class Moab::FileSignature
The fixity properties of a file, used to determine file content equivalence regardless of filename. Placing this data in a class by itself facilitates using file size together with the MD5 and SHA1 checksums as a single key when doing comparisons against other file instances. The Moab
design assumes that this file signature is sufficiently unique to act as a comparator for determining file equality and eliminating file redundancy.
The use of signatures for a compare-by-hash mechanism introduces a miniscule (but non-zero) risk that two non-identical files will have the same checksum. While this risk is only about 1 in 1048 when using the SHA1 checksum alone, it can be reduced even further (to about 1 in 1086) if we use the MD5 and SHA1 checksums together. And we gain a bit more comfort by including a comparison of file sizes.
Finally, the “collision” risk is reduced by isolation of each digital object’s file pool within an object folder, instead of in a common storage area shared by the whole repository.
Data Model¶ ↑
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{FileInventory} = container for recording information about a collection of related files
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{FileGroup} [1..*] = subset allow segregation of content and metadata files
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{FileManifestation} [1..*] = snapshot of a file’s filesystem characteristics
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{FileSignature} [1] = file fixity information
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{FileInstance} [1..*] = filepath and timestamp of any physical file having that signature
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{SignatureCatalog} = lookup table containing a cumulative collection of all files ever ingested
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{SignatureCatalogEntry} [1..*] = an row in the lookup table containing storage information about a single file
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{FileSignature} [1] = file fixity information
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{FileInventoryDifference} = compares two {FileInventory} instances based on file signatures and pathnames
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{FileGroupDifference} [1..*] = performs analysis and reports differences between two matching {FileGroup} objects
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{FileGroupDifferenceSubset} [1..5] = collects a set of file-level differences of a give change type
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{FileInstanceDifference} [1..*] = contains difference information at the file level
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{FileSignature} [1..2] = contains the file signature(s) of two file instances being compared
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@see searchstorage.techtarget.com/feature/The-skinny-on-data-deduplication @see www.ibm.com/developerworks/wikis/download/attachments/106987789/TSMDataDeduplication.pdf @see www.redlegg.com/pdf_file/3_1320410927_HowDataDedupeWorks_WP_100809.pdf @see www.library.yale.edu/iac/DPC/AN_DPC_FixityChecksFinal11.pdf
@note Copyright © 2012 by The Board of Trustees of the Leland Stanford
Junior University.
All rights reserved. See {file:LICENSE.rdoc} for details.
Constants
- KNOWN_ALGOS
Public Class Methods
# File lib/moab/file_signature.rb, line 69 def self.active_algos Moab::Config.checksum_algos end
@return [Hash<Symbol,String>] Key is type (e.g. :sha1), value is checksum names (e.g. [‘SHA-1’, ‘SHA1’])
# File lib/moab/file_signature.rb, line 191 def self.checksum_names_for_type { md5: ['MD5'], sha1: %w[SHA-1 SHA1], sha256: %w[SHA-256 SHA256] } end
@return [Hash<String, Symbol>] Key is checksum name (e.g. MD5), value is checksum type (e.g. :md5)
# File lib/moab/file_signature.rb, line 200 def self.checksum_type_for_name type_for_name = {} checksum_names_for_type.each do |type, names| names.each { |name| type_for_name[name] = type } end type_for_name end
Reads the file once for ALL (requested) algorithms, not once per. @param [Pathname] pathname @param [Array<Symbol>] one or more keys of KNOWN_ALGOS
to be computed @return [Moab::FileSignature] object populated with (requested) checksums
# File lib/moab/file_signature.rb, line 77 def self.from_file(pathname, algos_to_use = active_algos) raise(MoabRuntimeError, 'Unrecognized algorithm requested') unless algos_to_use.all? { |a| KNOWN_ALGOS.include?(a) } signatures = algos_to_use.to_h { |k| [k, KNOWN_ALGOS[k].call] } pathname.open('r') do |stream| while (buffer = stream.read(8192)) signatures.each_value { |digest| digest.update(buffer) } end end new(signatures.transform_values(&:hexdigest).merge(size: pathname.size)) end
Public Instance Methods
@api internal (see eql?
)
# File lib/moab/file_signature.rb, line 148 def ==(other) eql?(other) end
@return [Hash<Symbol,String>] A hash of the checksum data
# File lib/moab/file_signature.rb, line 108 def checksums @checksums ||= {}.tap do |checksum_hash| checksum_hash[:md5] = md5 unless md5.to_s.empty? checksum_hash[:sha1] = sha1 unless sha1.to_s.empty? checksum_hash[:sha256] = sha256 unless sha256.to_s.empty? end end
@return [Boolean] The signature contains all of the 3 desired checksums
# File lib/moab/file_signature.rb, line 117 def complete? checksums.size == 3 end
@api internal @param other [FileSignature] The other file signature being compared to this signature @return [Boolean] Returns true if self and other have comparable fixity data.
# File lib/moab/file_signature.rb, line 130 def eql?(other) return false if size.to_i != other.size.to_i self_checksums = checksums other_checksums = other.checksums matching_keys = self_checksums.keys & other_checksums.keys return false if matching_keys.empty? matching_keys.each do |key| return false if self_checksums[key] != other_checksums[key] end true rescue NoMethodError false end
@api internal @return [Hash<Symbol => String>] A hash of fixity data from this signataure object
# File lib/moab/file_signature.rb, line 123 def fixity { size: size.to_s }.merge(checksums) end
@api internal @return [Fixnum] Compute a hash-code for the fixity value array.
Two file instances with the same content will have the same hash code (and will compare using eql?).
@note The hash and eql? methods override the methods inherited from Object.
These methods ensure that instances of this class can be used as Hash keys. See * {http://www.paulbutcher.com/2007/10/navigating-the-equality-maze/} * {http://techbot.me/2011/05/ruby-basics-equality-operators-ruby/} Also overriden is {#==} so that equality tests in other contexts will also return the expected result.
# File lib/moab/file_signature.rb, line 160 def hash @size.to_i end
@api internal @param pathname [Pathname] The location of the file whose full signature will be returned @return [FileSignature] The full signature derived from the file, unless the fixity is inconsistent with current values
# File lib/moab/file_signature.rb, line 181 def normalized_signature(pathname) sig_from_file = FileSignature.new.signature_from_file(pathname) return sig_from_file if eql?(sig_from_file) # The full signature from file is consistent with current values, or... # One or more of the fixity values is inconsistent, so raise an exception raise(MoabRuntimeError, "Signature inconsistent between inventory and file for #{pathname}: #{diff(sig_from_file).inspect}") end
@param type [Symbol,String] The type of checksum @param value [String] The checksum value @return [void] Set the value of the specified checksum type
# File lib/moab/file_signature.rb, line 94 def set_checksum(type, value) case type.to_s.downcase.to_sym when :md5 @md5 = value when :sha1 @sha1 = value when :sha256 @sha256 = value else raise ArgumentError, "Unknown checksum type '#{type}'" end end
@deprecated this method is a holdover from an earlier version. use the class method .from_file going forward. @api external @param pathname [Pathname] The location of the file to be digested @return [FileSignature] Generate a FileSignature
instance containing size and checksums for a physical file
# File lib/moab/file_signature.rb, line 169 def signature_from_file(pathname) file_signature = self.class.from_file(pathname) self.size = file_signature.size self.md5 = file_signature.md5 self.sha1 = file_signature.sha1 self.sha256 = file_signature.sha256 self end