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From "Matthieu Morel (Commented) (JIRA)" <>
Subject [jira] [Commented] (S4-22) Adaptor
Date Tue, 13 Dec 2011 15:51:31 GMT


Matthieu Morel commented on S4-22:

bq. I argue that for App2 to receive events from App1, we want App2 to have access to a local
reference of App1. (Do we agree on this? otherwise I may be missing something.)

This constraint implies tight coupling between apps. Maybe your requirement for full symmetry
comes from that (deploying all apps on all nodes for allowing them to share code?).

We should rather strive for loose coupling between apps. Indeed, apps are already deployed
through different classloaders, and they may even be deployed in different VMs (nodes).

To enable loose coupling we need to remove dependencies to 
1. specialized event types defined in apps
2. specialized KeyFinder implementations

The most convenient way is probably to use a generic approach (only required for inter-app
1. generic event types for inter-app communications. 
2. a generic and parameterizable KeyFinder implementation (e.g. string keys)

And my understanding is that Leo already undertook this approach in his prototype! 

Note that the KeyFinder parameters could be retrieved directly from the stream configuration
stored in Zookeeper. This relates to S4-27 .
> Adaptor
> -------
>                 Key: S4-22
>                 URL:
>             Project: Apache S4
>          Issue Type: Improvement
>    Affects Versions: 0.5
>            Reporter: Leo Neumeyer
>            Assignee: Bruce Robbins
>             Fix For: 0.5
>         Attachments: s4-subclusters.pdf
> Need an adaptor for v0.5
> Idea I posted earlier:
> What do you think of this idea for a simple adaptor:
> - Adaptor extends App
> - Adaptor can send events but not receive (for now)
> - Adaptor is deployed as a regular App to the S4 cluster and as an
> Adaptor type  in a host (separate from the S4 cluster).
> - Adaptor, unlike regular apps, can accept event data (in any format)
> directly, not via comm layer.
> - Input data is transformed into S4 events using a modular approach
> and by providing standard modules such as JSON.
> - Output events are exposed using EventSource and consumed by other
> apps without even knowing that they are Adaptors (only the App type is
> exposed in the cluster).
> - S4 events can be processed locally using PEs and Streams as usual.
> (We kind of need to get a local Sender for the local PEs and a
> standard cluster Sender for the EventSource object.)
> So why this approach?
> The GOOD:
> - Seems to be the least disruptive way to inject external events
> - Apps can easily consume the events in a modular way without any
> dependencies. Getting events from an adaptor or from another app is
> identical.
> - The adaptor would be packaged and deployed to the cluster as if it
> was an App (no incremental cost)
> - The adaptor can do preprocessing using the same programming model
> and can reuse PEs.
> -  We need to also deploy the Adaptor in a separate host. On the other
> hand, this is inevitable. At least we use the same approach instead of
> creating a different system.
> -  The Adaptor will need to be integrated with ZK to get the physical addresses.
> -  We need to deal with two senders.
> for later: two-way communication and adapter clusters.
> thoughts?

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