[Hawkular-dev] start of message bus REST client

[Lukas Krejci]

Ok, with the Jinn out of the lamp, let's move the discussion we've had 
privately amongst the team-leads to the public forum :)

With the ability to send rest messages to the bus we need to solve 1 
fundamental question:

*Who is supposed be sending what kind of messages?*

It is clear, that any interested party should be able to listen to any 
message.

But I am concerned about the "source" of messages. 

For example I think it is wrong to send the message about metric being 
collected to the bus prior to it being stored. It could happen that the system 
reacts to something that we will have no record about later on.

But with more "involved" components that do more work than just store data 
upon arrival, the consequences of being out of sync between what's stored and 
what's being reacted upon are potentially worse.

As an example, consider this:
* someone sends a "resource inventoried" message to the bus - what should 
inventory think about it? Did the author make sure to do everything exactly 
like inventory would if it sent the message itself?
* someone sends an "alert fired" message to the bus - what should alerts do 
about it?
* someone DDoSes the the bus

I may be old fashioned in this, but I prefer consistency over eventual 
consistency where possible. I don't think it would be too much of a 
performance problem to architect the message flow like this:

1) anyone can listen on the bus
2) only Hawkular components can send messages to the bus (or rather the 
hawkular glue code built atop the standalone components)
3) 3rd parties (including feeds) can send data only to the component endpoints 
(which will upon processing generate appropriate messages on the bus)

I am no expert in either distributed architectures or in microservices, so 
this view may be naive and I'd love to be proven wrong about it, but I think 
the concerns I raise are real and we should have a solid answer to them 
whatever architecture we choose.

Cheers,

Lukas

On Thursday, April 02, 2015 17:51:31 John Mazzitelli wrote:
> I created a very simple REST client that you can use to send any json string
> as a bus message.
> 
> It requires no dependencies other than apache httpclient and jboss logging.
> 
> See:
> https://github.com/hawkular/hawkular-bus/tree/master/hawkular-bus-rest-clie
> nt
> 
>    new
> org.hawkular.bus.restclient.RestClient(url).postTopicMessage(topicName,
> jsonPayload, headers);
> 
> My agent uses it to send metrics to the hawkular bus's metric topic. I use
> REST rather than the bus-common API because (AFAIK) the thinking is this
> agent can be deployed in something like WildFly Core where JMS API isn't
> even available.
> 
> So right now, this agent should be sending data just like the Pinger - its
> sending metrics directly to Metrics REST interface for storage and sending
> those same metrics to the message bus's metric topic so other things like
> alerts can process it. Unfortunately, the kettle UI can't show any of it
> since it is tightly related to the Pinger's metrics and nothing else. But I
> think its working :) _______________________________________________
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