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Class: DBM

Relationships & Source Files
Super Chains via Extension / Inclusion / Inheritance
Instance Chain:
self, Enumerable
Inherits: Object
Defined in: ext/dbm/dbm.c

Overview

Introduction

The DBM class provides a wrapper to a Unix-style dbm or Database Manager library.

Dbm databases do not have tables or columns; they are simple key-value data stores, like a Ruby Hash except not resident in RAM. Keys and values must be strings.

The exact library used depends on how Ruby was compiled. It could be any of the following:

  • The original ndbm library is released in 4.3BSD. It is based on dbm library in Unix Version 7 but has different API to support multiple databases in a process.

  • Berkeley DB versions 1 thru 5, also known as BDB and Sleepycat DB, now owned by Oracle Corporation.

  • Berkeley DB 1.x, still found in 4.4BSD derivatives (FreeBSD, OpenBSD, etc).

  • / gdbm, the GNU implementation of dbm.

  • qdbm, another open source reimplementation of dbm.

All of these dbm implementations have their own Ruby interfaces available, which provide richer (but varying) APIs.

Cautions

Before you decide to use DBM, there are some issues you should consider:

  • Each implementation of dbm has its own file format. Generally, dbm libraries will not read each other's files. This makes dbm files a bad choice for data exchange.

  • Even running the same OS and the same dbm implementation, the database file format may depend on the CPU architecture. For example, files may not be portable between PowerPC and 386, or between 32 and 64 bit Linux.

  • Different versions of Berkeley DB use different file formats. A change to the OS may therefore break DBM access to existing files.

  • Data size limits vary between implementations. Original Berkeley DB was limited to 2GB of data. Dbm libraries also sometimes limit the total size of a key/value pair, and the total size of all the keys that hash to the same value. These limits can be as little as 512 bytes. That said, gdbm and recent versions of Berkeley DB do away with these limits.

Given the above cautions, DBM is not a good choice for long term storage of important data. It is probably best used as a fast and easy alternative to a Hash for processing large amounts of data.

Example

require 'dbm'
db = DBM.open('rfcs', 0666, DBM::WRCREAT)
db['822'] = 'Standard for the Format of ARPA Internet Text Messages'
db['1123'] = 'Requirements for Internet Hosts - Application and Support'
db['3068'] = 'An Anycast Prefix for 6to4 Relay Routers'
puts db['822']

Constant Summary

  • NEWDB =

    Indicates that dbm_open() should open the database in read/write mode, create it if it does not already exist, and delete all contents if it does already exist.

    # File 'ext/dbm/dbm.c', line 1112 
    INT2FIX(O_RDWR|O_CREAT|O_TRUNC|RUBY_DBM_RW_BIT)
  • READER =

    Indicates that dbm_open() should open the database in read-only mode

    # File 'ext/dbm/dbm.c', line 1098 
    INT2FIX(O_RDONLY|RUBY_DBM_RW_BIT)
  • VERSION =

    • “ndbm (4.3BSD)”

    • “Berkeley DB 4.8.30: (April 9, 2010)”

    • “Berkeley DB (unknown)” (4.4BSD, maybe)

    • “GDBM version 1.8.3. 10/15/2002 (built Jul 1 2011 12:32:45)”

    • “QDBM 1.8.78”

    # File 'ext/dbm/dbm.c', line 1158 
    Identifies ndbm library version.

Examples
  • WRCREAT =

    Indicates that dbm_open() should open the database in read/write mode, and create it if it does not already exist

    # File 'ext/dbm/dbm.c', line 1106 
    INT2FIX(O_RDWR|O_CREAT|RUBY_DBM_RW_BIT)
  • WRITER =

    Indicates that dbm_open() should open the database in read/write mode

    # File 'ext/dbm/dbm.c', line 1101 
    INT2FIX(O_RDWR|RUBY_DBM_RW_BIT)

Class Method Summary

Instance Attribute Summary

Instance Method Summary

Constructor Details

.new(filename[, mode[, flags]]) ⇒ DBM

Open a dbm database with the specified name, which can include a directory path. Any file extensions needed will be supplied automatically by the dbm library. For example, Berkeley DB appends '.db', and GNU gdbm uses two physical files with extensions '.dir' and '.pag'.

The mode should be an integer, as for Unix chmod.

Flags should be one of READER, WRITER, WRCREAT or NEWDB.

Class Method Details

.open(filename[, mode[, flags]]) ⇒ DBM .open(filename[, mode[, flags]]) {|dbm| ... }

Open a dbm database and yields it if a block is given. See also .new.

Instance Attribute Details

#closed? ⇒ Boolean (readonly)

Returns true if the database is closed, false otherwise.

#empty? ⇒ Boolean (readonly)

Returns true if the database is empty, false otherwise.

Instance Method Details

#[](key) ⇒ string value?

Return a value from the database by locating the key string provided. If the key is not found, returns nil.

#store(key, value) ⇒ value #[]=(key, value)
Also known as: #store

Stores the specified string value in the database, indexed via the string key provided.

#clear

Deletes all data from the database.

#close

#delete(key)

Deletes an entry from the database.

#reject! {|key, value| ... } ⇒ self #delete_if {|key, value| ... } ⇒ self

Alias for #reject!.

#each_pair {|key, value| ... } ⇒ self Also known as: #each_pair

Calls the block once for each [key, value] pair in the database. Returns self.

#each_key {|key| ... } ⇒ self

Calls the block once for each key string in the database. Returns self.

#each_pair {|key, value| ... } ⇒ self #each_pair {|key, value| ... } ⇒ self

Alias for #each.

#each_value {|value| ... } ⇒ self

Calls the block once for each value string in the database. Returns self.

#fetch(key[, ifnone]) ⇒ value

Return a value from the database by locating the key string provided. If the key is not found, returns ifnone. If ifnone is not given, raises IndexError.

#include?(key) ⇒ Boolean #has_key?(key) ⇒ Boolean #member?(key) ⇒ Boolean #key?(key) ⇒ Boolean

Alias for #key?.

#has_value?(value) ⇒ Boolean #value?(value) ⇒ Boolean

Alias for #value?.

#include?(key) ⇒ Boolean #has_key?(key) ⇒ Boolean #member?(key) ⇒ Boolean #key?(key) ⇒ Boolean

Alias for #key?.

#invert ⇒ Hash

Returns a Hash (not a DBM database) created by using each value in the database as a key, with the corresponding key as its value.

#key(value) ⇒ String

Returns the key for the specified value.

#include?(key) ⇒ Boolean #has_key?(key) ⇒ Boolean #member?(key) ⇒ Boolean #key?(key) ⇒ Boolean
Also known as: #include?, #has_key?, #member?

Returns true if the database contains the specified key, false otherwise.

#keys ⇒ Array

Returns an array of all the string keys in the database.

#length ⇒ Integer #size ⇒ Integer
Also known as: #size

Returns the number of entries in the database.

#include?(key) ⇒ Boolean #has_key?(key) ⇒ Boolean #member?(key) ⇒ Boolean #key?(key) ⇒ Boolean

Alias for #key?.

#reject {|key, value| ... } ⇒ Hash

Converts the contents of the database to an in-memory Hash, then calls Hash#reject with the specified code block, returning a new Hash.

#reject! {|key, value| ... } ⇒ self #delete_if {|key, value| ... } ⇒ self
Also known as: #delete_if

Deletes all entries for which the code block returns true. Returns self.

#replace(obj)

Replaces the contents of the database with the contents of the specified object. Takes any object which implements the each_pair method, including Hash and DBM objects.

#select {|key, value| ... } ⇒ Array

Returns a new array consisting of the [key, value] pairs for which the code block returns true.

#shift ⇒ Array, value

Removes a [key, value] pair from the database, and returns it. If the database is empty, returns nil. The order in which values are removed/returned is not guaranteed.

#length ⇒ Integer #size ⇒ Integer

Alias for #length.

#store(key, value) ⇒ value #[]=(key, value)

Alias for #[]=.

#to_a ⇒ Array

Converts the contents of the database to an array of [key, value] arrays, and returns it.

#to_hash ⇒ Hash

Converts the contents of the database to an in-memory Hash object, and returns it.

#update(obj)

Updates the database with multiple values from the specified object. Takes any object which implements the each_pair method, including Hash and DBM objects.

#has_value?(value) ⇒ Boolean #value?(value) ⇒ Boolean
Also known as: #has_value?

Returns true if the database contains the specified string value, false otherwise.

#values ⇒ Array

Returns an array of all the string values in the database.

#values_at(key, ...) ⇒ Array

Returns an array containing the values associated with the given keys.