NAME¶

Bio::DB::IndexedBase - Base class for modules using indexed sequence files

SYNOPSIS¶

  use Bio::DB::XXX; # a made-up class that uses Bio::IndexedBase
  # 1/ Bio::SeqIO-style access
  # Index some sequence files
  my $db = Bio::DB::XXX->new('/path/to/file');    # from a single file
  my $db = Bio::DB::XXX->new(['file1', 'file2']); # from multiple files
  my $db = Bio::DB::XXX->new('/path/to/files/');  # from a directory
  # Get IDs of all the sequences in the database
  my @ids = $db->get_all_primary_ids;
  # Get a specific sequence
  my $seq = $db->get_Seq_by_id('CHROMOSOME_I');
  # Loop through all sequences
  my $stream = $db->get_PrimarySeq_stream;
  while (my $seq = $stream->next_seq) {
    # Do something...
  }
  # 2/ Access via filehandle
  my $fh = Bio::DB::XXX->newFh('/path/to/file');
  while (my $seq = <$fh>) {
    # Do something...
  }
  # 3/ Tied-hash access
  tie %sequences, 'Bio::DB::XXX', '/path/to/file';
  print $sequences{'CHROMOSOME_I:1,20000'};

DESCRIPTION¶

Bio::DB::IndexedBase provides a base class for modules that want to index and read sequence files and provides persistent, random access to each sequence entry, without bringing the entire file into memory. This module is compliant with the Bio::SeqI interface and both. Bio::DB::Fasta and Bio::DB::Qual both use Bio::DB::IndexedBase.

When you initialize the module, you point it at a single file, several files, or a directory of files. The first time it is run, the module generates an index of the content of the files using the AnyDBM_File module (BerkeleyDB preferred, followed by GDBM_File, NDBM_File, and SDBM_File). Subsequently, it uses the index file to find the sequence file and offset for any requested sequence. If one of the source files is updated, the module reindexes just that one file. You can also force reindexing manually at any time. For improved performance, the module keeps a cache of open filehandles, closing less-recently used ones when the cache is full.

Entries may have any line length up to 65,536 characters, and different line lengths are allowed in the same file. However, within a sequence entry, all lines must be the same length except for the last. An error will be thrown if this is not the case!

This module was developed for use with the C. elegans and human genomes, and has been tested with sequence segments as large as 20 megabases. Indexing the C. elegans genome (100 megabases of genomic sequence plus 100,000 ESTs) takes ~5 minutes on my 300 MHz pentium laptop. On the same system, average access time for any 200-mer within the C. elegans genome was <0.02s.

DATABASE CREATION AND INDEXING¶

The two constructors for this class are new() and newFh(). The former creates a Bio::DB::IndexedBase object which is accessed via method calls. The latter creates a tied filehandle which can be used Bio::SeqIO style to fetch sequence objects in a stream fashion. There is also a tied hash interface.

$db = Bio::DB::IndexedBase->new($path [,%options])

Create a new Bio::DB::IndexedBase object from the files designated by $path $path may be a single file, an arrayref of files, or a directory containing such files.

After the database is created, you can use methods like get_all_primary_ids() and get_Seq_by_id() to retrieve sequence objects.

$fh = Bio::DB::IndexedBase->newFh($path [,%options])

Create a tied filehandle opened on a Bio::DB::IndexedBase object. Reading from this filehandle with <> will return a stream of sequence objects, Bio::SeqIO style. The path and the options should be specified as for new().

$obj = tie %db,'Bio::DB::IndexedBase', '/path/to/file' [,@args]

Create a tied-hash by tieing %db to Bio::DB::IndexedBase using the indicated path to the files. The optional @args list is the same set used by new(). If successful, tie() returns the tied object, undef otherwise.

Once tied, you can use the hash to retrieve an individual sequence by its ID, like this:

  my $seq = $db{CHROMOSOME_I};
    

The keys() and values() functions will return the sequence IDs and their sequences, respectively. In addition, each() can be used to iterate over the entire data set:

 while (my ($id,$sequence) = each %db) {
    print "$id => $sequence\n";
 }
    

When dealing with very large sequences, you can avoid bringing them into memory by calling each() in a scalar context. This returns the key only. You can then use tied(%db) to recover the Bio::DB::IndexedBase object and call its methods.

 while (my $id = each %db) {
    print "$id: $db{$sequence:1,100}\n";
    print "$id: ".tied(%db)->length($id)."\n";
 }
    

In addition, you may invoke the FIRSTKEY and NEXTKEY tied hash methods directly to retrieve the first and next ID in the database, respectively. This allows one to write the following iterative loop using just the object-oriented interface:

 my $db = Bio::DB::IndexedBase->new('/path/to/file');
 for (my $id=$db->FIRSTKEY; $id; $id=$db->NEXTKEY($id)) {
    # do something with sequence
 }
    

INDEX CONTENT¶

Several attributes of each sequence are stored in the index file. Given a sequence ID, these attributes can be retrieved using the following methods:

offset($id)

Get the offset of the indicated sequence from the beginning of the file in which it is located. The offset points to the beginning of the sequence, not the beginning of the header line.

strlen($id)

Get the number of characters in the sequence string.

length($id)

Get the number of residues of the sequence.

linelen($id)

Get the length of the line for this sequence. If the sequence is wrapped, then linelen() is likely to be much shorter than strlen().

headerlen($id)

Get the length of the header line for the indicated sequence.

header_offset

Get the offset of the header line for the indicated sequence from the beginning of the file in which it is located. This attribute is not stored. It is calculated from offset() and headerlen().

alphabet($id)

Get the molecular type (alphabet) of the indicated sequence. This method handles residues according to the IUPAC convention.

file($id)

Get the the name of the file in which the indicated sequence can be found.

INTERFACE COMPLIANCE NOTES¶

Bio::DB::IndexedBase is compliant with the Bio::DB::SeqI and hence with the Bio::RandomAccessI interfaces.

Database do not necessarily provide any meaningful internal primary ID for the sequences they store. However, Bio::DB::IndexedBase's internal primary IDs are the IDs of the sequences. This means that the same ID passed to get_Seq_by_id() and get_Seq_by_primary_id() will return the same sequence.

Since this database index has no notion of sequence version or namespace, the get_Seq_by_id(), get_Seq_by_acc() and get_Seq_by_version() are identical.

BUGS¶

When a sequence is deleted from one of the files, this deletion is not detected by the module and removed from the index. As a result, a "ghost" entry will remain in the index and will return garbage results if accessed.

Also, if you are indexing a directory, it is wise to not add or remove files from it.

In case you have changed the files in a directory, or the sequences in a file, you can to rebuild the entire index, either by deleting it manually, or by passing -reindex=>1 to new() when initializing the module.

SEE ALSO¶

DB_File

Bio::DB::Fasta

Bio::DB::Qual

AUTHOR¶

Lincoln Stein <lstein@cshl.org>.

Copyright (c) 2001 Cold Spring Harbor Laboratory.

Florent Angly (for the modularization)

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself. See DISCLAIMER.txt for disclaimers of warranty.

APPENDIX¶

The rest of the documentation details each of the object methods. Internal methods are usually preceded with a _

 Title   : new
 Usage   : my $db = Bio::DB::IndexedBase->new($path, -reindex => 1);
 Function: Initialize a new database object
 Returns : A Bio::DB::IndexedBase object
 Args    : A single file, or path to dir, or arrayref of files
           Optional arguments:
 Option        Description                                         Default
 -----------   -----------                                         -------
 -glob         Glob expression to search for files in directories  *
 -makeid       A code subroutine for transforming IDs              None
 -maxopen      Maximum size of filehandle cache                    32
 -debug        Turn on status messages                             0
 -reindex      Force the index to be rebuilt                       0
 -dbmargs      Additional arguments to pass to the DBM routine     None
 -index_name   Name of the file that will hold the indices
 -clean        Remove the index file when finished                 0

  $db = Bio::DB::IndexedBase->new('file.fa', -makeid => \&extract_gi);
  sub extract_gi {
      # Extract GI from GenBank
      my $header = shift;
      my ($id) = ($header =~ /gi\|(\d+)/m);
      return $id || '';
  }

 >gi|352962132|ref|NG_030353.1| Homo sapiens sal-like 3 (Drosophila) (SALL3)

 my $seq = $db->get_Seq_by_id(352962132);

 Title   : newFh
 Usage   : my $fh = Bio::DB::IndexedBase->newFh('/path/to/files/', %options);
 Function: Index and get a new Fh for a single file, several files or a directory
 Returns : Filehandle object
 Args    : Same as new()

 Title   : dbmargs
 Usage   : my @args = $db->dbmargs;
 Function: Get stored dbm arguments
 Returns : Array
 Args    : None

 Title   : glob
 Usage   : my $glob = $db->glob;
 Function: Get the expression used to match files in directories
 Returns : String
 Args    : None

 Title   : index_dir
 Usage   : $db->index_dir($dir);
 Function: Index the files that match -glob in the given directory
 Returns : Hashref of offsets
 Args    : Dirname
           Boolean to force a reindexing the directory

 Title   : get_all_primary_ids, get_all_ids, ids
 Usage   : my @ids = $db->get_all_primary_ids;
 Function: Get the IDs stored in all indexes. This is a Bio::DB::SeqI method
           implementation. Note that in this implementation, the internal
           database primary IDs are also the sequence IDs.
 Returns : List of ids
 Args    : None

 Title   : index_file
 Usage   : $db->index_file($filename);
 Function: Index the given file
 Returns : Hashref of offsets
 Args    : Filename
           Boolean to force reindexing the file

 Title   : index_files
 Usage   : $db->index_files(\@files);
 Function: Index the given files
 Returns : Hashref of offsets
 Args    : Arrayref of filenames
           Boolean to force reindexing the files

 Title   : index_name
 Usage   : my $indexname = $db->index_name($path);
 Function: Get the full name of the index file
 Returns : String
 Args    : None

 Title   : path
 Usage   : my $path = $db->path($path);
 Function: When a single file or a directory of files is indexed, this returns
           the file directory. When indexing an arbitrary list of files, the
           return value is the path of the current working directory.
 Returns : String
 Args    : None

 Title   : get_PrimarySeq_stream
 Usage   : my $stream = $db->get_PrimarySeq_stream();
 Function: Get a SeqIO-like stream of sequence objects. The stream supports a
           single method, next_seq(). Each call to next_seq() returns a new
           PrimarySeqI compliant sequence object, until no more sequences remain.
           This is a Bio::DB::SeqI method implementation.
 Returns : A Bio::DB::Indexed::Stream object
 Args    : None

 Title   : get_Seq_by_id, get_Seq_by_acc, get_Seq_by_version, get_Seq_by_primary_id
 Usage   : my $seq = $db->get_Seq_by_id($id);
 Function: Given an ID, fetch the corresponding sequence from the database.
           This is a Bio::DB::SeqI and Bio::DB::RandomAccessI method implementation.
 Returns : A sequence object
 Args    : ID

 Title   : _calculate_offsets
 Usage   : $db->_calculate_offsets($filename, $offsets);
 Function: This method calculates the sequence offsets in a file based on ID and
           should be implemented by classes that use Bio::DB::IndexedBase.
 Returns : Hash of offsets
 Args    : File to process
           Hashref of file offsets keyed by IDs.

 Title   : offset
 Usage   : my $offset = $db->offset($id);
 Function: Get the offset of the indicated sequence from the beginning of the
           file in which it is located. The offset points to the beginning of
           the sequence, not the beginning of the header line.
 Returns : String
 Args    : ID of sequence

 Title   : strlen
 Usage   : my $length = $db->strlen($id);
 Function: Get the number of characters in the sequence string.
 Returns : Integer
 Args    : ID of sequence

 Title   : length
 Usage   : my $length = $db->length($id);
 Function: Get the number of residues of the sequence.
 Returns : Integer
 Args    : ID of sequence

 Title   : linelen
 Usage   : my $linelen = $db->linelen($id);
 Function: Get the length of the line for this sequence.
 Returns : Integer
 Args    : ID of sequence

 Title   : headerlen
 Usage   : my $length = $db->headerlen($id);
 Function: Get the length of the header line for the indicated sequence.
 Returns : Integer
 Args    : ID of sequence

 Title   : header_offset
 Usage   : my $offset = $db->header_offset($id);
 Function: Get the offset of the header line for the indicated sequence from
           the beginning of the file in which it is located.
 Returns : String
 Args    : ID of sequence

 Title   : alphabet
 Usage   : my $alphabet = $db->alphabet($id);
 Function: Get the molecular type of the indicated sequence: dna, rna or protein
 Returns : String
 Args    : ID of sequence

 Title   : file
 Usage   : my $file = $db->file($id);
 Function: Get the the name of the file in which the indicated sequence can be
           found.
 Returns : String
 Args    : ID of sequence