--- CBOR-XS/README 2013/10/29 15:56:31 1.7 +++ CBOR-XS/README 2013/11/22 16:18:59 1.9 @@ -25,11 +25,10 @@ DESCRIPTION WARNING! This module is very new, and not very well tested (that's up to you to do). Furthermore, details of the implementation might change - freely before version 1.0. And lastly, the object serialisation protocol - depends on a pending IANA assignment, and until that assignment is - official, this implementation is not interoperable with other - implementations (even future versions of this module) until the - assignment is done. + freely before version 1.0. And lastly, most extensions depend on an IANA + assignment, and until that assignment is official, this implementation + is not interoperable with other implementations (even future versions of + this module) until the assignment is done. You are still invited to try out CBOR, and this module. @@ -44,6 +43,19 @@ (JSON often compresses better than CBOR though, so if you plan to compress the data later you might want to compare both formats first). + To give you a general idea about speed, with texts in the megabyte + range, "CBOR::XS" usually encodes roughly twice as fast as Storable or + JSON::XS and decodes about 15%-30% faster than those. The shorter the + data, the worse Storable performs in comparison. + + As for compactness, "CBOR::XS" encoded data structures are usually about + 20% smaller than the same data encoded as (compact) JSON or Storable. + + In addition to the core CBOR data format, this module implements a + number of extensions, to support cyclic and self-referencing data + structures (see "allow_sharing"), string deduplication (see + "allow_stringref") and scalar references (always enabled). + The primary goal of this module is to be *correct* and the secondary goal is to be *fast*. To reach the latter goal it was written in C. @@ -74,7 +86,7 @@ The mutators for flags all return the CBOR object again and thus calls can be chained: - #TODO my $cbor = CBOR::XS->new->encode ({a => [1,2]}); + my $cbor = CBOR::XS->new->encode ({a => [1,2]}); $cbor = $cbor->max_depth ([$maximum_nesting_depth]) $max_depth = $cbor->get_max_depth @@ -115,6 +127,113 @@ See SECURITY CONSIDERATIONS, below, for more info on why this is useful. + $cbor = $cbor->allow_unknown ([$enable]) + $enabled = $cbor->get_allow_unknown + If $enable is true (or missing), then "encode" will *not* throw an + exception when it encounters values it cannot represent in CBOR (for + example, filehandles) but instead will encode a CBOR "error" value. + + If $enable is false (the default), then "encode" will throw an + exception when it encounters anything it cannot encode as CBOR. + + This option does not affect "decode" in any way, and it is + recommended to leave it off unless you know your communications + partner. + + $cbor = $cbor->allow_sharing ([$enable]) + $enabled = $cbor->get_allow_sharing + If $enable is true (or missing), then "encode" will not + double-encode values that have been referenced before (e.g. when the + same object, such as an array, is referenced multiple times), but + instead will emit a reference to the earlier value. + + This means that such values will only be encoded once, and will not + result in a deep cloning of the value on decode, in decoders + supporting the value sharing extension. + + It is recommended to leave it off unless you know your communication + partner supports the value sharing extensions to CBOR + (http://cbor.schmorp.de/value-sharing). + + Detecting shared values incurs a runtime overhead when values are + encoded that have a reference counter large than one, and might + unnecessarily increase the encoded size, as potentially shared + values are encode as sharable whether or not they are actually + shared. + + At the moment, only targets of references can be shared (e.g. + scalars, arrays or hashes pointed to by a reference). Weirder + constructs, such as an array with multiple "copies" of the *same* + string, which are hard but not impossible to create in Perl, are not + supported (this is the same as for Storable). + + If $enable is false (the default), then "encode" will encode + exception when it encounters anything it cannot encode as CBOR. + + This option does not affect "decode" in any way - shared values and + references will always be decoded properly if present. + + $cbor = $cbor->allow_stringref ([$enable]) + $enabled = $cbor->get_allow_stringref + If $enable is true (or missing), then "encode" will try not to + encode the same string twice, but will instead encode a reference to + the string instead. Depending on your data format. this can save a + lot of space, but also results in a very large runtime overhead + (expect encoding times to be 2-4 times as high as without). + + It is recommended to leave it off unless you know your + communications partner supports the stringref extension to CBOR + (http://cbor.schmorp.de/stringref). + + If $enable is false (the default), then "encode" will encode + exception when it encounters anything it cannot encode as CBOR. + + This option does not affect "decode" in any way - string references + will always be decoded properly if present. + + $cbor = $cbor->filter ([$cb->($tag, $value)]) + $cb_or_undef = $cbor->get_filter + Sets or replaces the tagged value decoding filter (when $cb is + specified) or clears the filter (if no argument or "undef" is + provided). + + The filter callback is called only during decoding, when a + non-enforced tagged value has been decoded (see "TAG HANDLING AND + EXTENSIONS" for a list of enforced tags). For specific tags, it's + often better to provide a default converter using the + %CBOR::XS::FILTER hash (see below). + + The first argument is the numerical tag, the second is the (decoded) + value that has been tagged. + + The filter function should return either exactly one value, which + will replace the tagged value in the decoded data structure, or no + values, which will result in default handling, which currently means + the decoder creates a "CBOR::XS::Tagged" object to hold the tag and + the value. + + When the filter is cleared (the default state), the default filter + function, "CBOR::XS::default_filter", is used. This function simply + looks up the tag in the %CBOR::XS::FILTER hash. If an entry exists + it must be a code reference that is called with tag and value, and + is responsible for decoding the value. If no entry exists, it + returns no values. + + Example: decode all tags not handled internally into + CBOR::XS::Tagged objects, with no other special handling (useful + when working with potentially "unsafe" CBOR data). + + CBOR::XS->new->filter (sub { })->decode ($cbor_data); + + Example: provide a global filter for tag 1347375694, converting the + value into some string form. + + $CBOR::XS::FILTER{1347375694} = sub { + my ($tag, $value); + + "tag 1347375694 value $value" + }; + $cbor_data = $cbor->encode ($perl_scalar) Converts the given Perl data structure (a scalar value) to its CBOR representation. @@ -176,23 +295,11 @@ numbers 1 and 0 (for true and false) or to throw an exception on access (for error). See the Types::Serialiser manpage for details. - CBOR tag 256 (perl object) - The tag value 256 (TODO: pending iana registration) will be used to - deserialise a Perl object serialised with "FREEZE". See OBJECT - SERIALISATION, below, for details. - - CBOR tag 55799 (magic header) - The tag 55799 is ignored (this tag implements the magic header). - - other CBOR tags - Tagged items consists of a numeric tag and another CBOR value. Tags - not handled internally are currently converted into a - CBOR::XS::Tagged object, which is simply a blessed array reference - consisting of the numeric tag value followed by the (decoded) CBOR - value. + tagged values + Tagged items consists of a numeric tag and another CBOR value. - In the future, support for user-supplied conversions might get - added. + See "TAG HANDLING AND EXTENSIONS" and the description of "->filter" + for details. anything else Anything else (e.g. unsupported simple values) will raise a decoding @@ -233,11 +340,12 @@ other blessed objects Other blessed objects are serialised via "TO_CBOR" or "FREEZE". See - "OBJECT SERIALISATION", below, for details. + "TAG HANDLING AND EXTENSIONS" for specific classes handled by this + module, and "OBJECT SERIALISATION" for generic object serialisation. simple scalars - TODO Simple Perl scalars (any scalar that is not a reference) are - the most difficult objects to encode: CBOR::XS will encode undefined + Simple Perl scalars (any scalar that is not a reference) are the + most difficult objects to encode: CBOR::XS will encode undefined scalars as CBOR null values, scalars that have last been used in a string context before encoding as CBOR strings, and anything else as number value: @@ -378,10 +486,10 @@ There is no way to distinguish CBOR from other formats programmatically. To make it easier to distinguish CBOR from other formats, the CBOR specification has a special "magic string" that can be prepended to any - CBOR string without changing it's meaning. + CBOR string without changing its meaning. This string is available as $CBOR::XS::MAGIC. This module does not - prepend this string tot he CBOR data it generates, but it will ignroe it + prepend this string to the CBOR data it generates, but it will ignore it if present, so users can prepend this string as a "file type" indicator as required. @@ -443,6 +551,86 @@ CBOR::XS::tag 24, encode_cbor [1, 2, 3]; +TAG HANDLING AND EXTENSIONS + This section describes how this module handles specific tagged values + and extensions. If a tag is not mentioned here and no additional filters + are provided for it, then the default handling applies (creating a + CBOR::XS::Tagged object on decoding, and only encoding the tag when + explicitly requested). + + Tags not handled specifically are currently converted into a + CBOR::XS::Tagged object, which is simply a blessed array reference + consisting of the numeric tag value followed by the (decoded) CBOR + value. + + Future versions of this module reserve the right to special case + additional tags (such as base64url). + + ENFORCED TAGS + These tags are always handled when decoding, and their handling cannot + be overriden by the user. + + (perl-object, ) + These tags are automatically created (and decoded) for serialisable + objects using the "FREEZE/THAW" methods (the Types::Serialier object + serialisation protocol). See "OBJECT SERIALISATION" for details. + + , (sharable, sharedref, L + ) + These tags are automatically decoded when encountered, resulting in + shared values in the decoded object. They are only encoded, however, + when "allow_sharable" is enabled. + + , (stringref-namespace, stringref, L + ) + These tags are automatically decoded when encountered. They are only + encoded, however, when "allow_stringref" is enabled. + + 22098 (indirection, ) + This tag is automatically generated when a reference are encountered + (with the exception of hash and array refernces). It is converted to + a reference when decoding. + + 55799 (self-describe CBOR, RFC 7049) + This value is not generated on encoding (unless explicitly requested + by the user), and is simply ignored when decoding. + + NON-ENFORCED TAGS + These tags have default filters provided when decoding. Their handling + can be overriden by changing the %CBOR::XS::FILTER entry for the tag, or + by providing a custom "filter" callback when decoding. + + When they result in decoding into a specific Perl class, the module + usually provides a corresponding "TO_CBOR" method as well. + + When any of these need to load additional modules that are not part of + the perl core distribution (e.g. URI), it is (currently) up to the user + to provide these modules. The decoding usually fails with an exception + if the required module cannot be loaded. + + 2, 3 (positive/negative bignum) + These tags are decoded into Math::BigInt objects. The corresponding + "Math::BigInt::TO_CBOR" method encodes "small" bigints into normal + CBOR integers, and others into positive/negative CBOR bignums. + + 4, 5 (decimal fraction/bigfloat) + Both decimal fractions and bigfloats are decoded into Math::BigFloat + objects. The corresponding "Math::BigFloat::TO_CBOR" method *always* + encodes into a decimal fraction. + + CBOR cannot represent bigfloats with *very* large exponents - + conversion of such big float objects is undefined. + + Also, NaN and infinities are not encoded properly. + + 21, 22, 23 (expected later JSON conversion) + CBOR::XS is not a CBOR-to-JSON converter, and will simply ignore + these tags. + + 32 (URI) + These objects decode into URI objects. The corresponding + "URI::TO_CBOR" method again results in a CBOR URI value. + CBOR and JSON CBOR is supposed to implement a superset of the JSON data model, and is, with some coercion, able to represent all JSON texts (something that