1. Introduction to Distributed Systems Slides for CSCI 3171 Lectures E. W
Introduction to Distributed Systems Slides for CSCI 3171 Lectures E. W. Grundke

2. References A. Tanenbaum and M. van Steen (TvS)
Distributed Systems: Principles and Paradigms
Prentice-Hall
G. Coulouris, J. Dollimore and T. Kindberg (CDK)
Distributed System: Concepts and Design
Addison-Wesley
Acknowledgment: Some slides from
TvS:
CDK:

3. What is a Distributed System?
A collection of independent computers that appears to its users as a single coherent system.
Examples:
Distributed object-based systems (CORBA, DCOM) Distributed file systems (NFS) etc.
TvS 1.2

4. Heterogeneity Applies to all of the following: networks
Internet protocols mask the differences between networks
computer hardware
e.g. data types such as integers can be represented differently
operating systems
e.g. the API to IP differs from one OS to another
programming languages
data structures (arrays, records) can be represented differently
implementations by different developers
they need agreed standards so as to be able to interwork
CDK Ch. 1.4

5. Transparency in a Distributed System
Description
Access
Hide differences in data representation and how a resource is accessed
Location
Hide where a resource is located
Migration
Hide that a resource may move to another location
Relocation
Hide that a resource may be moved to another location while in use
Replication
Hide that a resource may be shared by several competitive users
Concurrency
Failure
Hide the failure and recovery of a resource
Persistence
Hide whether a (software) resource is in memory or on disk
Different forms of transparency in a distributed system.
TvS 1.4

6. Layers, interfaces, and protocols in the OSI model.
Layered Protocols: IP
Layers, interfaces, and protocols in the OSI model.

7. Layered Protocols: OSI
Layers, interfaces, and protocols in the OSI model.
2-1
TvS 2.2

8. An adapted reference model for networked communication.
Middleware Protocols
2-5
An adapted reference model for networked communication.
TvS 2.6

9. Middleware A software layer that masks the heterogeneity of systems
provides a convenient programming abstraction
provides protocols for providing general-purpose services to more specific applications, eg.
authentication protocols
authorization protocols
distributed commit protocols
distributed locking protocols
high-level communication protocols
remote procedure calls (RPC)
remote method invocation (RMI)

10. General structure of a distributed system as middleware.
1-22
TvS 1.24

11. Middleware and Openness
1.23
In an open middleware-based distributed system, the protocols used by each middleware layer should be the same, as well as the interfaces they offer to applications.
TvS 1.25

12. Middleware programming models
Remote Calls
remote Procedure Calls (RPC)
distributed objects and Remote Method Invocation (RMI)
eg. Java RMI
Common Object Request Broker Architecture (CORBA)
cross-language RMI
Other programming models
remote event notification
remote SQL access
distributed transaction processing
CDK Ch 1

13. External Data Representation
See Coulouris, Dollimore and Kindberg (CDK), Sec. 4.3

14. Motivation
Data in running programs: Not just primitives, but arrays, pointers, lists, trees, etc.
In general: complex graphs of interconnected structures or objects
Data being transmitted:
Sequential! Pointers make no sense. Structures must be flattened.
All the heterogeneities must be masked! (endian, binary formats, etc.)
CDK 4.3

15. Motivation
Data in running programs: Not just primitives, but arrays, pointers, lists, trees, etc.
In general: complex graphs of interconnected structures or objects
Data being transmitted:
Sequential! Pointers make no sense. Structures must be flattened.
All the heterogeneities must be masked! (endian, binary formats, etc.)
CDK 4.3

16. What is an External Data Representation?
“An agreed standard for the representation of data structures and primitive values.”
Internal to external: “marshalling”
External to internal: “unmarshalling”
Examples:
CORBA’s Common Data Representation (CDR) Java Object Serialization Sun XDR (RFC 1832)

17. CORBA CDR Defined in CORBA 2.0 in 1998 Primitive types:
Standard data types, both big/little endian, conversion by the receiver.
Constructed types:
sequence, string, array, struct, enumerated, union
(not objects)
Data types are not specified in the external format: receiver is assumed to have access to the definition (via IDL).
(unlike Java Object Serialization!)
CDK 4.3

18. the heterogeneity of hardware was masked
CORBA CDR
only defined in CORBA 2.0 in 1998, before that, each implementation of CORBA had an external data representation, but they could not generally work with one another. That is:
the heterogeneity of hardware was masked
but not the heterogeneity due to different programmers (until CORBA 2)
CORBA CDR represents simple and constructed data types (sequence, string, array, struct, enum and union)
note that it does not deal with objects
it requires an IDL specification of data to be serialised
CDK 4.3

19. CORBA CDR Example 0-3 5 Length of string 4-7 ”Smit” “Smith” 8-11
Index in sequence bytes wide Notes
of bytes
0-3 5
Length of string
4-7
”Smit”
“Smith”
8-11
”h____”
12-15 6
16-19
”Lond”
“London”
20-23
”on__”
24-27
1934
Unsigned long
The flattened form represents a Person struct with value: {”Smith”, ”London”, 1934}
CDK 4.3

20. Remote Procedure Calls (RPC)

21. What is RPC? Call a procedure (function, subroutine, method, …)
in a program
running on a remote machine,
while hiding communication details from the programmer.
Note: Think C, not java! We deal with objects later!

22. Conventional Procedure Call
Parameter passing in a local procedure call: the stack before the call to read
The stack while the called procedure is active
TvS 2.7

23. Parameter Passing Techniques
Call-by-value
Call-by-reference
Call-by-copy/restore

24. Client and Server Stubs
Principle of RPC between a client and server program.
TvS 2.8

25. Steps of a Remote Procedure Call
Client procedure calls client stub in normal way
Client stub builds message, calls local OS
Client's OS sends message to remote OS
Remote OS gives message to server stub
Server stub unpacks parameters, calls server
Server does work, returns result to the stub
Server stub packs it in message, calls local OS
Server's OS sends message to client's OS
Client's OS gives message to client stub
Stub unpacks result, returns to client
TvS 2.9

26. Passing Value Parameters
Steps involved in doing remote computation through RPC
2-8
TvS 2.10

27. Passing Value Parameters: Data Representation Issues
Original message on the Pentium
The message after receipt on the SPARC
The message after being inverted. The little numbers in boxes indicate the address of each byte BUT: This is not usually a problem with strings! (E.W.G.)
TvS 2.11

28. Parameter Specification and Stub Generation
A procedure
The corresponding message.
TvS 2.12

29. Passing Reference Parameters
Reference variables (pointers): pointers to arrays pointers to structures (objects without methods)
What if the structure contains other pointers?
The server may need a whole “graph” of structures!
“Parameter marshalling”
Interface Definition Language (IDL):
Specifies types, constants, procedures and parameter data types, compiled into client and server stubs.

30. Asynchronous RPC
2-12
The interconnection between client and server in a traditional RPC
The interaction using asynchronous RPC
TvS 2.14

31. Asynchronous RPC: Deferred Synchronous RPC
A client and server interacting through two asynchronous RPCs
TvS 2.15

32. Distributed Computing Environment (DCE)
A middleware system
Developed by The Open Group (previously OSF)
Includes
distributed file service directory service security service distributed time service
Adopted by Microsoft for distributed computing

33. DCE: Binding a Client to a Server
2-15
TvS 2.17

34. Remote Method Invocation (RMI)

35. What is RMI? RPC to a method in an object on another machine.
Note: Now think Java!

36. Object Orientation: Remote Method Invocation (RMI)
An object encapsulates
State (fields or instance variables)
Methods (often described by an interface)
Distributed object:
An interface known locally may describe an object on another machine.

37. Distributed Objects
Common organization of a remote object with client-side proxy.
2-16
TvS 2.18

38. Binding a Client to an Object
Distr_object* obj_ref; //Declare a systemwide object reference obj_ref = …; // Initialize the reference to a distributed object obj_ref-> do_something(); // Implicitly bind and invoke a method
(a)
Distr_object objPref; //Declare a systemwide object reference Local_object* obj_ptr; //Declare a pointer to local objects obj_ref = …; //Initialize the reference to a distributed object obj_ptr = bind(obj_ref); //Explicitly bind and obtain a pointer to the local proxy obj_ptr -> do_something(); //Invoke a method on the local proxy
(b)
An example with implicit binding using only global references
An example with explicit binding using global and local references
TvS 2.19

39. The situation when passing an object by reference or by value.
Parameter Passing
The situation when passing an object by reference or by value.
2-18
TvS 2.20

40. The DCE Distributed-Object Model
Distributed dynamic objects in DCE.
Distributed named objects
TvS 2.21