Google's Protocol Buffers Library
Jeffrey Rosenwald
E-mail: JeffRose@acm.org
Abstract
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/decode your data structure to/from 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.

# Table of Contents

## 1 Google's Protocol Buffers

### 1.1 Overview

Protocol Buffers ("protobufs") 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 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.

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

The idea behind Google's Protocol Buffers is that you define your structured messages using a domain-specific language. This takes the form of a ".proto" source file. You pass this file through a Google provided tool that generates source code for a target language, creating an interpreter that can encode/decode your structured data. You then compile and build this interpreter into your application program. Depending on the platform, the underlying runtime support is provided by a Google supplied library that is also bound into your program.

### 1.2 Processing protobufs with Prolog

There are two ways you can use protobufs in Prolog: with a compiled ".proto" file and protobuf_parse_from_codes/3 and protobuf_serialize_to_codes/3; or with a lower-level interface protobuf_message/2, which allows you to define your own domain-specific language for parsing and serliazing protobufs.

### 1.3 protoc

A protobuf ".proto" file can be processed by the protobuf compiler (`protoc`), using a Prolog specific plugin. You can do this by either adding `/usr/lib/swi-prolog/library/protobufs` to your `PATH` or by specifying the option `--plugin=protoc-gen-swipl=/usr/lib/swi-prolog/library/protobufs/protoc-gen-swipl`. You specify where the generated files go with the `--swipl_out` option, which must be an existing directory.

When using `protoc`, it's important to specify the `--protopath` (or `-I`) and files correctly. The idea of `protopath` is that it gives a list of source "roots", and the files are specified relative to that. If you want to include the current directory, you must also specify it (e.g., `protoc -I. swipl_out=. foo.proto`). For example, when bootstrapping the "swipl" plugin, these are used:

    protoc -I/usr/include --swipl_out=protoc_gen_prolog_pb google/include/descriptor.proto google/include/compiler/plugin.proto

which creates these files:

    protoc_gen_prolog_pb/google/protobuf/descriptor_pb.pl
    protoc_gen_prolog_pb/google/protobuf/compiler/plugin_pb.pl

The `plugin_pb` is used by:

    :- use_module(protoc_gen_prolog_pb/google/protobuf/compiler/plugin_pb)

which has this (import is relative to the current module):

    :- use_module('../descriptor_pb').

Each `X.proto` file generates a `X_pb.pl` file in the directory specified by `--swipl_out`. The file contains a module name `X`, some debugging information, and meta-data facts that go into the `protobufs` module (all the facts start with "`proto_meta_`") -- protobuf_parse_from_codes/3 uses these facts to parse the wire form of the message into a Prolog term and protobuf_serialize_to_codes/3 uses them to serialize the data to wire form.

The generated code does not rely on any Google-supplied code.

You must compile all the ".proto" files separately but you only need to load the top-level generated file -- it contains the necessary load directives for things that it uses. You can find out the dependencies for a .proto file by running `PATH="$PATH:/usr/lib/swipl/library/protobufs" protoc -I... --dependency_out=FILE --swipl_out=. SRC.proto`

#### 1.3.1 protobuf_serialize_to_codes/3

The Prolog term corresponding to a protobuf `message` is a dict, with the keys corresponding to the field names in the `message` (the dict tag is treated as a comment). Repeated fields are represented as lists; enums are looked up and converted to atoms; bools are represented by `false` and `true`; strings are represented by Prolog strings or atoms; bytes are represented by lists of codes.

`TODO`: Add an option to omit default values (this is the `proto3` behavior).

When serializing, the dict tag is treated as a comment and is ignored. So, you can use any dict tags when creating data for output. For example, both of these will generate the same output:

    protobuf_serialize_to_codes(_{people:[_{id:1234,name:"John Doe"}]}, 'tutorial.AddressBook', WireCodes).
    protobuf_serialize_to_codes('tutorial.AddressBook'{people:['tutorial.Person'{name:"John Doe",id:1234}]}, 'tutorial.AddressBook', WireCodes).

NOTE: if the wire codes can't be parsed, protobuf_parse_from_codes/3 fails. One common cause is if you give an incorrect field name. Typically, this shows up in a call to protobufs:field_segment/3, when protobufs:proto_meta_field_name/4 fails.

#### 1.3.2 protobuf_parse_from_codes/3

This is the inverse of protobuf_serialize_to_codes/3 -- it takes a wire stream (list of codes) and creates a dict. The dict tags are the fully qualified names of the messages. Repeated fields that aren't in the wire stream get set to the value `[]`; other fields that aren't in the wire stream get their default value (typically the empty string or zero, depending on type). Embedded messages and groups are omitted if not in the wire stream; you can test for their presence using get_dict/3. Enums are looked up and converted to atoms; bools are represented by `false` and `true`; strings are represented by Prolog strings (not atoms); bytes are represented by lists of codes.

There is no mechanism for determining whether a field was in the wire stream or not (that is, there is no equivalent of the Python implementation's `HasField`).

The "oneof" feature causes a slightly different behavior. Only the field that's in the wire stream gets set; the other fields are omitted. And if none of the fields in the "oneof" are set, then none of the fields appears. You can check which field is set by using get_dict/3.

Currently, there is no special support for the protobuf "map" feature. It is treated as an ordinary message field. The convenience predicates protobuf_field_is_map/3 and protobuf_map_pairs/3 can be used to convert between a "map" field and a key-value list, which gives you the freedom to use any kind of association list for the map. See also Issue #12 For example:

    message MapMessage {
      map<string, sint64> number_ints = 5;
    }

is treated as if it is

    message MapMessage {
      message KeyValue {
        optional string  Key = 1;
        optional sint64  Value = 2;
      }
      repeated KeyValue number_ints = 5;
    }

You can handle this on input by

    protobuf_parse_from_codes(WireCodes, 'MapMessage', Term),
    protobuf_map_pairs(Term.number_ints, _, Pairs).

and on output by

    protobuf_map_pairs(TermNnumberInts, _, Pairs),
    protobuf_serialize_to_codes(_{number_ints:TermNumberInts}, WireCodes).

#### 1.3.3 addressbook example

The Google documentation has a tutorial example of a simple addressbook: https://developers.google.com/protocol-buffers/docs/tutorials The Prolog equivalent is in `/usr/lib/swi-prolog/oc/packages/examples/protobufs/interop/addressbook.pl` and you can run it by `make run_addressbook`, which will run `protoc` to generate the _`pb.pl` files and then run the example. The resulting file is `addressbook.wire`.

### 1.4 The low-level SWI-Prolog Implementation

For most users, protobuf_serialize_to_codes/3 and protobuf_parse_from_codes/3 suffice. However, if you need greater control, or wish to define your own domain-specific language that maps to protobufs, you can use protobuf_message/2.

The wire stream interpreter is embodied in the form of a Definite Clause Grammar (DCG). It has a small underlying C-support library that loads when the Prolog module loads. This implementation does not depend on any code that is provided by Google and thus is not bound by its license terms.

On the Prolog side, you define your message template as a list of predefined Prolog terms that correspond to production rules in the DCG. The process is not unlike specifiying the format of a regular expression. To encode a message, `X`, to wire-stream, `Y`, you pass a grounded template, `X`, and a variable, `Y`, to protobuf_message/2. To decode a wire-stream, `Y`, to template, `X`, 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.

An example template is:

    protobuf([
            unsigned(1, 100),
            string(2, "abcd"),
            repeated(3, atom([foo, bar])),
            boolean(4, true),
            embedded(5, protobuf([integer(1, -666), string(2, "negative 666")])),
            repeated(6, embedded([
                protobuf([integer(1, 1234), string(2, "onetwothreefour")]),
                protobuf([integer(1, 2222), string(2, "four twos")])])),
            repeated(7, integer([1,2,3,4])),
            packed(8, integer([5,6,7,8]))
        ])

This corresponds to a message created with this .proto definition (using proto2 syntax):

    syntax = "proto2";
    package my.protobuf;
    message SomeMessage {
      optional int32 first = 1;  // example template also works with int64, uint32, uint64
      optional string second = 2;
      repeated string third = 3;
      optional bool fourth = 4;
      message NestedMessage {
        optional sint32 value = 1;
        optional string text = 2;
      }
      optional NestedMessage fifth = 5;
      repeated NestedMessage sixth = 6;
      repeated sint32 seventh = 7;
      repeated sint32 eighth = 8 [packed=true];
    }

The wire format message can be displayed:

    $ protoc --decode=my.protobuf.SomeMessage some_message.proto <some_message.wire
    first: 100
    second: "abcd"
    third: "foo"
    third: "bar"
    fourth: true
    fifth {
      value: -666
      text: "negative 666"
    }
    sixth {
      value: 1234
      text: "onetwothreefour"
    }
    sixth {
      value: 2222
      text: "four twos"
    }
    seventh: 1
    seventh: 2
    seventh: 3
    seventh: 4
    eighth: 100
    eighth: -200
    eighth: 1000

and the actual message would be created in Python by code similar to this:

    import some_message_pb2

    msg = some_message_pb2.SomeMessage()
    msg.first = 100
    msg.second = "abcd"
    msg.third[:] = ["foo", "bar"]
    msg.fourth = True
    msg.fifth.value = -666
    msg.fifth.text = "negative 666"

    m1 = msg.sixth.add()
    m1.value = 1234
    m1.text = "onetwothreefour"
    msg.sixth.append(msg.NestedMessage(value=2222, text="four twos"))
    msg.seventh.extend([1,2,3,4])
    msg.eighth.extend([100,-200,1000])

or

    msg2 = some_message_pb2.SomeMessage(
        first = 100,
        second = "abcd",
        third = ["foo", "bar"],
        fourth = True,
        fifth = some_message_pb2.SomeMessage.NestedMessage(value=-666, text="negative 666"),
        sixth = [some_message_pb2.SomeMessage.NestedMessage(value=1234, text="onetwothreefour"),
                 some_message_pb2.SomeMessage.NestedMessage(value=2222, text="four twos")],
        seventh = [1,2,3,4],
        eighth = [100,-200,1000],
        )

Note that the fields can be in any order (they are disambiguated by their tags) and if there is no value for a field, it would be simply omitted in the template. The field names and message names can be changed without any change to the wire format.

#### 1.4.1 Wiretypes

The wire-stream consists of six primitive payload types, two of which have been deprecated. A primitive in the wire-stream is a multi-byte string that provides three pieces of information: a wire-type, a user-specified tag (field number), and the raw payload. Except for the tag and its wire-type, protobuf payloads are not instantaneously recognizable because the wire-stream contains no payload type information. The interpreter uses the tag to associate the raw payload with a local host type specified by the template. Hence, the message can only be properly decoded using the template that was used to encode it. Note also that the primitive is interpreted according to the needs of a local host. Local word-size and endianness are dealt with at this level.

The following table shows the association between the types in the .proto file and the primitives used in the wire-stream. For how these correspond to other programming languages, such as C++, Java, etc. see Protocol Buffers Scalar Value Types, which also has advice on how to choose between the various integer types. (Python3 types are also given here, because Python is used in some of the interoperability tests.)

> Prolog | Wirestream | .proto file | C++ | Python3 | Notes
> double | fixed64 | double | double | float |
> unsigned64 | fixed64 | fixed64 | uint64 | int |
> integer64 | fixed64 | sfixed64 | int64 |  |
> float | fixed32 | float | float | float |
> unsigned32 | fixed32 | fixed32 | uint32 | int |
> integer32 | fixed32 | sfixed32 | int32 |  |
> integer | varint | sint32 | int32 | int | 1, 2, 9
> integer | varint | sint64 | int64 | int | 1, 2, 9
> signed32 | varint | int32 | int32 | int | 2, 3, 10
> signed64 | varint | int64 | int64 | int | 2, 3, 10
> unsigned | varint | uint32 | uint32 | int | 2, 3
> unsigned | varint | uint64 | uint64 | int | 2, 3
> boolean | varint | bool | bool | bool | 2, 8
> enum | varint | (enum) | (enum) | (enum) |
> atom | length delimited | string |  | str (unicode) |
> codes | length delimited | bytes |  | bytes |
> utf8_codes | length delimited | string |  | str (unicode) |
> string | length delimited | string | string | str (unicode) |
> embedded | length delimited | message |  | (class) | 5
> repeated | length delimited | repeated |  | (list) | 6
> repeated_embedded | length delimited | repeated |  | (list) | 11
> packed | length delimited | packed repeated |  | (list) |

Notes:

1. Encoded using a compression technique known as zig-zagging, which is more efficient for negative values, but which is slightly less efficient if you know the values will always be non-negative.
2. Encoded as a modulo 128 string. Its length is proportional to its magnitude. The intrinsic word length is decoupled between parties. If zig-zagging is not used (see note 1), negative numbers become maximum length.
3. SWI-Prolog has unbounded integers, so an unsigned integer isn't a special case (it is range-checked and an exception thrown if its representation would require more than 32 or 64 bits).
4. Encoded as UTF8 in the wire-stream.
5. Specified as `embedded(Tag,protobuf([...]))`.
6. Specified as `repeated(Tag,Type([...,...]))`, where Type is =unsigned, `integer`, `string`, etc.
7. `repeated ... [packed=true]` in proto2. Can not contain "length delimited" types.
8. Prolog `boolean(Tag,false)` maps to 0 and `boolean(Tag,true)` maps to 1.
9. Uses "zig-zag" encoding, which is more space-efficient for negative numbers.
10. The documentation says that this doesn't use "zig-zag" encoding, so it's less space-efficient for negative numbers. In particular, both C++ and Python encode negative numbers as 10 bytes, and this implementation does the same for wire-stream compatibility (note that SWI-Prolog typically uses 64-bit integers anyway). Therefore, signed64 is used for both .proto types `int32` and `int64`.
11. Specified as `repeated_embedded(Tag,protobuf([...]),Fields)`

#### 1.4.2 Tags (field numbers)

A tag (or field number) is a small integer that is present in every wire-stream primitive. The tag is the only means that the interpreter has to synchronize the wire-stream with its template. Tags are user defined for each term in each message of the wire-stream. The protobuf specification requires that each field within a message has a unique field number; the protobuf compiler (`protoc`) will produce an error if a field number is reused (field numbers are unique only within a message; an embedded message can use the same field numbers without ambigituity).

#### 1.4.3 Basic Usage

A protobuf wire-stream is a byte string that is comprised of zero or more of the above multi-byte wire-stream primitives. Templates are lists of Prolog terms. Each term corresponds to a production rule in the DCG. The purpose of the template is to provide a recipe and value set for encoding and decoding a particular message. Each term in the template has an arity of two. The term's functor is the local "host type". Argument 1 is its tag (field number), which must always be ground, and argument 2 is its associated value, which may or may not be ground.

A protobuf "message" is a list of fields and is encoded in the template as `protobufs([Field1, Field2, ...])`, where each field is of the form `Type(Tag,Value` and `Type` can be any scalar or compound type.

Scalar fields are encoded as `Type(Tag,Value)`. For example, if a field is defined in a .proto file by `optional string some_field = 10`, then it could be encoded by `string(10,"some field's contents")` or by `atom(10, 'some field\'s contents')`.

Repeated fields are done by `repeated(Tag,Type([Value1,Value2,...])`, where `Type` is any type.

Embedded messages are done by `embedded(Tag,protobuf([Field1,Field2,...]))` (this is the same =`protobuf(...)`= as is used at the top level).

Repeated embedded messages are done by `repeated_embedded(Tag,protobuf([Field1,Field2,...]),Fields)`, which gets repeated items and combines them into a list. For example, `repeated_embedded(Tag, protobuf([string(1,_Key),string(2,_Value)]), Fields)` could unify `Fields` to `[protobuf([string(1,"key1"),string(2,"value1")]), protobuf([string(1,"key2"),string(2,"value2")])]`. Note that the variables in the `protobuf` part of the term do not get instantiated: they are similar to the `Template` in findall/3 and similar.

Note: It is an error to attempt to encode a message using a template that is not ground. Decoding a message into a template that has unbound variables has the effect of unifying the variables with their corresponding values in the wire-stream.

Assume a .proto definition:

    message Command {
      optional string msg_type = 1;
      optional string command  = 2;
      optional int32  x        = 3;
      optional int32  y        = 4;
    }

Map a Prolog structure to a Protocol Buffer:

    %! command(+Term, -Proto) is det.
    % Map a Prolog term to a corresponding protobuf term.
    command(add(X,Y), Proto) :-
       freeze(X, must_be(integer, X)),  % for debugging
       freeze(Y, must_be(integer, Y)),  % for debugging
       Proto = protobuf([atom(1, command),
                         atom(2, add),
                         integer(3, X),
                         integer(4, Y)
                        ]).

Later on:

       ... prepare X, Y for command/2 ...

       command(add(X,Y), Proto),
       protobuf_message(Proto, WireCodes),

       % send the message
       open('filename', write, Stream, [encoding(octet),type(binary)]),
       format(Stream, '~s', [WireCodes]),
       close(Stream)

`Proto` is the protobuf template. Each template describes exactly one message. `WireCodes` is the wire-stream, which encodes bytes (values between 0 and 255 inclusive). If you are interworking with other systems and languages, then the protobuf templates that you supply to protobuf_message/2 must be equivalent to those described in the .proto file that is used on the other side.

#### 1.4.4 Alternation, Aggregation, Encapsulation, and Enumeration

Alternation

The protobuf grammar provides a reserved word, `optional`, that indicates that the production rule that it refers to may appear once or not at all in a protobuf message. Since Prolog has its own means of alternation, this reserved word is not supported on the Prolog side. It is anticipated that customary Prolog mechanisms for nondeterminism (e.g. backtracking) will be used to generate and test alternatives.

Note that `required` and `optional` have been removed from the proto3 specification, making all fields optional. This has been partially revised in releases 3.12 and later. In general, you should not expect any field to exist, nor can you expect a repeated field to have at least one item.

Also note that the handling of missing fields is slightly different in proto2 and proto3 -- proto2 allows specifying a default value but proto3 uses 0 and =""= as defaults for numbers and strings and omits encoding any field that has one of those default values.

`TODO`: determine correct behvaior for `oneof` with a default field value.Aggregation

It is possible to specify homogeneous vectors of things (e.g. lists of numbers) using the `repeated` attribute. You specify a repeated field as follows:

        repeated(22, float([1,2,3,4])),
        repeated(23, enum(tank_state([empty, half_full, full]))).

The first clause above will cause all four items in the list to be encoded in the wire-stream as IEEE-754 32-bit floating point numbers, all with tag 22. The decoder will aggregate all items in the wire-stream with tag 22 into a list as above. Likewise, all the items listed in the second clause will be encoded in the wire-stream according to the mapping defined in an enumeration (described below) tank_state/2, each with tag 23.

You can also encode vectors of embedded messages using `repeated_embedded`. This uses a "template" for the individual messages and a list of messages in the wire stream. For example: `repeated_embedded(Tag, protobuf([string(1,_Key),string(2,_Value)]), Fields)` where `Fields` gets a list (possibly empty), with each item of the form `protobuf([string(1,_Key),string(2,_Value)])`.

Notes:

Beware that there is no explicit means to encode an empty set. The protobuf specification provides that a `repeated` field may match a tag zero or more times. The empty set, while legal, produces no output on encode. While decoding a `repeated` term, failure to match the specified tag will yield an empty set of the specified host type.

An omitted `optional` field is handled the same way as a `repeated` field with an empty set.

The protobuf grammar provides a variant of the `repeated` field known as "packed." This is represented similar to `repeated`, e.g.:

        packed(22, float([1,2,3,4])),
        packed(23, enum(tank_state([empty, half_full, full]))).

Handling missing fields

For input, you can wrap fields in `repeated`, so that if a field is there, it gets a length-1 list and if it's missing, an empty list:

    ?- Codes = [82,9,105,110,112,117,116,84,121,112,101],
       protobuf_message(protobuf([embedded(10, protobuf([repeated(13, integer64(I))]))]),  Codes),
       protobuf_message(protobuf([embedded(10, protobuf([repeated(13, double(D))]))]),  Codes),
       protobuf_message(protobuf([repeated(10, string(S))]), Codes).
    I = [7309475598860382318],
    D = [4.272430685433854e+180],
    S = ["inputType"].

    ?- Codes = [82,9,105,110,112,117,116,84,121,112,101],
          protobuf_message(protobuf([repeated(10, string(S)),
                                     repeated(11, integer64(I))]), Codes).
    S = ["inputType"],
    I = [].

This technique can also be used for output - a missing field simply produces nothing in the wire format:

    ?- protobuf_message(protobuf([repeated(10, string([]))]), Codes).
    Codes = [].
    ?- protobuf_message(protobuf([repeated(10, string(S))]), []).
    S = [].

Encapsulation and Enumeration

It is possible to embed one protocol buffer specification inside another using the `embedded` term. The following example shows a vector of numbers being placed in an envelope that contains a command enumeration.

Enumerations are a compact method of sending tokens from one system to another. Most occupy only two bytes in the wire-stream. An enumeration requires that you specify a callable predicate like commands/2, below. The first argument is an atom specifying the name of token, and the second is an integer that specifies the token's value. These must of course, match a corresponding enumeration in the .proto file.

Note: You must expose this predicate to the protobufs module by assigning it explicitly.

    protobufs:commands(Key, Value) :-
        commands(Key, Value).

    commands(square, 1).
    commands(decimate, 2).
    commands(transform, 3).
    commands(inverse_transform, 4).

    basic_vector(Type, Proto) :-
        vector_type(Type, Tag),
        Proto = protobuf([ repeated(Tag, Type) ]).

    send_command(Command, Vector, WireCodes) :-
        basic_vector(Vector, Proto1),
        Proto = protobuf([enum(1, commands(Command)),
                          embedded(2, Proto1)]),
        protobuf_message(Proto, WireCodes).

Use it as follows:

    ?- send_command(square, double([1,22,3,4]), WireCodes).
    WireCodes = [8, 1, 18, 36, 17, 0, 0, 0, 0, 0, 0, 240, 63, 17, 0, 0, 0, 0, 0,
    0, 54, 64, 17, 0, 0, 0, 0, 0, 0, 8, 64, 17, 0, 0, 0, 0, 0, 0, 16, 64].

    ?- send_command(Cmd, V, $WireCodes).
    Cmd = square,
    V = double([1.0, 22.0, 3.0, 4.0]).

Compatibility Note: The protobuf grammar (protobuf-2.1.0) permits enumerations to assume negative values. This requires them to be encoded as integers. Google's own Golden Message unit-test framework has enumerations encoded as regular integers, without the "zigzag" encoding. Therefore, negative values are space-inefficient, but they are allowed.

An earlier version of protobuf_message/2 assumed that enumeration values could not be zero, and there might still be incorrect assumptions in the code, resulting in either exceptions or silent failure.Heterogeneous Collections

Using Protocol Buffers, it is easy to specify fixed data structures and homogeneous vectors like one might find in languages like C++ and Java. It is however, quite another matter to interwork with these languages when requirements call for working with compound structures, arrays of compound structures, or unstructured collections (e.g. bags) of data.

At bottom, a wire-stream is nothing more than a concatenated stream of primitive wire type strings. As long as you can associate a tag with its host type in advance, you will have no difficulty in decoding the message. You do this by supplying the structure. Tell the parser what is possible and let the parser figure it out on its own, one production at a time. An example may be found in the appendix.

#### 1.4.5 Groups (deprecated)

Protocol Buffer Groups provide a means for constructing unitary messages consisting of ad-hoc lists of terms. The following protobuf fragment shows the definition of a group carrying a complex number.

         Proto = group(2, [ double(1, Real_part), double(2, Img_part) ]).

Groups have been replaced by embedded messages, which are slightly less expensive to encode.

#### 1.4.6 Advanced Topics

Precompiled Messages

Performance can be improved using a strategy of precompiling the constant portions of your message. Enumerations for example, are excellent candidates for precompilation. Using protobuf_message/3, the precompiled portion of the message is inserted directly in the wire-stream on encode, and is unified with, and removed from the wire-stream on decode. The following shows how the "send_command" example above, can be converted to precompiled form:

    send_precompiled_command(Command, Vector, WireCodes) :-
        basic_vector(Vector, Proto1),
        % precompiled_message/3 is created by term_expansion
        precompiled_message(commands(Command), WireCodes, Tail),
        protobuf_message(protobuf([embedded(3, Proto1)]), Tail).

    term_expansion(precompile_commands, Clauses) :-
        findall(precompiled_message(commands(Key), WireCodes, Tail),
                (   protobufs:commands(Key, _),
                    Proto = protobuf([atom(1, command),
                                      enum(2, commands(Key))]),
                    protobuf_message(Proto, WireCodes, Tail)
                ),
                Clauses).

    *
    *
    *
    precompile_commands.  % Trigger the term-expansion precompilation

Supplying Your Own Host Type Message Sequences

You can extend the parser to support your own compound host types. These are treated as first class entities by the parser. That is they can be used either by themselves, or in `repeated` and `embedded` clauses just as any other host type would be. You do this by hooking into the parser and adding your own `message_sequence` productions. Your hook eventually calls back into the parser with your substitution/expansion protobuf, which is then embedded in the wire stream. Recursive structures can be defined this way. A simple example of a recursive XML like structure is shown in the appendix.

## 2 Appendix

### 2.1 Example: A Simple XML Like Structure

This is an example of using the low-level interface for implementing a domain-specific language that maps to protobufs.

In this example we demonstrate managing a recursive structure like XML. The structure shown in xml_proto/1 below, is similar to the structure returned by load_xml_file/2, which is part of the SGML library. We supply three `message_sequence` decorators: `kv_pair`, `xml_element`, and `aux_xml_element`. These are treated as first class host types.

    :- multifile protobufs:message_sequence//3.

    protobufs:message_sequence(Type, Tag, Value)  -->
        { my_message_sequence(Type, Value, Proto) },
        protobufs:message_sequence(embedded, Tag, Proto), !.
    %
    % On encode, the value type determines the tag. And on decode
    % the tag to determines the value type.
    %

    guard(Type, Value) :-
        ( nonvar(Value) -> is_of_type(Type, Value); true ).

    my_message_sequence(kv_pair, Key=Value, Proto) :-
        Proto = protobuf([atom(30, Key), X]),
        ( ( guard(integer, Value), X = integer(31, Value) )
        ; ( guard(float, Value),   X = double(32,  Value) )
        ; ( guard(atom, Value),    X = atom(33,    Value)) ).

    my_message_sequence(xml_element,
                        element(Name, Attributes, Contents), Proto) :-
        Proto = protobuf([ atom(21, Name),
                           repeated(22, kv_pair(Attributes)),
                           repeated(23, aux_xml_element(Contents))]).

    my_message_sequence(aux_xml_element, Contents, Proto) :-
        Contents = element(_Name, _Attributes, _ElementContents),
        Proto = protobuf([xml_element(40, Contents)]).

    my_message_sequence(aux_xml_element, Contents, Proto) :-
        Proto = protobuf([atom(43, Contents)]).

    xml_proto([element(space1,
                       [foo='1', bar='2'],
                       [fum,
                        bar,
                        element(space2,
                                [fum=3.1415, bum= -14],
                                ['more stuff for you']),
                        element(space2b,
                                [],
                                [this, is, embedded, also]),
                        to,
                        you])]).

    test_xml(X, Y) :-
        Proto = protobuf([repeated(20, xml_element(X))]),

        protobuf_message(Proto, Y).

    % And test it:

    ?- xml_proto(X), test_xml(X,Y), test_xml(Z,Y), Z == X.
    X = Z,
    Z = [element(space1,
                 [foo='1', bar='2'],
                 [fum,
                  bar,
                  element(space2,
                          [fum=3.1415, bum= -14],
                          ['more stuff for you']
                        ),
                  element(space2b,
                          [],
                          [this, is|...]
                         ),
                  to,
                  you])],
    Y = [162, 1, 193, 1, 170, 1, 6, 115, 112|...],

A protobuf description that is compatible with the above wire stream follows:

    message kv_pair {
      required string key = 30;
      optional sint64  int_value = 31;
      optional double float_value  = 32;
      optional string atom_value = 33;
    }

    message aux_xml_element {
      optional string atom = 43;
      optional xml_element element = 40;
    }

    message xml_element {
      required string name = 21;
      repeated kv_pair attributes = 22;
      repeated aux_xml_element contents = 23;
    }

    message XMLFile {
      repeated xml_element elements = 20;
    }

Verify the wire stream using the protobuf compiler's decoder:

    $ protoc --decode=XMLFile pb_vector.proto <tmp98.tmp
    elements {
      name: "space1"
      attributes {
        key: "foo"
        atom_value: "1"
      }
      attributes {
        key: "bar"
        atom_value: "2"
      }
      contents {
        atom: "fum"
      }
      contents {
        atom: "bar"
      }
      contents {
        element {
          name: "space2"
          attributes {
            key: "fum"
            float_value: 3.1415
          }
          attributes {
            key: "bum"
            int_value: -14
          }
          contents {
            atom: "more stuff for you"
          }
        }
      }
      contents {
        element {
          name: "space2b"
          contents {
            atom: "this"
          }
          contents {
            atom: "is"
          }
          contents {
            atom: "embedded"
          }
          contents {
            atom: "also"
          }
        }
      }
      contents {
        atom: "to"
      }
      contents {
        atom: "you"
      }
    }

### 2.2 Example: Vectors of Numbers

This is an example of using the low-level interface.

In the Prolog client:

    vector_type(double(_List), 2).
    vector_type(float(_List), 3).
    vector_type(integer(_List), 4).
    vector_type(integer64(_List), 5).
    vector_type(integer32(_List), 6).
    vector_type(unsigned(_List), 7).
    vector_type(codes(_List), 8).
    vector_type(atom(_List), 9).
    vector_type(string(_List), 10).

    vector(Type, B):-
        vector_type(Type, Tag),
        Proto = protobuf([ repeated(Tag, Type) ]),
        protobuf_message(Proto, B).

A protobuf description that is compatible with the above wire stream follows:

      message Vector {
      repeated double double_values     = 2;
      repeated float float_values       = 3;
      repeated sint32 integer_values    = 4;
      repeated fixed64 integer64_values = 5;
      repeated fixed32 integer32_values = 6;
      repeated uint32 unsigned_values   = 7;
      repeated bytes bytes_values       = 8;
      repeated string atom_values       = 9;
      repeated string string_values     = 10;
      }

A typical application might consist of an abstract adapter class along with a collection of concrete subclasses that refine an abstract behavior in order to hide the interaction with the underlying protobuf interpreter. An example of such a class written in C++ may be found in the demos.

On the Prolog side:

      :- meta_predicate ~>(0,0).
      :- op(950, xfy, ~>).

      ~>(P, Q) :-
        setup_call_cleanup(P, (true; fail), assertion(Q)).

      write_as_proto(Vector) :-
        vector(Vector, WireStream),
        open('tmp99.tmp', write, S, [encoding(octet),type(binary)])
          ~> close(S),
        format(S, '~s', [WireStream]), !.

      testv1(V) :-
        read_file_to_codes('tmp99.tmp', Codes, [encoding(octet),type(binary)]),
        vector(V, Codes).

Run the Prolog side:

    ?- X is pi,
       write_as_proto(double([-2.2212, -7.6675, X, 0, 1.77e-9, 2.54e222])).
    X = 3.14159.

    ?- testv1(Vector).
    Vector = double([-2.2212, -7.6675, 3.14159, 0.0, 1.77e-09, 2.54e+222])
    ?-

Verify the wire stream using the protobuf compiler's decoder:

    $ protoc --decode=Vector pb_vector.proto <tmp99.tmp
    double_values: -2.2212
    double_values: -7.6675
    double_values: 3.1415926535897931
    double_values: 0
    double_values: 1.77e-09
    double_values: 2.5400000000000002e+222

### 2.3 Example: Heterogeneous Collections

This is an example of using the low-level interface.

The following example shows how one can specify a Protocol Buffer message that can deal with variable-length, unstructured bags of numbers:

    compound_protobuf(complex(Real, Img), group(12, [double(1, Real), double(2, Img)])).
    compound_protobuf(float(Val), float(13, Val)).
    compound_protobuf(double(Val), double(14, Val)).
    compound_protobuf((Num rdiv Den), group(15, [integer(1, Num), integer(2, Den)])).
    compound_protobuf(integer(Val), integer(16, Val)).

    protobuf_bag([], []).

    protobuf_bag([ Type | More], WireCodes) :-
        compound_protobuf(Type, X),
        Proto = protobuf([embedded(1, protobuf([X]))]),
        protobuf_message(Proto, WireCodes, WireCodes1),
        protobuf_bag(More, WireCodes1), !.

Use it as follows:

    ?- protobuf_bag([complex(2,3), complex(4,5),
                     complex(6,7), 355 rdiv -113, integer(11)], X).

    X = [10, 20, 99, 9, 0, 0, 0, 0, 0|...].

    ?- protobuf_bag(Y, $X).
    Y = [complex(2.0, 3.0), complex(4.0, 5.0),
         complex(6.0, 7.0), 355 rdiv -113, integer(11)].

A protobuf description that is compatible with the above wire stream follows:

    message compound_protobuf {
    optional group Complex = 12 {
        required double real = 1;
        required double img = 2;
    };
    optional group Fraction = 15 {
        required sint64 num = 1;
        required sint64 den = 2;
    };
    optional float float = 13;
    optional double double = 14;
    optional sint32 integer = 16;
    }

    message protobuf_bag {
        repeated compound_protobuf bag = 1;

Verify the wire stream using the protobuf compiler's decoder:

    $ protoc --decode=protobuf_bag pb_vector.proto <tmp96.tmp
    bag {
      Complex {
        real: 2
        img: 3
      }
    }
    bag {
      Complex {
        real: 4
        img: 5
      }
    }
    bag {
      Complex {
        real: 6
        img: 7
      }
    }
    bag {
      Fraction {
        num: 355
        den: -113
      }
    }
    bag {
      integer: 11
    }

## 3 library(protobufs): Google's Protocol Buffers ("protobufs")

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

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 (section 1) 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`

[semidet]protobuf_parse_from_codes(+WireCodes:list(int), +MessageType:atom, -Term)
  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.

  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 (section 1.3.1)
  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).
[det]protobuf_serialize_to_codes(+Term:dict, -MessageType:atom, -WireCodes:list(int))
  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).

  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 (section 1.3.1)
  bug
    - `map` fields don't get special treatment (but see protobuf_map_pairs/3).
    - `oneof` is not checked for validity.
[semidet]protobuf_message(?Template, ?WireStream)
[nondet]protobuf_message(?Template, ?WireStream, ?Rest)
  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.

  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
[semidet]protobuf_field_is_map(+MessageType, +FieldName)
  Succeeds if `MessageType`’s `FieldName` is defined as a map<...> in the .proto file.
[det]protobuf_map_pairs(+ProtobufTermList:list, ?DictTag:atom, ?Pairs)
  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.

# Index

?
protobuf_field_is_map/2
protobuf_map_pairs/3
protobuf_message/2
protobuf_message/3
protobuf_parse_from_codes/3
protobuf_serialize_to_codes/3