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\documentstyle[12pt,pocs-header]{article}
\Scribe{Umesh Maheshwari}
\Lecturer{Bill Weihl}
\LectureNumber{1}
\LectureDate{September 11, 1991}
\include{macros} % used to include figures in this document
\begin{document}
\MakeScribeTop

\section{Course Outline}

This course is a graduate level introduction to principles of computer
systems.  It will focus on concurrent and distributed systems, but to
begin with, we shall develop some fundamental understanding in the
simpler context of single-site systems. It differs from the course
{\em Computer Systems (6.853)} in that it is not based on a series of
case studies.

The general theme of the course is set around these topics:

\begin{itemize}
\item {\bf Specifications} --- which are used to abstract the visible behavior
 of a system
from the details of its implementation. 

\item {\bf Reasoning techniques} --- aimed at proving the correctness of a
system.

\item {\bf Performance} --- including issues in 
design tradeoffs, and ways to enhance performance.

\end{itemize}


We shall discuss these issues in the context of some specific areas
and techniques listed below :
\begin{itemize}

\item {\bf Caching}, as a method to improve performance through the use of
a {\em faster}, {\em smaller}, and {\em nearer} memory to store
frequently used data.

\item {\bf Fault-tolerance}, which is about building systems that keep
    working in the wake of faults. 

\item {\bf Concurrency}, implying that several processes are
allowed to run in parallel and interact with each other. We will discuss
problems involved in  specifying and reasoning about concurrency. This course
does {\em not}
cover a study of parallel algorithms or applications for solving {\em specific}
problems. 

\item {\bf Distributed systems}, which are spread across multiple nodes
connected by a network. This introduces some new issues like communication
delays and independent failure of the nodes.

\item {\bf Transactions}, as a mechanism for building fault-tolerant,
distributed, and concurrent systems. 

\item {\bf Naming}, the issues involved in mapping names to resources and the
naming problems introduced by distributed systems.

\item {\bf Security}, involving matters like access-control and authentication.
\end{itemize}

\section{Principles of Computer System Design}

A list of principles useful in the design of computer systems could
conceivably be very long. As noted earlier, we shall focus on 
three factors: abstractions,  reasoning techniques, and tradeoffs
between performance, functionality and complexity.

{\em Abstraction} refers to the process of using specifications to
separate the functionality of a system from the details of its
implementation.  In other words, specifications tell {\em what} a
system should do, without trying to dictate {\em how} it should be
done. As an example, the specification for a $SORT$ routine may state
merely that it permutes the input sequence of numbers such that the
output sequence has them in increasing order. The particular sorting
algorithm used to achieve this is irrelevant.  Actually, the
distinction between what is and is not a part of a specification is
subjective, and depends upon the which aspects of the system's
behavior its users find interesting.  For instance, if the $SORT$
routine is required to run in $O(n \log n)$ time, then this fact would
appear in the specifications, and less efficient implementations like
$InsertionSort$ would be ruled out.

The use of abstractions in design has some  obvious advantages :
\begin{itemize}

\item It provides modularity and modifiability. That is, the
implementation of a module can be replaced without changing its
specification, and therefore, other modules using it remain unaffected.

\item A related point is that it allows the designer to avoid pre-mature
optimization. For example, one could employ an inefficient but simple-and-sure
implementation to begin with, and worry about performance only after the
integration of the system is complete.

\item It is a good way to document functionality.

\end{itemize}

The behavior of modules is usually well-understood in a sequential
world, but has more complicated meaning for concurrent and distributed
systems.  Specifications for a concurrent system are often {\em
weaker} than those for a sequential system. This results from the
non-determinacy allowed by the presence of concurrency. Forcing
determinism in such a system may result in degradation of performance.

As an illustration, consider a network of file servers and clients
(Fig~\ref{netfig}). If {\em Client A} updates a file before {\em
Client B} reads it, the most straightforward semantics would guarantee
that the modifications are reflected in the data read by {\em Client
B}. However, performance would improve tremendously if {\em Client B}
could use a cached (potentially old) copy of the file.  In this
scenario, modifications to the file are propagated to {\em Client B}
shortly after they occur.  The extra performance requires that the
semantics be relaxed; thus, there are tradeoffs between good
performance and the desirable functionality with simpler
specifications.


\begin{figure}
\centerline{\psfig{width=3in,figure=net.idraw}}
\caption{Performance/functionality tradeoff in a Distributed System.}
\label{netfig}
\end{figure}

\section{Writing Specifications}
In this course, both specifications and implementations will be
written using one language --- {\em Spec}. Spec is based on Edsger
Dijkstra's {\em Guarded Commands}\/\footnote{See Dijkstra's paper
called {\em Guarded Commands, Nondeterminacy and Formal Derivation of
Programs} in the Aug'75 issue of {\em Communications of the ACM.}},
extended by Greg Nelson\footnote{See Nelson's {\em A Generalization of
Dijkstra's Calculus}, ACM Transactions on Programming Languages and
Systems, 11(4):517--561, October 1989}, and augmented with constructs
for concurrency and atomicity by Butler Lampson.\footnote{[Neither of
these references will help students learn Spec; they are provided only
for historical interest -- Eric]}


Using only one language allows successive refinement of a
specification into a full-fledged implementation. On the other hand,
this has some pitfalls of its own. It is easy for one to lapse into
using the implementation style while coding the specifications, which
makes the specifications {\em overconstrained}.  Indeed, an
implementation {\em is} a plausible specification.
Conversely, one might be tempted to use some of the succinct
constructs provided for specifications while writing the
implementation, which may not translate into realistic or
efficient code. Thus, we need to exercise some
`self-restraint' in using Spec for the two different purposes.


We now turn to what the specification for a procedure might look like.
It could conceivably be a function mapping the input arguments and the
initial state to the results returned and the final state. Here, the results
encapsulate both the values returned normally and the exceptions raised. 
However, the use of a {\em function} for the specification fails to
capture the non-determinsitic behavior of some procedures. That is, for
the same input and initial state, a number of output values or state may
be acceptable. Some examples of such procedures are:
\begin{itemize}
\item $SQRT (x)$ which might be allowed to return any value within $\sqrt{x}
\pm \epsilon$ for want of better precision.

\item $SEARCH (item:T, s:SEQ[T])$ which returns any index $j$ in $s$,
such that $s[j] = item$, or raises a $NotFound$ exception if none
exists.

\item The storage allocater, which might return any of the several
available free blocks on request for a chunk of memory.
\end{itemize}

Thus the specification for a procedure should be expressed as a {\em
relation} between the input arguments plus the initial state, and the results
plus the final state --- this allows one-to-many mapping. In fact,
even a particular {\em implementation} of the procedure may be
nondeterminstic (e.g., if it involves concurrecny), and then
it too must be denoted by a relation rather than a function. 

Suppose that the relation dictated by the specification is $R_{S}$ and
that realized by the implementation is $R_{I}$. The requirement that
the implementation should satisfy the specification translates as
\[ R_{I} \subseteq R_{S} \]
In words, all possible outcomes of invoking the implementation on a
given set of arguments and initial state must be included in the
outcomes allowed by the specification. It is possible that $R_{I}$ is
actually a function while $R_{S}$ is not. For instance, if an
implementation of the $SEARCH$ routine discussed above always returns
the first location (if any) of the item in the sequence, then the
implementation is indeed deterministic. [Implicit in the subset
relation is the requirement that the domain of $R_{I}$ be the same
as the domain of $R_{S}$, otherwise the implementation ignores some
legal inputs -- Eric]

Before winding-up, we take a look at issues of {\em termination}. It
is possible for a procedure to not terminate at all for some inputs.
We can denote this case by a special state $\bot$, pronounced {\em
bottom}. An implementation is said to be {\em partially correct} if it
satisfies the specification whenever it terminates.  It is said to be
{\em totally correct} if it is partially correct and always
terminates.

\end{document}
