Piqi for Erlang User’s Manual

Table of Contents

 1. Overview

 2. Piqi compiler and generated Erlang code

      2.1. Command-line parameters

 3. Piqi to Erlang mapping

      3.1. Modules

           3.1.1. Includes

           3.1.2. Imports

      3.2. Primitive types

      3.3. User-defined types

      3.4. Custom Erlang types

      3.5. Piqi extensions

 4. Examples

 5. Limitations

 6. Supported Erlang and Protocol Buffers versions

1. Overview

Piqi includes a data serialization system for Erlang. It can be used for serializing Erlang values in 4 different formats: Google Protocol Buffers, JSON, XML and Piq.

A typical Piqi usage scenario involves the following steps:

1. Include Piqi as a rebar depedency : add this entry to your rebar.config file:

{deps, [
    ...
    {piqi, "", {git, "git://github.com/alavrik/piqi-erlang.git", {branch, "master"}}},
    ...
]}.
2. Describe data structures using the Piqi data definition language

The Piqi data definition language can describe many Erlang types, both primitive and user-defined. This includes integers, floats, booleans, strings, binaries, lists, records and variants (i.e. {tag, Value} tuples).

In addition to types supported by default, Piqi has a mechanism for adding support for arbitrary Erlang types. It can be used, for example, to add support for Erlang’s bigints or any().

Refer to the "Piqi to Erlang mapping" section below for details.

3. Call the Piqi compiler to generate Erlang type definitions and serialization code

See the next section for detailed description.

4. Use generated serializes/deserializers in a user’s program

the desired serialization format can be specified at runtime.

The Examples section contains links to several sample Erlang projects that use Piqi for data serialization and demonstrate steps 2–4.

2. Piqi compiler and generated Erlang code

Piqi compiler for Erlang generates Erlang header files and Erlang code for Protocol Buffers, JSON, XML and Piq serialization.

piqic-erlang command takes a Piqi module <dir path>/<file>.piqi and produces two files in the current directory: <erlang-module>.erl and <erlang-module>.hrl.

For each specified, imported or included module <m>.piqi, the compiler tries to load and automatically include <m>.erlang.piqi. This mechanism is called _Extension Modules_. It is described in detail in the Piqi language section.

By default, <erlang-module> is set to <file>_piqi. It can also be specified explicitly (see below). Output directory may be overridden by using the -C command-line option.

Generated <erlang-module>.hrl file contains Erlang type and record definitions.

Generated <erlang-module>.erl file includes <erlang-module>.hrl and contains functions for serializing and deserializing Erlang values.

Generated <erlang-module>.hrl file contains Erlang type definitions and functions for serializing and deserializing Erlang values.

For each defined data type <typename>, piqic-erlang will produce several functions:

  • gen_<typename> — for serializing a value of this type to Protocol Buffers

  • gen_<typename>/2 — for serializing a value of this type to Protobuf, JSON, XML and Piq

  • gen_<typename>/3 — same as gen_<typename>/2 but with extra options

  • parse_<typename> — for deserializing from Protocol Buffers

  • parse_<typename>/2 — for deserializing from Protobuf, JSON, XML and Piq

  • parse_<typename>/3 — same as parse_<typename>/2 but with extra options

  • default_<typename> — returns the minimal serializable instance of this type

The parse_<typename>/2 and gen_<typename>/2 functions from this module take an additional parameter specifying which serialization format to use:

-type input_format() :: bp | json | xml | piq.
-type output_format() :: input_format() | 'json_pretty' | 'xml_pretty'.

There are two variants of parse and gen functions. The simple variant accept two arguments as described above. parse_<typename>/3 and gen_<typename>/3 accept an additional third argument representing a list of serialization options:

% Serialization options to be passed as an argument to * gen_<typename>/3 and
% parse_<typename>/3 functions
%
% pretty_print
%
%      Pretty-print generated JSON and XML output (default = true)
%
% json_omit_missing_fields
%
%      Omit missing optional and empty repeated fields from JSON
%      output instead of representing them as {"field_name": null} and
%      {"field_name", []} JSON fields (default = true)
%
% use_strict_parsing
%
%      Treat unknown and duplicate fields as errors when parsing JSON,
%      XML and Piq formats (default = false)
%
% piq_frameless_output
%
%      Print a frame (i.e. :<typename> []) around a single output Piq object
%      (default=false)
%
% piq_frameless_input
%
%      Expect a frame around a single input Piq object (default=false)
%
% piq_relaxed_parsing
%
%      Parse Piq format using "relaxed" mode (default=false);
%
%      For instance, when set to `true`, single-word string literals don't have
%      to be quoted
%
-type piqi_convert_option() ::
       'pretty_print'
    | {'pretty_print', boolean()}
    |  'json_omit_missing_fields'
    | {'json_omit_missing_fields', boolean()}
    |  'use_strict_parsing'
    | {'use_strict_parsing', boolean()}
    |  'piq_frameless_output'
    | {'piq_frameless_output', boolean()}
    |  'piq_frameless_input'
    | {'piq_frameless_input', boolean()}
    |  'piq_relaxed_parsing'
    | {'piq_relaxed_parsing', boolean()}.

2.1. Command-line parameters

piqic-erlang accepts the following command-line parameters.

  • -C <dir> — specify output directory for the generated .erl and .hrl files.

  • -I <dir> — add directory to the list of imported .piqi search paths

  • -e <name> — try including extension for all loaded modules (can be used several times)

  • --normalize-names true|false — convert "CamelCase"-style identifiers from the original type spec into "camel-case" Erlang names. When the argument is false, the original identifiers will be lowercased without performing any additional transformations, e.g. "CamelCase" turns into "camelCase". The default value is true.

  • --gen-preserve-unknown-fields — generate code that preserves unknown Protobuf fields when they are serialized back. When enabled, unknown (unrecognized) Protobuf fields are captured during de-serialization in a special ‘piqi_unknown_pb’ field and automatically written back when the record is serialized to Protobuf.

  • --trace — turn on tracing (verbose output)

  • --no-warnings — don’t print warnings

  • -h, --help — print command-line options help

3. Piqi to Erlang mapping

The following sections describe how different Piqi constructs such as modules and types are mapped to Erlang.

3.1. Modules

The name of Erlang module is derived from Piqi module name, unless overridden by erlang-module top-level field.

If Piqi module’s name is "example.com/foo/bar", then "bar" (the last part of Piqi module name) will be used as Erlang module name. It is possible to override such default name assignment by specifying .erlang-module "<other-name>" in the Piqi module.

All type and record names in generated .hrl file are prefixed with Erlang module’s name to avoid name conflicts between types defined in different modules. For example, if a Piqi module named mod defines type foo, the resulting .hrl file will contain this type defined as mod_foo.

Sometimes prefixing type names with long module names can produce really long identifiers, which may be inconvenient, — especially for record names. Piqi provides a mechanism for shortening such prefixes.

Instead of using module names as prefixes, it is possible to specify custom prefix using top-level erlang-type-prefix property. For example, if we have a some-long-modname module we can defined a shorted type prefix:

.erlang-type-prefix "m_"

That will result in shorter Erlang names for types defined in that module, as they will be prefixed by "m_" instead "some_long_modname_".

It is also possible to specify an empty prefix which may be useful for short standalone programs.

3.1.1. Includes

There is no direct representation of Piqi includes in Erlang. Piqic takes all "include" directives of a Piqi module, resolves them internally and produces a compound Piqi module which is then mapped to the resulting Erlang module.

3.1.2. Imports

Piqi "import" directives are mapped to Erlang includes in the following way.

For example, we have two Piqi modules: a.piqi and b.piqi, and Piqi module b imports module a using Piqi "import" directive:

.import [ .module b ]

After running piqic-erlang on each of these modules, we will get 4 files: a_piqi.hrl, a_piqi.erl, b_piqi.hrl, b_piqi.erl, where b_piqi.hrl includes a_piqi.hrl and b_piqi.erl uses functions from a_piqi.erl to serialize and deserialize values of types that were defined in module a.

Local Piqi import names defined using name attribute of import directive are ignored as Erlang doesn’t have correspondent constructs.

3.2. Primitive types

The table below represents correspondence between Piqi primitive types and Erlang types.

(Mapping between Piqi and Protocol Buffers primitive type is documented here).

Piqi type(s) Erlang type Protobuf type(s)
bool boolean() bool
string string() | binary() string
binary binary() bytes
int, int32, int64, int32-fixed, int64-fixed, protobuf-int32, protobuf-int64 integer() sint32, sing64, sfixed32, sfixed64, int32, int64
uint, uint32, uint64, uint32-fixed, uint64-fixed non_negative_integer() uint32, uing64, fixed32, fixed64
float, float64, float32 number() double, float

string and float types are treated slightly differently during serialization and deserialization. For instance, when serializing float value, it can be passed as Erlang number(), that is can be either float() or integer(). However, when deserializing float values, they are always returned as Erlang float().

Similarly, when serializing string value, it can be passed either as Erlang string() (i.e. as a list of integers representing Unicode codepoints) or as Erlang binary() representing a valid utf8-encoded string.

When deserializing strings, they will be returned as binaries by default. This behavior can be changed to return lists by setting the following top-level property in the Piqi module:

.erlang-string-type.list

Note that the above property affects string deserialization behavior for all data types defined in the module along with their content (i.e. fields and options).

If there is a need to add serialization support for other Erlang types, such as atom(), reference() or "big" integers, refer to Custom Erlang types section which describes a method for mapping custom Erlang types to Piqi types.

3.3. User-defined types

  • Type names

    Each user-defined type is identified by its name. Piqi type names are converted to Erlang type name using the following rule:

    By default, Piqi identifiers are normalized and all hyphen characters are replaced by underscores. Normalization means converting "CamelCase" to "camel-case".

    If --normalize false command-line option is specified, then instead of full normalization, the first letter of the type name is uncapitalized.

    Sometimes it is necessary to override this rule and specify a custom Erlang name for a type. For example, Piqi type name can conflict with one of Erlang keywords. In such case, custom Erlang name can be specified using .erlang-name "<erlang name>" field next to the original .name <name> entry. (This feature also works for field names, option names and import names.)

    For those Piqi fields or options which do not specify names, Erlang name is derived from the name of the Piqi type for that field.

  • Records are mapped to Erlang records.

    For example, Piqi record

    .record [
        .name r
        .field [ .name a .type int ]
        .field [ .name b .type binary .repeated ]
    ]

    will be mapped to the following Erlang record:

    -record(r, { a :: integer(), b :: [ binary() ] }).

    required Piqi fields are mapped directly to Erlang record fields.

    optional Piqi fields of type <t> are mapped to fields with type <t>. If the field value for an optional field is missing (i.e. undefined), the value of such field is set to Erlang atom undefined.

    optional Piqi fields without type (i.e. "flags") are mapped to boolean() Erlang fields. The value of the field will be set to true if flag is present in the record.

    repeated Piqi fields of type <t> are mapped to Erlang fields with type [ <t> ].

    In addition to Erlang record definitions, Erlang type aliases will be generated for each record type. For example, for the above record definition, .hrl file will also include the following Erlang type alias:

    -type(r :: #r{}).

    These type alases are used instead of the original record types in other type definitions. Overall, this mechanism makes it possible to define recursive Erlang records.

  • Enums and Variants are mapped directly to Erlang union types.

    Depending on whether options have associated values or not, union subtypes are represented either by atoms or by {<atom-tag>, <value>} pairs.

    For example, these definitions:

    .enum [
        .name e
        .option [ .name a ]
        .option [ .name b ]
    ]
    .varint [
        .name v
        .option [ .type e ]
        .option [ .name f ]
        .option [ .name i .type int ]
    ]

    are mapped to:

    -type e() :: a | b.
    -type v() :: e() | f | {i, integer()}.
  • List type is mapped Erlang list type.

    For example,

    .list [
        .name l
        .type x
    ]

    is mapped to:

    -type l() :: [ x() ].
  • Aliases are mapped to Erlang type definitions.

    For example,

    .alias [
        .name a
        .type x
    ]

    is mapped to:

    -type a() :: x().

    For aliases, it is possible to specify an optional erlang-default string property. When present, it overrides the Piqi-default value for the type in the generated *_piqi:default_<typename>/0 functions. It is especially useful with custom Erlang types described in the following section.

    Example:

    .alias [
        .name positive-int64
        .type int64
        .erlang-type "piqirun_custom:pos_int64"
    
        % we need to specify a custom default value for this type to prevent
        % Dialyzer and runtime errors caused by the default value of 0
        %
        % NOTE: the value is an arbitrary Erlang expression
    
        .erlang-default "1"
    ]

3.4. Custom Erlang types

Piqi provides a way to define mappings between custom Erlang types and Piqi types. Such mechanism is useful when there is a need to automatically serialize an Erlang type using some relevant Piqi type, but there is no way to describe the desired Erlang type using Piqi.

Inability to use Piqi to define an Erlang type would mean that the Erlang type is either a primitive built-in type that has no direct equivalent in Piqi (e.g. atom or "big" integer), or it is some higher-order type (e.g. function closure or an arbitrary Erlang term).

The mapping mechanism works as follows. Suppose we need to add support for serializing Erlang’s term() type (i.e. any Erlang term) as Piqi binary. This can be done in a few steps:

  1. First, define Piqi alias for such mapping:

    .alias [
        % the new Piqi type
        .name erlang-term
    
        % the original Piqi type
        .type binary
    
        % Erlang type (should be point to the Erlang module with the mapping
        % implementation -- see below)
        .erlang-type "piqirun_custom:term_t"
    
        % optionally, define a custom Erlang name for this type
        % .erlang-name "term_t"
    ]

    (Note that we use term_t instead of term, because Erlang forbids reusing names of built-in types for user-defined types. In particular, it is not allowed to refer to a foreign type as piqirun_custom:term()).

  2. Second, implement runtime functions for mapping the custom Erlang type to the Piqi type:

    In module piqirun_custom.erl:

    -module(piqirun_custom).
    
    -export_type([term_t/0]).
    -export([term_t_to_binary/1, term_t_of_binary/1]).
    
    -type term_t() :: any().
    
    -spec term_t_of_binary/1 :: (binary()) -> any().
    -spec term_t_to_binary/1 :: (any()) -> binary().
    
    term_t_of_binary(X) -> binary_to_term(X).
    
    term_t_to_binary(X) -> term_to_binary(X).

After that, the only thing that’s left is to make piqirun_custom module accessible from your Erlang application.

More examples of how to map various Erlang types to Piqi types can be found here.

3.5. Piqi extensions

There is no direct notion of Piqi extensions in Erlang: Piqi extensions are all resolved and applied to Piqi types before generating Erlang types from them.

4. Examples

  • The first example is based on the "addressbook" example from Protocol Buffers source distribution. It contains Erlang implementation of two simple programs: for adding a record to an addressbook and for listing addressbook contents. The programs implement the same functionality as programs from the Protocol Buffers examples written in C++, Java and Python.

    examples/erlang

  • Data serialization in XML, JSON and Piq formats

    In the same directory, there is the io_json_xml_pb.erl Erlang module. It reads and writes the addressbook data structure from the previous example in various formats.

  • More complicated example demonstrating complex types and module imports

    Piq compiler for Erlang (piqic-erlang) produces Erlang parsers and generators from Piqi self-specification (piqi.piqi). After that, an Erlang program reads (and writes back) Piqi self-specification represented as a binary object.

    tests/erlang_piqi

  • Examples of serializing custom Erlang types using Piqi

    examples/erlang-custom-types

5. Limitations

  • No support for IEEE 754 floating point infinities and NaN.

    These special values are supported by Protocol Buffers, but not by the Erlang language.

    Although it is not hard to add support for serializing/deserializing such values, it is not obvious what would be the best way to represent them in Erlang.

    Currently, Piqi Erlang runtime will throw an exception when it encounters infinities or NaN during deserialization.

The way how Piqi records are mapped to Erlang records introduces several limitations:

  • Limited support for defaults.

    There is no way to tell if the value of an optional field came from the original serialized object or it is the default value.

  • No other dynamic properties.

    For example, in Protocol Buffer, there is a way to get the count of repeated fields and access them using field index.

  • JSON and XML serialization relies on external program (Erlang port).

    The external program is used for converting data between JSON or XML and natively supported Protocol Buffers format.

    Although implementation is fairly optimized and supports running a pool of workers on multi-core, communication over Unix pipe will still cause some latency for JSON or XML serialization.

Other limitations:

  • Piqi runtime library hasn’t been heavily optimized for performance yet.

  • No integer overflow checks in Piqi runtime library during serialization

    Currently, if an integer value doesn’t fit into the range of the specified integer type, it will be silently stripped down.

6. Supported Erlang and Protocol Buffers versions

Piqi works with Erlang >= R13B04 and Protocol Buffers >= 2.3.0