On Fri, Oct 2, 2015 at 4:24 PM, Michal Skrivanek <
On 2 Oct 2015, at 12:47, Roman Mohr wrote:
I am contributing to the engine for three months now. While I dug into the
started to wonder how to visualize what the engine is actually doing.
This is one of the main problems with large application, anything to help
to understand what's going on is very welcome
To get better insights I added hystrix to the engine. Hystrix is a
breaker library which was developed by Netflix and has one pretty
feature: Real time metrics for commands.
In combination with hystrix-dashboard it allows very interesting
You can easily get an overview of commands involved in operations, their
performance and complexity. Look at  and the attachments in  and 
screenshots to get an Impression.
I want to propose to integrate hystrix permanently because from my
the results were really useful and I also had some good experiences with
in past projects.
A first implementation can be found on gerrit.
# Where is it immediately useful?
During development and QA.
An example: I tested the hystrix integration on /api/vms and /api/hosts
endpoints and immediately saw that the number of command exectuions grew
lineary whit the number of vms and hosts. The bug reports  and  are
# How to monitor the engine?
It is as easy as starting a hystrix-dashboard  with
$ git clone https://github.com/Netflix/Hystrix.git
$ cd Hystrix/hystrix-dashboard
$ ../gradlew jettyRun
and point the dashboard to
# Other possible benefits?
* Live metrics at customer site for admins, consultants and support.
* Historical metrics for analysis in addition to the log files.
The metrics information is directly usable in graphite . Therefore
it would be
possible to collect the json stream for a certain time period and
later like in . To do that someone just has to run
curl --user admin@internal:engine
for as long as necessary. The results can be analyzed later.
it's a great idea and when properly documented so even a BFU can do that
it would allow us to get much better idea when something is not working or
working too slow on a system we don't have access to, but it\'s
reproducible elsewhere. Just ask for "hey, run this thingie while you are
reproducing the issue and send us the result"
# Possible architectural benefits?
In addition to the live metrics we might also have use for the real
* Circuit Breaker
* Bulk execution of commands
* De-dublication of commands (Caching)
* Synchronous and asynchronous execution support
Our commands do already have a lot of features, so I don't think that
some quick wins, but maybe there are interesting opportunities for infra.
eh..I would worry about that much later. First we should understand what
are we actually doing and why (as we all know the engine is likely doing a
lot of useless stuff;-)
In  the netflix employees describe their results regarding the overhead
wrapping every command into a new instance of a hystrix command.
They ran their tests on a standard 4-core Amazon EC2 server with a load of
request per second.
When using threadpools they measured a mean overhead of less than one
millisecond (so negligible). At the 90th percentile they measured an
of 3 ms. At the 99th percentile of about 9 ms.
This is likely good enough for backend commands and REST entry points (as
you currently did), but may need more careful examination if we would want
to add this to e.g. thread pool allocations
Don't get slowed down by that though, even for higher level stuff it is a
great source of information
When configuring the hystrix commands to use semaphores instead of
they are even faster.
# How to integrate?
A working implementation can be found on gerrit. These patch sets wrap
hystrix command around every VdcAction, every VdcQuery and every
This just required four small modifications in the code base.
In the provided patches the hystrix-metrics-servlet is accessible at
/ovirt-engine/api/hystrix.stream. It is protected by basic auth but
for everyone who can authenticate. We should probably restrict it to
that would be great if it doesn't require too much work. If it does then
we can start with enabling/disabling via JMX using Roy's recent patch 
The hystrix stream is now accessible in
and admin privileges are needed.
Further it can be enabled an disabled via JMX (disabled by default). @Juan,
@Roy thank you for your feedback on the code.
1) We do report failed actions with return values. Hystrix expects failing
commands to throw an exception. So on the dashboard almost every command
like a success. To overcome this, it would be pretty easy to throw an
exception inside the command and catch it immediately after it leaves the
at the beginning it's probably enough to see what stuff is getting called,
without differentiating between success or failure (we mostly do log
failures, so hopefully we know when stuff is broken this way)
Ok, I leave it disabled for now. But should really be just as easy as to
throw an exception if the command fails and immediately catching it
afterwards (not the nicest looking code then, but would work). And this can
be encapsulated in the command executor, so it would not pollute the
Do we want semaphores or a thread pool. When the thread pool, what size do
To answer this myself, I use semaphores, to be sure to support
multiple commands properly.