[zeromq-dev] Distributed Event Bus

[Ondrej Kupka]

On Feb 19, 2013, at 7:43 PM, Pieter Hintjens wrote:

> On Tue, Feb 19, 2013 at 6:40 PM, Ondrej Kupka <ondra.cap at gmail.com> wrote:
> 
>> ...
> 
> Basically you want to layer statefulness above ZRE, right? This is
> something I wanted to address but haven't had time for yet.
> 

My concern was more in the direction of a reliable event bus. An exemplar scenario could be to scatter a bunch of FSMs around the cluster and let them process the events inserted for example by a bunch of processes providing a RESTful entry point into the whole system. If you miss an event, it kinda breaks your FSM's internal state. That is why I was thinking about using this architecture of a network <-> daemon <-> PUB+PULL on every node. The daemon would make sure it got all the events from other nodes it knows and none got lost. Then there are other questions like what to do once you are partitioned for a long time (restart the FSMs?), but that is application-dependent and not that important in this discussion I guess...

Then other things could be implemented on top, like what you mention, but I haven't though about that much yet.

> The hardest part is ensuring consistency if you have multiple updates
> at the same time, to the same data. If you don't care about this, just
> hold a key=value table in each node, and when a new node joins, it can
> ask a random node for a state dump, much like the simple Clone
> versions. Easy to write on top of ZRE as an application.
> 
> If you want global consistency you have to first design a small
> protocol for electing a leader. Perhaps the oldest node. Then when a
> new node joins, it first discovers the leader, and then it asks the
> leader for a snapshot. Then, every node when it has a new key=value,
> sends this to the leader, which rebroadcasts it to everyone else.
> 
> In any case I'd build this on top of ZRE using the group pub-sub it
> provides you.
> 

Thanks,
Ondra Kupka

> -Pieter
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