--- CBOR-XS/README 2013/10/29 15:56:31 1.7 +++ CBOR-XS/README 2016/02/08 04:37:12 1.16 @@ -23,26 +23,31 @@ } 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. - - 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. + serialisation format that aims to use an (almost) superset of the JSON + data model, i.e. when you can represent something useful in JSON, you + should be able to represent it in CBOR. - In short, CBOR is a faster and very compact binary alternative to JSON, + In short, CBOR is a faster and quite compact binary alternative to JSON, with the added ability of supporting serialisation of Perl objects. (JSON often compresses better than CBOR though, so if you plan to - compress the data later you might want to compare both formats first). + compress the data later and speed is less important 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. + + Regarding 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 shared data structures (see + "allow_sharing" and "allow_cycles"), string deduplication (see + "pack_strings") 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 +79,7 @@ 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 +120,159 @@ 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. This also makes it possible + to encode cyclic data structures (which need "allow_cycles" to ne + enabled to be decoded by this module). + + It is recommended to leave it off unless you know your communication + partner supports the value sharing extensions to CBOR + (), as without decoder + support, the resulting data structure might be unusable. + + 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 shareable 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 with Storable). + + If $enable is false (the default), then "encode" will encode shared + data structures repeatedly, unsharing them in the process. Cyclic + data structures cannot be encoded in this mode. + + This option does not affect "decode" in any way - shared values and + references will always be decoded properly if present. + + $cbor = $cbor->allow_cycles ([$enable]) + $enabled = $cbor->get_allow_cycles + If $enable is true (or missing), then "decode" will happily decode + self-referential (cyclic) data structures. By default these will not + be decoded, as they need manual cleanup to avoid memory leaks, so + code that isn't prepared for this will not leak memory. + + If $enable is false (the default), then "decode" will throw an error + when it encounters a self-referential/cyclic data structure. + + FUTURE DIRECTION: the motivation behind this option is to avoid + *real* cycles - future versions of this module might chose to decode + cyclic data structures using weak references when this option is + off, instead of throwing an error. + + This option does not affect "encode" in any way - shared values and + references will always be encoded properly if present. + + $cbor = $cbor->pack_strings ([$enable]) + $enabled = $cbor->get_pack_strings + 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 + (), as without decoder support, + the resulting data structure might not be usable. + + If $enable is false (the default), then "encode" will encode strings + the standard CBOR way. + + This option does not affect "decode" in any way - string references + will always be decoded properly if present. + + $cbor = $cbor->validate_utf8 ([$enable]) + $enabled = $cbor->get_validate_utf8 + If $enable is true (or missing), then "decode" will validate that + elements (text strings) containing UTF-8 data in fact contain valid + UTF-8 data (instead of blindly accepting it). This validation + obviously takes extra time during decoding. + + The concept of "valid UTF-8" used is perl's concept, which is a + superset of the official UTF-8. + + If $enable is false (the default), then "decode" will blindly accept + UTF-8 data, marking them as valid UTF-8 in the resulting data + structure regardless of whether thats true or not. + + Perl isn't too happy about corrupted UTF-8 in strings, but should + generally not crash or do similarly evil things. Extensions might be + not so forgiving, so it's recommended to turn on this setting if you + receive untrusted CBOR. + + This option does not affect "encode" in any way - strings that are + supposedly valid UTF-8 will simply be dumped into the resulting CBOR + string without checking whether that is, in fact, true or not. + + $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. @@ -136,6 +294,64 @@ CBOR::XS->new->decode_prefix ("......") => ("...", 3) + INCREMENTAL PARSING + In some cases, there is the need for incremental parsing of JSON texts. + While this module always has to keep both CBOR text and resulting Perl + data structure in memory at one time, it does allow you to parse a CBOR + stream incrementally, using a similar to using "decode_prefix" to see if + a full CBOR object is available, but is much more efficient. + + It basically works by parsing as much of a CBOR string as possible - if + the CBOR data is not complete yet, the pasrer will remember where it + was, to be able to restart when more data has been accumulated. Once + enough data is available to either decode a complete CBOR value or raise + an error, a real decode will be attempted. + + A typical use case would be a network protocol that consists of sending + and receiving CBOR-encoded messages. The solution that works with CBOR + and about anything else is by prepending a length to every CBOR value, + so the receiver knows how many octets to read. More compact (and + slightly slower) would be to just send CBOR values back-to-back, as + "CBOR::XS" knows where a CBOR value ends, and doesn't need an explicit + length. + + The following methods help with this: + + @decoded = $cbor->incr_parse ($buffer) + This method attempts to decode exactly one CBOR value from the + beginning of the given $buffer. The value is removed from the + $buffer on success. When $buffer doesn't contain a complete value + yet, it returns nothing. Finally, when the $buffer doesn't start + with something that could ever be a valid CBOR value, it raises an + exception, just as "decode" would. In the latter case the decoder + state is undefined and must be reset before being able to parse + further. + + This method modifies the $buffer in place. When no CBOR value can be + decoded, the decoder stores the current string offset. On the next + call, continues decoding at the place where it stopped before. For + this to make sense, the $buffer must begin with the same octets as + on previous unsuccessful calls. + + You can call this method in scalar context, in which case it either + returns a decoded value or "undef". This makes it impossible to + distinguish between CBOR null values (which decode to "undef") and + an unsuccessful decode, which is often acceptable. + + @decoded = $cbor->incr_parse_multiple ($buffer) + Same as "incr_parse", but attempts to decode as many CBOR values as + possible in one go, instead of at most one. Calls to "incr_parse" + and "incr_parse_multiple" can be interleaved. + + $cbor->incr_reset + Resets the incremental decoder. This throws away any saved state, so + that subsequent calls to "incr_parse" or "incr_parse_multiple" start + to parse a new CBOR value from the beginning of the $buffer again. + + This method can be caled at any time, but it *must* be called if you + want to change your $buffer or there was a decoding error and you + want to reuse the $cbor object for future incremental parsings. + MAPPING This section describes how CBOR::XS maps Perl values to CBOR values and vice versa. These mappings are designed to "do the right thing" in most @@ -152,7 +368,7 @@ support, 64 bit integers will be truncated or otherwise corrupted. byte strings - Byte strings will become octet strings in Perl (the byte values + 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 @@ -176,23 +392,11 @@ 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 on which tags are handled how. anything else Anything else (e.g. unsupported simple values) will raise a decoding @@ -200,13 +404,14 @@ PERL -> CBOR The mapping from Perl to CBOR is slightly more difficult, as Perl is a - truly typeless language, so we can only guess which CBOR type is meant - by a Perl value. + typeless language. That means this module can only guess which CBOR type + is meant by a perl value. hash references Perl hash references become CBOR maps. As there is no inherent ordering in hash keys (or CBOR maps), they will usually be encoded - in a pseudo-random order. + in a pseudo-random order. This order can be different each time a + hahs is encoded. Currently, tied hashes will use the indefinite-length format, while normal hashes will use the fixed-length format. @@ -215,14 +420,17 @@ 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. + Other unblessed references will be represented using the indirection + tag extension (tag value 22098, + ). CBOR decoders are guaranteed + to be able to decode these values somehow, by either "doing the + right thing", decoding into a generic tagged object, simply ignoring + the tag, or something else. 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. You cna use + value will be encoded as appropriate for the value. You must use "CBOR::XS::tag" to create such objects. Types::Serialiser::true, Types::Serialiser::false, @@ -233,11 +441,12 @@ 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: @@ -247,7 +456,7 @@ encode_cbor [-3.0e17] # yields [-3e+17] my $value = 5; encode_cbor [$value] # yields [5] - # used as string, so dump as string + # used as string, so dump as string (either byte or text) print $value; encode_cbor [$value] # yields ["5"] @@ -261,6 +470,16 @@ $x .= ""; # another, more awkward way to stringify print $x; # perl does it for you, too, quite often + You can force whether a string ie encoded as byte or text string by + using "utf8::upgrade" and "utf8::downgrade"): + + utf8::upgrade $x; # encode $x as text string + utf8::downgrade $x; # encode $x as byte string + + Perl doesn't define what operations up- and downgrade strings, so if + the difference between byte and text is important, you should up- or + downgrade your string as late as possible before encoding. + You can force the type to be a CBOR number by numifying it: my $x = "3"; # some variable containing a string @@ -279,10 +498,15 @@ might suffer loss of precision. OBJECT SERIALISATION + This module implements both a CBOR-specific and the generic + Types::Serialier object serialisation protocol. The following + subsections explain both methods. + + ENCODING This module knows two way to serialise a Perl object: The CBOR-specific way, and the generic way. - Whenever the encoder encounters a Perl object that it cnanot serialise + Whenever the encoder encounters a Perl object that it cannot serialise directly (most of them), it will first look up the "TO_CBOR" method on it. @@ -297,12 +521,17 @@ The "FREEZE" method can return any number of values (i.e. zero or more). These will be encoded as CBOR perl object, together with the classname. + These methods *MUST NOT* change the data structure that is being + serialised. Failure to comply to this can result in memory corruption - + and worse. + If an object supports neither "TO_CBOR" nor "FREEZE", encoding will fail with an error. - Objects encoded via "TO_CBOR" cannot be automatically decoded, but - objects encoded via "FREEZE" can be decoded using the following - protocol: + DECODING + Objects encoded via "TO_CBOR" cannot (normally) be automatically + decoded, but objects encoded via "FREEZE" can be decoded using the + following protocol: When an encoded CBOR perl object is encountered by the decoder, it will look up the "THAW" method, by using the stored classname, and will fail @@ -333,7 +562,7 @@ my ($self) = @_; my $uri = "$self"; # stringify uri utf8::upgrade $uri; # make sure it will be encoded as UTF-8 string - CBOR::XS::tagged 32, "$_[0]" + CBOR::XS::tag 32, "$_[0]" } This will encode URIs as a UTF-8 string with tag 32, which indicates an @@ -378,10 +607,10 @@ 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 +672,109 @@ 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. + + 26 (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. + + 28, 29 (shareable, sharedref, ) + These tags are automatically decoded when encountered (and they do + not result in a cyclic data structure, see "allow_cycles"), + resulting in shared values in the decoded object. They are only + encoded, however, when "allow_sharing" is enabled. + + Not all shared values can be successfully decoded: values that + reference themselves will *currently* decode as "undef" (this is not + the same as a reference pointing to itself, which will be + represented as a value that contains an indirect reference to itself + - these will be decoded properly). + + Note that considerably more shared value data structures can be + decoded than will be encoded - currently, only values pointed to by + references will be shared, others will not. While non-reference + shared values can be generated in Perl with some effort, they were + considered too unimportant to be supported in the encoder. The + decoder, however, will decode these values as shared values. + + 256, 25 (stringref-namespace, stringref, + ) + These tags are automatically decoded when encountered. They are only + encoded, however, when "pack_strings" 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. + + 0, 1 (date/time string, seconds since the epoch) + These tags are decoded into Time::Piece objects. The corresponding + "Time::Piece::TO_CBOR" method always encodes into tag 1 values + currently. + + The Time::Piece API is generally surprisingly bad, and fractional + seconds are only accidentally kept intact, so watch out. On the plus + side, the module comes with perl since 5.10, which has to count for + something. + + 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 @@ -507,6 +839,14 @@ Strict mode and canonical mode are not implemented. +LIMITATIONS ON PERLS WITHOUT 64-BIT INTEGER SUPPORT + On perls that were built without 64 bit integer support (these are rare + nowadays, even on 32 bit architectures, as all major Perl distributions + are built with 64 bit integer support), support for any kind of 64 bit + integer in CBOR is very limited - most likely, these 64 bit values will + be truncated, corrupted, or otherwise not decoded correctly. This also + includes string, array and map sizes that are stored as 64 bit integers. + THREADS This module is *not* guaranteed to be thread safe and there are no plans to change this until Perl gets thread support (as opposed to the