The document discusses three RPC communication protocols: 1. The R protocol uses asynchronous communication with only request messages and no reply. This improves performance. 2. The RR protocol fits requests and replies into a single packet and caches replies to reduce overhead. 3. The RRA protocol adds acknowledgements to replies to make communication more reliable as it ensures replies are received.

2. RPC communication protocol
Following are different communication protocols used in
RPC communication:
1. The request protocol (R protocol)
2. The request/reply protocol(RR protocol)
3. The request/reply/acknowledge(RRA protocol)

3. The R protocol:
 In request protocol, the server has nothing to return to the client
process. In this protocol, there is no reply or acknowledgement; hence
in each call only one message is there, called request message.
 The client doesn’t wait after sending the request message as there is no
reply expected. It proceeds to its next execution immediately.
 The RPC R protocol is also referred as asynchronous RPC.IT helps in
improving the performance since client doesn’t need any reply form
server, thus reducing network communication overheads.
 The working of R protocol is illustrated :
 As R protocol is an asynchronous RPC, it is mostly use in the case
where periodic updates like clock synchronisation are required.

4. The RR protocol
 In request/reply protocol, the RPC has the arguments
and the result as well fitted into a single packet buffer
and interval between two calls and its duration is very
short.
 The server keeps the history of reply messages, so that
if client request the same procedure, he server need
not to reexecute it, but simply it retransmits the reply
and thus reducing execution overhead.

5. The RRA protocol
 To overcome the limitation of RR protocol, the concept of
request/reply/acknowledge-reply protocol has been
implemented .
 The server needs an acknowledge in response to its reply
message has been delivered to the client process successfully.
 The RRA protocol consists of these messages in each RPC call .
 The RRA approach is supposed to be the fully reliable because in
RR protocol sometimes the reply messages sent from server may
not reach to client and server do not know about this message
failure, since client don’t send any acknowledgement.
 In RRA protocol client the client sends an acknowledgement in
response to the reply message sent by server and thus making it
more reliable .

6. Client-server Binding
 In a RPC communication between a client and a server
, the client stub should know well in advance the
location of server to make a remote procedure call.
 The process in which client becomes associated with
the location of server to make a remote procedure call.
 The process in which client becomes associated with
server to establish a remote call is referred for the as
“client-server binding” .
 The server first registers itself with a binding agent to
announce its availability .

7.  Before a remote call can be made , the client requests
the binding agent for the address of server location .
 Since binding agent mechanism has drawbacks , since
it involves huge overhead. Also the binding agent must
be robust against failures since it can become a single
point of failure.
 The binding agent should not become a bottleneck
problem due to large number of requests and replies.

8. Other issues related to RPC
mechanism
 In addition to various issues discussed so far, the RPC
mechanism has following two important issues:
 Security
 Exception handling
 Some of the RPC implementation include client and server
authentication and also provide encrypted message to
handle security issues.
 The RPC mechanism must have a strong exception-
handling mechanism to handle errors and failures occurred
during message transmission and report failures to client as
well as server.

9. DIFFERENCE BETWEEN THREADS
AND PROCESSES

10. THREADS PROCESSES
1. Threads are the unit of execution in a
process: A virtualized processor, a stack,
and program state. Processes are running
binaries and threads are the smallest unit of
execution schedulable by an operating
system's process scheduler.
1. Processes are the abstraction of
running programs: A binary image,
virtualized memory, various kernel
resources, an associated security
context, and so on.
2. A thread is an entity within a process
that can be scheduled for execution. All
threads of a process share its virtual address
space and system resources. In addition,
each thread maintains exception handlers,
a scheduling priority, thread local storage, a
unique thread identifier, and a set of
structures the system will use to save the
thread context until it is scheduled.
2. Each process provides the resources
needed to execute a program. A process
has a virtual address space, executable
code, open handles to system objects, a
security context, a unique process
identifier, environment variables, a
priority class, minimum and maximum
working set sizes, and at least one
thread of execution.

11. THREADS PROCESSES
3. Threads, since they share the same
address space are interdependent, so
caution must be taken so that different
threads don't step on each other.
3. Processes are independent of each
other.
4. Threads exist within a process and
every process has at least one thread.
4. A process has a self contained
execution environment that means it
has a complete, private set of basic run
time resources purticularly each process
has its own memory space.
5. Threads are considered lightweight
because they use far less resources than
processes.
5. A process can consist of multiple
threads.

12. Kernel level threads
 Kernel-Level Threads
 To make concurrency cheaper, the execution aspect of process is
separated out into threads. As such, the OS now manages threads and
processes. All thread operations are implemented in the kernel and the
OS schedules all threads in the system. OS managed threads are called
kernel-level threads or light weight processes.
 NT: Threads
 Solaris: Lightweight processes(LWP).
 In this method, the kernel knows about and manages the threads. No
runtime system is needed in this case. Instead of thread table in each
process, the kernel has a thread table that keeps track of all threads in
the system. In addition, the kernel also maintains the traditional
process table to keep track of processes. Operating Systems kernel
provides system call to create and manage threads.

13. ADVANTAGES AND
DISADAVNTAGES
 Advantages:

 Because kernel has full knowledge of all threads, Scheduler may decide to give more time
to a process having large number of threads than process having small number of
threads.
 Kernel-level threads are especially good for applications that frequently block.

 Disadvantages:

 The kernel-level threads are slow and inefficient. For instance, threads operations are
hundreds of times slower than that of user-level threads.
 Since kernel must manage and schedule threads as well as processes. It require a full
thread control block (TCB) for each thread to maintain information about threads. As a
result there is significant overhead and increased in kernel complexity.

14. USER-LEVEL THREADS
 User-Level Threads
 Kernel-Level threads make concurrency much cheaper than process because,
much less state to allocate and initialize. However, for fine-grained
concurrency, kernel-level threads still suffer from too much overhead. Thread
operations still require system calls. Ideally, we require thread operations to be
as fast as a procedure call. Kernel-Level threads have to be general to support
the needs of all programmers, languages, runtimes, etc. For such fine grained
concurrency we need still "cheaper" threads.
 To make threads cheap and fast, they need to be implemented at user level.
User-Level threads are managed entirely by the run-time system (user-level
library).The kernel knows nothing about user-level threads and manages them
as if they were single-threaded processes . User-Level threads are small and
fast, each thread is represented by a PC , register , stack, and small thread
control block. Creating a new thread, switching between threads, and
synchronizing threads are done via procedure call. i.e. no kernel involvement.
User-Level threads are hundred times faster than Kernel-Level threads.

15. ADVANTAGES AND
DISADVANTAGES
 Advantages:
 The most obvious advantage of this technique is that a user-level threads
package can be implemented on an Operating System that does not support
threads.
 User-level threads does not require modification to operating systems.
 Simple Representation: Each thread is represented simply by a PC, registers,
stack and a small control block, all stored in the user process address space.
 Simple Management: This simply means that creating a thread, switching
between threads and synchronization between threads can all be done without
intervention of the kernel.
 Fast and Efficient: Thread switching is not much more expensive than a
procedure call.

16. DISADVANTAGES:
 Disadvantages:
 User-Level threads are not a perfect solution as with everything else, they are a
trade off. Since, User-Level threads are invisible to the OS they are not well
integrated with the OS. As a result, Os can make poor decisions like scheduling
a process with idle threads, blocking a process whose thread initiated an I/O
even though the process has other threads that can run and unscheduling a
process with a thread holding a lock. Solving this requires communication
between kernel and user-level thread manager.
 There is a lack of coordination between threads and operating system kernel.
Therefore, process as whole gets one time slice irrespect of whether process has
one thread or 1000 threads within. It is up to each thread to relinquish control
to other threads.
 User-level threads requires non-blocking systems call i.e., a multithreaded
kernel. Otherwise, entire process will blocked in the kernel, even if there are
runnable threads left in the processes. For example, if one thread causes a page
fault, the process blocks.

17. Happened Before Relationship
 In case of ordering events according to time, Lamport proposed a
scheme using logical clocks.
 Since there is no perfect synchronisation in local time b/w the 2
processes , due to unavailability of a global clock, events cant be
ordered based on time. What Lamport proposed that is possible to
order events b/w the processes based on the behaviour of that specific
computation.
 He used the notion of “happened before ” relation.
 It captures the casual dependencies b/w events i.e. 2 relations are
related or not . The relation ->is defined as:
 1. a->b i.e. a occurred before b 2. a->b
 Here a is sending message to b. Both a and b are from different
processes.
 3. If a->b and b->c then a->c
 i.e. -> relation is transitive.

18.  In a distributed system, the ordering of the events
occurring in the processes involved has impact on the
outcome of events yields different outcomes. Hence
designing and understanding the sequence of execution for
a computation is very important.
 So event that are ordered by -> relation are called as
casually related events and this effect is called as casual
effects.
 Therefore, event ‘a’ casually affects event ‘b’, if a->b. Both
the events a and b are said to be casually related events .
 2 distinct events a and b are called as concurrent events and
are denoted by a||b.

19. Logical clocks:
 Lamport introduced logical clocks to realize the relation “->”. The clock say Ci a each process P(i)
works on the concept of timestamp.
 Let Ci(a) is the clock value for event a. The value cia is called timestamp of event a at pi.
 The values assigned to the clocks don’t have any relation to the actual time.
 The logical clocks have increasing values and thy can be implemented with the help of counters .i.e.
1,2,3,4 and so on.
 Conditions satisfied by logical
 For any events a and b,
 If a->b
 Then C(a)<C(b)
 The relation -> can be realized by using logical clock if following 2 conditions are met.
 Condition 1
 For any events a and b in a process pi, if a occurs before b,
 Then ci (a) <cib
 Condition 2
 If event a is sending , event a message m in process pi to the event b in process pj,
 Then cia <cjb

20. IMPLEMENTATION:
 Following two implementation rules guarantee that the clocks satisfy
condition 1 and condition 2:
 Implementation rule 1
 The clock ci is incremented b/w 2 successive events in a process pi:
 Ci = ci +d(d>0)
 If a and b are two successive events in pi and a -> b, then it becomes:
 Ci(b)=ci(a )+d
 Implementation rule 2
 If event a is sending a message m by a process pi to event b by a process
pj and the timestamp for the message m is tm them :
 Tm = ci (a) -> timestamp of event a in clock ciOn receiving the message
the clock cj is set to a value greater than or equal or equal to its present
value and greater than tm.
 Cj=max(cj,tm+d)
 Where d>0

21. Effects of RMI execution
 In java ,the difference b/w a local method invocation
and remote method invocation is kept hidden from
the user.
 After marshalling (i.e. convert the object into required
format for presentation ) the object is passed as a
parameter to RMI in Java.
 A java remote object is built from two main classes :
 1. Server class 2. Client class
 The server-class contains the implementation of
server-side code that runs on server and client-class
contains the implementation of client-side code.

22.  In computing, the Java Remote Method Invocation (Java
RMI) is a Java APIthat performs remote method
invocation, the object-oriented equivalent of remote
procedure calls (RPC), with support for direct transfer
of serialized Java classes and distributed garbage-
collection.
 The original implementation depends on Java Virtual
Machine (JVM) class-representation mechanisms and it
thus only supports making calls from one JVM to another.
The protocol underlying this Java-only implementation is
known as Java Remote Method Protocol (JRMP). In order
to support code running in a non-JVM context,
programmers later developed a CORBA version.

24.  Usage of the term RMI may denote solely the
programming interface or may signify both the API
and JRMP , IIOP , or another implementation, whereas the
term RMI-IIOP (read: RMI over IIOP) specifically denotes
the RMI interface delegating most of the functionality to
the supporting CORBA implementation.
 The basic idea of Java RMI, the distributed garbage-
collection (DGC) protocol, and much of the architecture
underlying the original Sun implementation, come from
the "network objects" feature of Modula-3.