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          13 rfc7464                                                    
json                                                         N. Williams
Internet-Draft                                              Cryptonector
Intended status: Standards Track                            May 23, 2014
Expires: November 24, 2014

            JavaScript Object Notation (JSON) Text Sequences


   This document describes the JSON text sequence format and associated
   media type.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on November 24, 2014.

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   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.      Introduction and Motivation  . . . . . . . . . . . . . . .  3
   1.1.    JSON Parser Types  . . . . . . . . . . . . . . . . . . . .  3
   1.2.    Conventions used in this document  . . . . . . . . . . . .  3
   2.      JSON Text Sequence Format  . . . . . . . . . . . . . . . .  4
   2.1.    Ambiguities  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.1.1.  Ambiguities Resulting from Partial Texts . . . . . . . . .  4
   2.2.    Rationale for Choice of LF as the Text Separator . . . . .  5
   3.      Use for Logfiles, or How to Resynchronize Following
           Truncated entries  . . . . . . . . . . . . . . . . . . . .  6
   4.      Security Considerations  . . . . . . . . . . . . . . . . .  8
   5.      IANA Considerations  . . . . . . . . . . . . . . . . . . .  9
   6.      Acknowledgements . . . . . . . . . . . . . . . . . . . . . 10
   7.      Normative References . . . . . . . . . . . . . . . . . . . 11
           Author's Address . . . . . . . . . . . . . . . . . . . . . 12

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1.  Introduction and Motivation

   The JavaScript Object Notation (JSON) [RFC7159] is a very handy
   serialization format.  However, when serializing a large sequence of
   values as an array, or a possibly indeterminate-length or never-
   ending sequence of values, JSON becomes difficult to work with.

   Consider a sequence of one million values, each possibly 1 kilobyte
   when encoded, which would be roughly one gigabyte.  It is often
   desirable to process such a dataset in an incremental manner: without
   having to first read all of it before beginning to produce results.
   Traditionally the way to do this with JSON is to use a "streaming"
   parser (see Section 1.1), but these are neither widely available,
   widely used, nor easy to use.

   This document describes the concept and format of "JSON text
   sequences", which are specifically not JSON texts themselves but are
   composed of JSON texts.  JSON text sequences can be parsed (and
   produced) incrementally without having to have a streaming parser
   (nor encoder).

1.1.  JSON Parser Types

   For the purposes of this document we shall classify JSON parsers as

   Streaming  Consumes a text incrementally, outputs values
      incrementally (e.g., as (path, leaf value) pairs).

   Online  Consumes a text incrementally.

   Off-line  Consumes only complete texts.

1.2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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2.  JSON Text Sequence Format

   The ABNF [RFC5234] for the JSON text sequence format is as given in
   Figure 1.  Note that this ABNF does not work if we assume greedy
   matching.  Therefore, in prose, a JSON text sequence is a sequence of
   zero or more JSON texts, each surrounded by any number of JSON
   whitespace characters and always followed by a newline.

     JSON-sequence = ws *(JSON-text ws LF ws)
     LF = <given by RFC5234>
     ws = <given by RFC7159>
     JSON-text = <given by RFC7159>

                     Figure 1: JSON text sequence ABNF

   As long as a JSON text sequence consist of complete JSON texts, the
   only requirement is that whitespace separate any non-object, array,
   string top-level values from neighboring texts.  The simplest way to
   ensure this is to require such whitespace, and furthermore it is
   convenient to use a newline, as we'll see in Section 2.1.  Therefore
   we impose one requirement:

   o  JSON text sequence encoders MUST emit a newline after any JSON

2.1.  Ambiguities

   Otherwise An input of 'truefalse' is not a valid sequence of two JSON
   values, true and false!  Neither is 'true0' a valid sequence of true
   and zero.  Some existing JSON parsers that might be used to construct
   sequence parsers might in fact accept such sequences, resulting in
   erroneous parsing of sequences of two or more numbers.  E.g., a
   sequence of two numbers, 4 and 2, encoded without the required
   whitespace between them would parse incorrectly as the number 42.

   Such ambiguities is resolved by requiring that encoders emit a
   whitespace separator (specifically: a newline) after each text.

2.1.1.  Ambiguities Resulting from Partial Texts

   Another kind of ambiguity arises when a JSON text sequence contains
   partial texts.  Such a sequence can result when using "append writes"
   to write to a file.  For example, many systems might commit partial
   writes to stable storage then fail to complete the remainder of a
   write as a result of, e.g., power failures; upon recovery the file
   may then end with a partial JSON text.

   [[anchor1: Perhaps we should add a note about what POSIX requires

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   w.r.t.  O_APPEND, and how POSIX is agnostic as to power failures and
   so on.  The point being that even where a standard imposes strong
   atomicity requirements as to append writes, there are good reasons
   why that might be difficult to obtain under exceptional

   Consider a portion of a JSON text sequence such as:

                                { "foo":
                                { "bar": 42 }

   How can we tell that the first line isn't part of an incomplete JSON
   text?  We can't, especially if the third line were missing.

   In the common case JSON text sequence parsers assume every text is
   complete, and abort processing if any one text fails to parse.
   However, for logfiles, there is value is being able to recover from
   such situations.  Recovery is described in Section 3.

2.2.  Rationale for Choice of LF as the Text Separator

   A variety of characters or character sequences (even non-whitespace
   characters) could have been used as the JSON text separator in JSON
   text sequences.  The rationale for using newline (LF) as the
   separator is as follows:

   o  it matches the 'ws' ABNF rule in [RFC7159] (as do CR, HTAB, and

   o  it is always escaped in encoded JSON strings, therefore it is safe
      remove LFs (or replace then with other JSON whitespace characters)
      from any JSON text (this is also true of CR and HTAB, but not SP);

   o  it is generally understood as the end-of-line marker by line-
      oriented tools;

   o  at least one JSON text sequence implementation exists and has
      existed for some time [XXX add external informative reference to
      https://stedolan.github.com/jq], and it uses LF as the JSON text

   Note that JSON text sequence writers may (and should) use CR LF as
   the text separator where the end-of-line marker is expected to be CR

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3.  Use for Logfiles, or How to Resynchronize Following Truncated

   The JSON Text Sequence format is useful for logfiles, as those are
   generally (and atomically) appended to on an ongoing basis.  I.e.,
   logfiles are of indeterminate length, at least right up until they
   are closed.

   The partial-write ambiguities described in Section 2.1.1 come up in
   the case of logfiles.

   As long as all texts in the logfile sequence are followed by a
   newline, it is possible to detect a subsequent JSON text written
   after an entry that fails to parse: either the first or the second
   subsequent, complete JSON texts.  Figure 2 shows an ABNF rule for
   detecting the boundary between a non-truncated [and some truncated]
   JSON text and the next JSON text in a sequence.  This rule assumes
   that only valid JSON texts are written to a sequence.

     boundary = endchar *text-sep *ws startchar
     text-sep = *(SP / HTAB / CR) LF ; these are from RFC5234
     endchar = ( "}" / "]" / DQUOTE / "e" / "l" / DIGIT )
     startchar =  ( "{" / "[" / DQUOTE / "t" / "f" / "n" / "-" / DIGIT )
     ws = <given by RFC7159>

                   Figure 2: ABNF for resynchronization

   To resynchronize after failing to parse a JSON text, simply search
   for a boundary as described in figure 2.  A boundary found this way
   might be the boundary between the truncated entry and the subsequent
   entry, or it might be a subsequent boundary.

   This method does not support scanning backwards for boundaries.

   To make resynchronization reliable, and work both forwards and
   backwards, the writer MUST first ensure that the JSON text being
   written is valid, and SHOULD apply either (or both) of the following:

   1.  Remove internal newlines (not including escaped newlines in
       strings) from any JSON text being written.

   2.  Prefix any JSON text with a null value and a newline.  The append
       write must still be atomic (one write), and contain both texts.

   Method #1 permits scanning for newlines (in either direction) as the
   resynchronization method.

   Method #2 permits scanning for "null" LF (in either direction) as the

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   resynchronization method.

   Consider a JSON text sequence such as:

                           { "foo":"hello world" }
                           "a broken writenull
                           "a complete write"

   Resynchronization methods #1 and #2 will correctly detect that the
   third line is an incomplete JSON text, and that the next complete
   text starts at the fourth line.  We can't tell which of method #1 or
   #2 the writer was using, but either method works for the parser.  The
   parser SHOULD know which method the writer was using, as to know
   whether to discard the nulls, and whether to attempt
   resynchronization at all.

   Method #1 is RECOMMENDED for JSON text sequence logfile writers.

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4.  Security Considerations

   All the security considerations of JSON [RFC7159] apply.

   There is no end of sequence indicator.  This means that "end of
   file", "end of transmission", and so on, can be indistinguishable
   from a logical end of sequence.  Applications where this matters
   should denote end of sequence by convention (e.g., Content-Length in

   The resynchronization ABNF heuristic is imperfect and might skip a
   valid entry following a truncated one.  Purposefully appending a
   truncated (or invalid) JSON text to a JSON text sequence logfile can
   cause the subsequent entry to be invisible.

   JSON text sequence writers MUST validate (parse) any JSON text inputs
   from untrusted third parties.

   JSON text sequence logfile writers SHOULD apply one of the
   resynchronization methods described in Figure 2, preferably method

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5.  IANA Considerations

   The MIME media type for JSON text sequences is application/json-seq.

   Type name: application

   Subtype name: json-seq

   Required parameters: n/a

   Optional parameters: n/a

   Encoding considerations: binary

   Security considerations: See <this document, once published>,
   Section 4.

   Interoperability considerations: Described herein.

   Published specification: <this document, once published>.

   reference.RFC.2119.xml>ions that use this media type: JSON text
   sequences have been used in applications written with the jq
   programming language.

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6.  Acknowledgements

   Phillip Hallam-Baker proposed the use of JSON text sequences for
   logfiles and pointed out the need for resynchronization.  James
   Manger contributed the ABNF for resynchronization.

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7.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC7159]  Bray, T., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, March 2014.

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Author's Address

   Nicolas Williams
   Cryptonector, LLC

   Email: nico@cryptonector.com

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