Lesson 2: Middleware

Question 1

Why would we choose middleware? Give 4 arguments

Answer 1

• It’s mostly invisible
• It provides a standard way of doing things
• It ties together parts of complex systems
• It lets you focus on the core task

Question 2

State the requirements for remote invocation, client-side

Answer 2

• creating sockets
• converting arguments to bit stream
• interpreting received bit stream
• fault tolerance measures
• multi-threading
• closing sockets

Question 3

State the requirements for remote invocation, server-side

Answer 3

• creating sockets
• interpreting received bit stream
• converting return values, exception to bit stream
• fault tolerance measures
• multi- threading
• closing sockets

Question 4

Why do we need middleware?

Answer 4

There is no OS architecture for a multicomputer hardware architecture
(multiple computers, no direct memory access). Middleware enables communication between the nodes in this multicomputer architecture.

Question 5

Describe method invocation for local objects regarding: object, object reference and interface

Answer 5

Object: consists of a set of data and a set of methods.

Objects reference: an identifier via which objects can be accessed.

Interface: signatures of methods, no implementation

Question 6

Describe method invocation for remote objects regarding: object, object reference and interface

Answer 6

Remote objects: objects that can receive remote invocation.

Remote object reference:
An identifier that can be used globally throughout a distributed system to refer to a particular unique remote object

Remote interface:
Every remote object has a remote interface that specifies which of its methods can be invoked remotely.

Question 7

What is the purpose of a remote object reference?

Answer 7

To create unique IDs for objects in a distributed system.
Uniqueness over time, uniqueness over space.

Question 8

What is contained in a remote object reference?

Answer 8

Host, process, object and object type.

Question 9

Give differences between local and remote invocations.

Answer 9

Local:
- use object reference
- any public method
- invocation: exactly once

Remote:
- use remote object reference
- limited access, remote interface
- invocation: unknown

Question 10

What are the middleware layers?

Answer 10

• RPCs and RMIs
• Request Reply protocol
• External Data Representation

Question 11

Give some fault tolerance techniques

Answer 11

Retry-request message
Duplicate request filtering
Retransmission of results:
-> re-execute call or
-> history table of results, retransmit reply

Question 12

What is marshalling?

Answer 12

Marshalling is the term used to transform an object in memory to a bitstream.

You can use an external standard (external data representation) or you can use a standard and send this standard along.

Question 13

What is a proxy and what is it responsible for?

Answer 13

A proxy is responsible for making RMI transparent to clients by behaving like a local object to the invoker. It implements the methods in the interface of the remote object that it represents but instead of executing an an invocation, the proxy forwards it to a remote object.

Question 14

What is a skeleton and what is it responsible for?

Answer 14

A skeleton is the reverse of a proxy at the server-side. It is responsible for making RMI transparent to servers by behaving like a local invoker to the object.

Question 15

How does the middleware provide transparency?

Answer 15

It makes the invocation syntax similar to the local invocation.
It hides the underlying complexity by using the same invocation syntax. The use of a remote interface will reflect when an object is remote.

Question 16

What does the communication module do?

Answer 16

• implements the invocation semantics
• runs a request-reply protocol
• ensures communication between proxy & server side

Question 17

What is the dispatcher? What does it do?

Answer 17

It receives all request messages from the communication module. For each request message, it uses the method id to select the appropriate method in the appropriate skeleton, passing on the request message.

Question 18

What is the responsibility of the remote reference module?

Answer 18

It is responsible for translating between local and remote object references and for creating remote object references

Question 19

How does the remote object table of the remote reference module look?

Answer 19

At the server side it contains an entry for each remote object held by any process. At the client side, it has an entry for each local proxy.

Question 20

When filling the remote object table, how do you get the initial reference? (Binding service)

Answer 20

Option 1: Contact the server.
Go to the server and ask for the name

Option 2: Contact a binding server. This is a dictionary service for middleware (or DNS for RPC)