
>> as json list. Lowers transactional load but would push some load onto
>> the API servers and such to parse these out, and would make pagination
>> challenging. Blobs also can be a drain on DB server performance.
>> 2) Write a purge script. Delete old ones. Pros: No code change, just
>> new code to do purging. Cons: same as 1, plus more vulnerability to an
>> aggressive attacker who can fit a lot of data in between purges. Also
>> large scale deletes can be really painful (see: keystone sql token
>> backend).
>> 3) Log events to Swift. I can't seem to find information on how/if
>> appending works there. Tons of tiny single-row files is an option, but I
>> want to hear from people with more swift knowledge if that is a viable,
>> performant option. Pros: Scale to the moon. Can charge tenant for usage
>> and let them purge events as needed. Cons: Adds swift as a requirement
>> of Heat.
>> 4) Provide a way for users to receive logs via HTTP POST. Pros: Simple
>> and punts the problem to the users. Cons: users will be SoL if they
>> don't have a place to have logs posted to.
>> 5) Provide a way for users to receive logs via messaging service like
>> Marconi.  Pros/Cons: same as HTTP, but perhaps a little more confusing
>> and ambitious given Marconi's short existence.
>> 6) Provide a pluggable backend for logging. This seems like the way most
>> OpenStack projects solve these issues, which is to let the deployers
>> choose and/or provide their own way to handle a sticky problem. Pros:
>> Simple and flexible for the future. Cons: Would require writing at least
>> one backend provider that does what the previous 5 options suggest.
>> To be clear: Heat cannot really exist without this, as it is the only way
>> to find out what your stack is doing or has done.
> btw Clint I have ditched that "Recorder" patch as Ceilometer is
> getting a Alarm History api soon, so we can defer to that for that
> functionality (alarm transitions).
> But we still need a better way to record events/logs for the user.
> So I make this blueprint a while ago:
> https://blueprints.launchpad.net/heat/+spec/user-visible-logs
> I am becomming more in favor of user options rather than deployer
> options if possible. So provide resources for Marconi, Meniscus and
> what ever...
> Although what is nice about Marconi is you could then hook up what
> ever you want to it.

Logs are one thing (and Meniscus is a great choice for that), but events
are the very thing CM is designed to handle. Wouldn't it make sense to
push them back into there?

> -Angus
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