json N. Williams
Internet-Draft Cryptonector
Intended status: Standards Track May 23, 2014
Expires: November 24, 2014
JavaScript Object Notation (JSON) Text Sequences
draft-ietf-json-text-sequence-04
Abstract
This document describes the JSON text sequence format and associated
media type.
Status of this Memo
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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
follows:
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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
text.
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
circumstances.]]
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
SP);
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
separator.
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
LF.
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3. Use for Logfiles, or How to Resynchronize Following Truncated
entries
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:
null
{ "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
HTTP).
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
#1.
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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>.
Applicat<http://xml2rfc.tools.ietf.org/public/rfc/bibxml/
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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