Google Data APIs Protocol Reference

This document describes the protocol used by the Google Data APIs, including information about what a query looks like, what results look like, and so on.

For more information about the Google Data APIs, see the Google Data Developer's Guide document and the Protocol Guide.

Audience

This document is intended for anyone wanting to understand the details of the XML format and protocol used by the Google Data APIs.

If you just want to write code that uses the Google Data client APIs, then you don't need to know these details; instead, you can use the language-specific client libraries.

But if you want to understand the protocol, read this document. For example, you may want to read this document to help you with any of the following tasks:

  • evaluating the Google Data architecture
  • coding using the protocol without using the provided Google Data client libraries
  • writing a client library in a new language

This document assumes that you understand the basics of XML, namespaces, syndicated feeds, and the GET, POST, PUT, and DELETE requests in HTTP, as well as HTTP's concept of a "resource." For more information about those things, see the Additional resources section of this document.

This document doesn't rely on any particular programming language; you can send and receive Google Data messages using any programming language that lets you issue HTTP requests and parse XML-based responses.

Protocol details

This section describes the Google Data document format and query syntax.

Document format

Google Data, Atom, and RSS 2.0 all share the same basic data model: a container that holds both some global data and any number of entries. For each protocol, the format is defined by a base schema, but it can be extended using foreign namespaces.

The Google Data APIs can use either the Atom syndication format (for both reads and writes) or the RSS format (for reads only).

Atom is Google Data's default format. To request a response in RSS format, use the /alt=rss/ parameter; for more information, see Query requests.

When you request data in RSS format, Google Data supplies a feed (or other representation of the resource) in RSS format. If there's no equivalent RSS property for a given Google Data property, Google Data uses the Atom property, labeling it with an appropriate namespace to indicate that it's an extension to RSS.

Note: Most Google Data feeds in Atom format use the Atom namespace as the default namespace by specifying an xmlns attribute on the feed element; see the examples section for examples of how to do that. Thus, the examples in this document don't explicitly specify atom: for elements in an Atom-format feed.

The following tables show the Atom and RSS representations of the elements of the schema. All data not mentioned in these tables is treated as plain XML and shows up the same in both representations. Unless indicated otherwise, the XML elements in a given column are in the namespace corresponding to that column. This summary uses standard XPath notation: in particular, slashes show the element hierarchy, and an @ sign indicates an attribute of an element.

In each of the following tables, the highlighted items are required.

The following table shows the elements of a Google Data feed:

Feed Schema Item Atom Representation RSS Representation
Feed Title /feed/title /rss/channel/title
Feed ID /feed/id /rss/channel/atom:id
Feed HTML Link /feed/link[@rel="alternate"]\
[@type="text/html"]/@href		 /rss/channel/link
Feed Description /feed/subtitle /rss/channel/description
Feed Language /feed/@xml:lang /rss/channel/language
Feed Copyright /feed/rights /rss/channel/copyright
Feed Author

/feed/author/name
/feed/author/email

(Required in certain cases; see Atom specification.)

 /rss/channel/managingEditor
Feed Last Update Date /feed/updated
(RFC 3339 format)		 /rss/channel/lastBuildDate
(RFC 822 format)
Feed Category /feed/category/@term /rss/channel/category
Feed Category Scheme /feed/category/@scheme /rss/channel/category/@domain
Feed Generator /feed/generator
/feed/generator/@uri		 /rss/channel/generator
Feed Icon /feed/icon /rss/channel/image/url (unless there's also a logo, in which case the icon isn't included in the feed)
Feed Logo /feed/logo /rss/channel/image/url

The following table shows the elements of a Google Data search-results feed. Note that Google Data exposes some of the OpenSearch 1.1 Response elements in its search-results feeds.

Search Result Feed Schema Item Atom Representation RSS/OpenSearch Representation
Number of Search Results /feed/openSearch:totalResults /rss/channel/openSearch:totalResults
Search Result Start Index /feed/openSearch:startIndex /rss/channel/openSearch:startIndex
Number of Search Results Per Page /feed/openSearch:itemsPerPage /rss/channel/openSearch:itemsPerPage

The following table shows the elements of a Google Data entry:

Entry Schema Item Atom Representation RSS Representation
Entry ID /feed/entry/id /rss/channel/item/guid
Entry Version ID Optionally embedded in EditURI (see the Optimistic concurrency section of this document). —
Entry Title /feed/entry/title /rss/channel/item/title
Entry Link /feed/entry/link /rss/channel/item/link
/rss/channel/item/enclosure
/rss/channel/item/comments
Entry Summary

/feed/entry/summary

(Required in certain cases; see Atom specification.)

 /rss/channel/item/atom:summary
Entry Content

/feed/entry/content

(If no content element, then entry must contain at least one <link rel="alternate"> element.)

 /rss/channel/item/description
Entry Author

/feed/entry/author/name
/feed/entry/author/email

(Required in certain cases; see Atom specification.)

 /rss/channel/item/author
Entry Category /feed/entry/category/@term /rss/channel/item/category
Entry Category Scheme /feed/entry/category/@scheme /rss/channel/item/category/@domain
Entry Publication Date /feed/entry/published
(RFC 3339)		 /rss/channel/item/pubDate
(RFC 822)
Entry Update Date /feed/entry/updated
(RFC 3339)		 /rss/channel/item/atom:updated
(RFC 3339)

Queries

This section describes how to use the query system.

Query model design tenets

The query model is intentionally very simple. The basic tenets are:

  • Queries are expressed as HTTP URIs, rather than as HTTP headers or as part of the payload. One benefit of this approach is that you can link to a query.
  • Predicates are scoped to a single item. Thus, there's no way to send a correlation query such as "find all emails from people who sent me at least 10 emails today."
  • The set of properties that queries can predicate on is very limited; most queries are simply full text search queries.
  • Result ordering is up to the implementation.
  • The protocol is naturally extensible. If you want to expose additional predicates or sorting in your service, you can do so easily through the introduction of new parameters.

Query requests

A client queries a Google Data service by issuing an HTTP GET request. The query URI consists of the resource's URI (called FeedURI in Atom) followed by query parameters. Most query parameters are represented as traditional ?name=value[&...] URL parameters. Category parameters are handled differently; see below.

For example, if the FeedURI is http://www.example.com/feeds/jo, then you might send a query with the following URI:

http://www.example.com/feeds/jo?q=Darcy&updated-min=2005-04-19T15:30:00Z

Google Data services support HTTP Conditional GET. They set the Last-Modified response header based upon the value of the <atom:updated> element in the returned feed or entry. A client can send this value back as the value of the If-Modified-Since request header to avoid retrieving the content again if it hasn't changed. If the content hasn't changed since the If-Modified-Since time, then the Google Data service returns a 304 (Not Modified) HTTP response.

A Google Data service must support category queries and alt queries; support for other parameters is optional. Passing a standard parameter not understood by a given service results in a 403 Forbidden response. Passing an unsupported nonstandard parameter results in a 400 Bad Request response. For information on other status codes, see the HTTP status codes section of this document.

The standard query parameters are summarized in the following table. All parameter values need to be URL encoded.

Parameter Meaning Notes
q Full-text query string
  • When creating a query, list search terms separated by spaces, in the form q=term1 term2 term3. (As with all of the query parameter values, the spaces must be URL encoded.) The Google Data service returns all entries that match all of the search terms (like using AND between terms). Like Google's web search, a Google Data service searches on complete words (and related words with the same stem), not substrings.
  • To search for an exact phrase, enclose the phrase in quotation marks: q="exact phrase".
  • To exclude entries that match a given term, use the form q=-term.
  • The search is case-insensitive.
  • Example: to search for all entries that contain the exact phrase "Elizabeth Bennet" and the word "Darcy" but don't contain the word "Austen", use the following query: ?q="Elizabeth Bennet" Darcy -Austen
/-/category Category filter
  • List each category as if it were part of the resource's URI, in the form /categoryname/—this is an exception to the usual name=value form.
  • List all categories before any other query parameters.
  • Precede the first category with /-/ to make clear that it's a category. For example, if Jo's feed has a category for entries about Fritz, you could request those entries like this: http://www.example.com/feeds/jo/-/Fritz. This allows the implementation to distinguish category-predicated query URIs from resource URIs.
  • You can query on multiple categories by listing multiple category parameters, separated by slashes. The Google Data service returns all entries that match all of the categories (like using AND between terms). For example: http://www.example.com/feeds/jo/-/Fritz/Laurie returns entries that match both categories.
  • To do an OR between terms, use a pipe character (|), URL-encoded as %7C. For example: http://www.example.com/feeds/jo/-/Fritz%7CLaurie returns entries that match either category.
  • An entry matches a specified category if the entry is in a category that has a matching term or label, as defined in the Atom specification. (Roughly, the "term" is the internal string used by the software to identify the category, while the "label" is the human-readable string presented to a user in a user interface.)
  • To exclude entries that match a given category, use the form /-categoryname/.
  • To query for a category that has a scheme—such as <category scheme="urn:google.com" term="public"/>—you must place the scheme in curly braces before the category name. For example: /{urn:google.com}public. If the scheme contains a slash character (/) it should be URL-encoded as %2F. To match a category that has no scheme, use an empty pair of curly braces. If you don't specify curly braces, then categories in any scheme will match.
  • The above features can be combined. For example: /A%7C-{urn:google.com}B/-C means (A OR (NOT B)) AND (NOT C).
category Category filter
  • An alternative way to perform a category filter. The two methods are equivalent.
  • To do an OR between terms, use a pipe character (|), URL-encoded as %7C. For example: http://www.example.com/feeds?category=Fritz%7CLaurie returns entries that match either category.
  • To do an AND between terms, use a comma character (,). For example: http://www.example.com/feeds?category=Fritz,Laurie returns entries that match both categories.
author Entry author
  • The service returns entries where the author name and/or email address match your query string.
alt Alternative representation type
  • If you don't specify an alt parameter, the service returns an Atom feed. This is equivalent to alt=atom.
  • alt=rss returns an RSS 2.0 result feed.
  • alt=json returns a JSON representation of the feed. More information
  • alt=json-in-script Requests a response that wraps JSON in a script tag. More information
updated-min, updated-max Bounds on the entry update date
  • Use the RFC 3339 timestamp format. For example: 2005-08-09T10:57:00-08:00.
  • The lower bound is inclusive, whereas the upper bound is exclusive.
published-min, published-max Bounds on the entry publication date
  • Use the RFC 3339 timestamp format. For example: 2005-08-09T10:57:00-08:00.
  • The lower bound is inclusive, whereas the upper bound is exclusive.
start-index 1-based index of the first result to be retrieved
  • Note that this isn't a general cursoring mechanism. If you first send a query with ?start-index=1&max-results=10 and then send another query with ?start-index=11&max-results=10, the service cannot guarantee that the results are equivalent to ?start-index=1&max-results=20, because insertions and deletions could have taken place in between the two queries.
max-results Maximum number of results to be retrieved For any service that has a default max-results value (to limit default feed size), you can specify a very large number if you want to receive the entire feed.
entryID ID of a specific entry to be retrieved
  • If you specify an entry ID, you can't specify any other parameters.
  • The form of the entry ID is determined by the Google Data service.
  • Unlike most of the other query parameters, entry ID is specified as part of the URI, not as a name=value pair.
  • Example: http://www.example.com/feeds/jo/entry1.

About category queries

We decided to specify a slightly unusual format for category queries. Instead of a query like the following:

http://example.com/jo?category=Fritz&category=2006

we use:

http://example.com/jo/-/Fritz/2006

This approach identifies a resource without using query parameters, and it produces cleaner URIs. We chose this approach for categories because we think that category queries will be the most common queries.

The drawback to this approach is that we require you to use /-/ in category queries, so that Google Data services can distinguish category queries from other resource URIs, such as http://example.com/jo/MyPost/comments.

Query responses

Queries return an Atom feed, an Atom entry, or an RSS feed, depending on the request parameters.

Query results contain the following OpenSearch elements directly under the <feed> element or the <channel> element (depending on whether results are Atom or RSS):

openSearch:totalResults
The total number of search results for the query (not necessarily all present in the results feed).
openSearch:startIndex
The 1-based index of the first result.
openSearch:itemsPerPage
The maximum number of items that appear on one page. This allows clients to generate direct links to any set of subsequent pages. However, for a possible pitfall in using this number, see the note regarding start-index in the table in the Query requests section.

The Atom response feed and entries may also include any of the following Atom and Google Data elements (as well as others listed in the Atom specification):

<link rel="http://schemas.google.com/g/2005#feed" type="application/atom+xml" href="..."/>
Specifies the URI where the complete Atom feed can be retrieved.
<link rel="http://schemas.google.com/g/2005#post" type="application/atom+xml" href="..."/>
Specifies the Atom feed's PostURI (where new entries can be posted).
<link rel="self" type="..." href="..."/>
Contains the URI of this resource. The value of the type attribute depends on the requested format. If no data changes in the interim, sending another GET to this URI returns the same response.
<link rel="previous" type="application/atom+xml" href="..."/>
Specifies the URI of the previous chunk of this query result set, if it is chunked.
<link rel="next" type="application/atom+xml" href="..."/>
Specifies the URI of the next chunk of this query result set, if it is chunked.
<link rel="edit" type="application/atom+xml" href="..."/>
Specifies the Atom entry's EditURI (where you send an updated entry).

Here's a sample response body, in response to a search query:

<?xml version="1.0" encoding="UTF-8"?>
<feed xmlns:atom="http://www.w3.org/2005/Atom"
        xmlns:openSearch="http://a9.com/-/spec/opensearchrss/1.0/">
  <id>http://www.example.com/feed/1234.1/posts/full</id> 
  <updated>2005-09-16T00:42:06Z</updated> 
  <title type="text">Books and Romance with Jo and Liz</title> 
  <link rel="alternate" type="text/html" href="http://www.example.net/"/> 
  <link rel="http://schemas.google.com/g/2005#feed"
    type="application/atom+xml"
    href="http://www.example.com/feed/1234.1/posts/full"/> 
  <link rel="http://schemas.google.com/g/2005#post"
    type="application/atom+xml"
    href="http://www.example.com/feed/1234.1/posts/full"/> 
  <link rel="self" type="application/atom+xml"
    href="http://www.example.com/feed/1234.1/posts/full"/> 
  <author>
    <name>Elizabeth Bennet</name> 
    <email>liz@gmail.com</email> 
  </author>
  <generator version="1.0"
    uri="http://www.example.com">Example Generator Engine</generator> 
  <openSearch:totalResults>2</openSearch:totalResults> 
  <openSearch:startIndex>0</openSearch:startIndex> 
  <entry>
    <id>http://www.example.com/feed/1234.1/posts/full/4521614025009481151</id> 
    <published>2005-01-09T08:00:00Z</published> 
    <updated>2005-01-09T08:00:00Z</updated> 
    <category scheme="http://www.example.com/type" term="blog.post"/> 
    <title type="text">This is the title of entry 1009</title> 
    <content type="xhtml">
      <div
        xmlns="http://www.w3.org/1999/xhtml">This is the entry body of entry 1009</div> 
    </content>
    <link rel="alternate" type="text/html"
      href="http://www.example.com/posturl"/> 
    <link rel="edit" type="application/atom+xml"
      href="http://www.example.com/feed/1234.1/posts/full/4521614025009481151"/> 
    <author>
      <name>Elizabeth Bennet</name> 
      <email>liz@gmail.com</email> 
    </author>
  </entry>
  <entry>
    <id>http://www.example.com/feed/1234.1/posts/full/3067545004648931569</id> 
    <published>2005-01-07T08:00:00Z</published> 
    <updated>2005-01-07T08:02:00Z</updated> 
    <category scheme="http://www.example.com/type" term="blog.post"/> 
    <title type="text">This is the title of entry 1007</title> 
    <content type="xhtml">
      <div
        xmlns="http://www.w3.org/1999/xhtml">This is the entry body of entry 1007</div> 
    </content>
    <link rel="alternate" type="text/html"
      href="http://www.example.com/posturl"/> 
    <link rel="edit" type="application/atom+xml"
      href="http://www.example.com/feed/1234.1/posts/full/3067545004648931569"/> 
    <author>
      <name>Elizabeth Bennet</name> 
      <email>liz@gmail.com</email> 
    </author>
  </entry>
</feed>

If the requested feed is in the Atom format, if no query parameters are specified, and if the result doesn't contain all the entries, the following element is inserted into the top-level feed: <link rel="next" type="application/atom+xml" href="..."/>. It points to a feed containing the next set of entries. Subsequent sets contain a corresponding <link rel="previous" type="application/atom+xml" href="..."/> element. By following all the next links, a client can retrieve all entries from a feed.

HTTP status codes

The following table describes what various HTTP status codes mean in the context of the Google Data services.

Code Explanation
200 OK No error.
201 CREATED Creation of a resource was successful.
304 NOT MODIFIED The resource hasn't changed since the time specified in the request's If-Modified-Since header.
400 BAD REQUEST Invalid request URI or header, or unsupported nonstandard parameter.
401 UNAUTHORIZED Authorization required.
403 FORBIDDEN Unsupported standard parameter, or authentication or authorization failed.
404 NOT FOUND Resource (such as a feed or entry) not found.
409 CONFLICT Specified version number doesn't match resource's latest version number.
500 INTERNAL SERVER ERROR Internal error. This is the default code that is used for all unrecognized errors.

Optimistic concurrency (versioning)

Sometimes it is important to ensure that multiple clients don't inadvertently overwrite one another's changes. This is most easily accomplished by ensuring that the current version of an entry that a client is modifying is the same as the version that the client is basing its modifications on. If a second client makes an update before the first client does, then the first client's update is denied, because the first client is no longer basing its modifications on the latest version.

In Google Data feeds that support versioning, we achieve these semantics by appending a version ID to each entry's EditURI. Note that only the EditURI is affected, not the entry ID. In this scheme, each update changes the entry's EditURI, thus guaranteeing that subsequent updates based on the original version fail. Deletes, of course, are identical to updates with respect to this feature; if you send a delete with an old version number, the delete fails.

Not all Google Data feeds support optimistic concurrency. In a feed that does support it, if the server detects a version conflict on PUT or DELETE, the server responds with 409 Conflict. The body of the response contains the current state of the entry (an Atom entry document). The client is advised to resolve the conflict and resubmit the request, using the EditURI from the 409 response.

Motivation and design notes

This approach to optimistic concurrency allows us to implement the semantics we want without requiring new markup for version IDs, which makes Google Data's responses compatible with non-Google Data Atom endpoints.

Instead of specifying version IDs, we could have chosen to look at the update timestamp on each entry (/atom:entry/atom:updated). However, there are two problems with using the update timestamp:

  • It only works for updates and not deletions.
  • It forces applications to use date/time values as version IDs, which would make it harder to retrofit Google Data on top of many existing data stores.

Authentication

When a client tries to access a service, it may need to provide the user's credentials to the service, to demonstrate that the user has the authority to perform the action in question.

The approach that a client should use for authentication depends on the type of client:

In the ClientLogin system, the desktop client asks the user for their credentials, and then sends those credentials to the Google authentication system.

If authentication succeeds, then the authentication system returns a token that the client subsequently uses (in an HTTP Authorization header) when it sends Google Data requests.

If authentication fails, then the server returns a 403 Forbidden status code, along with a WWW-Authenticate header containing a challenge applicable to the authentication.

The AuthSub system works similarly, except that instead of asking the user for their credentials, it connects the user to a Google service that requests credentials. The service then returns a token that the web application can use; the advantage of this approach is that Google (rather than the web front end) securely handles and stores the user's credentials.

For details about these authentication systems, see the Google Data Authentication Overview or the Google Account Authentication documentation.

Session state

Many business logic implementations require session stickiness—keeping track of the state of a user's session.

Google tracks session state in two ways: using cookies, and using a token that can be sent as a query parameter. Both methods achieve the same effect. We recommend that clients support one of these session-state tracking methods (either one is sufficient). If a client doesn't support either of these methods, then that client will still work with Google Data services, but performance may suffer compared to clients that do support these methods. Specifically, if a client doesn't support these methods, then every request results in a redirect, and therefore every request (and any associated data) is sent to the server twice, which affects the performance of both the client and the server.

The Google client libraries handle session state for you, so if you use our libraries, you don't have to do anything to get session state support.

Additional resources

You may find the following third-party documents useful:

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