1.264 Lecture 16. Legacy Middleware

Transcription

1 1.264 Lecture 16 Legacy Middleware

2 What is legacy middleware? Client (user interface, local application) Client (user interface, local application) How do we connect clients and servers? Middleware Network Pre-Web, but post-internet!! Server (database, application, etc.) Server (database, application, etc.)

3 Why middleware? Development of Internet Wide area networks with high bandwidth, many users connected Cheap, powerful PCs Many more users in enterprises and on Internet Network and computing leading to business on the Net Purchases, transactions, supply chain, collaborative design, project management, Millions of pre-web servers connected at high bandwidths Pre-Web applications with trillions of distributed transactions Traditional client-server design is not scalable Client-server means one server Fine for departmental application on a local area net (LAN) Need multiple servers to handle applications in our new world Need client-server middleware

4 Why middleware, cont Applications are not scaling well Excel went from 4MB to 32MB of RAM from 1990 to 2000 Lotus went from 0.5MB to 11MB from 1982 to 1997 Ditto for Word, PowerPoint, Access. And growing Many share common features: graphics, tables, formatting, mail merge, etc. Components allow shared code across applications (application middleware) Shortage and high expense of software developers Diseconomies of software development as size of project increases Legacy middleware doesn t scale very well either Same middleware concepts used to share code in applications and across the network between clients and servers: Applications must communicate with other applications that provide some of the needed shared features, whether on same computer or a different one

5 Why not SQL? SQL manages only data, not processes OK for returning list of carriers in a state Not ok for returning optimized truck routes for deliveries SQL performance across networks is poor Sequences of SQL statements and query returns have high delay Much faster to have stored procedures on database server invoked by a function call, execute as a group and return a single result set Execute procedure Client Server Stored procedure Return results DB Insert, delete Client Server Update DB

6 Distributed objects How do you find all the pieces of an application when you can t or don t have them all on your PC or workstation? Distributed objects = components Object: piece of application within a single application (data, methods) Component: a piece of application that can interoperate across operating systems, networks, languages, applications, tools, hardware Several competing standards: Remote procedure call (RPC): synchronous Common Object Request Broker Architecture (CORBA): consortium ActiveX (COM, DCOM, OLE): Microsoft Extensible markup language (XML): consortium Simple Object Application Protocol (SOAP): based on XML Message oriented middleware (MOM); asynchronous XML, BizTalk (Microsoft prefers MOM vs RPC as simpler) Transaction processing monitors (TP monitors) Proprietary, for high volume heterogeneous databases

7 Remote procedure call middleware Two competing legacy RPC standards Common Object Request Broker Architecture (CORBA) ActiveX/COM/DCOM Components live on distributed object buses (ActiveX/COM or CORBA) Object buses provide system support: services that components inherit at build or run time to collaborate with other components Components are architected to work with other components from start

8 Components (legacy and WSDL) Basic requirements: Marketable Not a complete application Usable in unpredictable combinations (similar to database!) Well-specified interface Interoperable across languages, networks, operating systems, Desirable features: Security: protect itself, authenticate itself to clients, audit trails Licensing, versioning, metadata: provide data about itself Event notification: notify interested parties when something happens Configuration and property management, scripting Support for open toolboxes

9 COM Component model with widest use Bottom up architecture: no overall architecture, features added from release to release as feasible Fundamental object model on which ActiveX rests COM allows an object to expose its functionality to other components and to host applications. COM defines both how the object exposes itself and how this exposure works across processes and across networks. COM also defines the object's life cycle. Examples: ActiveX controls can talk with each other and Visual Basic forms All meet the ActiveX interface standard Custom controls are usually built to ActiveX standards Visio, Quicken, Oracle, etc. provide controls

10 COM Concepts Interfaces: Mechanism through which an object exposes its functionality. Table of (pointers to) functions implemented by the object. Replaced by UDDI, WSDL and XML in Internet middleware Discovery (Iunknown): Basic interface on which all others are based. Query interface: Method used to query an object for a given interface. Either returns (pointer to) function or notifies client that it doesn t exist. Reference counting: Technique by which an object (or, strictly, an interface) decides when it is no longer being used and is therefore free to remove itself. Counts number of references from clients; releases when 0. J2EE and ASP.NET handle this in Internet middleware Marshaling: Mechanism that enables objects to be used across thread, process, and network boundaries, allowing for location independence. COM provides code to pack a method's parameters into a format that can be moved across processes (as well as across a network to processes running on other machines) and to unpack those parameters at the other end. SOAP (XML) does this across the Internet Aggregation: Allow one object to make use of another Not implemented on Internet

11 COM Interfaces COM interfaces have a globally unique ID (GUID) 128 bit number, generated by utility to prevent collisions COM is language independent No notion of how the underlying component (object) is implemented Only interface and COM marshalling standard is exposed This is key to reuse. If any internals are exposed, they can and will create problems if they ever change Local/remote transparency. COM object may be: Within same process (.EXE) On same machine On remote machine Use MS registry to locate components on network Must potentially install an entry for every component on every computer: scalability, security, reliability, performance problems COM is essentially at a dead-end, though in wide use

12 ActiveX/COM component-based application Buy ActiveX intfc Chemical ActiveX component Manufacturer ActiveX component Build ActiveX intfc Order entry ActiveX component Buy, configure ADO connection Chem Prod DB Oracle ADO connection Order DB Sybase Databases on servers in corporate network

13 ActiveX Data Objects DB server Web server Browser

14 ActiveX application, cont Components can all be on one machine, or each component can be on a different machine One, two or three tier architecture Chemical catalog and order entry components can have user interface if on client PC. Alternatively, they may be server components, and the vendor of these components will provide separate UI components to be included in the PC client. Remember the client-server SQL performance problems. Forms and navigation can be configured. Databases and components must be in corporate net This won t work over Internet: performance, security, protocol

15 CORBA CORBA is open standard middleware that establishes remote relationships between objects Client can transparently invoke method on server object in network Object appears to be local to the client application Object Request Broker (ORB) is key CORBA object: Intercepts call from client to server Is responsible for finding an object that can implement the request Passes it the parameters Invokes its method Returns the results to the client Client is unaware of where object is located, its programming language, operating system CORBA provides interoperability in heterogeneous environments COM is essentially Microsoft-only, though extensions exist

16 CORBA Defined by Object Management Group (OMG), founded in 1989 as consortium of object vendors (now 800 members). Web site is Current version is CORBA 2.x in the field; CORBA 3.0 spec is out Object Request Broker (ORB) Allows clients to invoke methods on remote objects If remote object has component interface, the calling object binds to a stub to call its methods Otherwise, consult Interface Repository to discover methods Communication between ORBs via tcp/ip (Internet) backbone Object bus has object services Standards for how objects are created, stored, defined and named on bus In process: event service, transactions, relationships, query, licensing... Example: air freight booking Show carriers, flights, space available, book space, pay,

17 CORBA ORB Client Object Implementation Dynamic Invocation Client IDL Stubs ORB Interface Dynamic Static Skeleton Skeletons Invocation Object Request Broker Core Object Adapter Interface Repository Implementation Repository

18 Client ORB components Client IDL stubs: (Static binding) Stubs invoke services actually resident on servers However, from client perspective, server object appears to be local Stub acts as a proxy for the distant object Header files (function names and arguments) for invoking methods locally from programming languages are made available locally Stub performs messaging (marshalling): encode and decode parameters into messages to and from the server Dynamic invocation interface: (Dynamic binding) Standard interface to Interface Repository metadata that defines all server objects Allows lookup, parameter generation/listing, issuing remote request and getting results back Interface repository Contains information on classes (components) supported

19 Server ORB components Server IDL stubs or skeletons: Same as client IDL stubs Server object is not really being called locally on the client but its skeleton makes it seem that way Dynamic skeleton interface: Same as dynamic invocation interface on client Message is in more generic format, processed differently Object adapter (manager): Not needed on client, which has fewer objects or processes Accepts requests to server objects, assigns object IDs, registers classes supported on server (very different than ActiveX) Implementation repository Similar to client Interface Repository that provides run-time information on classes supported, objects instantiated on server

20 CORBA Interface Definition Language (IDL) Interfaces to objects are key Defined using Interface Definition Language (IDL) IDL is compiled into all the pieces on the previous page (Similar to COM s table of (pointers to) functions Java now generates IDL automatically C++ Java Cobol C++ Java Cobol IDL IDL IDL IDL IDL IDL Client Server ORB

21 Inter-ORB communication IIOP is the Internet Inter-ORB Protocol

22 ORB problems Commercial ORBs are still somewhat slow and inefficient No load balancing, fault tolerance, scaling to millions of transactions Discovery of objects across ORBs at a large scale is unclear: Broadcast directories, central database,? Server code is not very portable, though client code is Specifications late on server side Events, transactions, etc. not widely available yet Interoperability problems between competing ORBs Visigenic (Inprise), Iona are leading implementations CORBA is top-down design: good architecture, limited services ActiveX/COM is bottom-up: awkward architecture, lots of services

23 CORBA component-based application ORB IIOP ORB Chem prod application Buy component ORB IIOP Chem DB Manuf application component ORB IIOP Build Order entry appl component ORB IIOP Order DB Buy, configure Databases on servers in corporate network Oracle Sybase

24 CORBA application, cont Components can all be on one machine, or each component can be on a different machine One, two or three tier architecture Same options, with even easier flexibility than COM/ActiveX One ORB per computer. If some components are on the same server, there is just one ORB on that server Databases and components must be in corporate net This won t work over Internet: performance, security, protocol Java and CORBA, however, do work well over the Net. Need the Java security model More difficult to develop CORBA than ActiveX More costly tools, but more scalable with UNIX servers

25 Typical CORBA usage example Mainframe storage: Orders, Product Availability Unsupported Not Y2K Only program intfc to mainframe! UNIX system: Order taking, Processing Separate SQL Query for each field on form!

26 Typical CORBA usage, cont Replace EZBridge with CORBA ORBs Only need 6 objects Use CORBA ORBs Between UNIX and Windows PCs

27 ActiveX/COM vs CORBA ActiveX: Separates object interface from object implementation, as in CORBA Has IDL, not the same as CORBA IDL and does not interoperate ActiveX objects are not true objects; they have methods but no attributes ActiveX objects are presented as a group of related functions Clients are given a pointer (handle) to access functions When reconnecting, the attributes are not preserved from the last time COM Type Library is essentially the CORBA Interface Repository Clients query COM Type Library to discover object interfaces and parameters COM Registry is similar to CORBA Implementation Repository

28 Summary ActiveX/COM Dominant in desktop and for PC-centric applications Some technical restrictions (attributes, discovery) Components on market for desktops: graphs, data, display, etc. Bottom-up approach: incremental improvements in interfaces between desktop interfaces CORBA Used in UNIX and server world IIOP built into Netscape Components on market for large systems: billing, finance, etc. Top-down approach: overall architecture leading to specific services Both are superseded by SOAP, UDDI, WSDL and ebxml