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From Aled Sage <>
Subject UsageListener: notified of promotion to master?
Date Wed, 22 Mar 2017 17:11:11 GMT
Hi all,

I'm writing a UsageListener [1] that is going to record the Brooklyn 
instance's apps in an external inventory. This is mostly 
straight-forward (except needing the fix at [2]).

The tricky bit is when a Brooklyn standby instance is promoted to master 
- I want my listener to be notified so it can record the identity of the 
new master Brooklyn for each of the pre-existing app.

I don't see anywhere I can get a notification of this. Is there?


Possible solutions include:

 1. Add an additional application event (e.g. state
    "MANAGEMENT_CHANGED") - the UsageListener would receive this event
    for every app when it successfully rebinds.
 2. Register a new kind of listener, to be notified of the management
    context's state - on becoming "master", my listener could iterate
    over all the apps.

I don't particularly like either of those options.

For (1), it isn't really to do with app "Usage" like the other events.

For (2), my listener code would be fiddly - I'd need to worry about 
receiving concurrent notifications via the normal UsageListener while 
I'm iterating over the pre-existing apps (assuming the new listener is 
called asynchronously, rather than blocking the management context's 

It feels like (2) is a more generally useful feature to add (for 
programmatic support, at least). Does it sound useful for things beyond 
my use-case?




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