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1473Format & interpretation of URL fragments for JSON resources

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  • Jacob Davies
    Feb 26, 2010
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      I have a question regarding the use of URL fragments (the part after
      the # (hash) character in a standard URL) for navigating JSON
      resources. So far as I can see from some searches & investigation,
      there does not seem to be a firm consensus on the format and
      interpretation of them, and there is a fairly major problem with the
      most common suggestion I've seen, which is the interpretation of the
      fragment as a series of dot-delimited, URL-encoded keys to be used to
      navigate through a set of nested JSON objects and arrays.

      So, an example. The fragment:


      when used to navigate the JSON resource:

          "foo" : {
              "bar" : [

      would refer to the value "xyz".

      This has the attractive feature of looking like the Javascript or Java
      dot-notation for navigating objects.

      The problem is that dot/period is explicitly included in the list of
      non-reserved characters in URL-encoding:


      "For consistency, percent-encoded octets [...] period (%2E) [...]
      should not be created by URI producers"

      So the simple statement of the format ("dot-delimited, URL-encoded
      keys") is either ambiguous or cannot accommodate keys containing

      A simple example to illustrate:

      "foo" : {
      "bar" : "xyz"
      "foo.bar" : "abc"

      Does the fragment #foo.bar refer to the value "xyz" or "abc".

      Obviously it is straightforward to replace the periods in keys with %2E
      and therefore distinguish between these fragments:

      #foo.bar - intended to refer to "xyz"
      #foo%2Ebar - intended to refer to "abc"

      But, there are some problems with this procedure, two minor, one major.

      The first minor problem is that standard URL-encoding routines do not
      replace dots with the %2E escape. The second minor problem is that it
      makes it awkward to construct fragments by hand that refer to keys that
      contain dots.

      The major problem is that this method of interpretation of a URL is
      explicitly disallowed. Quoting again from RFC 3986:

      "URIs that differ in the replacement of an unreserved character with
      its corresponding percent-encoded US-ASCII octet are equivalent: they
      identify the same resource."

      Clearly this is not true in the above example. Replacement of %2E with
      a period changes the interpretation of the fragment. Note that the
      word "unreserved" is significant in the above quote - the
      replacement of a reserved character by its URL-encoded counterpart IS
      allowed to make a difference in distinguishing between resources.

      So, I have a suggestion for an alternative format and interpretation,
      which is:

      "URL fragments contain a slash-delimited, URL-encoded list of keys
      used to navigate a JSON structure from the root".

      So, given the JSON resource:

      "foo" : {
      "bar" : "xyz"
      "foo.bar" : "abc",
      "foo/bar" : "123"

      the contained values can be unambiguously referred to using the

      #foo/bar - "xyz"
      #foo.bar - "abc"
      #foo%2Fbar - "123"

      Slash IS a reserved character for URL-encoding, which means,
      firstly, that we can legitimately distinguish between the first and
      last examples there as referring to different resources; secondly,
      that standard URL-encoding routines will correctly escape it, and
      the wording of the format is unambiguous; and thirdly, that keys
      containing dots can be easily used in URLs - in my experience such
      keys are far more common than keys containing slashes, and there
      have been several recent suggestions for using reversed domain names
      in dotted keys as an ad-hoc namespace mechanism in JSON similar to the
      use for Java package names, for instance:

      "org.itemscript.Name" : "Jacob"

      One final note: the use of an initial slash to indicate that the value
      is rooted at the top level of the JSON structure seems unnecessary,
      since fragment identifiers by definition are global to a given resource
      or document.

      Anyway, just some thoughts. I know that the dot-delimited fragment
      format already has some momentum, but I had to make a decision about
      which format to use for something I was working on recently, and after
      thinking about it (and using the dot-delimited format for a while) I
      found that the problems with dot-delimited were significant enough that
      I didn't use it. I do think a consistent interpretation of URL fragments
      in JSON resources would be quite useful though.

      Jacob Davies
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