gdbm.md

GDBM

GNU dbm (GDBM) is a simple database for storing key value pairs in a file. Keys and values are strings and a database can only be accessed by either many readers or exactly one writer at a time.

Ruby's standard library includes a gdbm C extension that wraps around the library and let's us use it like a Hash.

Writing

Writing key value pairs:

require 'gdbm'

gdbm = GDBM.new('database.db')
gdbm['foo'] = 'bar'
gdbm.close

Reading

gdbm = GDBM.new('database.db')
gdbm['foo'] # => 'bar'
gdbm.close

As GDBM includes the Enumerable module, all its goodies are available.

Block Shortcut

Instead of .new we can use .open which will open a database file, yield the GDBM object and close the database when it's done:

GDBM.open('database.db') do |gdbm|
  gdbm['foo'] # => 'bar'
end

Read-Only Access

The default access is GDBM::WRITER which can read and write data. If we only want to read data, we can open a database as a GDBM::READER:

read_only_gdbm = GDBM.new('database.db', mode = 0666, GDBM::READER)
read_only_gdbm['foo'] # => 'bar'

… which will raise when trying to write:

read_only_gdbm['foo'] = 'baz' # => GDBMError: Reader can't store

Concurrent Access

As stated, only many readers or exactly one writer can access a database at a time. But it seems the wrapper is clever for a single process, as:

gdbm_writer = GDBM.new('database.db', mode = 0666, GDBM::WRITER)
gdbm_reader = GDBM.new('database.db', mode = 0666, GDBM::READER)

… works, but this doesn't:

gdbm_writer = GDBM.new('database.db', mode = 0666, GDBM::WRITER)

fork do
  gdbm_reader = GDBM.new('database.db', mode = 0666, GDBM::READER) # raises Errno::EAGAIN: Resource temporarily unavailable - database.db
end