David Barbour wrote:
Going back to this post (to avoid distraction), I note that
Aggregate Level Simulation Protocol
and its successor
High Level Architecture
Both provide "time management" to achieve consistency, i.e. "so that
the times for all simulations appear the same to users and so that
event causality is maintained – events should occur in the same
sequence in all simulations."
and Alan Kay wrote:
Yes "time management" is a good idea.
Looking at the documentation here I see no mention of the (likely)
inventor of the idea -- John McCarthy ca 1962-3, or the most
adventurous early design to actually use the idea (outside of AI
robots/agents work) -- David Reed in his 1978 MIT thesis "A Network
Operating System".
Viewpoints implemented a strong "real-time enough" version of Reed's
ideas about 10 years ago -- "Croquet"
The ALSP blurb on Wikipedia does mention the PARC Pup Protocol and
Netword (the "Internet" before the Internet).
Actually, last time I looked (admittedly about 4 years, when I last
worked for a firm that sold low-level libraries for simulation
protocols, both DIS and HLA), larger network sims (e.g., the Air Force's
Distributed Mission Training net) used DIS (Distributed Internet
Simulation) rather than HLA.
The DIS protocol is essentially distributed information stuffed into
multi-cast UDP packets. HLA is a an object oriented middleware - basic
model is that objects are replicated across sims, with background
protocols keeping those copies in sync - essentially publish-subscribe
applied to replicated objects. As I remember, HLA just proves to
complicated and injects too much overhead as the number of players on
the net grows.
As I recall, DIS PDUs contain time stamps referenced to exercise time,
which is generally synchronized with GPS time. Part of the problem with
using HLA for larger sims has to do with time synchronization,
particularly if using the time management service for synchronization
(one now has to wait for the time management events to traverse the
network) - and there are also various options the different simulators
can/do use for synchronizing with exercise time. It all becomes a
management nightmare. Lots of papers on the topic, though.
Miles
--
In theory, there is no difference between theory and practice.
In practice, there is. .... Yogi Berra
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