Network Working Group A. Phillips, Ed.
Internet-Draft Yahoo! Inc.
Expires: August 23, 2008 February 20, 2008
The record-jar Format
draft-phillips-record-jar-02
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Copyright (C) The IETF Trust (2008).
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Abstract
The record-jar format provides a method of storing multiple records
with a variable repertoire of fields in a text format. This document
provides a description of the format. Comments are solicited and
should be addressed to the mailing list 'record-jar@yahoogroups.com'
and/or the author.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Format and Grammar . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Folding of Field Values . . . . . . . . . . . . . . . . . 5
2.2. Comments . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3. Characters, Encodings, and Escapes . . . . . . . . . . . . 7
3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Normative References . . . . . . . . . . . . . . . . . . . 11
4.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Introduction
The record-jar format was originally described by The Art of Unix
Programming [AOUP]. This format is useful for storing information in
a human-readable text form, while making the data available for
machine processing. It is a flexible format, since it provides for
an arbitrary range of fields in any given record and can be used to
store data with variable length and content.
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. Format and Grammar
The record-jar format is described by the following ABNF ([RFC4234]):
record-jar = [encodingSig] [separator] *record
record = 1*field separator
field = ( field-name field-sep field-body CRLF )
field-name = 1*character
field-sep = *SP ":" *SP
field-body = *(continuation 1*character)
continuation = ["\"] [[*SP CRLF] 1*SP]
separator = [blank-line] *("%%" [comment] CRLF)
comment = SP *69(character)
character = SP / ASCCHAR / UNICHAR / ESCAPE
encodingSig = "%%encoding" field-sep
*(ALPHA / DIGIT / "-" / "_") CRLF
blank-line = WSP CRLF
; ASCII characters except %x26 (&) and %x5C (\)
ASCCHAR = %x21-25 / %x27-5B / %x5D-7E
; Unicode characters
UNICHAR = %x80-10FFFF
ESCAPE = "\" ("\" / "&" / "r" / "n" / "t" )
/ "&#x" 2*6HEXDIG ";"
record-jar ABNF
The record-jar format uses plain-text to represent data values. A
record-jar document consists of a sequence of records, each of which
contains one or more fields. Each record is separated from other
records by at least one line beginning with the sequence "%%"
(%x25.25). A record MAY contain as many or as few fields as are
necessary to convey the necessary data. Empty records and blank
lines are ignored.
A field is a single, logical line of characters from the Universal
Character Set (Unicode) [Unicode]. Each field is comprised of three
parts: the field-name, the field-separator, and the field body.
The field-name is an identifer. Field-names consist of a sequence of
Unicode characters. Whitespace characters and colon (":", %x3A) are
not permitted in a field-name.
An application can impose additional restrictions on field-names.
For example, they might be restricted to the characters permitted in
identifiers according to Unicode Standards Annex #31 (UAX#31)
[UAX31]. Or they might be restricted to a sequence of letters and
digits from the US-ASCII [ISO646] character repertoire.
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Field-names are case sensitive. Upper and lowercase letters are
often used to visually break up the name, for example using
CamelCase. It is a common convention that field names use an initial
capital letter, although this is not enforced.
The field separator (field-sep) is the colon character (":", %x3A).
The separator MAY be surrounded on either side by any amount of
horizontal whitespace (tab or space characters). The normal
convention is one space on each side.
The field-body contains the data value. Logically, the field-body
consists of a single line of text using any combination of characters
from the Universal Character Set followed by a CRLF (newline). The
carriage return, newline, and tab characters, when they occur in the
data value stored in the field-body, are represented by their common
backslash escapes ("\r", "\n", and "\t" respectively). See
Section 2.3 for more information on escape sequences.
2.1. Folding of Field Values
Some protocols limit total line length. For example, many Internet
plain-text protocols limit lines to 72 total bytes. To accommodate
such limits or for readability and presentational purposes, the
field-body portion of a field can be split into a multiple-line
representation; this is called "folding".
Successive lines in the same field-body begin with one or more
whitespace characters. When processing the record-jar format, the
linear whitespace (including the newline and any preceeding spaces)
is consumed by the processor and the two parts of the field-body
joined to form a single, logical line. For example:
Eulers-Number : 2.718281828459045235360287471
352662497757247093699959574966967627724076630353547
5945713821785251664274274663919320030599218174135...
Figure 2: Example of Folding
Note that imposing a line length limit effectively limits the length
of the field-name, since the field separator MUST appear on the same
line with the field-name and the field-name MUST NOT be folded.
Also, when imposing a line length limit, note that some encodings
(including the Unicode encodings) can use a variable number of bytes
per character or commonly use more than one byte per character.
Characters MUST NOT be folded in the middle of a byte sequence.
It is RECOMMENDED that folding not occur between characters inside a
Unicode grapheme cluster (since this will alter the display of
characters in the file and might result in unintentional alteration
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of the file's semantics). Information on grapheme clusters can be
found in [UAX29]
In some cases, the field-body contains spaces that are important to
the data. To accurately preserve whitespace in the document, an
optional line-continuation character (backslash, %x5C) MAY be
included to delimit and separate whitespace to be preserved from
whitespace that will be removed by the processor. The line-
continuation character and any whitespace that follows it (including
whitespace at the beginning of the continuing field-body on the next
line) MUST be consumed by the processor when reading the file.
Whitespace appearing before the line-continuation MUST NOT be
consumed. Use of the line continuation character makes the
whitespace visible in the file.
In other cases, the field-body might contain natural language text,
and, while it is readily apparent that many languages use spaces to
separate words, others, such as Japanese or Thai, do not.
Implementations MAY, in the absence of line continuation characters,
replace the continuation sequence (the line break and surrounding
whitespace) in a folded line with a single ASCII space (%x20),
however, implementations SHOULD just remove the continuation sequence
altogether in order to avoid causing unnatural breaks in the text.
Here are some examples:
SomeField : This is some running text \
that is continued on several lines \
and which preserves spaces between \
the words.
%%
AnotherExample: There are three spaces \
between 'spaces' and 'between' in this record.
%%
SwallowingExample: There are no spaces between \
the numbers one and two in this example 1\
2.
%%
Figure 3: Example of Folding with Preserved Whitespace
Note that entirely blank continuation lines are not permitted. That
is, this record is illegal, since the field-body of "SomeText" would
be the empty string:
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%%
SomeText: \
\
\
%%
Figure 4: Whitespace Folding Example
2.2. Comments
Comments MAY be included in the body of the record-jar document by
placing them at the end of a separator line. The comment MUST be
separated by at least one space from the "%%" sequence that
introduces the record separator.
Multiple record separators (including comment lines) MAY appear
between records. Logically this appears to result in records that
contain no fields: records containing no fields MUST be ignored by a
processor.
Folding of comments is not permitted; instead multiple comment lines
MUST be used. Comments can not appear in the body of a record. For
example:
%% this is a comment.
Record: goes here
%%
%% here is another sequence of comments
%% that appear on multiple lines
Record: another record
%% a final comment
%%
Figure 5: Comment example
Although comments are not associated with any particular record in
the file, processors that preserve comments sometimes treat the
comments as if they were associated with the record just following
them. Reserialization of a record-jar file would thus restore the
comments to their logical position in the file. In many cases,
processing a record-jar file loses comment information associated
with the file.
2.3. Characters, Encodings, and Escapes
By default, a file containing a record-jar archive uses the UTF-8
character encoding (see [RFC3629]). If an application, protocol, or
specification permits a character encoding other than UTF-8 to be
used in the file, it SHOULD also support reading the character
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encoding from the encoding signature.
The encoding signature, when present, MUST be the very first line of
the file. If the encoding signature is not present, an application
or protocol MAY attempt to infer the character encoding using other
means. Record-jar files SHOULD always include an encoding signature,
even if one is not required, whenever the application, protocol, or
specification permits one.
A file that uses the UTF-16 or UTF-32 encoding MAY also include a
Byte Order Mark (U+FEFF) as the first sequence of two octets (in the
case of UTF-16) or four octets (in the case of UTF-32) in the file,
just preceeding the encoding signature.
Some applications, protocols, or specifications require that the
record-jar file use some other, non-Unicode, legacy character
encoding. In particular, some applications, protocols, or
specifications only support the US-ASCII character set ([ISO646]).
Here is an example of the encoding signature for the UTF-8 encoding
of Unicode:
%%encoding:UTF-8
Figure 6: Example of an Encoding Signature
Printable ASCII characters excepting backslash ("\") and ampersand
("&") are represented as themselves.
Non-ASCII values MAY be included in a record-jar file in several
ways. For portability, the best mechanism is to use escape sequences
in the field-body. Exclusive use of escape sequences results in a
pure ASCII text file.
Non-ASCII characters MAY be represented using the character's Unicode
value represented using the Numeric Character Reference format
adapted from XML; the sequence "&#x" (%x26.23.78) is followed by the
character's Unicode scalar value in hex followed directly by the
semi-colon character (";", %x3B). Leading zeroes MAY be omitted.
For example, the EURO SIGN is U+20AC and could be represented as
"€".
Non-ASCII characters MAY also be represented as their associated
octet sequence in the file's character encoding. For example, the
EURO SIGN would be represented as the octet sequence %xE2.82.AC,
since those three bytes encode that character in UTF-8.
The characters for carriage return, newline, and tab when considered
as part of the data (and not the file format itself) are represented
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by the traditional escape sequences "\r" (%x5C.72), "\n" (%x5C.6E),
and "\t" (%x5C.74) respectively. The character backslash is
represented by "\\" (%x5C.5C), while the ampersand character is
represented by "\&" (%x5C.26). A single backslash at the end of a
line indicates continuation, as discussed in Section 2.1. Otherwise
a single backslash followed by some other character in the data is an
error, although a record-jar processor MAY choose to interpret it as
a backslash.
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3. Examples
Here is the canonical example from [AOUP]:
Planet: Mercury
Orbital-Radius: 57,910,000 km
Diameter: 4,880 km
Mass: 3.30e23 kg
%%
Planet: Venus
Orbital-Radius: 108,200,000 km
Diameter: 12,103.6 km
Mass: 4.869e24 kg
%%
Planet: Earth
Orbital-Radius: 149,600,000 km
Diameter: 12,756.3 km
Mass: 5.972e24 kg
Moons: Luna
A more complete example showing more of the various features in the
format is described in [RFC4646]. The data shown here is taken from
the Language Subtag Registry defined that document:
%%
Type: language
Subtag: ia
Description: Interlingua (International Auxiliary Language \
Association)
Added: 2005-08-16
%%
Type: language
Subtag: id
Description: Indonesian
Added: 2005-08-16
Suppress-Script: Latn
%%
Type: language
Subtag: nb
Description: Norwegian Bokmål
Added: 2005-08-16
Suppress-Script: Latn
%%
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4. References
4.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", draft-crocker-abnf-rfc2234bis-00
(work in progress), October 2005,
<ftp://ftp.rfc-editor.org/in-notes/rfc4234.txt>.
[UAX31] Davis, M., "Unicode Standard Annex #31: Identifier and
Pattern Syntax", 09 2006.
[Unicode] Unicode Consortium, "The Unicode Consortium. The Unicode
Standard, Version 5.0, (Boston, MA, Addison-Wesley, 2003.
ISBN 0-321-49081-0)", January 2007.
4.2. Informative References
[AOUP] Raymond, E., "The Art of Unix Programming", 2003,
<urn:isbn:0-13-142901-9>.
[ISO646] International Organization for Standardization, "ISO/IEC
646:1991, Information technology -- ISO 7-bit coded
character set for information interchange.", 1991.
[RFC4646] Phillips, A., Ed. and M. Davis, Ed., "Tags for the
Identification of Languages", September 2006,
<http://www.ietf.org/rfc/rfc4646.txt>.
[UAX29] Davis, M., "Unicode Standard Annex #29: Text Boundaries",
10 2006, <UAX29>.
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Appendix A. Acknowledgements
Thanks to Eris S. Raymond for his gracious permission to both
reference and quote The Art of Unix Programming in this document.
Without his work, this document would likely not exist.
Contributors to this document include: Stephane Bortzmeyer, John
Cowan, Frank Ellerman, Doug Ewell.
The IETF LTRU working group adopted record-jar format on John Cowan's
suggestion. That effort required record-jar to be documented and
many people in that group contributed to this work there: the author
thanks everyone who participated in that effort, even though names
cannot be mustered here.
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Author's Address
Addison Phillips (editor)
Yahoo! Inc.
Email: addison@inter-locale.com
URI: http://www.inter-locale.com
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