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(Strategies for Lightweight Databases)

Length: 3 hours

Prerequisites: Basic familiarity with Perl is required.

Many programs need cheap, convenient access to small amounts of data. There are two commonly used solutions: Flat text files and DBM files. This class will look at these in detail. Whether you're looking for a good solution for storage of your own data, or you have to deal with data stored in one of these formats by another program, this class will equip you with valuable tools for solving your problems.

In the first section, we'll look at techniques for managing flat text databases and the systems programming that underlies these. We'll examine the tradeoffs of variable vs. fixed-length records and sorted vs. unsorted files. In the second section, we'll take a detailed look at Tie::File, a new standard module that provides easy access to text databases.

The third section will be an overview of Perl's 'DBM' feature, including a comparison of the standard DBM modules. We'll see several extremely useful but little-known features of DB_File, the only one of these standard modules that doesn't have serious defects.

• Text Files
  • Rotating log file; deleting a user
  • Copy the File
    • -i.bak
    • Using -i inside a program
    • Problems with -i
    • Atomicity issues
  • Essential problem with files; fundamental operations; seeking
  • Sorted files
  • In-place modification of records
    • Overwriting records
    • Bytes vs. positions
    • Gappy Files
    • Fixed-length records
    • Numeric indices
    • Case study: lastlog
  • Indexing
    • Void fields
    • Generic text indices
    • Packed offsets
• Tie::File
  • Tie::File Examples
  • delete_user revisited
  • uppercase_username revisited
  • Rotating log file revisited
  • Most important thing to know about Tie::File
  • Indexing with Tie::File
  • Tie::File Internals
    • Caching
    • Record modification
    • Immediate vs. Deferred Writing
    • Autodeferring
  • Miscellaneous Features
• DBM
  • Common DBM Implementations
  • What DBM Does
  • Small DBMs: ODBM, NDBM, and SDBM
  • GDBM
  • DB_File
    • Indexing revisited
    • Ordered hashes
    • Partial matching
    • Sequential access
    • Multiple values
    • Filters
    • BerkeleyDB

Samples

Sample slides coming soon. Watch this space.

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