| json | N. Williams |
| Internet-Draft | Cryptonector |
| Intended status: Standards Track | May 08, 2014 |
| Expires: November 09, 2014 |
JavaScript Object Notation (JSON) Text Sequences
draft-ietf-json-text-sequence-01
This document describes the JSON text sequence format and associated media type.
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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. If processing such a dataset requires first parsing it entirely, then the result is very inefficient and the processing will be limited by virtual memory. “Online” (a.k.a., “streaming”) parsers help, but they are neither widely available or widely used, nor are they easy to use.
Ideally such datasets could be parsed and processed one element at a time. Even if each element must be parsed in a not-online manner due to local choice of parser, the result will usually be sufficiently online: limited by the size of the biggest element in the sequence rather than by the size of the sequence.
This document describes the concept and format of “JSON text sequences”, which are specifically not JSON texts themselves but are composed of JSON texts.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
The ABNF [RFC5234] for the JSON text sequence format is as follows:
JSON-sequence = *whitespace *(JSON-text 1*whitespace) whitespace = %x20 / %x09 / %x0A / %x0D JSON-text = <given by RFC7159>
Figure 1: JSON text sequence ABNF
A JSON text sequence is a sequence of zero or more JSON texts, each followed by JSON whitespace separator.
Requirements:
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. This ambiguity is resolved by requiring that encoders never omit the separator.
The JSON Text Sequence format is perfect for use with 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 closed.
A problem comes up with this use case: it is difficult to guarantee that append writes will complete. Therefore it's possible (if unlikely) to end up with truncated log entries -which may fail to parse as JSON texts- followed by other entries. The mechanics of such failures are not explained here (consider power failures though).
Fortunately, as long as all texts in the logfile sequence are followed by a newline, it is possible to detect a subsequent entry written after an entry that fails to parse. 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.
boundary = endchar *whitespace NL *whitespace startchar
endchar = ( "}" / "]" / %x22 / "e" / "l" / DIGIT )
startchar = ( "{" / "[" / %x22 / "t" / "f" / "n" / "-" / DIGIT )
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.
Applications SHOULD scan backwards (up to the start of the incomplete text) from such a boundary looking for a newline followed by a valid JSON text, otherwise valid entries following truncated entries can be missed by this rule.
Note that in order to enable resynchronization all JSON texts appended to a logfile MUST be followed by a newline.
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 form a logical end of sequence. Applications where this matters should denote end of sequence by convention (e.g., Content-Length in HTTP).
JSON text sequence parsers based on non-incremental, non-online JSON text parsers will not be able to efficiently parser JSON texts in which newlines appear; attempting to parse such sequences with non-incremental, non-online JSON text parsers creates a compute resource exhaustion vulnerability.
The first requirement given in Section 2 (otherwise-ambiguous JSON texts must be separated by whitespace) is critical and must be adhered to. It is best to always emit a whitespace separator after every JSON text emitted.
Purposefully appending a truncated (or invalid) JSON text to a JSON text sequence logfile can cause the subsequent entry to be ignored by tooling that does not scan backwards from resynchronization boundaries looking for otherwise missed complete JSON texts.
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>.
Applications that use this media type: JSON text sequences have been used in applications written with the jq programming language.
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.
| [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. |