cdb tools for fasta files

# CDB (Constant DataBase) indexing and retrieval tools for multi-FASTA files

This is a brief introduction to a couple of platform independent file-based hashing tools (cdbfasta and cdbyank) that can be used for creating indices for quick retrieval of any particular sequences from large multi-FASTA files. The last version has the option to compress data records in order to save space. The index files are now architecture independent, the same index file can be created and used on many different Unix platform (be it 32bit/64bit, big-endian or little-endian architectures) and even Windows.

Content:

1.Typical usage
2.Retrieving sequence ranges or deflines
3.Data compression option
4.Development notes

## 1. Typical usage

Use cdbfasta to create the index file for a multi-FASTA file and cdbyank to pull records based on that index file. An usage message is displayed if the commands cdbyank or cdbyank are run without any parameters (or with -h). In order to create an index file, the name of the fasta file to be indexed must be provided:

    cdbfasta <fasta_file>

The fasta file can be specified with the whole path (if it's not in the current directory), e.g.

    cdbfasta /usr/local/db/GUDB.human

By default cdbfasta creates an index file with the same path and name as the database file but with the .cidx suffix added to the original name. So in the example above, a file GUDB.human.cidx will be created in /usr/local/db/. The default usage considers the key for a FASTA record to be the first space-delimited token following the ">" starting character from the definition line. For example, if a FASTA record had a defline like this:

>AA141526

...then we can use the string 'AA141526' with cdbyank to retrieve the full FASTA record associated to that sequence name:

    cdbyank -a 'AA141526' /usr/local/db/GUDB.human.cidx

Sometimes all the space delimited tokens in the defline need to be declared as keys in the index file, pointing to the same fasta record. This can be accomplished by cdbfasta by using the "-m" switch.

For long and complex fastA file accessions (for example : EGAD|61|GP|186739|gb|AAA63210.1||M60828) there is a possibility to create the index file in such a way that there is no need to provide the full string to cdbyank in order to retrieve such a sequence, but only the first "<db>|<accession>" pair (i.e. a substring ending at the second '|' character) should be enough. (EGAD|61 in the example above). In order to enable this feature, there are two alternative options for cdbfasta:

- -c : the index file is built only by storing the "shortcut key" (the first "db|accession" pair found in the defline of each fasta record). In this case, cdbyank will only be able to accept these "shortcut" accessions for record retrieval.
- -C : the index file is built by storing both the "shortcut key" and the full keys (which are considered to end at the first space character in the defline). In this case, two strings are stored as keys for each fastA record so any of them can be used as an accession for retrieval of the same record with cdbyank.

In order to retrieve records from the database file, cdbyank should be provided with the name of the index file created previously with cdbfasta, e.g.:

    cdbyank -a 'human|Z98492' /usr/local/db/GUDB.human.cidx

A list of accessions is expected at stdin if -a option is not provided, e.g.:

    cat seq_list | cdbyank /usr/local/db/GUDB.human.cidx

This way the output will be a series a fasta records at stdout. By redirecting this output to a file a multifasta file is obtained. cdbyank locates the database file by stripping the '.cidx' suffix off the index filename. But this is not enforced, because by using the -d option, cdbyank can make use of a user-provided database to be used by the given index file. In the example above, if the index file "GUDB.human.cidx" is moved into another directory, a cdbyank command (in that other directory) can be issued like that:

    cdbyank -a 'human|Z98492' -d /usr/local/db/GUDB.human GUDB.human.cidx

The position of the index file in the list of arguments of cdbyank is not enforced. For the -a usage, the error status returned by cdbyank to the shell will be 1 if the given key was not found and 0 for success.

The total number of fasta records indexed and the list of the keys stored in a specific cdb index file can be retrieved with cdbyank's -n and -l switches, respectively. This information is obtained from the index file directly (the database file is not needed for that). There is also a -s option that displays a summary of the indexing information stored in the index at index time. These are the initial name of the fastA file, its size, how the index was created (e.g. was -m (multiple keys) option given ? was -c or -C (shortcut keys) option given?), the number of keys stored in the file as well as the number of fasta records indexed - the latter being the same with what -n option returns.

As an extra feature, cdbfasta and cdbyank can also be used for some special cases where databases may have different records but with the same key (non-unique keys). Although the performance will degrade a little, cdbfasta is able to index this kind of files, but by default cdbyank only outputs the first record found. If you want all the possible records sharing the same key (accession) to be retrieved and displayed, the -x option should be given to cdbyank.

## 2. Retrieving sequence ranges or only the defline

There are two cdbyank options added for convenience: -F option returns the definition line of each requested FASTA record (the first line for each record). The -R option of cdbyank is intended for FASTA files containing actual genetic sequences (nucleotide or protein) and expects each of the retrieval commands to have the following format (space delimited):

<key> <right_coordinate> <left_coordinate>

For example if we only want to retrieve the sequence range 24...178 (letter numbering starts at 1) from sequence with the name 'human|Z98492', then the cdbyank command would look like this:

    cdbyank -a 'human|Z98492 24 178' -R GUDB.human.cidx

Multiple sequence ranges can be extracted this way by providing a file having each line following the format above (key followed by the two coordinates). Then, as before, such file can be piped into cdbyank with -R option to pull specific sequence ranges for each of the sequences specified in the input file.

    cat seqlistranges | cdbyank -R GUDB.human.cidx

Note that this range option works by actually parsing and looping through the retrieved record characters internally - so the performance is poor when some terminal range is pulled from a very large record.

## 3. Data compression option

The indexing program cdbfasta has the -z <compressed_db> option which creates a compressed file <compress_db> from the data in the given input file and at the same time creates an index file for this new compressed database, named <compressed_db>.cidxz.The original input file can then be discarded -- as it can be recovered at any point later from the <compressed_db> file by using the -z option of cdbyank.
Because each record is compressed separately, compression is poor if the records are small. Compression is only advised when:

- data records are large enough for the compression algorithm to become efficient (at least 1KB per record, the more the better)
- only random access is needed to the data records (so the original file can be discarded)

The compression can be quite slow for large files and there is also some performance penalty for cdbyank as it has to decompress the retrieved records on the fly. The input data for cdbfasta compression can be collected from stdin if '-' is used instead of a file name:

    cat my_data_files* | cdbfasta - -z mydata.cdbz

This option is useful especially when the total size of input data files is extremely large (over the file-system limits or over the 4GB internal limit of cdbfasta) while the compressed output can be small enough to fall under such limits.
With compressed databases cdbyank can be used normally without extra options as it will auto-detect the compression (from the index file info) and activate on-the-fly decompression of the retrieved records. Only -F and -R options are not yet supported for compressed records.

## 4. Development notes

These tools were developed in C++, based on the publicly available cdb ("constant database") code written by D.J. Bernstein (http://cr.yp.to/djb.html). "Constant databases" are those that we don't need to add to or remove records from. The original C source was (rather crudely) wrapped into C++ classes and adjusted to automatically index fasta records and to create an external index instead of compacting the original data file like the original cdb library code does. Also the "endianness" is now checked at runtime and the bytes are swapped accordingly such that the file offsets and record sizes are always read/written in the same way in the index file.
The compression option uses zlib's "deflate" method. The program uses deflate() with Z_FULL_FLUSH after each record, such that random record decompression is possible after the first [dummy] record is decompressed internally.
The index file contains an info chunk (actually stored at the end of the file) which maintains a summary data and flags about the indexing process (the -s option of cdbyank shows this info). Since the compression option was added, cdbyank is always trying to read this information first (before opening the data file) in order to determine if the data records are compressed or not.

Please let me know if you notice any problems with these tools.

--
Geo Pertea
geo.pertea@gmail.com
06/09/2003