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Interest Management
• Objectives –
– Understand what is meant by the term interest management.
– Realise how interest management schemes may be deployed.
– Understand the difficulties encountered when trying to deploy
interest management.
Interest Management
• What is interest management?
– A mechanism by which the amount of information required by a
host to successfully enable their local objects to participate in a
virtual environment may be determined.
• Why use interest management?
– When a virtual world may consist of many entities and many
networked hosts are used in the deployment of the virtual
environment, interest management may considerably reduce the
amount of message passing over a network.
• Is interest management primarily used in networked
virtual environments?
– Yes, but its origins stem from determining a user’s view of a
virtual environment (e.g., no need to animate entities beyond a
user’s view).
The aura-nimbus model
Aura-nimbus
• Entity state information, say of E1, that may influence an entity, say
E2, should be made available to E2.
• Entity state information, say of E1, that does not influence an entity,
say E3, should not be propagated to E3.
• An entity, say E1, may influence another entity, say E2, when the
aura’s of E1 and E2 overlap.
– Or it may be that E2 itself must be within the aura of E1.
• An entity, say E1, does not influence another entity, say E3, when
the aura of E1 does not overlap with E3.
– Or E3 itself does not fall within the aura of E1.
• Complications:
– The aura describes the sending entity’s perceived range of influence.
– The nimbus describes the receiving entity’s perceived range of interest.
Message oriented approach
• What we are actually seeing is a variation of the publisher
subscriber paradigm.
– An entity may register and un-register their interest in the activities of
other entities throughout the lifetime of the networked virtual
environment.
– This is considered an asynchronous approach to message passing
(unlike synchronous RPC).
• There is a requirement that an entity may express their interest and
so reduce further the need for message passing.
– This may be accomplished via a subscription interest language.
• This level of filtering is a restricted version of the aura-nimbus
model.
– Increased scalability is derived with further application level knowledge.
– For example, even if entity E2 is in the aura of entity E1, E1 knows only
to send movement related information to E2.
Benefits of filtering
• Reduces processing requirements. Unwanted state
update messages are not processed by receivers.
• When IP level services (such as multicast and/or
broadcast) are unavailable point-to-point
communications are kept to a minimum.
• When client subscriptions change rapidly and underlying
group communication protocols cannot ensure group
dynamics (changes in group membership) in a timely
fashion.
Drawback of filtering
• Can be computationally expensive maintaining interest
groups.
– Application may inspect each message to determine appropriate
interest group (intrinsic filtering).
– Application does not inspect message content but destination
determined based on network properties (e.g., packet’s
destination address) (extrinsic filtering).
• When many entities are interested in the same
information the use of IP level multicast routing is more
desirable.
– Reduces the multiple sending of the same state update
message.
– Can’t really use IP level multicasting with filtering due to the
need to have many multicast groups (one for each filter).
Multicasting
• Multicasting is a mechanism by which a sender may
send a message to more than one receiver.
– Different to broadcast as a broadcast requires a sender to send
to all available receivers.
– Multicasting may be achieved at the IP routing level (IP multicast
exists in theory but is not widely available in practise).
• Multicasting is less expensive (in terms of network
usage) as it removes many duplicated message sends
associated to multiple unicast.
• The difficulty arises in networked virtual environments
when trying to map multicast groups to the interests of
entities.
– The worst case scenario is an entity interested in small amounts
of information from many interest groups.
Mapping multicast groups to entities
• Assign a multicast group to each entity.
– Allows each entity to receive information about all hosts that lie
within its nimbus.
– Entities subscribe to multicast groups of other entities that are of
interest.
– Subscription occurs on a per entity basis (not per message).
– Entities cannot specify their aura and have no influence over
which hosts will receive information.
• Assign multiple multicast groups to each entity.
– Allows each entity to send specific information to different
entities.
– This is expensive as there is only a finite number of multicast
groups available.
Mapping multicast groups to regions
• Each region of a virtual environment is assigned a
multicast group.
– Entities send state update messages to groups associated to
current location.
– Entities subscribe to groups corresponding to the regions it has
interest (usually the regions that are immediately adjacent to an
entity).
• Entities have control over their aura by permitting
information dissemination to be restricted to particular
regions on the virtual environment.
– Subscribers have limited control over their nimbi because they
can only filter based on a sender’s location.
Groups per region
• In the groups per entity approach an entity sends update
messages within the same group for the lifetime of the
entity.
• In the groups per region approach an entity may join and
leave different groups as it travels through the virtual
environment.
– When crossing into different regions an entity must realise the
multicast group associated to each region.
– These regions are usually static and defined at deployment time.
Appropriate regions
• Rectangle or hexagons are commonly used to divide the virtual world
into regions.
• Hexagons appear better as they present intersections between 3
regions whereas rectangles present intersection between 4
regions.