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124-542: The designers of distributed applications must determine the best placement of the application's programs and data in terms of the quantity and frequency of data to be transmitted, along with data management, security, and timeliness considerations. There are three client–server models for the design of distributed applications: The DDM architecture was initially designed to support the fat client model of distributed applications; it also supports whole-file transfers. The DDM architecture provides distributed applications with

248-401: A computer network . Typically, a client is a computer application , such as a web browser , that runs on a user 's local computer , smartphone , or other device, and connects to a server as necessary. Operations may be performed client-side because they require access to information or functionality that is available on the client but not on the server, because the user needs to observe

372-417: A computer security context, server-side vulnerabilities or attacks refer to those that occur on a server computer system, rather than on the client side, or in between the two . For example, an attacker might exploit an SQL injection vulnerability in a web application in order to maliciously change or gain unauthorized access to data in the server's database . Alternatively, an attacker might break into

496-412: A string literal . For example: If the sequence does not include whitespace or punctuation characters, this can also be written as: Arrays: defines an array of four integers. defines a seven element array whose first element is a literal array, second element a byte array, third element the string 'four', and so on. Many implementations support the following literal syntax for ByteArrays: defines

620-775: A user 's local computer , smartphone , or other device. Operations may be performed server-side because they require access to information or functionality that is not available on the client , or because performing such operations on the client side would be slow, unreliable, or insecure . Client and server programs may be commonly available ones such as free or commercial web servers and web browsers , communicating with each other using standardized protocols . Or, programmers may write their own server, client, and communications protocol which can only be used with one another. Server-side operations include both those that are carried out in response to client requests, and non-client-oriented operations such as maintenance tasks. In

744-412: A web server serves web pages and a file server serves computer files . A shared resource may be any of the server computer's software and electronic components, from programs and data to processors and storage devices . The sharing of resources of a server constitutes a service . Whether a computer is a client, a server, or both, is determined by the nature of the application that requires

868-459: A DDM client is connected to a known DDM server, such as a System/38 client to a System/38 server, DDM architecture can also be extended by adding Such extensions can be defined within DDM's object-oriented framework so that existing DDM message handling facilities can be used. In a purely object-oriented implementation of DDM, clients and servers and all of their contained managers and objects exist in

992-657: A Programmer's Guide summarizes DDM concepts for programmers implementing clients and servers. Three general file models are defined by DDM architecture: record-oriented files, stream-oriented files and hierarchical directories. The following services are provided by DDM architecture for managing remote files: Record-oriented files were designed to meet the data input, output, and storage requirements of third generation (3GL) programming languages, such as Fortran, Cobol, PL/I, and RPG. Rather than have each language provide its own support for these capabilities, they were incorporated into services provided by operating systems. A record

1116-506: A RQSDSS or a PRYDSS to accommodate as many objects as necessary. A DSS consists of the total length of the DSS, a flag byte identifying the type of DSS, a request identifier, and the linearized objects in the DSS. The request identifier ties an RQSDSS with subsequent OBJDSSes from the client, such as the records to be loaded into a file by the Load File command. The request identifier also ties

1240-462: A class's method dictionary. The part of the class hierarchy that defines classes can add new classes to the system. The system is extended by running Smalltalk-80 code that creates or defines classes and methods. In this way a Smalltalk-80 system is a "living" system, carrying around the ability to extend itself at run time. One can even extend the compiler at run-time; indeed this is how the Compiler

1364-440: A client is called a Source Server and a Server is called a Target Server .) DDM architecture is object-oriented . All entities defined by DDM are objects defined by self-defining Class objects. The messages, replies and data that flow between systems are serialized objects. Each object specifies its length, identifies its class by means of a DDM codepoint, and contains data as defined by its class. Further, its class specifies

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1488-501: A client-server or client-queue-client network, peers communicate with each other directly. In peer-to-peer networking, an algorithm in the peer-to-peer communications protocol balances load , and even peers with modest resources can help to share the load. If a node becomes unavailable, its shared resources remain available as long as other peers offer it. Ideally, a peer does not need to achieve high availability because other, redundant peers make up for any resource downtime ; as

1612-440: A client-side attack would normally be an attacker's only opportunity to gain access to the decrypted contents. For instance, the attacker might cause malware to be installed on the client system, allowing the attacker to view the user's screen, record the user's keystrokes, and steal copies of the user's encryption keys, etc. Alternatively, an attacker might employ cross-site scripting vulnerabilities to execute malicious code on

1736-418: A client/server protocol; that is, a client requests services from a server which interacts with its local resources to perform the requested service, the results of which, data and status indicators, are returned to the client. The above diagram illustrates the roles of DDM clients and servers in relation to local resources. (The common terminology of clients and servers is used here, but in DDM architecture,

1860-460: A code snippet by Ralph Johnson , demonstrating all the basic standard syntactic elements of methods: The following examples illustrate the most common objects which can be written as literal values in Smalltalk-80 methods. Numbers. The following list illustrates some of the possibilities. The last two entries are a binary and a hexadecimal number, respectively. The number before the 'r'

1984-476: A data set (a server-side operation) and sends it back to the client. The client then analyzes the data (a client-side operation), and, when the analysis is complete, displays it to the user (as with Google Earth) and/or transmits the results of calculations back to the server (as with SETI@home). In the context of the World Wide Web , commonly encountered computer languages which are evaluated or run on

2108-469: A larger program of Advanced Research Projects Agency (ARPA) funded research that in many ways defined the modern world of computing. In addition to Smalltalk, working prototypes of things such as hypertext , GUIs, multimedia , the mouse, telepresence , and the Internet were developed by ARPA researchers in the 1960s. Alan Kay (one of the inventors of Smalltalk) also described a tablet computer he named

2232-425: A mainframe and its suite of workstations, which were under the complete software control of the mainframe computer. Other communications between mainframes was also in terms of fixed connections used by software defined for specific purposes. As communication networks became more flexible and dynamic, generic peer-to-peer communications were desirable, in which a program on one computer could initiate and interact with

2356-440: A memory heap, with pointers (memory addresses) used to interconnect them. For example, a command object points to each of its parameter objects. But a command cannot be transmitted from a client to a server in this way; an isomorphic copy of the command must be created as a single, contiguous string of bits. In the heap, a command consists of the size of the command in the heap, a pointer to the command's class, and pointers to each of

2480-417: A number of client programs (e.g. most modern web browsers can request and receive data using both HTTP and FTP). In the case of more specialized applications, programmers may write their own server, client, and communications protocol which can only be used with one another. Programs that run on a user's local computer without ever sending or receiving data over a network are not considered clients, and so

2604-648: A number of facilities such as distributed Smalltalk where messages are exchanged between multiple Smalltalk systems, database interfaces where objects are transparently faulted out of a database, promises , etc. The design of distributed Smalltalk influenced such systems as CORBA . Smalltalk-80 syntax is rather minimalist, based on only a handful of declarations and reserved words. In fact, only six "keywords" are reserved in Smalltalk: true , false , nil , self , super , and thisContext . These are properly termed pseudo-variables , identifiers that follow

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2728-415: A program on a different computer. When IBM's SNA Advanced Program to Program Communications (APPC) architecture was defined in the early 1980s, it was also apparent that APPC could be used to provide operating system services on remote computers. An SNA workgroup pursued this idea and outlined several possible distributed services, such as file services, printer services, and system console services, but

2852-678: A prototype of a distributed RDB and they felt it was now time to turn it into marketable products. However, System/R* was based on System/R, a research prototype of a RDB, and could not be easily added to the IBM RDB products. See for a discussion of RDBs in a distributed processing environment. Roger Reinsch from the IBM Santa Theresa Programming Center lead a cross-product team to define a Distributed Relational Database Architecture (DRDA). He enlisted: In 1990, DDM Architecture Level 3 and DRDA were published at

2976-456: A record at a time and processed in batches. When direct access storage devices became available, programming languages added ways for programs to randomly access records one at a time, such as access by the values of key fields or by the position of a record in a file. All of the records in a file can be of the same format (as in a payroll file) or of varying formats (as in an event log). Some files are read-only in that their records, once written to

3100-438: A request, and the server returns a response. This exchange of messages is an example of inter-process communication . To communicate, the computers must have a common language, and they must follow rules so that both the client and the server know what to expect. The language and rules of communication are defined in a communications protocol . All protocols operate in the application layer . The application layer protocol defines

3224-403: A server farm. Each load balancer sits between client devices and backend servers, receiving and then distributing incoming requests to any available server capable of fulfilling them. In a peer-to-peer network, two or more computers ( peers ) pool their resources and communicate in a decentralized system . Peers are coequal, or equipotent nodes in a non-hierarchical network. Unlike clients in

3348-506: A server system using vulnerabilities in the underlying operating system and then be able to access database and other files in the same manner as authorized administrators of the server. In the case of distributed computing projects such as SETI@home and the Great Internet Mersenne Prime Search , while the bulk of the operations occur on the client side, the servers are responsible for coordinating

3472-417: A server. Clients, therefore, initiate communication sessions with servers, which await incoming requests. Examples of computer applications that use the client–server model are email , network printing, and the World Wide Web . The server component provides a function or service to one or many clients, which initiate requests for such services. Servers are classified by the services they provide. For example,

3596-453: A service is an abstraction of computer resources and a client does not have to be concerned with how the server performs while fulfilling the request and delivering the response. The client only has to understand the response based on the relevant application protocol , i.e. the content and the formatting of the data for the requested service. Clients and servers exchange messages in a request–response messaging pattern . The client sends

3720-489: A short period. A computer can only perform a limited number of tasks at any moment, and relies on a scheduling system to prioritize incoming requests from clients to accommodate them. To prevent abuse and maximize availability , the server software may limit the availability to clients. Denial of service attacks are designed to exploit a server's obligation to process requests by overloading it with excessive request rates. Encryption should be applied if sensitive information

3844-457: A spelling corrector. Each release consisted of a virtual image (platform-independent file with object definitions) and a virtual machine specification. ANSI Smalltalk has been the standard language reference since 1998. Two currently popular Smalltalk implementation variants are descendants of those original Smalltalk-80 images. Squeak is an open source implementation derived from Smalltalk-80 Version 1 by way of Apple Smalltalk. VisualWorks

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3968-514: A string: Two equal strings (strings are equal if they contain all the same characters) can be different objects residing in different places in memory. In addition to strings, Smalltalk has a class of character sequence objects named Symbol. Symbols are guaranteed to be unique—there can be no two equal symbols which are different objects. Because of that, symbols are very cheap to compare and are often used for language artifacts such as message selectors (see below). Symbols are written as # followed by

4092-525: A toolkit for developing collaborative applications Croquet Project , and the Open Cobalt virtual world application. GNU Smalltalk is a free software implementation of a derivative of Smalltalk-80 from the GNU project. Pharo Smalltalk is a fork of Squeak oriented toward research and use in commercial environments. As of 2016, a significant development that has spread across all Smalltalk environments

4216-410: A year of failure to sell the idea of DDM to other IBM system houses, Lawrence's arguments prevailed. Richard Sanders joined the DDM architecture team and worked with Lawrence and Demers to define the specific messages needed for System/36 DDM. Progress in the definition of DDM encouraged System/38 to also participate. This broadened the scope of DDM record-file support to meet many of the requirements of

4340-511: Is a distributed application structure that partitions tasks or workloads between the providers of a resource or service, called servers , and service requesters, called clients . Often clients and servers communicate over a computer network on separate hardware, but both client and server may be on the same device. A server host runs one or more server programs, which share their resources with clients. A client usually does not share any of its resources, but it requests content or service from

4464-442: Is a series of related data fields, such as the name, address, identification number and salary of a single employee, in which each field is encoded and mapped to a contiguous string of bytes. Early computers had limited input and output capabilities, typically in the form of stacks of 80 column punched cards or in the form of paper or magnetic tapes. Application records, such as employee data records, were sequentially read or written

4588-517: Is a structurally reflective system which structure is defined by Smalltalk-80 objects. The classes and methods that define the system are also objects and fully part of the system that they help define. The Smalltalk compiler, which is itself written in Smalltalk and exists alongside all the other code in the system, compiles textual source code into method objects, typically instances of CompiledMethod . These get added to classes by storing them in

4712-421: Is always private to that object. Other objects can query or change that state only by sending requests (messages) to the object to do so. Any message can be sent to any object: when a message is received, the receiver determines whether that message is appropriate. If the message is not understood by the object then the virtual machine sends the doesNotUnderstand: message with the original message as an argument, and

4836-414: Is any computer connected to a network. Whereas the words server and client may refer either to a computer or to a computer program, server-host and client-host always refer to computers. The host is a versatile, multifunction computer; clients and servers are just programs that run on a host. In the client–server model, a server is more likely to be devoted to the task of serving. An early use of

4960-621: Is derived from Smalltalk-80 version 2 by way of Smalltalk-80 2.5 and ObjectWorks (both products of ParcPlace Systems, a Xerox PARC spin-off company formed to bring Smalltalk to the market). As an interesting link between generations, in 2001, Vassili Bykov implemented Hobbes, a virtual machine running Smalltalk-80 inside VisualWorks. ( Dan Ingalls later ported Hobbes to Squeak.) During the late 1980s to mid-1990s, Smalltalk environments, including support, training and add-ons, were sold by two competing organizations: ParcPlace Systems and Digitalk, both California based. ParcPlace Systems tended to focus on

5084-425: Is developed and maintained. Since the classes are objects, they can be asked questions such as "what methods do you implement?" or "what fields/slots/instance variables do you define?". So objects can easily be inspected, copied, (de) serialized and so on with generic code that applies to any object in the system. Smalltalk-80 also provides computational reflection, the ability to observe the computational state of

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5208-574: Is implemented on top of the built-in constructs by the standard Smalltalk class library. (For performance reasons, implementations may recognize and treat as special some of those messages; however, this is only an optimization and is not coded into the language syntax.) The adage that "Smalltalk syntax fits on a postcard " may have originated in Alan Kay's original conception of the language, as related by him in practically every of tens or hundreds of public lectures, op. cit., or perhaps it could refer to

5332-535: Is implemented using this facility. One of the more interesting uses of this is in the Seaside web framework which relieves the programmer of dealing with the complexity of a Web Browser's back button by storing continuations for each edited page and switching between them as the user navigates a web site. Programming the web server using Seaside can then be done using a more conventional programming style. As with message sending Smalltalk-80 virtual machines optimize away

5456-471: Is now termed Smalltalk-72 and influenced the development of the Actor model . Its syntax and execution model were very different from modern Smalltalk variants. After significant revisions which froze some aspects of execution semantics to gain performance (by adopting a Simula -like class inheritance model of execution), Smalltalk-76 was created. This system had a development environment featuring most of

5580-641: Is summarized in the commonly heard phrase "In Smalltalk everything is an object", which may be more accurately expressed as "all values are objects", as variables are not. Since all values are objects, classes are also objects. Each class is an instance of the metaclass of that class. Metaclasses in turn are also objects, and are all instances of a class named Metaclass. Classes contain method dictionaries that map selectors (the equivalent of function procedure names in other languages) to method objects, objects that are executed to evaluate messages. Classes inherit from other classes, with either Object or ProtoObject at

5704-404: Is the radix or base. The base does not have to be a power of two; for example 36rSMALLTALK is a valid number equal to 80738163270632 decimal. Characters are written by preceding them with a dollar sign: Strings are sequences of characters enclosed in single quotes: To include a quote in a string, escape it using a second quote: Double quotes do not need escaping, since single quotes delimit

5828-419: Is the increasing usage of two web frameworks, Seaside and AIDA/Web , to simplify the building of complex web applications. Seaside has seen considerable market interest with Cincom, Gemstone, and Instantiations incorporating and extending it. Smalltalk was one of many object-oriented programming languages based on Simula . Smalltalk is also one of the most influential programming languages. Virtually all of

5952-486: Is the part of the system that implements the programming system itself, and developers can use the meta-model to do things like walk through, examine, and modify code in the running system, or find all the instances of a certain kind of structure (e.g., all instances of the Method class in the meta-model). Smalltalk-80 is a totally reflective system. Smalltalk-80 provides both structural and computational reflection. Smalltalk

6076-403: Is to be communicated between the client and the server. When a bank customer accesses online banking services with a web browser (the client), the client initiates a request to the bank's web server. The customer's login credentials may be stored in a database , and the webserver accesses the database server as a client. An application server interprets the returned data by applying

6200-577: The Dynabook which resembles modern tablet computers like the iPad. Smalltalk environments were often the first to develop what are now common object-oriented software design patterns. One of the most popular is the model–view–controller (MVC) pattern for user interface design. The MVC pattern enables developers to have multiple consistent views of the same underlying data. It's ideal for software development environments, where there are various views (e.g., entity-relation, dataflow, object model, etc.) of

6324-795: The Golden Gate project had followed the lead of the File Transfer Access and Management ( FTAM ) international standard for distributed files, but it was very abstract and difficult to map to local file services. In fact, this had been one of the barriers to acceptance by the IBM system houses. Kenneth Lawrence, the system architect responsible for System/36 file services, argued that it would be better to define messages that at least one IBM system could easily implement and then let other systems request whatever changes they needed. Naturally, he argued for support of System/36 requirements. After

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6448-636: The IBM Personal Computer running IBM PC DOS and the IBM RS/6000 running IBM AIX (IBM's version of Unix). See Stream-oriented files . DDM Architecture Level 2 was published in 1988. Jan Fisher and Sunil Gaitonde did most of the architecture work on DDM support for directories and stream files. In 1986, IBM marketed four different relational database (RDB) products, each built for a specific IBM operating system. Scientists at IBM's Almaden Research Laboratory had developed System/R*,

6572-603: The Stack Overflow Developer Survey in 2017, but it was not among the 26 most loved programming languages of the 2018 survey. There are a large number of Smalltalk variants. The unqualified word Smalltalk is often used to indicate the Smalltalk-80 language and compatible VM, the first version to be made publicly available and created in 1980. The first hardware-environments which ran the Smalltalk VMs were Xerox Alto computers. Smalltalk

6696-451: The 2000s, web applications matured enough to rival application software developed for a specific microarchitecture . This maturation, more affordable mass storage , and the advent of service-oriented architecture were among the factors that gave rise to the cloud computing trend of the 2010s. In addition to the client-server model, distributed computing applications often use the peer-to-peer (P2P) application architecture. In

6820-649: The DDM Reference Manual Level 3 is bulky, at over 1400 pages, and somewhat awkward to use, but an interactive version was also built using internal IBM communication facilities. Given the relatively slow speed of those communication facilities, it was primarily of use within the IBM Rochester laboratory. In addition to the DDM Reference Manual, a General Information document provide's executive level information about DDM, and

6944-526: The Digitalk products initially tried to reach a wider audience with a lower price. IBM initially supported the Digitalk product, but then entered the market with a Smalltalk product in 1995 named VisualAge/Smalltalk. Easel introduced Enfin at this time on Windows and OS/2. Enfin became far more popular in Europe, as IBM introduced it into IT shops before their development of IBM Smalltalk (later VisualAge). Enfin

7068-524: The IBM System/38. This model made it clear how DDM products could be implemented on various systems. See How DDM works . In 1982, the System/36 planners became convinced there was a sufficient market for DDM record-oriented file services. The generic format of DDM messages had already been designed, but what specific messages should be defined? The System/36 file system had been defined to meet

7192-602: The RQSDSS from the client with a RPYDSS or the OBJDSSes from the server to the client. The DDM Reference Manual consists of named Menu, Help, and Class objects. The subclasses of DDM class Class are described by variables that specify These objects can contain references to other named objects in text and specifications, thereby creating hypertext linkages among the pages of the DDM Reference Manual. Menu and Help pages form an integrated tutorial about DDM. The paper version of

7316-491: The Smalltalk programming environment. Having undergone significant industry development toward other uses, including business and database functions, Smalltalk is still in use today. When first publicly released, Smalltalk-80 presented numerous foundational ideas for the nascent field of object-oriented programming (OOP). Since inception, the language provided interactive programming via an integrated development environment . This requires reflection and late binding in

7440-697: The System/36 DDM server programming for use in the IBM Customer Information Control System (CICS) transaction processing environment, thereby making CICS a DDM server for both the MVS and VSE mainframe operating systems. Lawrence also worked with programmers from the IBM Cary, North Carolina laboratory to implement a DDM record-oriented client for IBM PC DOS . Level 1 of DDM Architecture was formally published in 1986. At

7564-489: The System/38's advanced file system. Files exist in a context provided by an operating system that provides services for organizing files, for sharing them with concurrent users and for securing them from unwarranted access. In level 1 of DDM, access to remote file directories was not supported beyond the transmission of the fully qualified name of the file to be used. Security and sharing, however, were required. Sanders did

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7688-510: The Unix/Sun microsystems market, while Digitalk focused on Intel-based PCs running Microsoft Windows or IBM's OS/2. Both firms struggled to take Smalltalk mainstream due to Smalltalk's substantial memory needs, limited run-time performance, and initial lack of supported connectivity to SQL -based relational database servers. While the high price of ParcPlace Smalltalk limited its market penetration to mid-sized and large commercial organizations,

7812-431: The availability and load capacity of peers change, the protocol reroutes requests. Both client-server and master-slave are regarded as sub-categories of distributed peer-to-peer systems. Smalltalk Smalltalk is a purely object oriented programming language (OOP) that was originally created in the 1970s for educational use, specifically for constructionist learning , but later found use in business. It

7936-402: The bank's business logic and provides the output to the webserver. Finally, the webserver returns the result to the client web browser for display. In each step of this sequence of client–server message exchanges, a computer processes a request and returns data. This is the request-response messaging pattern. When all the requests are met, the sequence is complete and the web browser presents

8060-430: The basic patterns of the dialogue. To formalize the data exchange even further, the server may implement an application programming interface (API). The API is an abstraction layer for accessing a service. By restricting communication to a specific content format , it facilitates parsing . By abstracting access, it facilitates cross-platform data exchange. A server may receive requests from many distinct clients in

8184-701: The business. In 1999, Seagull Software acquired the ObjectShare Java development lab (including the original Smalltalk/V and Visual Smalltalk development team), and still owns VisualSmalltalk, although worldwide distribution rights for the Smalltalk product remained with ObjectShare who then sold them to Cincom . VisualWorks was sold to Cincom and is now part of Cincom Smalltalk. Cincom has backed Smalltalk strongly, releasing multiple new versions of VisualWorks and ObjectStudio each year since 1999. Cincom , GemTalk, and Instantiations, continue to sell Smalltalk environments. IBM ended VisualAge Smalltalk, having in

8308-770: The central concept in Smalltalk-80 (but not in Smalltalk-72) is that of an object . An object is always an instance of a class . Classes are "blueprints" that describe the properties and behavior of their instances. For example, a GUI's window class might declare that windows have properties such as the label, the position and whether the window is visible or not. The class might also declare that instances support operations such as opening, closing, moving and hiding. Each particular window object would have its own values of those properties, and each of them would be able to perform operations defined by its class. A Smalltalk object can do exactly three things: The state an object holds

8432-520: The client and server agents to coordinate their processing. In DDM architecture, these envelopes are called Data Stream Structures (DSS). Commands are put into a Request DSS (RQSDSS), replies are put into a Reply DSS (RPYDSS), and other objects are put into an Object DSS (OBJDSS). There can be only one command in a RQSDSS and only one reply in RPYDSS, but many objects, such as records, can be put into an OBJDSS. Further many OBJDSSes can be chained to

8556-475: The client and specifies the managers the client requires and the level of DDM architecture at which support is required. The server responds by identifying itself and specifying at what level it supports the requested managers. A general rule is that a product that supports Level X of a DDM manager must also support Level X-1 so that new server products connect with older client products. Subsets of DDM can be implemented to meet varying product requirements: When

8680-543: The client side include: An early form of client–server architecture is remote job entry , dating at least to OS/360 (announced 1964), where the request was to run a job , and the response was the output. While formulating the client–server model in the 1960s and 1970s, computer scientists building ARPANET (at the Stanford Research Institute ) used the terms server-host (or serving host ) and user-host (or using-host ), and these appear in

8804-577: The client's system without needing to install any permanently resident malware. Distributed computing projects such as SETI@home and the Great Internet Mersenne Prime Search, as well as Internet-dependent applications like Google Earth , rely primarily on client-side operations. They initiate a connection with the server (either in response to a user query, as with Google Earth, or in an automated fashion, as with SETI@home), and request some data. The server selects

8928-411: The client, etc. In the context of the World Wide Web , commonly encountered server-side computer languages include: However, web applications and services can be implemented in almost any language, as long as they can return data to standards-based web browsers (possibly via intermediary programs) in formats which they can use. Client-side refers to operations that are performed by the client in

9052-483: The client-server model, the server is often designed to operate as a centralized system that serves many clients. The computing power, memory and storage requirements of a server must be scaled appropriately to the expected workload. Load-balancing and failover systems are often employed to scale the server beyond a single physical machine. Load balancing is defined as the methodical and efficient distribution of network or application traffic across multiple servers in

9176-402: The clients, sending them data to analyze, receiving and storing results, providing reporting functionality to project administrators, etc. In the case of an Internet-dependent user application like Google Earth , while querying and display of map data takes place on the client side, the server is responsible for permanent storage of map data, resolving user queries into map data to be returned to

9300-489: The command's parameter objects. Linearized, the command consists of the total length of the linearized command, a code point identifying the command's class, and each of its linearized parameter objects. DDM architecture assigns unique code points to each class of object. This straightforward technique is used for all objects transmitted between client's and servers, including commands, records, and reply messages. All of these linearized objects are put into envelopes that enable

9424-481: The commands that can be sent to its instances when an object resides in a DDM client or server, thereby encapsulating the object by a limited set of operations. Structurally, DDM architecture consists of hierarchical levels of objects, each level manifesting emergent properties at increasingly higher levels. While DDM architecture is object-oriented, the DDM products were implemented using the languages and methods typical of their host systems. A Smalltalk version of DDM

9548-448: The data to the customer. This example illustrates a design pattern applicable to the client–server model: separation of concerns . Server-side refers to programs and operations that run on the server . This is in contrast to client-side programs and operations which run on the client . (See below) "Server-side software" refers to a computer application , such as a web server , that runs on remote server hardware , reachable from

9672-471: The default implementation of doesNotUnderstand: raises an exception that if not caught opens the system's debugger. Alan Kay has commented that despite the attention given to objects, messaging is the most important concept in Smalltalk: "The big idea is 'messaging'—that is what the kernel of Smalltalk/Squeak is all about (and it's something that was never quite completed in our Xerox PARC phase)." Unlike most other languages, Smalltalk code can be modified while

9796-598: The design work in these areas. Sanders also defined specific protocols regarding the use of communication facilities, which were incorporated in a component called the DDM Conversational Communications Manager. Initially implemented using APPC, it was later implemented using TCP/IP . With the completion of the System/36 DDM product, Lawrence worked with programmers from the IBM Hursley Park, UK laboratory to adapt much of

9920-460: The early documents RFC 5 and RFC 4. This usage was continued at Xerox PARC in the mid-1970s. One context in which researchers used these terms was in the design of a computer network programming language called Decode-Encode Language (DEL). The purpose of this language was to accept commands from one computer (the user-host), which would return status reports to the user as it encoded the commands in network packets. Another DEL-capable computer,

10044-423: The expensive use of contexts internally, providing the illusion and flexibility of a spaghetti stack without most its costs. Essentially context objects are created lazily as required, for example when a message is sent to the thisContext variable. An example of how Smalltalk can use reflection is the mechanism for handling errors. When an object is sent a message that it does not implement, the virtual machine sends

10168-490: The file, can only be read, while other files allow their records to be updated. The DDM record-oriented file models consist of file attributes, such as its creation date, the date of last update, the size of its records, and slots in which records can be stored. The records can be of either fixed or varying length, depending on the media used to store the file's records. DDM defines four kinds of record-oriented files: Client%E2%80%93server model The client–server model

10292-436: The first being the sender of the former. In this way the stack is a linked list of context objects, and the debugger is essentially an inspector of this "spaghetti stack". By sending messages to thisContext a method activation can ask questions like "who sent this message to me". These facilities make it possible to implement coroutines or Prolog -like back-tracking without modifying the virtual machine. The exception system

10416-448: The following benefits: DDM architecture is a set of specifications for messages and protocols that enable data distributed throughout a network of computers to be managed and accessed. IBM's Systems Network Architecture (SNA) was initially designed to enable the hierarchical connection of workstations to IBM mainframe computers. The communication networks available at the time were rigidly designed in terms of fixed connections between

10540-447: The headquarters computers of a company to interact with the computers in its various warehouses. APPC was implemented on these systems and used by various customer applications. The idea of distributed operating system services was then revived as the Golden Gate project and an attempt made to justify its development. This attempt also failed; the whole idea of distributed services was too new for IBM product planners to be able to quantify

10664-496: The language execution of code . Later development has led to at least one instance of Smalltalk execution environment which lacks such an integrated graphical user interface or front-end. Smalltalk-like languages are in active development and have gathered communities of users around them. American National Standards Institute (ANSI) Smalltalk was ratified in 1998 and represents the standard version of Smalltalk. Smalltalk took second place for "most loved programming language" in

10788-646: The last two being vectors of bytes. Consequently Smalltalk can evaluate 52 factorial to produce 80658175170943878571660636856403766975289505440883277824000000000000. The transition from small to large integers is transparent to the programmer; variables do not require type declarations. This makes the system both concise and flexible. A programmer can change or extend (through subclassing ) the classes that implement what in other languages would be primitive values, so that new behavior can be defined for their instances—for example, to implement new control structures—or even so that their existing behavior will be changed. This fact

10912-534: The late 1990s decided to back Java instead and, as of 2005 , is supported by Instantiations, Inc. Instantiations renamed the product VA Smalltalk (VAST Platform) and continue to release new versions yearly. The open Squeak implementation has an active community of developers, including many of the original Smalltalk community, and was used to provide the Etoys environment on the One Laptop per Child (OLPC) project,

11036-648: The mid-range computer systems produced in Rochester. A primitive form of distributed file services, called Distributed Data File Facility (DDFF) had been implemented to connect the IBM System/3 , IBM System/34 , and IBM System/36 minicomputers. Further, the IBM System/36 and the IBM System/38 computers were being sold to customers in multiples and there was a clear need to enable, for example,

11160-625: The network, and a computer terminal has no operating system at all; it is only an input/output interface to the server. In contrast, a rich client , such as a personal computer , has many resources and does not rely on a server for essential functions. As microcomputers decreased in price and increased in power from the 1980s to the late 1990s, many organizations transitioned computation from centralized servers, such as mainframes and minicomputers , to rich clients. This afforded greater, more individualized dominion over computer resources, but complicated information technology management . During

11284-452: The now familiar tools, including a class library code browser/editor. Smalltalk-80 added metaclasses , to help maintain the "everything is an object" (except variables) paradigm by associating properties and behavior with individual classes, and even primitives such as integer and Boolean values (for example, to support different ways to create instances). Smalltalk-80 was the first language variant made available outside of PARC. In 1981, it

11408-411: The object the doesNotUnderstand: message with a reification of the message as an argument. The message (another object, an instance of Message ) contains the selector of the message and an Array of its arguments. In an interactive Smalltalk system the default implementation of doesNotUnderstand: is one that opens an error window (a Notifier) reporting the error to the user. Through this and

11532-476: The object-oriented languages that came after— Flavors , CLOS , Objective-C , Java , Python , Ruby , and many others—were influenced by Smalltalk. Smalltalk was also one of the most popular languages for agile software development methods, rapid application development (RAD) or prototyping, and software design patterns . The highly productive environment provided by Smalltalk platforms made them ideal for rapid, iterative development. Smalltalk emerged from

11656-426: The operations of such programs would not be termed client-side operations. In a computer security context, client-side vulnerabilities or attacks refer to those that occur on the client / user's computer system, rather than on the server side , or in between the two . As an example, if a server contained an encrypted file or message which could only be decrypted using a key housed on the user's computer system,

11780-502: The operations or provide input, or because the server lacks the processing power to perform the operations in a timely manner for all of the clients it serves. Additionally, if operations can be performed by the client, without sending data over the network, they may take less time, use less bandwidth , and incur a lesser security risk. When the server serves data in a commonly used manner, for example according to standard protocols such as HTTP or FTP , users may have their choice of

11904-531: The record-oriented needs of third generation programming languages (3GLs), such as Fortran , COBOL , PL/I , and IBM RPG , and so had the System/38 file system and the Virtual Storage Access Method (VSAM) file system of the IBM mainframe computers. And yet, their actual facilities and interfaces varied considerably, so what facilities and interfaces should DDM architecture support? See record-oriented files . The initial work on DDM by

12028-471: The reflective facilities the user can examine the context in which the error occurred, redefine the offending code, and continue, all within the system, using Smalltalk-80's reflective facilities. By creating a class that understands (implements) only doesNotUnderstand:, one can create an instance that can intercept any message sent to it via its doesNotUnderstand: method. Such instances are called transparent proxies. Such proxies can then be used to implement

12152-410: The root of the class hierarchy. Sending a message to an object at the most abstract involves fetching the class of the receiver (the object being sent the message) and looking up the message's selector in the class's method dictionary, followed by the superclass and so on until the method is found or doesNotUnderstand is sent. Smalltalk virtual machines use various techniques to speed up message lookup so

12276-462: The rules for variable identifiers but denote bindings that a programmer cannot change. The true , false , and nil pseudo-variables are singleton instances. self and super refer to the receiver of a message within a method activated in response to that message, but sends to super are looked up in the superclass of the method's defining class rather than the class of the receiver, which allows methods in subclasses to invoke methods of

12400-473: The same name in superclasses. thisContext refers to the current activation record. The only built-in language constructs are message sends, assignment, method return and literal syntax for some objects. From its origins as a language for children of all ages, standard Smalltalk syntax uses punctuation in a manner more like English than mainstream coding languages. The remainder of the language, including control structures for conditional evaluation and iteration,

12524-455: The same time. Both DDM and DRDA were designated as strategic components of IBM's Systems Application Architecture (SAA). DRDA was implemented by all four of the IBM RDB products and by other vendors. Awards were given to key participants in the design of DRDA. Richard Sanders received an Outstanding Contribution Award and Roger Reinsch and Richard Demers received Outstanding Innovation Awards . The Distributed File Management (DFM) project

12648-568: The same underlying specification. Also, for simulations or games where the underlying model may be viewed from various angles and levels of abstraction. In addition to the MVC pattern, the Smalltalk language and environment were influential in the history of the graphical user interface (GUI) and the what you see is what you get ( WYSIWYG ) user interface, font editors, and desktop metaphors for UI design. The powerful built-in debugging and object inspection tools that came with Smalltalk environments set

12772-406: The sense that they are instances of corresponding classes, and operations on them are invoked by sending messages. For efficiency and generality integers are implemented by four classes, Integer, the abstract superclass of all integers, SmallInteger, whose instances fit in a machine word, for example having a 61-bit signed range in a 64-bit implementation, LargePositiveInteger and LargeNegativeInteger,

12896-539: The server-host, received the packets, decoded them, and returned formatted data to the user-host. A DEL program on the user-host received the results to present to the user. This is a client–server transaction. Development of DEL was just beginning in 1969, the year that the United States Department of Defense established ARPANET (predecessor of Internet ). Client-host and server-host have subtly different meanings than client and server . A host

13020-423: The service functions. For example, a single computer can run a web server and file server software at the same time to serve different data to clients making different kinds of requests. The client software can also communicate with server software within the same computer. Communication between servers, such as to synchronize data, is sometimes called inter-server or server-to-server communication. Generally,

13144-423: The shared resources of other hosts. Centralized computing , however, specifically allocates a large number of resources to a small number of computers. The more computation is offloaded from client-hosts to the central computers, the simpler the client-hosts can be. It relies heavily on network resources (servers and infrastructure) for computation and storage. A diskless node loads even its operating system from

13268-449: The standard for all the integrated development environments , starting with Lisp Machine environments, that came after. Smalltalk uses several commands that rhyme with the "-ect" suffix. This was inspired by a line from the 1967 Arlo Guthrie monologue " Alice's Restaurant Massacree ," in which Guthrie underwent a battery of being "injected, inspected, detected, infected, neglected and selected." As in other object-oriented languages,

13392-502: The system is running. Live coding and applying fixes 'on-the-fly' is a dominant programming methodology for Smalltalk and is one of the main reasons for its productivity. Smalltalk is a "pure" object-oriented programming language, meaning that, unlike C++ and Java , there are no primitive types. All values are represented as objects and computation on integers uses message sending just like any other object. In Smalltalk, types such as integers, Booleans and characters are also objects, in

13516-465: The system provides both a simple consistent message binding mechanism and good efficiency. Code blocks —Smalltalk's way of expressing anonymous functions —are also objects. They have a very lightweight syntax and are used throughout the system to implement control structures, especially for the Collection hierarchy. Reflection is a feature of having a meta-model as Smalltalk does. The meta-model

13640-410: The system. In languages derived from the original Smalltalk-80 the current activation of a method is accessible as an object named via a pseudo-variable (one of the six reserved words), thisContext , which corresponds to a stack frame in conventional language implementations, and is called a "context". Sending a message is done within some context, and to evaluate the message another context is created,

13764-426: The tasks of defining DDM architecture and selling the idea of DDM to the IBM system houses. The first year of this effort was largely fruitless as the IBM system houses continued to demand up-front business cases and as they insisted on message formats isomorphic to the control block interfaces of their local file systems. Further, as Personal Computers began to be used as terminals attached to mainframe computers, it

13888-519: The time of this announcement, IBM presented an Outstanding Technical Achievement Award to Kenneth Lawrence, an Outstanding Contribution Award to Richard Sanders, and an Outstanding Innovation Award to Richard Demers. With the increasing importance of the IBM PC and the Unix operating system in network environments, DDM support was also needed for the hierarchical directories and stream-oriented files of

14012-552: The value of software that interconnected heterogeneous computers. However, one Golden Gate planner, John Bondy, remained convinced and persuaded management to create a department outside of the normal control of the Rochester laboratory so that there would be no immediate need for a predefined business case. Further, he narrowed its mission to include only support for Distributed Data Management (DDM), in particular, support for record-oriented files . He then convinced an experienced software architect, Richard A. Demers, to join him in

14136-597: The word client occurs in "Separating Data from Function in a Distributed File System", a 1978 paper by Xerox PARC computer scientists Howard Sturgis, James Mitchell, and Jay Israel. The authors are careful to define the term for readers, and explain that they use it to distinguish between the user and the user's network node (the client). By 1992, the word server had entered into general parlance. The client-server model does not dictate that server-hosts must have more resources than client-hosts. Rather, it enables any general-purpose computer to extend its capabilities by using

14260-477: Was argued that simply enhancing the 3270 data stream would enable PCs to access mainframe data. During this period, Demers designed an architectural model of DDM clients and servers, of their components, and of interactions between communicating computers. Further, he defined a generic format for DDM messages based on the principles of object-orientation as pioneered by the Smalltalk programming language and by

14384-563: Was created at Xerox PARC by Learning Research Group (LRG) scientists, including Alan Kay , Dan Ingalls , Adele Goldberg , Ted Kaehler , Diana Merry , and Scott Wallace. In Smalltalk, executing programs are built of opaque, atomic, so-called objects, which are instances of template code stored in classes. These objects intercommunicate by passing of messages, via an intermediary virtual machine environment (VM). A relatively small number of objects, called primitives, are not amenable to live redefinition, sometimes being defined independently of

14508-414: Was developed for the IBM PC by Object Technology International , with appropriate Smalltalk classes automatically created from the DDM Reference Manual. DDM is an open architecture. DDM products can implement subsets of DDM architecture; they can also create their own extensions. The DDM 'Exchange Server Attributes' command is the first command sent when a client is connected with a server. It identifies

14632-519: Was in November 1981. Xerox only distributed Version 1 to Apple, DEC, HP, and Tektronix, but these companies were allowed unrestricted redistribution via any system they built. This encouraged the wide spread of Smalltalk. Later, in 1983, Xerox released Smalltalk-80 Version 2. This version was generally available to the public, although under a restrictive license. Versions 1 and 2 were fairly similar, although Version 2 did have some added features such as

14756-719: Was initiated to add DDM services to IBM's MVS operating system to enable programs on remote computers to create, manage, and access VSAM files. John Hufferd, the manager of the DFM project looked to the DDM Architecture team for a means of converting the data fields in records as they flowed between systems. Richard Demers took the lead on this issue, aided by Koichi Yamaguchi from the DFM project. See Data description and conversion . The following additional services were defined by Richard Sanders, Jan Fisher and Sunil Gaitonde in DDM architecture at Level 4: DDM architecture level 4

14880-625: Was later acquired by Cincom Systems , and is now sold under the name ObjectStudio , and is part of the Cincom Smalltalk product suite. In 1995, ParcPlace and Digitalk merged into ParcPlace-Digitalk and then rebranded in 1997 as ObjectShare, located in Irvine, CA. ObjectShare ( NASDAQ : OBJS) was traded publicly until 1999, when it was delisted and dissolved. The merged firm never managed to find an effective response to Java as to market positioning, and by 1997 its owners were looking to sell

15004-579: Was published in 1992. Architecture work on DDM level 5 consisted of support for Jan Fisher was the architect responsible for DDM level 5, which was published by the Open Group , rather than IBM. Shortly thereafter, the IBM DDM architecture group was disbanded. DDM architecture is a formally defined and highly structured set of specifications. This section introduces key technical concepts that underlie DDM. [REDACTED] DDM architecture defines

15128-406: Was shared with Tektronix , Hewlett-Packard , Apple Computer , and DEC for review and debugging on their platforms. The August 1981 issue of Byte Magazine was devoted to Smalltalk-80 and brought its ideas to a large audience. Several books on Smalltalk-80 were also published. Smalltalk-80 became the basis for all future commercial versions of Smalltalk. The final release of Smalltalk-80 Version 1

15252-510: Was the product of research led by Alan Kay at Xerox Palo Alto Research Center (PARC); Alan Kay designed most of the early Smalltalk versions, Adele Goldberg wrote most of the documentation, and Dan Ingalls implemented most of the early versions. The first version, termed Smalltalk-71, was created by Kay in a few mornings on a bet that a programming language based on the idea of message passing inspired by Simula could be implemented in "a page of code". A later variant used for research work

15376-467: Was unable to initiate product development. APPC software was not yet available on mainframes and, more basically, mainframes were still viewed primarily as stand-alone systems. As a result, work on distributed services was suspended by the SNA work group. Members of the SNA work group from IBM's Rochester, Minnesota development laboratory were convinced that a business case existed for distributed services among

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