5 - Middleware

Question 1

Middleware

Answer 1

Middleware is a type of software used to manage and facilitate interactions between applications across computing platforms

Question 2

What is Middleware used for?

Answer 2

Middleware enables the distribution of applications to multiple computers in the network

Question 3

Middleware technologies (3)

Answer 3

• Common Object Request Broker Architecture (CORBA)
• Distributed Component Object Model (DCOM)
• Remote Method Invocation (RMI)
• Also: Web Services (extension of middleware concept)

Question 4

Abstractions of middleware

Answer 4

• Remote Procedure Call
• Distributed Objects
• Message Queues
• Web Services

Question 5

Remote Procedure Calls (RPC)

Answer 5

A remote procedure call is the synchronous language level transfer of control between programs in disjoint address spaces whose primary communication medium is a narrow channel.
Ein Remote-Prozeduraufruf ist die synchrone Übertragung der Steuerung auf Sprachebene zwischen Programmen in disjunkten Adressräumen, deren primäres Kommunikationsmedium ein schmaler Kanal ist.

Question 6

What are RPC used for?

Answer 6

• RPC allows functions to be called from other address spaces (frequently on another computer on a shared network)
• Usually, the called functions and the calling program are not executed on the same computer
• The general idea of a procedure call fits the request response pattern
  -> The client makes a request to some external application, sleeps, and finds the result after
  control is returned to the procedure

Question 7

Client Stub

Answer 7

1. Can be called via regular procedure call

2. Marshals (verpacken) arguments and sends request message to server

Question 8

Server stub

Answer 8

1. Receives request message and unmarshals (entpacken)
   arguments
2. Calls server procedure
3. Marshals results and sends response message to client

Question 9

Marshalling: How is problem of serialization of Arguments and return values of procedures solved?

Answer 9

• RPC defines application layer protocol above transport layer
  
  1. Format and sequence of messages (requests,
     response, binding, etc.)
  2. Mapping to transport protocol(s)
• Additionally, specifies encoding of language types (XDR, XML, ..) to message payload

-> Standard protocols and encodings are key for interoperability between different middleware systems

Question 10

Binding: How is problem of specifying what server the client want to bind to solved?

Answer 10

Static Binding: Client statically bound to address of
server
+ Simple, No Overhead
- No load balancing, No failover (Ausfallversicherung)

Dynamic Binding: Client dynamically locates server before (first) call
+ enables load balancing, redundant servers provide failover
- Requires additional service, Overhead of lookup

Question 11

Portmapping and dispatching: How is problem of dispatching requests to correct process and procedure solved?

Answer 11

Portmapping in RPC:

• Client specifies address, program, and version number of server
• Portmapper returns port of server process
• Server stub dispatches request message to correct method implementation

Question 12

What are the three types of Middleware?

Answer 12

1. Message-Oriented Middleware
2. Transaction-Oriented Middleware
3. Object-Oriented Middleware

Question 13

Message oriented middleware (MOM)

Answer 13

Message oriented middleware (MOM) is any middleware infrastructure that provides messaging capabilities . It provides a means to build distributed systems, where distributed processes communicate through messages exchanged via message queuing or message passing

Question 14

How are messages exchanged?

Answer 14

• Messages are exchanged peer to peer (mostly in XML format)
• Asynchronous Message Passing
  or
• Message queue as an intermediary (Message Queuing)

Question 15

Massage passing (Asynchronous)

Answer 15

• Client does not wait for response

- refers to transient communication between two processes that are active at the same time

Question 16

Message Queuing

Answer 16

• Message queue accepts messages to different programs; addressed programs are activated as needed, answers can be fetched later from the queue
• Queue manager can optimize the performance (e.g., prioritization, load allocation)
• Queue manager utilizes persistence and transaction support to protect against loss

-The message queuing model requires an additional intermediary component, which is called the message queue (essentially a mailbox independent of the receiver)

Question 17

Message Broker

Answer 17

• Sender does not need to be aware on which system the server is running
• Receiver can change location during the runtime location dependency is reduced
• Option to change to multicast or broadcast system (not deleting message after first deque operation) -> option for multiple receivers

Question 18

What are two MOM software examples

Answer 18

Microsoft Message Queue
- Used for simple integration of Windows applications

WebSphere MQ

• Available on almost all platforms
• Better scalable because available on more powerful platforms
• Higher fault tolerance, greater reliability of delivery of messages, higher security, better database access
• Broadcasting, publish/subscribe service
• Mature, tested
• Disadvantage: Installation consuming

Question 19

Transaction Oriented Middleware (TOM)

Answer 19

Transaction Oriented Middleware (TOM) is any middleware infrastructure that supports the execution of electronic transactions in a distributed setting

Question 20

Two-Phase commit

Answer 20

Prepare Phase:
A coordinator (usually the process that initiates the committing) obtains approval or denial to commit the data changes of all the processes involved

Commit Phase:
(When all participants agree) the coordinator decide to ‚Commit’. If the decision has been made, the coordinator informs the participants of the result in the second phase

Question 21

What does ACID (transactions) stand for?

Answer 21

atomicity, consistency, isolation, and durability

Question 22

Automicity

Answer 22

Each transaction is either fully executed or not at all (rollback if something fails)

Question 23

Consistency

Answer 23

Transaction leaves database in a consistent state given it was consistent before the transaction

Question 24

Isolation

Answer 24

Transactions cannot influence other transactions when

executed parallel

Question 25

Durability

Answer 25

Permanent storage in DB is guaranteed once a transaction is completed successfully

Question 26

Transaction Processing Monitor

Answer 26

coordinate the flow of requests between terminals or other devices and application programs that can process these requests - not developed for general program-to-program communication integration but provide solutions for transaction-type applications that utilize a database

Question 27

TP-Lite Monitor

Answer 27

- integrated in database (for small applications) - Stored procedure invoked and executed according to ACID principles - Better for periodic data synchronisation

Question 28

TP-Heavy Monitor

Answer 28

- Supports client server architecture - Allows for very complex transactions - Good when using multiple databases

Question 29

Transaction

Answer 29

a series of database operations that are executed either in full or not at all

Question 30

Object Oriented Middleware (OOM)

Answer 30

Object Oriented Middleware (OOM) is defined as a middleware infrastructure that offers object oriented principles for the development of distributed systems - Enables communication between objects within distributed systems

Question 31

Interface Description Language (IDL)

Answer 31

language syntax to describe a software component’s interfaces

Question 32

Common Object Request Broker Architecture (CORBA)

Answer 32

client can discover a service at runtime (Dynamic | Skeleton Interface)

Question 33

Components of TP Monitor

Answer 33

- Transaction Management - interface (API, presentation, authentication) - program flow - Registered programs - TP Services - Resources - router - communication manager