1. Distributed Object-based systems
Prof. Orhan Gemikonakli
Module Leader: Prof. Leonardo Mostarda
Università di Camerino
Prof. Orhan Gemikonakli - Camerino,

2. Prof. Orhan Gemikonakli - Camerino,
Last lecture
Basic concepts
Fault modelling
Failure masking
RPC Semantics in the Presence of Failures
Reliable multicast schemes
Virtual Synchrony
Two phase commit
|Three phase commit
Checkpointing and Message logging
Prof. Orhan Gemikonakli - Camerino,

3. Prof. Orhan Gemikonakli - Camerino,
Outline
Architecture
Processes
Communication
Naming
Synchronization
Consistency and Replication
Fault Tolerance
Tanenbaum & Van Steen, Distributed Systems: Principles and Paradigms, 2e, (c) 2007
Prentice-Hall, Inc
Prof. Orhan Gemikonakli - Camerino,

4. Prof. Orhan Gemikonakli - Camerino,
Learning outcomes
To understand the basic concepts related to distributed object-based systems
To describe and discuss
Architecture
Processes
Communication
Naming
Synchronization
Consistency and Replication
Fault Tolerance as they apply to distributed object-based systems
Prof. Orhan Gemikonakli - Camerino,

5. Distributed Object-based systems
A paradigm used to organise ditributed systems
The notion of object plays a key role in establishing distribution transparency
Everything is treated as objects
Clients are offered services and resources in the form of objects they can revoke.
Examples of object-based distributed systems
CORBA
Java-based systems
Globe
Prof. Orhan Gemikonakli - Camerino,

6. Prof. Orhan Gemikonakli - Camerino,
Architecture
Object orientation plays an important part in software development
Functionality can be added as independent components
1980s: Distributed Object-oriented systems started
An independent object hosted by a remote server
High degree of distribution transparency
An object
Encapsulates data (state)
Encapsulates operations on that data (methods)
May implement multiple interfaces
Prof. Orhan Gemikonakli - Camerino,

7. Prof. Orhan Gemikonakli - Camerino,
Distributed Objects
Distributed object: an interface at one machine while the object resides on another machine
When a client binds to a distributed object an implementation of the object’s interface called proxy is then loaded into the client’s address space.
A proxy marshals method invocations and unmarshalls reply messages (similar to a stub in RPC)
Skeleton: The server-side stub
Objects: compile-time objects and run-time objects
Prof. Orhan Gemikonakli - Camerino,

8. Prof. Orhan Gemikonakli - Camerino,
Distributed Objects
Figure Common organization of a remote object with client-side proxy.
Prof. Orhan Gemikonakli - Camerino,

9. Example: Enterprise Java Beans (EJBs))
Services: messaging, naming
DB access etc.
Figure General architecture of an EJB server.
Prof. Orhan Gemikonakli - Camerino,

10. Prof. Orhan Gemikonakli - Camerino,
Four Types of EJBs
The use of services is automated, but a programmer should make a distinction between types of EJBs:
Stateless session beans
Stateful session beans
Entity beans
Message-driven beans
Prof. Orhan Gemikonakli - Camerino,

11. Prof. Orhan Gemikonakli - Camerino,
Four Types of EJBs
Stateless session bean: a transient object; invoked once, does it work, gets discarded (e.g. an SQL query)
Stateful session bean: a persistent object; maintains client related state (e.g. electronic shopping cart). Yet, limited lifetime.
Entity bean: a long time persistent object (e.g. holding customer details in e-commerce)
Message-driven beans are used to program objects that should react incoming messages and send messages. Stateless.
Prof. Orhan Gemikonakli - Camerino,

12. Example: Globe Distributed Shared Objects (1)
Globe: an object-based distributed system
Scalability plays a central role
Can support large-scale wide-area systems and huge numbers of users.
Objects (distributed shared objects)
encapsulate state and operations on that state.
Also encapsulate the policies on the distribution of an object’s state over its replicas
Physically distributed and replicated (the states) (no remote object model)
Have local implementations (with or without states) – only methods are visible
Primitive or composite local object
Prof. Orhan Gemikonakli - Camerino,

13. Globe Distributed Shared Objects (2)
Figure The organization of a Globe distributed shared object.
Prof. Orhan Gemikonakli - Camerino,

14. Globe Distributed Shared Objects (3)
Figure The general organization of a local object for distributed shared objects in Globe.
Prof. Orhan Gemikonakli - Camerino,

15. Prof. Orhan Gemikonakli - Camerino,
Processes
Object servers play a key role in object-based distributed systems.
They are designed to host distributed objects
Services are implemented by objects and are easy to change
In multi-threaded object servers, each object can be assigned a thread, or each invocation can be assigned a thread.
Prof. Orhan Gemikonakli - Camerino,

16. Processes: Object Adapter
Activation: decisions on how to invoke an object
Servant: piece of code that forms the implementation of an object (e.g. a java bean is a servant.)
Object Adapter (Object Wrapper): A mechanism to group objects per policy.
Controls one or more objects
They are generic but configurable
Several object adapters can exist in a server
Invocation requests are first dispatched to the appropriate object adapters.
Prof. Orhan Gemikonakli - Camerino,

17. Prof. Orhan Gemikonakli - Camerino,
Object Adapter
Figure Organization of an object server supporting different activation policies.
Prof. Orhan Gemikonakli - Camerino,

18. Example: The Ice Runtime System
Ice is a distributed object system
An object server in Ice is a process that starts by initialising Ice runtime system (RTS)
Communicator: a runtime component that manages resources (e.g. pool of threads)
Can specify maximum message length, maximum number of retries etc.
Can create an object adapter
Multiple communicators possible for an object server.
E.g. managing different thread pools.
Prof. Orhan Gemikonakli - Camerino,

19. Prof. Orhan Gemikonakli - Camerino,
The Ice Runtime System
Figure Example of creating an object server in Ice.
Prof. Orhan Gemikonakli - Camerino,

20. Prof. Orhan Gemikonakli - Camerino,
Communication
The mechanism of a client invoking a remote object is largely based on RPCs.
Binding a client to an object
Object references can be passed to processes and used for method invocations
A process holding an object reference first binds to the referenced object.
Binding results in a proxy been placed into the clients address space
Implicit binding: provides the client with a simple mechanism to invoke methods directly
Explicit binding: client first needs to call a function before binding.
Prof. Orhan Gemikonakli - Camerino,

21. Binding a Client to an Object
Figure (a) An example with implicit binding using only global references. (b) An example with explicit binding using global and local references.
Prof. Orhan Gemikonakli - Camerino,

22. Prof. Orhan Gemikonakli - Camerino,
Parameter passing
When invoking a method, objects may be passed with invocation
Local objects (usually small) are passed as a whole
Remote objects are passed by reference
Prof. Orhan Gemikonakli - Camerino,

23. Prof. Orhan Gemikonakli - Camerino,
Parameter Passing
Figure The situation when passing an object by reference or by value.
Prof. Orhan Gemikonakli - Camerino,

24. Object-Based Messaging
Remote Method Invocation (RMI) is the preferred way of handling communication in object-based distributed systems
CORBA messaging combines method invocation and message-oriented communication
Task: Compare and contrast RMI and CORBA in the context of handling communication in object-based distributed systems
Prof. Orhan Gemikonakli - Camerino,

25. Prof. Orhan Gemikonakli - Camerino,
Naming
The interesting aspect of naming in object-based distributed systems evolves around the way that object references are supported.
As discussed, in Java, these object references correspond to portable proxy implementations.
Task: How are objects referenced in CORBA? Distinguish between referencing local, and remote objects.
Prof. Orhan Gemikonakli - Camerino,

26. Prof. Orhan Gemikonakli - Camerino,
Synchronisation
Problem: Implementation details are hidden behind interfaces
When a process invokes a remote object, it has no knowledge whther that invocation will lead to invoking other objects.
If an object is protected against concurrent access, we may have cascading set of locks the invoking process is unaware of (Fig 10-a)
When dealing with data resources such as files or database tables that are protected by locks, the pattern for the control flow is visible to the process. (Fig 10-b)
Prof. Orhan Gemikonakli - Camerino,

27. Prof. Orhan Gemikonakli - Camerino,
Synchronization
Figure Differences in control flow for locking objects
Prof. Orhan Gemikonakli - Camerino,

28. Consistency and Replication
General approach to replicated objects:
Traditional approaches (as discussed in Chap 7) treating them as containers of data with their own operations
Consistency: Data-centric consistency comes in the form of entry consistency
No multiple invocations on the same object allowed
Ensure that all the changes to a replicated object are the same.
Prof. Orhan Gemikonakli - Camerino,

29. Consistency and Replication
No multiple invocations on the same object allowed
Use locks to allow only one method execution at a time – use local locks
Recall that data is accessed through methods only, hence access to data is serialized
Ensure thatr all the changes to a replicated object are the same.
No two independent method invocations takes place on different replicas at the same time – order invocations so that each replica see them in same order. Use
Primary-based approach or
Totally ordered approach
Prof. Orhan Gemikonakli - Camerino,

30. Prof. Orhan Gemikonakli - Camerino,
Entry Consistency
Figure Deterministic thread scheduling for replicated object servers.
Prof. Orhan Gemikonakli - Camerino,

31. Replication Frameworks (1)
Invocations to objects are intercepted at three different points:
At the client side just before the invocation is passed to the stub – in case the caller is replicated
Inside the client’s stub, where the interception forms part of the replication algorithm – to forward or abort
At the server side, just before the object is about to be invoked.
Prof. Orhan Gemikonakli - Camerino,

32. Replication Frameworks (2)
Figure A general framework for separating replication algorithms from objects in an EJB environment.
Prof. Orhan Gemikonakli - Camerino,

33. Replicated Invocations (1)
Figure The problem of replicated method invocations.
Prof. Orhan Gemikonakli - Camerino,

34. Replicated Invocations (2)
Figure (a) Forwarding an invocation request from a replicated object to another replicated object.
Prof. Orhan Gemikonakli - Camerino,

35. Replicated Invocations (3)
Figure (b) Returning a reply from one replicated object to another.
Prof. Orhan Gemikonakli - Camerino,

36. Prof. Orhan Gemikonakli - Camerino,
Fault Tolerance
Similar mechanisms used for other distributed systems are used for object-based DSs too.
CORBA:
Replicates objects into object groups
Object groups are referenced as a single object offering the same interface as replicas it contains
Different replication strategies are used
Primary-backup replication
Active replication
Quorum-based replication
Object groups are referenced using an Interoperable Object Group Reference (IOGR)
Prof. Orhan Gemikonakli - Camerino,

37. Example: Fault-Tolerant CORBA
Figure A possible organization of an IOGR for an object group having a primary and backups.
Prof. Orhan Gemikonakli - Camerino,

38. An Example Architecture
Figure An example architecture of a fault-tolerant CORBA system.
Prof. Orhan Gemikonakli - Camerino,

39. Example: Fault-Tolerant Java
Causes for nondeterministic behavior:
JVM can execute native code, that is, code that is external to the JVM and provided to the latter through an interface.
Input data may be subject to nondeterminism.
In the presence of failures, different JVMs will produce different output revealing that the machines have been replicated.
Prof. Orhan Gemikonakli - Camerino,

40. Prof. Orhan Gemikonakli - Camerino,
Summary
Distributed Object-based Systems
Architecture
Processes
Communication
Naming
Synchronization
Consistency and Replication
Fault Tolerance
Prof. Orhan Gemikonakli - Camerino,

41. Prof. Orhan Gemikonakli - Camerino,
Next Lecture
Distributed File Systems
Architecture
Processes
Communication
Naming
Synchronization
Consistency and Replication
Fault Tolerance
Prof. Orhan Gemikonakli - Camerino,