Four categories of middleware

  Middleware Lou Somers

  April 24, 2003

  1 Contents Overview

  Definition, goals, requirements

Four categories of middleware

  Transactional, message oriented, procedural, object

Middleware examples

  XML-RPC, SOAP, Corba, (D)COM, .NET

Some applications

  Controller - engine communication Controller architecture Web based printer monitoring and control

  2 History

Mainframes

  Centralized computing Expensive devices

Client-server paradigm

  More functions performed by client PC’s Applications split into light client part and sharable server part

Distributed objects

  Distribution combined with objects Components can have both server and client roles

  3 Why distributed? Increased resource sharing Increased maintainability and reduced costs

  Install once, run anywhere Upgrade software on single machine only

Increased fault tolerance

  Replication

Performance

  Compared to single machine (-) Compared to client-server (+)

  4

  5 What is middleware? “The glue which connects objects which are distributed across multiple heterogeneous computer systems” “An extension of the operating system which provides a transparent communication layer to the applications” “A software layer that serves to shield the application of the heterogeneity of the underlying computer platforms and networks”

  6 Middleware goals Integrate existing components into a distributed system

  Components may be off-the-shelf Components may have incompatible requirements for hardware and OS platforms Scalability requires distribution (not centralized or client server).

Resolve heterogeneity

  Facilitate communication and coordination of distributed components Build systems distributed across a local area network, the internet Future: adaptive, reconfigurable Requirements for middleware / 1

Network communication

  Need higher level primitives than network operating system primitives Transport complex data structures over the network (marshalling / unmarshalling)

Coordination

  Three models:

  Synchronous: client waits for result Deferred synchronous: client asks for result (e.g. by polling) Asynchronous: server initiates result

  Group requests Component activation / deactivation Concurrent requests ₇ Requirements for middleware / 2

Reliability

  Error detection and correction mechanisms on top of network protocols

Scalability

  Access to a component independent of whether it is local or remote Migration transparency Replication transparency

Heterogeneity

  Primitive data encoding Different programming languages

  8 Middleware categories

Transactional middleware

  Offers a tuple abstraction (SQL) Distributed transaction processing (DTP protocol)

Message oriented (MOM)

  Offers a mailbox abstraction Asynchronous messages

Procedural (RPC)

  Offers a procedure abstraction Synchronous client / server interaction

Object and component

  Offers an object abstraction (A)synchronous client / server interaction ₉ Transactional middleware / 1

Transactions on distributed relational database

  Two-phase commit protocol to implement distributed transactions

Examples

  IBM CICS BEA Tuxedo LDAP (simple protocol to access remote directories, no transactions)

  10 Transactional middleware / 2

Network communication

  Client and servers may reside on different hosts

Coordination

  Synchronous and asynchronous

Reliability

  DTP (Distributed Transaction Protocol): two phase commit ACID properties:

  Atomic: transaction is either complete or not Consistent: system always in consistent state Isolation: transaction is independent of other transactions Durable: committed transaction survives system failures

Scalability

  Load balancing and replication of server components

Heterogeneity

  Different hardware and operating systems platforms No data heterogeneity ₁₁ Message oriented middleware / 1

Exchange messages between components

  “Mailbox”

Examples

  Java Message Queue

  IBM MQSeries

  12 Message oriented middleware / 2

Network communication

  Client sends message, server replies with result Well suited for event notification and publish / subscribe

Coordination

  Asynchronous Synchronous has to be coded by client

Reliability

  Message queues are stored on persistent memory At-least-once semantics possible

Scalability

  Local / remote differs

Heterogeneity

  Marshalling code has to be written by hand ₁₃ Procedural middleware / 1

Remote procedure calls (RPC)

  Procedures can be called across the network

Examples

  Unix RPC’s DCE RPC (Distributed Computing Environment) Windows RPC’s

  XML-RPC

  14 Procedural middleware / 2

Network communication

  Server exports parameterized procedures Clients call these across the network Marshalling and unmarshalling by client and server stubs

  (generated by the compiler)

Coordination

  Synchronous interaction between one client and one server Startup on demand possible

  (daemon needs a table that maps RPC names to program locations in the file system)

Reliability

  At-most-once semantics (exception if RPC fails)

No scalability Heterogeneity

  Can be used between different programming languages Across different hardware and OS platforms ₁₅ Object middleware / 1

Objects are available across the network Examples

  Corba COM, DCOM Java RMI, Enterprise Java Beans

  16 Object middleware / 2

Network communication

  Client objects call methods of exported server objects Marshalling and unmarshalling by client and server stubs

  (generated by the compiler)

Coordination

  Default: synchronous Corba 3.0: also deferred synchronous and asynchronous

Reliability

  Default: at-most-once semantics (exceptions on failure) Usually requests may be clustered into transactions

  (Object Transaction Server, Microsoft Transaction Server, Java Transaction Service)

Scalability

  Support for load-balancing, replication is rather limited

Heterogeneity

  Corba, COM: multi-language binding Corba / RMI and Corba / COM may interoperate ₁₇ Middleware examples

XML-RPC, SOAP

  RPC on top of HTTP

Corba

  Distributed objects

COM / DCOM

  Microsoft’s Component Object Model

  .NET

  Microsoft’s answer to J2EE

  18 XML-RPC protocol / 1

Uses XML to transmit and receive RPC’s

  Defines the bare minimum to get RPC’s across the network

Based on HTTP with the POST method

  Request is an XML document containing a method name and parameters Response is an XML document with returned values _{Client Server XML}

  XML

SOAP (Simple Object Access protocol)

  Successor of XML-RPC ₁₉ XML-RPC protocol / 2

HTTP Header

  Content-Type: text/xml Content-Length must be specified and correct User agent must be specified Host must be specified

Document content

  Root <methodCall> Contains name of method: <methodName> Contains list of parameters: <params> Each parameters is a pair <param>, <value>

Data types supported

  String, integer, float, date, binary, boolean Array, struct ₂₀ Corba “Common Object Request Broker Architecture” Defined by Object Management Group (OMG)

  > 800 companies: www.omg.org

Goal: enable interoperability

  Languages, implementations, platforms

  Conceptual view: _{Client Server} Object Request Broker (ORB)

  21 Corba: overall architecture _{Client Server Invocation Static ORB Skeleton Skeleton Adapter} Dynamic _{Interface Invocation Interface} ^{IDL Stubs: General Static IDL Dynamic Object} _{Object Request Broker (ORB)} Static interfaces

  Specific for each object type Dynamic interfaces

  Same interface independent of target object Object adapter Object references, object (de)activation, method invocation, ... ₂₂ Corba: general interaction scheme _{Client Server} Method call Method implementation _{(proxy) (server-side Stub Skeleton}

_stub)

Object Request Broker (ORB)

  Stubs and skeletons Generated from the Interface Definition (in IDL) Using appropriate language mappings

  Interface Definition Language Purely descriptive, not an implementation language Strongly typed ₂₃ Corba: object model

Object

  Identifiable entity Provides services to a client May be created and destroyed The interface repository contains type information

Service requests

  Consist typically of: operation, target object, arguments May be specified statically or formed dynamically

Values

  Non-object values and object references

Inheritance

  An interface may inherit from another interface Multiple inheritance Object: base of the entire hierarchy ₂₄ Corba: object request broker More realistic view: _{Client Server}

  ORB ORB _{Device X Device Y}

Protocol between ORB’s

  General InterORB Protocol (GIOP)

GIOP on top of TCP/IP

  Internet InterORB Protocol (IIOP)

Future: on top of SOAP

  ₂₅ COM / DCOM (Distributed) Component Object Model

  Functionality like Corba Internally, (D)COM uses RPC’s

Difference between interface and implementation

  Objects may implement more than one interface Interface is immutable Objects may be implemented in “any” language

Objects register themselves in the Windows registry

  Using a 128-bit Global “Unique” Identifier (GUID)

IDL compiler produces a type library

  Binary description Usable from all COM aware languages Registered into the registry ₂₆

  .NET

Common Language Runtime

  Virtual machine like Java

MSIL

  Intermediate language, like Java byte code

JIT compiler Cross-language interoperability

  VB, C#, Jscript, C++, third party (Perl, Python, Cobol)

Framework class library

  Written in C# Any CLR language can access it

Web services

  Accessed via SOAP and defined by XML ₂₇

  .NET: remote objects

COM and .NET components are interchangeable Remote objects

  Single call object (serves only one request) Singleton object (serves multiple clients) Client activated object

Managed / unmanaged code

  Using / not using Common Language Runtime

Inter process communication via

  HTTP/SOAP TCP/binary COM is used for managed/unmanaged communication ₂₈ Some middleware applications

Controller - engine communication

  Via message oriented middleware

Controller architecture

  Components interact via COM

Web based printer monitoring and control

  ASP’s use COM

  29 Controller - engine communication _{SNMP, HTTP Remote control User Interface Client Controller Device (PC) (PC) (embedded} Print jobs Control _{(pcl, postscript, pdf, ...) LPR, AppleTalk, IPP, … FTP, HTTP Bitmap scans Scans (tiff, pdf)}

_{Bitmaps to print}

_{CSL software)}

Control via “CSL” middleware (Client Server Layer)

  Message oriented Synchronous (post message) Asynchronous (send message, subscribe) ₃₀ Controller architecture _{Controller PC Scans Print jobs Export client Splitter Scanner control Remote client … workers Print client RIP worker Printer control} … clients _{Control Bitmaps Control Blackboard COM} Bitmaps

Components interact via COM with blackboard

  ₃₁ Web based printer monitoring / control _{Intra Logic PC ADP component RPC controllers EFI clients HTTP Web Web EFI plug-in ASP’s server} COM _{SNMP plug-in SNMP 8400 HP … plug-in … 31x5 plug-in SNMP} 31x5 Active Server Pages interact via COM with “business objects” RPC and SNMP used for interaction with printers ₃₂