--- CBOR-XS/README 2013/11/22 16:18:59 1.9 +++ CBOR-XS/README 2013/12/01 17:10:42 1.12 @@ -23,38 +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, 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. - 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. - As for compactness, "CBOR::XS" encoded data structures are usually about - 20% smaller than the same data encoded as (compact) JSON or Storable. + 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 self-referencing data - structures (see "allow_sharing"), string deduplication (see - "allow_stringref") and scalar references (always enabled). + 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. @@ -149,48 +142,89 @@ 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. + 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 - (http://cbor.schmorp.de/value-sharing). + (), 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 sharable whether or not they are actually + 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 for Storable). + supported (this is the same as with Storable). - If $enable is false (the default), then "encode" will encode - exception when it encounters anything it cannot encode as CBOR. + 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 = $cbor->allow_stringref ([$enable]) - $enabled = $cbor->get_allow_stringref + $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. + + This option does not affect "encode" in any way - shared values and + references will always be decoded 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 + 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). + (), as without decoder support, + the resulting data structure might not be usable. - If $enable is false (the default), then "encode" will encode - exception when it encounters anything it cannot encode as CBOR. + 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 $cb is @@ -220,7 +254,7 @@ returns no values. Example: decode all tags not handled internally into - CBOR::XS::Tagged objects, with no other special handling (useful + "CBOR::XS::Tagged" objects, with no other special handling (useful when working with potentially "unsafe" CBOR data). CBOR::XS->new->filter (sub { })->decode ($cbor_data); @@ -271,7 +305,7 @@ 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 @@ -299,7 +333,7 @@ Tagged items consists of a numeric tag and another CBOR value. See "TAG HANDLING AND EXTENSIONS" and the description of "->filter" - for details. + for details on which tags are handled how. anything else Anything else (e.g. unsupported simple values) will raise a decoding @@ -307,13 +341,14 @@ 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. @@ -322,14 +357,17 @@ 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, @@ -355,7 +393,7 @@ 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"] @@ -369,6 +407,16 @@ $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 $x = "3"; # some variable containing a string @@ -387,10 +435,15 @@ might suffer loss of precision. OBJECT SERIALISATION + This module implements both a CBOR-specific and the generic + Types::Serialier object serialisation protocol. The following + subsections explain both methods. + + ENCODING This module knows two way to serialise a Perl object: The CBOR-specific way, and the generic way. - Whenever the encoder encounters a Perl object that it cnanot serialise + Whenever the encoder encounters a Perl object that it cannot serialise directly (most of them), it will first look up the "TO_CBOR" method on it. @@ -405,12 +458,17 @@ The "FREEZE" method can return any number of values (i.e. zero or more). These will be encoded as CBOR perl object, together with the classname. + These methods *MUST NOT* change the data structure that is being + serialised. Failure to comply to this can result in memory corruption - + and worse. + If an object supports neither "TO_CBOR" nor "FREEZE", encoding will fail with an error. - Objects encoded via "TO_CBOR" cannot be automatically decoded, but - objects encoded via "FREEZE" can be decoded using the following - protocol: + DECODING + Objects encoded via "TO_CBOR" cannot (normally) be automatically + decoded, but objects encoded via "FREEZE" can be decoded using the + following protocol: When an encoded CBOR perl object is encountered by the decoder, it will look up the "THAW" method, by using the stored classname, and will fail @@ -441,7 +499,7 @@ my ($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 @@ -570,21 +628,34 @@ These tags are always handled when decoding, and their handling cannot be overriden by the user. - (perl-object, ) + 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. - , (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. + 28, 29 (shareable, sharedref, L ) + 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. - , (stringref-namespace, stringref, L + 256, 25 (stringref-namespace, stringref, L ) These tags are automatically decoded when encountered. They are only - encoded, however, when "allow_stringref" is enabled. + encoded, however, when "pack_strings" is enabled. 22098 (indirection, ) This tag is automatically generated when a reference are encountered @@ -608,6 +679,16 @@ 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 @@ -695,6 +776,13 @@ 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), 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