Fundamental file structure concepts & managing files of records
- 1. 1 Fundamental File Structure Concepts & Managing Files of Records
- 2. 2 Outline I: Fundamental File Structure Concepts • Stream Files • Field Structures • Reading a Stream of Fields • Record Structures • Record Structures that use a length indicator
- 3. 3 Outline II: Managing Files of Records • Record Access • More About Record Structures • File Access and File Organization • More Complex File Organization and Access • Portability and Standardization
- 4. 4 Field and Record Organization: Overview • The basic logical unit of data is the field which contains a single data value. • Fields are organized into aggregates, either as many copies of a single field (an array) or as a list of different fields (a record). • When a record is stored in memory, we refer to it as an object and refer to its fields as members. • In this lecture, we will investigate the many ways that objects can be represented as records in files.
- 5. 5 Stream Files • Mary Ames • 123 Maple • Stillwater, OK 74075 • Alan Mason • 90 Eastgate • Ada, OK 74820 n Stream Files, the information is written as a eam of bytes containing no added information: mesMary123 MapleStillwaterOK74075MasonAlan90 EastgateAdaOK7 Problem: There is no way to get the information ck in the organized record format.
- 6. 6 Field Structures • There are many ways of adding structure to files to maintain the identity of fields: – Force the field into a predictable length – Begin each field with a length indicator – Use a “keyword = value” expression to identify each field and its content.
- 7. 7 Reading a Stream of Fields • A Program can easily read a stream of fields and output ===> • This time, we do preserve the notion of fields, but something is missing: Rather than a stream of fields, these should be two records Last Name: ‘Ames’ First Name: ‘Mary’ Address: ‘123 Maple’ City: ‘Stillwater State: ‘OK’ Zip Code: ‘74075’ Last Name: ‘Mason’ First Name: ‘Alan’ Address: ‘90 Eastgate’ City: ‘Ada’ State: ‘OK’ Zip Code: ‘74820’
- 8. 8 Record Structure I • A record can be defined as a set of fields that belong together when the file is viewed in terms of a higher level of organization. • Like the notion of a field, a record is another conceptual tool which needs not exist in the file in any physical sense. • Yet, they are an important logical notion included in the file’s structure.
- 9. 9 Record Structures II • Methods for organizing the records of a file include: – Requiring that the records be a predictable number of bytes in length. – Requiring that the records be a predictable number of fields in length. – Beginning each record with a length indicator consisting of a count of the number of bytes that the record contains. – Using a second file to keep track of the beginning byte address for each record. – Placing a delimiter at the end of each record to separate it from the next record.
- 10. 10 Record Structures that Use a Length Indicator • The notion of records that we implemented are lacking something: none of the variability in the length of records that was inherent in the initial stream file was conserved. • Implementation: – Writing the variable-length records to the file – Representing the record length – Reading the variable-length record from the file.
- 11. 11 Record Access: Keys • When looking for an individual record, it is convenient to identify the record with a key based on the record’s content (e.g., the Ames record). • Keys should uniquely define a record and be unchanging. • Records can also be searched based on a secondary key. Those do not typically uniquely identify a record.
- 12. 12 Sequential Search • Evaluating Performance of Sequential Search. • Improving Sequential Search Performance with Record Blocking. • When is Sequential Search Useful?
- 13. 13 Direct Access • How do we know where the beginning of the required record is? It may be in an Index (discussed in a different lecture) We know the relative record number (RRN) • RRN are not useful when working with variable length-records: the access is still sequential. • With fixed-length records, however, they are useful.
- 14. 14 Record Structure • Choosing a Record Structure and Record Length within a fixed-length record. 2 approaches: – Fixed-Length Fields in record (simple but problematic). – Varying Field boundaries within the fixed- length record. • Header Records are often used at the beginning of the file to hold some general info about a file to assist in future use of the file.
- 15. 15 File Access and File Organization: A Summary • File organization depends on what use you want to make of the file. • Since using a file implies accessing it, file access and file organization are intimately linked. • Example: though using fixed-length records makes direct access easier, if the documents have very variable lengths, fixed-length records is not a good solution: the application determines our choice of both access and organization.
- 16. 16 Beyond Record Structure • Abstract Data Models for File Access • Headers and Self-Describing File • Metadata • Color Raster Images • Mixing Object Types in One File • Representation-Independent File Access • Extensibility
- 17. 17 Portability and Standardization • Factors Affecting Portability – Differences among Operating Systems – Differences among Languages – Differences in Machine Architectures • Achieving Portability – Agree on a Standard Physical Record Format and Stay with it – Agree on a Standard Binary Encoding for Data Elements – Number and Text Conversion – File Structure Conversion – File System Differences – Unix and Portability