Distributed Systems Distributed Systems- COMMUNICATION.pptx

DistributedSystems
Asst.Prof.Dr.Satea Hikmat Alnajjar
Chapter Four: COMMUNIC ATION
By: Mustafa Saad

Layered Protocols (1)
Figure 4-1. Layers, interfaces, and protocols in the OSI
model.

Layered Protocols (2)
Figure 4-2. A typical message as it appears on the
network.

Middleware Protocols
Figure 4-3. An adapted reference model for
networked communication.

Types of Communication
Figure 4-4.Viewing middleware as an intermediate (distributed)
service in application-level communication.

Types of Communication
1) Client/Server computing is generally based on a model of transient
synchronous
communication:
•Client and server have to be active at time of communication.
•Client issues request and blocks until it receives reply.
•Server essentially waits only for incoming requests, and subsequently processes
them
Drawbacks of synchronous communication:
•Client cannot do any other work while waiting for reply.
•Failures have to be handled immediately: the client is waiting.
•The model may simply not be appropriate (mail, news)
2) Message-oriented middleware: asynchronous communication: Processes
send each other messages, which are queued
•Sender need not wait for immediate reply, but can do other things Middleware often
ensures fault tolerance

Conventional Procedure Call
Figure 4-5. (a) Parameter passing in a local procedure call: the stack before the
call to read. (b) The stack while the called procedure is active
tead(fd,buf,nbytes)

Basic RPC Operation
Figure 4-6. Principle of RPC between a client and server program.
RPC = Remote Procedure Calls

RPC Operation
Figure 4-7.The steps involved in a doing a remote computation through RPC.

Remote Procedure Calls (1)
A remote procedure call occurs in the following steps:
1. The client procedure calls the client stub.
2.The client stub builds a message and calls the local
operating system.
3. The client’s OS sends the message to the remote OS.
4. The remote OS gives the message to the server stub.
5.The server stub unpacks the parameters and calls the
server.
Continued …

Remote Procedure Calls (2)
6.The server does the work and returns the result to the
stub.
7.The server stub packs it in a message and calls its local
OS.
8. The server’s OS sends the message to the client’s OS.
9. The client’s OS gives the message to the client stub.
10.The stub unpacks the result and returns to the client.
Client - client stub - client OS – remote OS - server stub – server

Passing Value Parameters (2)
Figure 4-8. (a) The original message on the Pentium.

Passing by Reference
• Like passing a pointer or an array or by reference.
•The pointer or address has meaning within the same address
space.
•Call by reference is replaced by a copy/restore.
•Difference between input and output parameters.
•A remote reference mechanism enhances access
transparency
 Remote reference offers unified access to remote data (URI
in restful API)
 Remote references can be passed as parameter in
RPCs(passed in same URL)
 Note: stubs can sometimes be used as such references

Parameter Specification and Stub Generation
Figure 4-9. (a) A procedure. (b) The corresponding
message.

Asynchronous RPC (1)
Figure 4-10. (a) The interaction between client and server in a traditional RPC.

Figure 4-10. (b) The interaction using asynchronous RPC.

Figure 4-11. A client and server interacting through
two asynchronous RPCs.

Distributed Systems Distributed Systems- COMMUNICATION.pptx

Writing a Client and a Server (2)
Three files output by the IDL compiler:
•A header file (e.g., interface.h, in C terms).
•The client stub.
•The server stub.

Binding a Client to a Server (1)
•Registration of a server makes it possible for a client to
locate the server and bind to it.
• Server location is done in two steps:
1. Locate the server’s machine.
2. 2. Locate the server on that machine.
• The server also registers with the directory service by providing it
the network address of the server's machine and a name under
which the server can be looked up.
• How does the client stub find the server stub?
• Needs to know remote IP address/port no.

Binding a Client to a Server (2)
Figure 4-13. Client-to-server binding in DCE.
DCE = Distributed Computing Environment
RPC = Remote Procedure Calls
• Port mapper
• Daemon on server machine maintaining server
bindings
• Listens on a well-known port
• Server stub registers its port no. and service
name with port mapper
• Client gets this binding by querying port
mapper

Berkeley Sockets
Figure 4-14. The socket primitives for TCP/IP
.

Distributed Systems Distributed Systems- COMMUNICATION.pptx

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