• shows how a system works at runtime as a collaboration of components
• runtime system quality attributes:
  • performance
  • availability
  • reliability
  • scalability
  • integrability

Elements of the View

• Components
  • processes, threads, clients, servers, services, peers
• Connectors
  • shared memory, remote procedure call, messages, pipe service invocation

Distributed Programming

• Components distributed over a network
  • client/server
• concurrency
• communication and coordination
• built on top of a basic messaging system
  • sockets

Sockets

• Operating System communication mechanism
  • part of POSIX specification
  • inter-process communication API
  • Identified by IP/Port
  • Byte Streams Messages

Distributed Programming

• Abstraction of communication
  • localization
  • communication errors handling
  • repetitive programming
• heterogeneity
• security

Middle-ware

• Layer between applications and operating systems
  • hides distribution
  • hides heterogeneity
  • high-level API
  • common services

Middle-Ware Technologies

• Remote Procedure Call (RPC)
• Message Oriented Middle-ware (MOM)
• Distributed Object-Oriented Components
• Service Oriented Architecture (SOA)

RPC

• Interface definition language (IDL)
  • specify remote procedures
  • generate stubs and skeletons
  • language heterogeneity
  • evolution, portability

IDL Example

• gRPC interface definition
• protocol buffers

// ProductInfo.proto
syntax = "proto3";
package ecommerce;
service ProductInfo {
 rpc addProduct(Product) returns (ProductID);
 rpc getProduct(ProductID) returns (Product);
}
message Product {
 string id = 1;
 string name = 2;
 string description = 3;
}
message ProductID {
 string value = 1;
}

Call Semantics

Types of guarantees provided to Caller

• at least once semantics

  At least once:
  request_received = false
  while not request_received:
      send RPC
      wait for acknowledgement with timeout
      if acknowledgment received and acknowledgement.is_successful:
          request_received = true     

• at most once semantics

  At most once:
  request_sent = false
  while not request_sent:
      send RPC
      request_sent = true
      wait for acknowledgement with timeout
      if acknowledgment received and not acknowledgement.is_successful:
          request_sent = false
  

RPC Advantages / Disadvantages

• make distributed programming easier
• hides semantic differences between local/remote calls
• coupling related to interfaces

Message-Oriented Middleware

• message broker between producers and consumers
• asynchronous messages
• point-to-point -> unicast communication
• publish/subscribe -> broadcast communication

RabbitMQ

• open source message oriented Middleware
  • cross-platform, cross-language
  • implements advanced message queuing protocol (AMQP)
    • AMQP is a standard message broker specification
      • interoperability
    • various interaction modes

AMQ Model

• private/shared

• durable/temporary

• server-named/client-named

• Direct Exchange

  • point-to-point communication

• Fanout Exchange

  • broadcast communication

• Topic Exchange

  • public-subscribe communication

Spring AMQP

• spring support for AMQP
• Springboot starter: spring-boot-starter-amqp
  • autoconfiguration
  • RabbitMQ integration

Advantages/Drawbacks of MOM

• lower coupling

• better reliability

• better interoperability

• lower-lever abstraction

Distributed Object-Oriented Components

• Evolution of RPC model to Object-Oriented Paradigm
  • Transparent invocation of distributed objects
  • Based on
    • proxy pattern
      • substitute for a component
      • abstract communication
      • location transparency
      • a broker for each host (broker registers to server)
    • broker pattern

Middle-ware Advantages / Drawbacks

• Speed up development
• software quality (reliability, robustness)
• performance penalty
• must be learned
• vendor lock-in

Types of communication

Synchronous communication

Caller is blocked until Callee returns

• behaviour traceability (debugging, understandably)
• tight coupling
• connection overhead
• handling failures is hard
• performance low

Asynchronous communication

Caller continues work, gets notified

Component Based Software Engineering

• software development based on assembly of software components
• based on component models