Paper evals

Carnevali $ilvio (
Wed, 26 Apr 2000 19:33:58 PDT


The main objective of this paper is presenting the basic concepts of a
Mobility-oriented OS, along with the main
features of the language supporting it. The growing importance of
network-based applications is the main reason
for the usefulness of such a system; the redefinition of a data object was
also necessary to reach this goal.

In order to obtain a fine grain information mobility, the notion of object
has been generalized: it does not
correspond to a process anymore, but can be any collection of data as well
as processes; thus, the unit of
mobility can be very small. The use of a language supporting mobility was
intended to ease the design of such
a System.
The System was optimized for execution in both local and remote mode; since
remote execution is always slower
than local, special care was dedicated to a good placement of global objects
in order to reduce remote calls.
This requires constant relocation of objects that are thus moved from node
to node depending on the needs, which
is a potential source of problems: some pointers may need to be translated,
we need to keep links and reference
counts on objects for future accesses and garbage collection.
The relocation strategy can be either imposed by the programmer (using the
call-by-move/visit features) or done
statically during compilation time. In this case, the compiler can use 3
different object implementations:
- Global Objects.
- Local Objects.
- Direct Objects.

The experimental results proved that the main goals were successfully met by
Emerald, which is probably the reason
why future developments are not mentioned.
I think the paper gives good details about the architecture of the system,
thus giving a clear idea of it's
mobility features. This was quite helpful for me as I gained a clearer
vision on the execution of programs across
a network.


This paper describes the structure of a distributed system providing a
variety of network services, the main
application being message delivery. The adaptability of the system to a
variety of OS platforms is one of the
good features that makes it fit for managing message communication among an
heterogeneous network.

The system uses internet protocols as the mean of communication between
servers distributed across a network.
Each server provides two services:
- Message service: takes care of message delivery and queing.
- Registration service: keeps the database with informations about
registered users.
For security reasons, the messages and user databases are kept in at least
two servers, in order to reduce the
risk of data loss and delays (when a server becomes unavailable, the data
can be obtain from another one).
Furthermore, the user database is distributed among several servers, in
order to speed up the user authentification
and delivery process. Thus, a growing number of users can be faced with the
addition of new servers, which
preserves the overall efficiency of the distributed system. Sharing common
messages is also a good way to reduce
storage problems and delivery time.
Three main problems were successfully faced by the system:
- new user information update
- remote authentication delay
- remote message reading with no deletion from Inbox
Finally, the possibility of remote operation enabled system experts to fix
most problems without having to be
physically present.

This paper helped me understand the organization of a modern message
delivery system. Most features introduced
here are still used today in message protocols. I think this paper is a good
presentation of the features of a
well organized messaging system that has been running for years; one of the
future improvements seems to be the
generalization of the naming hierarchy, in order to provide access to a much
larger number of users.

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