--- CBOR-XS/README 2013/10/26 11:08:34 1.3 +++ CBOR-XS/README 2013/10/28 21:28:14 1.6 @@ -10,15 +10,40 @@ # OO-interface $coder = CBOR::XS->new; - #TODO + $binary_cbor_data = $coder->encode ($perl_value); + $perl_value = $coder->decode ($binary_cbor_data); + + # prefix decoding + + my $many_cbor_strings = ...; + while (length $many_cbor_strings) { + my ($data, $length) = $cbor->decode_prefix ($many_cbor_strings); + # data was decoded + substr $many_cbor_strings, 0, $length, ""; # remove decoded cbor string + } DESCRIPTION - WARNING! THIS IS A PRE-ALPHA RELEASE! IT WILL CRASH, CORRUPT YOUR DATA - AND EAT YOUR CHILDREN! + 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. + + You are still invited to try out CBOR, and this module. + + This module converts Perl data structures to the Concise Binary Object + Representation (CBOR) and vice versa. CBOR is a fast binary + serialisation format that aims to use a superset of the JSON data model, + i.e. when you can represent something in JSON, you should be able to + represent it in CBOR. - This module converts Perl data structures to CBOR and vice versa. Its - primary goal is to be *correct* and its secondary goal is to be *fast*. - To reach the latter goal it was written in C. + In short, CBOR is a faster and very compact binary alternative to JSON, + with the added ability of supporting serialisation of Perl objects. + + 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. See MAPPING, below, on how CBOR::XS maps perl values to CBOR values and vice versa. @@ -120,15 +145,56 @@ refers to the abstract Perl language itself. CBOR -> PERL - True, False - These CBOR values become "CBOR::XS::true" and "CBOR::XS::false", + integers + CBOR integers become (numeric) perl scalars. On perls without 64 bit + support, 64 bit integers will be truncated or otherwise corrupted. + + byte strings + Byte strings will become octet strings in Perl (the byte values + 0..255 will simply become characters of the same value in Perl). + + UTF-8 strings + UTF-8 strings in CBOR will be decoded, i.e. the UTF-8 octets will be + decoded into proper Unicode code points. At the moment, the validity + of the UTF-8 octets will not be validated - corrupt input will + result in corrupted Perl strings. + + arrays, maps + CBOR arrays and CBOR maps will be converted into references to a + Perl array or hash, respectively. The keys of the map will be + stringified during this process. + + null + CBOR null becomes "undef" in Perl. + + true, false, undefined + These CBOR values become "Types:Serialiser::true", + "Types:Serialiser::false" and "Types::Serialiser::error", respectively. They are overloaded to act almost exactly like the - numbers 1 and 0. You can check whether a scalar is a CBOR boolean by - using the "CBOR::XS::is_bool" function. + numbers 1 and 0 (for true and false) or to throw an exception on + access (for error). See the Types::Serialiser manpage for details. - Null, Undefined - CBOR Null and Undefined values becomes "undef" in Perl (in the - future, Undefined may raise an exception). + 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. + + In the future, support for user-supplied conversions might get + added. + + anything else + Anything else (e.g. unsupported simple values) will raise a decoding + error. PERL -> CBOR The mapping from Perl to CBOR is slightly more difficult, as Perl is a @@ -140,29 +206,36 @@ ordering in hash keys (or CBOR maps), they will usually be encoded in a pseudo-random order. + Currently, tied hashes will use the indefinite-length format, while + normal hashes will use the fixed-length format. + array references - Perl array references become CBOR arrays. + Perl array references become fixed-length CBOR arrays. other references Other unblessed references are generally not allowed and will cause an exception to be thrown, except for references to the integers 0 - and 1, which get turned into "False" and "True" in CBOR. + and 1, which get turned into false and true in CBOR. - CBOR::XS::true, CBOR::XS::false - These special values become CBOR True and CBOR False values, - respectively. You can also use "\1" and "\0" directly if you want. - - blessed objects - Blessed objects are not directly representable in CBOR. TODO See the - "allow_blessed" and "convert_blessed" methods on various options on - how to deal with this: basically, you can choose between throwing an - exception, encoding the reference as if it weren't blessed, or - provide your own serialiser method. + CBOR::XS::Tagged objects + Objects of this type must be arrays consisting of a single "[tag, + value]" pair. The (numerical) tag will be encoded as a CBOR tag, the + value will be encoded as appropriate for the value. + + Types::Serialiser::true, Types::Serialiser::false, + Types::Serialiser::error + These special values become CBOR true, CBOR false and CBOR undefined + values, respectively. You can also use "\1", "\0" and "\undef" + directly if you want. + + other blessed objects + Other blessed objects are serialised via "TO_CBOR" or "FREEZE". See + "OBJECT SERIALISATION", below, for details. 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 - scalars as CBOR "Null" values, scalars that have last been used in a + 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: @@ -195,14 +268,110 @@ Tell me if you need this capability (but don't forget to explain why it's needed :). - Note that numerical precision has the same meaning as under Perl (so - binary to decimal conversion follows the same rules as in Perl, - which can differ to other languages). Also, your perl interpreter - might expose extensions to the floating point numbers of your - platform, such as infinities or NaN's - these cannot be represented - in CBOR, and it is an error to pass those in. + Perl values that seem to be integers generally use the shortest + possible representation. Floating-point values will use either the + IEEE single format if possible without loss of precision, otherwise + the IEEE double format will be used. Perls that use formats other + than IEEE double to represent numerical values are supported, but + might suffer loss of precision. + + OBJECT SERIALISATION + This module knows two way to serialise a Perl object: The CBOR-specific + way, and the generic way. + + Whenever the encoder encounters a Perl object that it cnanot serialise + directly (most of them), it will first look up the "TO_CBOR" method on + it. + + If it has a "TO_CBOR" method, it will call it with the object as only + argument, and expects exactly one return value, which it will then + substitute and encode it in the place of the object. + + Otherwise, it will look up the "FREEZE" method. If it exists, it will + call it with the object as first argument, and the constant string + "CBOR" as the second argument, to distinguish it from other serialisers. + + The "FREEZE" method can return any number of values (i.e. zero or more). + These will be encoded as CBOR perl object, together with the classname. + + If an object supports neither "TO_CBOR" nor "FREEZE", encoding will fail + with an error. + + Objects encoded via "TO_CBOR" cannot be automatically decoded, but + objects encoded via "FREEZE" can be decoded using the following + protocol: + + When an encoded CBOR perl object is encountered by the decoder, it will + look up the "THAW" method, by using the stored classname, and will fail + if the method cannot be found. + + After the lookup it will call the "THAW" method with the stored + classname as first argument, the constant string "CBOR" as second + argument, and all values returned by "FREEZE" as remaining arguments. + + EXAMPLES + Here is an example "TO_CBOR" method: + + sub My::Object::TO_CBOR { + my ($obj) = @_; + + ["this is a serialised My::Object object", $obj->{id}] + } - MAGIC HEADER + When a "My::Object" is encoded to CBOR, it will instead encode a simple + array with two members: a string, and the "object id". Decoding this + CBOR string will yield a normal perl array reference in place of the + object. + + A more useful and practical example would be a serialisation method for + the URI module. CBOR has a custom tag value for URIs, namely 32: + + sub URI::TO_CBOR { + my ($self) = @_; + my $uri = "$self"; # stringify uri + utf8::upgrade $uri; # make sure it will be encoded as UTF-8 string + CBOR::XS::tagged 32, "$_[0]" + } + + This will encode URIs as a UTF-8 string with tag 32, which indicates an + URI. + + Decoding such an URI will not (currently) give you an URI object, but + instead a CBOR::XS::Tagged object with tag number 32 and the string - + exactly what was returned by "TO_CBOR". + + To serialise an object so it can automatically be deserialised, you need + to use "FREEZE" and "THAW". To take the URI module as example, this + would be a possible implementation: + + sub URI::FREEZE { + my ($self, $serialiser) = @_; + "$self" # encode url string + } + + sub URI::THAW { + my ($class, $serialiser, $uri) = @_; + + $class->new ($uri) + } + + Unlike "TO_CBOR", multiple values can be returned by "FREEZE". For + example, a "FREEZE" method that returns "type", "id" and "variant" + values would cause an invocation of "THAW" with 5 arguments: + + sub My::Object::FREEZE { + my ($self, $serialiser) = @_; + + ($self->{type}, $self->{id}, $self->{variant}) + } + + sub My::Object::THAW { + my ($class, $serialiser, $type, $id, $variant) = @_; + + $class- $type, id => $id, variant => $variant) + } + +MAGIC HEADER 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 @@ -213,8 +382,18 @@ if present, so users can prepend this string as a "file type" indicator as required. - CBOR and JSON - TODO +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 + other "binary JSON" formats such as BSON generally do not support). + + CBOR implements some extra hints and support for JSON interoperability, + and the spec offers further guidance for conversion between CBOR and + JSON. None of this is currently implemented in CBOR, and the guidelines + in the spec do not result in correct round-tripping of data. If JSON + interoperability is improved in the future, then the goal will be to + ensure that decoded JSON data will round-trip encoding and decoding to + CBOR intact. SECURITY CONSIDERATIONS When you are using CBOR in a protocol, talking to untrusted potentially @@ -287,6 +466,9 @@ The JSON and JSON::XS modules that do similar, but human-readable, serialisation. + The Types::Serialiser module provides the data model for true, false and + error values. + AUTHOR Marc Lehmann http://home.schmorp.de/