Protocol buffers are Google's language-neutral, platform-neutral, extensible mechanism for serializing structured data -- think XML, but smaller, faster, and simpler. You define how you want your data to be structured once. This takes the form of a template that describes the data structure. You use this template to encode and decode your data structure into wire-streams that may be sent-to or read-from your peers. The underlying wire stream is platform independent, lossless, and may be used to interwork with a variety of languages and systems regardless of word size or endianness. Techniques exist to safely extend your data structure without breaking deployed programs that are compiled against the "old" format.

The idea behind Google's Protocol Buffers is that you define your structured messages using a domain-specific language and tool set. Further documentation on this is at https://developers.google.com/protocol-buffers.

There are two ways you can use protobufs in Prolog:

• with a compiled .proto file: protobuf_parse_from_codes/3 and protobuf_serialize_to_codes/3.
• with a lower-level interface protobuf_message/2, which allows you to define your own domain-specific language for parsing and serializing protobufs.

The protobuf_parse_from_codes/3 and protobuf_serialize_to_codes/3 interface translates between a "wire stream" and a Prolog term. This interface takes advantage of SWI-Prolog's dict. There is a protoc plugin (protoc-gen-swipl) that generates a Prolog file of meta-information that captures the .proto file's definition in the protobufs module:

• proto_meta_normalize(Unnormalized, Normalized)
• proto_meta_package(Package, FileName, Options)
• proto_meta_message_type( Fqn, Package, Name)
• proto_meta_message_type_map_entry( Fqn)
• proto_meta_field_name( Fqn, FieldNumber, FieldName, FqnName)
• proto_meta_field_json_name( FqnName, JsonName)
• proto_meta_field_label( FqnName, LabelRepeatOptional) % 'LABEL_OPTIONAL', 'LABEL_REQUIRED', 'LABEL_REPEATED'
• proto_meta_field_type( FqnName, Type) % 'TYPE_INT32', 'TYPE_MESSAGE', etc
• proto_meta_field_type_name( FqnName, TypeName)
• proto_meta_field_default_value( FqnName, DefaultValue)
• proto_meta_field_option_packed( FqnName)
• proto_meta_enum_type( FqnName, Fqn, Name)
• proto_meta_enum_value( FqnName, Name, Number)
• proto_meta_field_oneof_index( FqnName, Index)
• proto_meta_oneof( FqnName, Index, Name)

The protobuf_message/2 interface allows you to define your message template as a list of predefined Prolog terms that correspond to production rules in the Definite Clause Grammar (DCG) that realizes the interpreter. Each production rule has an equivalent rule in the protobuf grammar. The process is not unlike specifiying the format of a regular expression. To encode a template to a wire-stream, you pass a grounded template, X, and variable, Y, to protobuf_message/2. To decode a wire-stream, Y, you pass an ungrounded template, X, along with a grounded wire-stream, Y, to protobuf_message/2. The interpreter will unify the unbound variables in the template with values decoded from the wire-stream.

For an overview and tutorial with examples, see library(protobufs): Google's Protocol Buffers Examples of usage may also be found by inspecting test_protobufs.pl and the demo directory, or by looking at the "addressbook" example that is typically installed at /usr/lib/swi-prolog/doc/packages/examples/protobufs/interop/addressbook.pl

author

- Jeffrey Rosenwald (JeffRose@acm.org)

- Peter Ludemann (peter.ludemann@gmail.org)

See also

- https://developers.google.com/protocol-buffers

- https://developers.google.com/protocol-buffers/docs/encoding

Compatibility

- SWI-Prolog

 protobuf_parse_from_codes(+WireCodes:list(int), +MessageType:atom, -Term) is semidet

Process bytes (list of int) that is the serialized form of a message (designated by MessageType), creating a Prolog term.

Protoc must have been run (with the --swipl_out= option and the resulting top-level _pb.pl file loaded. For more details, see the "protoc" section of the overview documentation.

Fails if the message can't be parsed or if the appropriate meta-data from protoc hasn't been loaded.

All fields that are omitted from the WireCodes are set to their default values (typically the empty string or 0, depending on the type; or [] for repeated groups). There is no way of testing whether a value was specified in WireCodes or given its default value (that is, there is no equivalent of the Python implementation's =HasField`). Optional embedded messages and groups do not have any default value -- you must check their existence by using get_dict/3 or similar. If a field is part of a "oneof" set, then none of the other fields is set. You can determine which field had a value by using get_dict/3.

Arguments:

WireCodes	- Wire format of the message from e.g., read_stream_to_codes/2. (The stream should have options encoding(octet) and type(binary), either as options to read_file_to_codes/3 or by calling set_stream/2 on the stream to read_stream_to_codes/2.)
MessageType	- Fully qualified message name (from the .proto file's package and message). For example, if the package is google.protobuf and the message is FileDescriptorSet, then you would use '.google.protobuf.FileDescriptorSet' or 'google.protobuf.FileDescriptorSet'. If there's no package name, use e.g.: 'MyMessage or '.MyMessage'. You can see the packages by looking at protobufs:proto_meta_package(Pkg,File,_) and the message names and fields by protobufs:proto_meta_field_name('.google.protobuf.FileDescriptorSet', FieldNumber, FieldName, FqnName) (the initial '.' is not optional for these facts, only for the top-level name given to protobuf_serialize_to_codes/3).
Term	- The generated term, as nested dicts.

Errors

- version_error(Module-Version) you need to recompile the Module with a newer version of protoc.

See also

- library(protobufs): Google's Protocol Buffers

bug

- Ignores .proto extensions.

- map fields don't get special treatment (but see protobuf_map_pairs/3).

- Generates fields in a different order from the C++, Python, Java implementations, which use the field number to determine field order whereas currently this implementation uses field name. (This isn't stricly speaking a bug, because it's allowed by the specification; but it might cause some surprise.)

To be done

- document the generated terms (see library(http/json) and json_read_dict/3)

- add options such as true and value_string_as (similar to json_read_dict/3)

- add option for form of the dict tags (fully qualified or not)

- add option for outputting fields in the C++/Python/Java order (by field number rather than by field name).

 protobuf_serialize_to_codes(+Term:dict, -MessageType:atom, -WireCodes:list(int)) is det

Process a Prolog term into bytes (list of int) that is the serialized form of a message (designated by MessageType).

Protoc must have been run (with the --swipl_out= option and the resulting top-level _pb.pl file loaded. For more details, see the "protoc" section of the overview documentation.

Fails if the term isn't of an appropriate form or if the appropriate meta-data from protoc hasn't been loaded, or if a field name is incorrect (and therefore nothing in the meta-data matches it).

Arguments:

Term	- The Prolog form of the data, as nested dicts.
MessageType	- Fully qualified message name (from the .proto file's package and message). For example, if the package is google.protobuf and the message is FileDescriptorSet, then you would use '.google.protobuf.FileDescriptorSet' or 'google.protobuf.FileDescriptorSet'. If there's no package name, use e.g.: 'MyMessage or '.MyMessage'. You can see the packages by looking at protobufs:proto_meta_package(Pkg,File,_) and the message names and fields by protobufs:proto_meta_field_name('.google.protobuf.FileDescriptorSet', FieldNumber, FieldName, FqnName) (the initial '.' is not optional for these facts, only for the top-level name given to protobuf_serialize_to_codes/3).
WireCodes	- Wire format of the message, which can be output using format('~s', [WireCodes]).

Errors

- version_error(Module-Version) you need to recompile the Module with a newer version of protoc.

- existence_error if a field can't be found in the meta-data

See also

- library(protobufs): Google's Protocol Buffers

bug

- map fields don't get special treatment (but see protobuf_map_pairs/3).

- oneof is not checked for validity.

 protobuf_message(?Template, ?WireStream) is semidet

 protobuf_message(?Template, ?WireStream, ?Rest) is nondet

Marshals and unmarshals byte streams encoded using Google's Protobuf grammars. protobuf_message/2 provides a bi-directional parser that marshals a Prolog structure to WireStream, according to rules specified by Template. It can also unmarshal WireStream into a Prolog structure according to the same grammar. protobuf_message/3 provides a difference list version.

Arguments:

Template	- is a protobuf grammar specification. On decode, unbound variables in the Template are unified with their respective values in the WireStream. On encode, Template must be ground.
WireStream	- is a code list that was generated by a protobuf encoder using an equivalent template.

bug

- The protobuf specification states that the wire-stream can have the fields in any order and that unknown fields are to be ignored. This implementation assumes that the fields are in the exact order of the definition and match exactly. If you use protobuf_parse_from_codes/3, you can avoid this problem.o

 protobuf_field_is_map(+MessageType, +FieldName) is semidet

Succeeds if MessageType's FieldName is defined as a map<...> in the .proto file.

 protobuf_map_pairs(+ProtobufTermList:list, ?DictTag:atom, ?Pairs) is det

Convert between a list of protobuf map entries (in the form DictTag{key:Key, value:Value} and a key-value list as described in library(pairs). At least one of ProtobufTermList and Pairs must be instantiated; DictTag can be uninstantiated. If ProtobufTermList is from a term created by protobuf_parse_from_codes/3, the ordering of the items is undefined; you can order them by using keysort/2 (or by a predicate such as dict_pairs/3, list_to_assoc/2, or list_to_rbtree/2.