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Problem
There are multiple parties like
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event-driven
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workflows
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systems,
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monitoring
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system,
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usage
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&
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billing
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system
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would
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be
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interested
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in
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getting
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notification
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of
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a
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events
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in
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the
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cloud
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in
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near
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instantaneous
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manner.
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In
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CloudStack
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event
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could
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be
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state
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change
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of
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virtual
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or
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psychical
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resources,
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an
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action
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performed
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by
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user
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(action
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events),
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policy
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based
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events
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(alerts).
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Currently
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CloudStack
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has
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no
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notification
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mechanism
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to
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let
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the
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interested
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parties
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know
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of
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the
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occurrence
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of
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an
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event.
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Polling
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by
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invoking
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listEvents
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and
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listAlerts
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is
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the
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only
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way
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to
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know
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occurrence
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of
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events.
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Also
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there
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are
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no
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externsion
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hooks
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one
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can
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implement
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to
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get
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the
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event
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notification.
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Though
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there
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is
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StateListener
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interface
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which
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has
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both
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pre
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and
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post
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state
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transition
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event
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call
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back
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methods,
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only
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user
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VM
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resource
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has
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state
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listener
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implemented.
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Most
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of
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the
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resorces
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volume,
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network,
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nic,
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etc
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has
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states
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but
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does
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not
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have
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state
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machine
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and
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StateListener.
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So
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for
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a
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component
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interested
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getting
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notification
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need
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to
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get
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tightly
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coupled
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into
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orchestration
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workflow.
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For
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e.g,
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code
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to
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generate
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usage
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date
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(usage
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events)
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is
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spread
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across
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the
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orchestration
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components.
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Proposal
This proposal is to introduce an in-process
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event
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bus
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in
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the
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management
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server
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and
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by
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which
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achieve
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event
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notifications.
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Scope
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of
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the
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event
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bus
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is
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only
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with
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in
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the
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management
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server,
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so
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only
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CloudStack
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components
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and
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extension
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plug-in's
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can
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be
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subscribers
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to
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the
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events.
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Goal of
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this
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proposal
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is
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to
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have
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bare
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minimum
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mechanism
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in
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the
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CloudStack
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code,
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using
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which
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more
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powerful
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notification
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mechanism
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(like
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SMS,
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etc)
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can
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be
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built
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as
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CloudStack
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extensions.
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Ideal
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requirements
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for
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such
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event
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bus
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is
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to
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be
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- light
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- weight
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- (a
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- library)
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- provide
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- simple
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- interface
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- decouple
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- the
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- publisher
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- and
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- subscriber
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- so
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- that
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- they
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- need
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- not
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- be
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- aware
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- of
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- other
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- powerful
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- enough
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- to
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- provide
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- filters
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- for
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- events
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- of
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- a
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- type
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- to
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- wildcard
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- (all
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- events)
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In
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order
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for
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CloudStack
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be
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able
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to
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generate
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rich
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set
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of
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events
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and
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have
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clean
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mechanism
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to
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generate
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events,
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its
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important
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to
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have
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state
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machine
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associated
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with
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every
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resource
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that
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is
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managed
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by
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CloudStack.
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So
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another
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goal
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of
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this
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proposal
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is
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to
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associate
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with
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every
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resource
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(User
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VM,
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Volume,
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Nic,
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Network,
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public
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IP,
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snapshot,
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template
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etc)
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with
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a
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state
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machine
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and
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generate
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events
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as
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part
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of
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the
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state
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change.
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So the
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publishers
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of
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events
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in
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CloudStack
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will
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become
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that
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StateListeners
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associated
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with
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the
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resource.
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For e.g.
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UserVMStateListener
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which
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is
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listener
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for
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the
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state
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changes
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of
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user
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VM's
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would
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create
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event
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object
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and
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post
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on
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to
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the
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event
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bus
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as
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below.
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public boolean postStateTransitionEvent(State
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oldState,
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Event
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event,
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State
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newState,
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VirtualMachine
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vo,
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boolean
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status,
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Object
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opaque)
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{
_eventBus.post(new UserVmStateChangeEventVO(....));
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}
A subscriber interested in the user VM state change would register to the event and get the events as below
_eventBus.register(this)
@Subscribe
public void userVMStateChangetHandler(UserVmStateChangeEventVO event)
Use cases
Following are some use cases demonstrated in the diagram below
- Usage/billing engines: a 3rd party Cloud Usage solution can implement a plug-in to CloudStack that subscribes to CloudStack events and generates usage data, that is consumed by their usage software
- AMQP plugin can put all the events on the a message queue, a AMQP message broker then can provide topic based notification to subscribers
- pub/sub notification service can be implemented as Pluggable service in to CloudStack that provides rich API for event notifications (topics based subscription and notification) and deal with multi-tenancy, authentication&authorization issues
Discussion topics
- Google Guava Event bus library which is under Apache License 2.0 seems to be good fit for implementing event bus in CloudStack. Its light-wieght and has simple interface but very powerful. But any other alternatives could be evaluated
- If there is clustered management server setup then subscribers need to be present on the both management servers. Using a messaging queue servers like RabbitMQ can be used for event bus, but i thinks will add complexity into CloudStack. Are there any scenarios where having in-process event bus proposed will fall short in clustered management server setups?
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