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Retrieving DAS2 genomic sequence and annotation feature records

This document describes the DAS2 retrieval protocol for annotation servers and genomic sequence reference servers. There are four main sets of documents. The sources documents describe the available data sources, the segments documents describe the reference genomic sequences, the features documents contain the features on the sequences, and the types documents characterize the different feature types. The formal schema for the XML responses is a RelaxNG schema in .rnc format.

Genome DAS/2 uses half-open intervals to specify regions along a nucleotide sequence. See Segment ranges for more information.

Every document is identified with a URI, which should be an HTTP URL. A document is retrieved by doing a GET request on its URL. The following table summarizes the various types of GET requests used within the genome domain.

Request Documentation Information Retrieved Default Response Type
sources Sources Specification Information about all data sources or a specific data source application/x-das-sources+xml
segments Segments Specification Description of the regions on which features are located application/x-das-segments+xml
types Types Specification Description of all feature types or a single type application/x-das-types+xml
features Features Specification One or more genomic features, including sequence alignments application/x-das-features+xml

General Information

This section contains information pertaining to the DAS/2 specification as a whole.


This specification makes extensive use of URIs, and more specifically HTTP URLs. While other URLs and URIs are possible the exchange protocol uses concepts like request action and headers, response code and headers, and query string construction which only make sense in the context of HTTP and related protocols like HTTPS.

Content-Type header

Each of the five new formats has its own MIME type. These are


A server should include the correct MIME type in the Content-Type header of the response. If not it must respond with "application/xml" and must not respond with text/xml. Character encoding is determined as per RFC 3023. We recommend that server implementers either not include the charset parameter in the Content-Type header or ensure that it is identical to the encoding in the document's XML declaration.

For use during specification development a server may include a "version" value so clients can determine which version of the spec is implemented by the server. Unless others can convince me otherwise this will be removed in the final specification.


Content-Type: application/x-das-types+xml; version=300

The list of versions is as follows:

  • 100 - the version as of 2006/02/07
  • 200 - the version as of 2006/02/10
    • (changed the feature query language format)
    • (using "prop-" instead of "att" for property searches)
  • 300 - the version as of 2006/03/10, which includes
    • the updates from the first sprint. This document
    • describes version 300.

If not present the client may assume the format is in the most recent version.

ISO dates

Several elements have 'created' and 'modified' attributes. These dates are formatted in a subset of <a href="http://www.w3.org/TR/NOTE-datetime">ISO 8601</a>. Data providers must write the date using one of the following forms

  • Complete date:
    YYYY-MM-DD (e.g. 1997-07-16)
  • Complete date plus hours and minutes:
    YYYY-MM-DDThh:mmTZD (e.g. 1997-07-16T19:20+01:00)
  • Complete date plus hours, minutes and seconds:
    YYYY-MM-DDThh:mm:ssTZD (eg 1997-07-16T19:20:30+01:00)
          YYYY = four-digit year
          MM   = two-digit month (01=January, etc.)
          DD   = two-digit day of month (01 through 31)
          hh   = two digits of hour (00 through 23) (am/pm NOT allowed)
          mm   = two digits of minute (00 through 59)
          ss   = two digits of second (00 through 59)
          TZD  = time zone designator (optional; one of the formats "Z", +hh:mm, +hhmm, -hh:mm, or -hhmm)

If the timezone designator is not specified a parser may assume 'Z'. If the seconds are not specified then a parser may assume 0. If the time is not specified then a parser may assume 12:00:00Z.

Here are some examples of valid dates


Global sequence identifiers

Segments of genomic sequence referenced by a DAS/2 document are identified by a set of community agreed upon identifiers. Global sequence identifiers permit DAS/2 clients to merge annotations from different servers if they both provide segments having the same name.

The canonical list of global sequence identifiers is publically maintained and accessible via this wiki page: http://www.biodas.org/wiki/GlobalSeqIDs.

Segment ranges

Segment locations are used in three places in DAS: feature locations, range-based feature filters, and sequence retrieval.

Every location is on a segment, named using its URI. This is either the primary URI given in the segments document or the reference URI either specified by the coordinate system or by the "reference" attribute in the segments document.

Segment ranges are given by start and end positions specified using half-open, zero-based intervals (a.k.a. interbase coordinates). The interval is half-open meaning that the interval from 'start' to 'end' includes the residues from position 'start' up to but not including position 'end'. The first residue in the sequence is at position zero and is specified as the range (0,1). The length of an interval is always equal to end minus start (length=end-start).

This scheme is sometimes called "interbase coordinates" because the numbering system labels the positions inbetween the bases (residues) rather than the bases themselves.

For example, the range (3,6) includes the fourth, fifth, and sixth residues not second or seventh:

      0   1   2   3   4   5   6   7
                    C---C---G     (range="3:6" from GATCCGA)

The end coordinate of a range is never less than the start position. The range (5,6) covers the residue at position 5 while (5,5) has size of zero and refers to the point between positions 4 and 5. Cleavage site annotations may use zero size annotations like the latter.

Features may optionally be located on a strand. "1" denotes the positive strand, "-1" denotes the negative strand, "0" denotes both strands. If the strand is not specified then the strand is unknown or meaningless for the given feature or sequence type.

Features ranges are specified using a compact notation. Given the start and end positions the range is

    <start> + ":" + <end>

When the strand is allowed in a range field, it looks like:

    <start> + ":" + <end> + ":" + <strand>

Here are a few examples of how these look like in feature locations

  • all residues of Chr1
       <LOC segment="http://biodas.org/Chr1" />
  • the 1st and 2nd residues of Chr2
       <LOC segment="http://biodas.org/Chr2" range="0:2" />
  • the site between the 19th and 20th residues of Chr3
       <LOC segment="http://biodas.org/Chr3" range="20:20" />
  • the negative strand of Chr1 (assuming a length of 245522847)
       <LOC segment="http://ensembl.org/Homo_sapiens/Chr1" range="0:245522847:-1" />

Here are a few examples of how they look like in feature filters. Note that feature filters do not support strand information in the query range. The query parameters in both cases were encoded for use as a URL query parameter.

  • All features on http://ensemble.org/Homo_sapiens/Chr1 or Chr2
  • All features that overlap residues 200-300 of Chr1
    Sequence retrieval queries work directly on the segment URI.  To get
    the sequence for a subrange, pass the range using the "range" key of
    the query, as in the following:
      <li>The sequence for Contig4392
    <pre class="speclist">
  • The sequence for the first 10 residues of Contig4392 ("range=0:10")

The link element

The LINK element connects a document or a feature record to some other resource identified by a URL. The LINK element is modeled on the "link" element in HTML 4.0 and has many of the same attributes, with identical meanings.

The optional 'title' attribute is an advisory title providing a hint about what the element links to. The optional 'href' attribute is the URL being linked to. The optional 'type' attribute contains an advisory MIME type describing the expected document content-type. For example, a feature may link to an image in both GIF and a PNG formats, letting the client chose a prefered format.

The 'rel' and 'rev' attributes contain space separated terms which characterize the type of the forward and reverse links, respectively. Likely most links will be forward links and so use the 'rel' attribute. This specification does not reserve any link terms. We expect de facto types to evolve through common use.

Here are a few examples

  • a segments, type, or feature record might refer to an "official" sources record:
    <LINK rel="das2-sources" href="http://example.com/das2/human/v35/sources.xml"
      type="application/x-das-sources+xml" />
  • a feature filter response might link to an RSS feed. Subscribe to the feed to watch for any new results.
    <LINK rel="alternate" title="RSS feed for this search
      href="http://example.com/das2/human/v35/rss?search_id=68302" />
  • a feature element may link to a bibliographic record
    <LINK title="Primary reference" rel="pubmed-ref" 
      href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&list_uids=8744570" />

Formats and extensibility

This specification defines a base set of XML document formats. A data provider may want to return an alternate format in addition to the base formats. For examples, if the XML overhead is too high a data provider may want to develop a more compact binary feature format, or if the feature data is too rich for the simple DAS property table then it would be better represented in an alternate format.

Once implemented the alternative format support is announced through the FORMAT elements of the CAPABILITY element.

For example, the following says that the server implements three formats. The format name "das2xml" is reserved for the formats defined by this specification and must be supported by the server even if not listed. The other two format names are for example only:

<CAPABILITY type="features" query_uri="http://example.com/das/features.xml">
  <FORMAT name="das2xml" />
  <FORMAT name="das3xml" />
  <FORMAT name="compact-binary" />

To request an alternate format the client must add a "format=<name>" field to the query string of the URL. For example, to request all of the features from the above example but in "das3xml" the client makes a request for:


while to get all the features on http://biodas.org/segment/Chr3 in "compact-binary" format the client makes a request for


Servers may extend the <a href="#feature_filters">features filter language</a> to add new capabilities as long as those terms do not affect queries without those fields. A server may list support for a query extension using the SUPPORTS tag. In the following the server says it supports the "curation-search" as well as the das2xml and compact-binary formats:

<CAPABILITY type="features" query_uri="http://example.com/das/features.xml">
  <SUPPORTS name="curation-search" />
  <FORMAT name="das2xml" />
  <FORMAT name="compact-binary" />

The client implementer must use some other means to discover what additional filters are available for a "curation-search".

A server may support additional capabilities not defined by this specification and list support for it through a new CAPABILITY item. For example, in the following the hypothetical server implements an alternative query language based on XQuery.

<CAPABILITY type="xquery-features" query_uri="http://example.com/das-search" />

The contents of the non-DAS2 CAPABILITY elements is determined by the server implementer and a client implementer must look elsewhere to discover what it means.