Visual Basic An Object Oriented Approach 9 Persistence

Visual Basic: An Object Oriented Approach 9 - Persistence

Persistence in software n Information entered into a program is volatile n n n Can switch computer off Power can be removed accidentally Power-cuts System could crash We need to make data persist beyond a single run of a program

Files v File - an orderly line v v v In admin, a file is used to keep like items together in an order In a computer, a file is a storage mechanism, usually associated with long-term storage on magnetic media A computer file has an order v v items are written to the file in strict sequence items are read from the file in the same sequence

Computer file mechanisms v In a computer, files are handled by the operating system v v v Provides facilities to open, read items from, write items to and close files to maintain a link between a program and an open file, the O/S assigns a file handle; a number by which the file can be identified Programs use operating system services v Normally facilities are added to programming languages to access these services

Creating a file v To create a computer file v v v Ask the O/S for a file handle Open the file for write access Send data item variables to the file in some order Close the file The order of the file is defined at the point of creation v Care required to prevent file errors due to upset sequence

Code to create a file Ask for handle Close file Sub Create. File( ) Dim f As Integer ‘ File handle Dim a. Name As String Dim a. Number As Integer f = Free. File Open “My. File. dat” For Output As #f a. Name = Input. Box(“Name? ”) Write #f, a. Name a. Number = Input. Box(“Number? ”) Write #f, a. Number Close #f End Sub Open File Send data

Retrieving data from a file v To retrieve data from a file v v v Ask the O/S for a file handle Open the file for read access Read data items from the file - they will be in the same order Close the file File problems generally due to sequence problem v e. g. program reads a string when expecting a number

Code to retrieve data from a file Ask for handle Close file Sub Read. File( ) Dim f As Integer ‘ File handle Dim name As String Dim number As Integer f = Free. File Open “My. File. dat” For Input As #f Input #f, name Msg. Box name Input #f, number Msg. Box number Close #f End Sub Open File Read data

Types of file v Files can be organised in a number of ways v v v Tabular Character based text Text with line-breaks Binary (sequential) Binary (Random access) File organisation is determined by. . . v v v the program code that writes the file the variable types stored decisions made by the programmer in the interests of efficiency and ease of use

File formats Tabular Character Text with Line breaks “Fred Bloggs”, “ 1 High Street”, £ 200 “Jane Smith”, “ 55 Glen Road”, £ 28. 55 The key feature of this file is that it is composed of a continu ous sequence of text characters with no format beyond the la nguage used. Strictly, it should be seen as one long line of te xt characters. This type of file is distinguished by the use of line breaks (special, non-printing characters) to preserve the format of the text. Each line ends with a line-break and so need not be a fixed length.

File formats II Binary (this is Hexadecimal which is a representation of binary) Random access is similar, but each record is the same number of characters long E 9 26 0 D 01 00 33 DB B 8 00 44 CD 21 F 6 D 2 F 6 C 2 80 75 03 F 6 C 2 02 C 3 8 B C 4 40 40 A 3 D 4 13 5 B 89 1 E D 6 13 F 8 FF E 3 8 B 26 D 4 13 8 B 1 E D 6 13 F 9 FF E 3 52 56 8 B F 0 AC 0 A C 0 74 1 E 3 C 0 A 74 F 7 3 C 0 D 74 08 8 A D 0 B 4 02 CD 21 EB EB B 2 0 D B 4 02 CD 21 B 2 0 A B 4 02 CD 21 EB DD 5 E 5 A C 3 53 51 52 BB D 2 13 B 9 0 A 00 33 D 2 F 7 F 1 80 C 2 30 4 B 88 17 0 B C 0 75 F 2 8 B C 3 E 8 BA FF 5 A 59 5 B C 3 50 B 8 D 8 13 E 8 AF FF 58 E 8 AB FF B 8 64 16 E 8 A 5 FF C 3 E 8 EB FF B 4 4 C CD 21 80 3 E CA 13 FF 74 49 B 8 65 14 80 3 E F 3 13 00 75 03 B 8 6 B 16 F 7 06 7 A 16 00 02 74 1 E E 8 7 E FF A 1 7 A 16 25 8 E 00 3 D 02 00 74 08 B 2 09 B 4 02 CD 21 EB 1 E B 8 64 16 E 8 65 FF EB 16 50 B 8 5 D 16 E 8 5 C FF 58 E 8 58 FF B 8 63 16 E 8 52 FF C 6 06 CA 13 FF C 3 0 B ED 74 12 F 7 06 7 A 16 88 00 75 08 F 7 06 7 A 16 06 00 75 02 F 8 C 3 F 9 C 3 A 0 C 9 13

Files and objects n We send objects to a file by serializing them n n The data in each object is written out as a sequence of values, either in binary of character form The class is responsible for sending and retrieving object data Consider a serialised file of objects to be a stream Can also use streams to transmit and receive objects over a number of different media

Streams n Provide an object with Save and Load methods n n By keeping the code within the class, objects manage serialization with no help from client programs Complex objects (e. g. structured hierarchies) can manage themselves hierarchically

Sending an object hierarchy to a stream ‘ Save to Stream pseudo code… Write each member variable to the stream Write the number of child objects to the stream For each child object Repeat the process Next n n n Recursive nature of this means a complex hierarchy does not require complex code Only ever need to deal with this object and its direct children (which deals with itself and its children etc. ) The count of child objects is the key to being able to retrieve the while hierarchy from the stream

Retrieving an object hierarchy ‘ Load from Stream pseudo code… Read each member variable from the stream Read the number of child objects from the stream For index From 1 To Number of children Create a new child object Get it to retrieve itself from the stream Next n More complex that sending hierarchy to stream n Need to create new child objects before each can read data from the stream n Need to build up hierarchy

Objects and Databases n n Streams of objects have an inherent problem n Need to send and retrieve the whole hierarchy n No chance of reading one or two items back n OK for a few hundred or thousand items n Not possible for millions In this case, a database provides the solution n Structured random access n Fast retrieval of objects and sets of objects

Pure object databases n n n Transparent in operation Linked tightly to the language and the operating system Automatic retrieval of objects as needed n n Try to access an object currently not in memory, and it will be fetched automatically Tends to be domain specific n n CAD systems Large structured inventories

Relational object databases n Make use of existing relational database technology n n Common, cheap, well understood Can fit into any domain that works with a database n n Legacy systems Common computer storage mechanisms n Data, Mail, Documents etc.

Relational object modelling n n n One table per class Relationships between records map relationships between objects Class methods can hide relational database layer n n n Automatically fetch objects as references to them are made Automatically flush objects to database on destruction Coding can become awkward and complex n Rewards in flexibility and legacy compatibility

Tiered system models n Three (or more) tiers n n n Presentation Business Data access

Tiers and responsibilities n Presentation layer n n n Business logic layer n n n Handles user-interactions Connects to business logic only Enforces rules on use of data and access to it Models the way the business uses information Data access layer n n Enforces data security and integrity Maintains connections to various data sources and presents them in a homogenous way to the business layer

Visual Basic data access n Active. X Data Objects (ADO) n n (Bizarrely, Microsoft says the X is silent) A simple, uniform layer of services for accessing almost any type of structured data n n Relational databases (various products) Mail servers Flat-file data from spreadsheets, old style databases Filing system resources

ADO Object Model n Connection class n n Recordset class n n Creates and maintains links to data sources Presents data as tables of records Using these two classes, possible to work with any database n n Connection opens and closes links and sends Commands to database engine Recordset allows new records to be created, records manipulated, records deleted