31 July 2011

Storing some SNPs using the leveldb library. My notebook.

In this post I'll describe how I've used the leveldb API, a C++ key-value database, to store some SNPs (key=rs,value=sequence). "LevelDB is a fast key-value storage library written at Google that provides an ordered mapping from string keys to string values.". A benchmark for this engine is available here: http://code.google.com/p/leveldb/source/browse/trunk/doc/benchmark.html.

Download & install

$ svn checkout http://leveldb.googlecode.com/svn/trunk/ leveldb-read-only
$ cd leveldb-read-only/
$ make

Open & close a leveldb database

#include "leveldb/db.h"
(...)
leveldb::DB* db=NULL;
leveldb::Options options;
options.comparator=&rs_comparator;/* custom comparator for ordering the keys, see later */
options.create_if_missing = true;
clog << "[LOG]opening database" << endl;
leveldb::Status status = leveldb::DB::Open(options,db_home, &db);
//check status
if (!status.ok())
{
cerr << "cannot open " << db_home << " : " << status.ToString() << endl;
return EXIT_FAILURE;
}
(...)
/* use the database */
(...)
delete db;

A custom comparator for ordering the keys

Here, the keys are the rs-ids ordered on their numerical value. Both keys and values have a type "leveldb::Slice".
class RsComparator : public leveldb::Comparator
{
private:
/* parses the rsId: rs[0-9]+ */
int rs(const leveldb::Slice& s) const
{
int n=0;
for(size_t i=2;i< s.size();++i)
{
n=n*10+s[i]-'0';
}
return n;
}
public:
virtual int Compare(const leveldb::Slice& a, const leveldb::Slice& b) const
{
return rs(a)-rs(b);
}
(...)
}
(...)
RsComparator rs_comparator;
options.comparator=&rs_comparator;

Inserting a FASTA sequence


(...)
std::string name;
std::string sequence;
(...)
leveldb::Status status = db->Put(leveldb::WriteOptions(), name, sequence);
if(!status.ok())
{
cerr << "Cannot insert "<< name << " "
<< status.ToString() << endl;
return EXIT_FAILURE;
}

Searching for a rs###

(...)
std::string name(argv[optind]);
std::string sequence;
(...)
leveldb::Status status = db->Get(leveldb::ReadOptions(),seqname, &sequence);
if(!status.ok())
{
cerr << "Cannot find " << seqname<< " in " << db_home << endl;
continue;
}
printFasta(seqname,sequence);
(...)

Dumping all the SNPs using an iterator

(...)
leveldb::Iterator* it = db->NewIterator(leveldb::ReadOptions());
for (it->SeekToFirst(); it->Valid(); it->Next())
{
printFasta(it->key(),it->value());
}
delete it;
(...)

Examples

Reading the SNPs:
$ curl -s "ftp://ftp.ncbi.nih.gov/snp/organisms/human_9606/rs_fasta/rs_chMT.fas.gz" |\
./rsput -D snp.db

[LOG]opening database
[LOG] added rs8896
[LOG] added rs8936
[LOG] added rs9743
(...)
[LOG]closing database

$ du -h snp.db
336K snp.db
Dump the snps

$ ./rsget  -D snp.db

[LOG]opening database
>rs8896
GGTGTTGGTTCTCTTAATCTTTAACTTAAAAGGTTAATGCTAAGTTAGCTTTACAGTGGG
CTCTAGAGGGGGTAGAGGGGGTGYTATAGGGTAAATACGGGCCCTATTTCAAAGATTTTT
AGGGGAATTAATTCTAGGACGATGGGCATGAAACTGTGGTTTGCTCCACAGATTTCAGAG
CATT
>rs8936
ACTACGGCGGACTAATCTTCAACTCCTACATACTTCCCCCATTATTCCTAGAACCAGGCG
ACCTGCGACTCCTTGACGTTGACAATCGAGTAGTACTCCCGATTGAAGCCCCCATTCGTA
TAATAATTACATCACAAGACGTCTTGCACTCATGAGCTGTCCCCACATTAGGCTTAAAAA
CAGATGCAATTCCCGGACGTHTAAACCAAACCACTTTCACCGCTACACGACCGGGGGTAT
ACTACGGTCAATGCTCTGAAATCTGTGGAGCAAACCACAGTTTCATGCCCATCGTCCTAG
AATTAATTCCCCTAAAAATCTTTGAAATAGGGCCCGTATTTACCCTATAGCACCCCCTCT
ACCCCCTCTAGAGCCCACTGTAAAGCTAACTTAGCATTAAC
>rs9743
CCATGTGATTTCACTTCCACTCCATAACGCTCCTCATACTAGGCCTACTAACCAACACAC
TAACCATATACCAATGATGNCGCGATGTAACACGAGAAAGCACATACCAAGGCCACCACA
CACCACCTGTCCAAAAAGGCCTTCGATACGGGATAATCCTATTTATTACCTCAGAANTTT
TTTTCTTCGCAGGATTTTTCTGAGCCTTTTACCACTCCAGCCTAGCCCCTACCCCCCAAN
(...)
[LOG]closing database
Search for some SNPs:
$ ./rsget -D snp.db rs78894381 rs72619361 rs25
[LOG]opening database
[LOG]searching for rs78894381
>rs78894381
CTACTAATCTCATCAACACAACCCCCGCCCATCCTACCCAGCACACACACACCGCTGCTA
ACCCCATACCCCGAACCAACCAAACCCCAAAGACACCCCCNCACAGTTTATGTAGCTTAC
CTCCTCAAAGCAATACACTGAAAATGTTTAGACGGGCTCACATCACCCCATAAACAAATA
GGTTTGGTCCTAGCCTTTCTA
[LOG]searching for rs72619361
>rs72619361
ATGCATTTGGTATTTTAATCTGGGGGGTGTGCACGCGATAGCATTGTGAAACGCTGGCCC
CAGAGCACCCTATGTCGCAGTGTCTGTCTTTGATTCCTGCCYCATCCCATTATTGATCAC
ACCTACATTCAATATCCCAGGCGAGCATACCTATCACAAGGTGTTAATTAATTAATGCTT
GTAGGACATAACAATCAGTAAAC
[LOG]searching for rs25
Cannot find rs25 in snp.db
[LOG]closing database

Source code






That's it,

Pierre

1 comment:

adamtha said...

Thanks, looks promising
I'm always looking for a persistent and friendly ways to store some key-value pairs
Performance wise looks very nice though tests presented were a bit small so ill just grab it and have a look
again thanks for sharing and for your cool posts!

regards
adam