Piqi-RPC: an RPC-over-HTTP system for Erlang

Table of Contents

This is a homepage of Piqi-RPC — an RPC-over-HTTP system for Erlang.

1. Overview

Piqi-RPC gives Erlang developers a convenient and reliable way to connect services written in Erlang with non-Erlang clients using JSON, XML or Google Protocol Buffers over HTTP.

Basically, Piqi-RPC can be viewed as an HTTP gateway for Erlang functions. Any standard HTTP client can communicate with them not knowing anything about Erlang or Piqi-RPC. And the opposite is also true: Erlang functions work with idiomatic Erlang data structures and don’t have to be aware of HTTP or even Piqi-RPC (note that it is still possible to get details of HTTP request from application code).

In addition to calling Erlang functions using standard HTTP, Piqi-RPC also comes with piqi call — a native client for calling Piqi-RPC Erlang functions using command-line interface. As in case of HTTP, Erlang implementation remains completely oblivious about command-line interface and the particular method used for its invocation.

In other words, Piqi-RPC system automates several areas where Erlang developers usually need to write a lot of plumbing code:

Parsing and validating input parameters

Piqi-RPC automatically parses XML, JSON, Protocol Buffers and command-line arguments, validates them and converts to idiomatic Erlang data structures.

Generating output parameters

Again, Piqi-RPC automatically generates XML, JSON and Protocol Buffers from native Erlang representation.

HTTP communication

Piqi-RPC provides fully-compliant HTTP communication layer. It receives and validates HTTP requests, sends responses, handles "Content-Type" and "Accept" headers and generates around 10 different meaningful HTTP response types for various error conditions that can occur during request execution.

Piqi-RPC also handles service registration and dispatching which allows many services to run concurrently using different HTTP URLs.

In the current Piqi-RPC implementation, services are stateless. That is Piqi-RPC doesn’t maintain state between calls and doesn’t perform any request synchronization. If one needs their service to have a state or prevent several functions from being executed concurrently, he would need to implement such functionality manually.

Piqi-RPC can be used for automating only Erlang servers — currently, it doesn’t have support for writing clients.

One of Piqi-RPC fundamental features is the ability to make changes in protocol while staying backward compatible with exiting clients and even servers. Such changes may include adding a new parameter, turning a scalar parameter to a list, or making a required parameter into an optional one.

Although Piqi-RPC currently supports only HTTP, other transports can be easily added if needed. Core Piqi-RPC interfaces are transport-agnostic.

2. Piqi-RPC user guide

General Piqi-RPC usage scenario for an Erlang service implementer works as follows.

  1. Add piqi_rpc Erlang application as a rebar dependency.

  2. Describe service data and functions using the Piqi data definition language.

  3. Call piqic-erlang-rpc Piqi compiler to generate serialization/ deserialization functions and server stubs for Erlang.

  4. Implement server callback functions (i.e. the actual service implementation).

  5. Start the piqi_rpc Erlang application and register the new Piqi-RPC service.

The following sections describe each step in detail.

2.1. Getting Piqi-RPC

2.1.1. Piqi-RPC source code

Piqi-RPC source code and rebar package are available here: https://github.com/alavrik/piqi-rpc.

2.2. Defining a Piqi-RPC service

Refer to the Piqi documentation for information how define Piqi functions and data types.

There is an example of service definition in the Examples section below.

2.3. Calling piqic-erlang-rpc Piqi compiler

piqic-erlang-rpc Piqi compiler should be used in exactly the same way as the basic piqic erlang compiler which is documented here.

In addition to the output Erlang files produced by piqic erlang, piqic-erlang-rpc generates two additional files: <Mod>_piqi_rpc.erl and <Mod>_piqi_impl.hrl.

The first generated file contains Piqi-RPC server stubs.

The second file contains Erlang function specifications (i.e. -spec ...) for the service implementation functions.

2.4. Registering a Piqi-RPC service

Before registering a Piqi-RPC service, make sure that the piqi_rpc Erlang application is running. It can be started by calling


Piqi-RPC service registration is performed by calling the piqi_rpc:add_service(<service-tuple>) function, where <service-tuple> has the following format:

{ImplMod, RpcMod, UrlPath}


{ImplMod, RpcMod, UrlPath, Options}

ImplMod contains implementation of the Piqi-RPC service defined by RpcMod. Note that ImplMod can contain implementations of more than one Piqi-RPC service.

RpcMod is a module generated by the piqic-erlang-rpc compiler from Piqi specification. The module is usually named as <Mod>_piqi_rpc.

UrlPath is a base URL path of the HTTP resource for the Piqi-RPC service. For example, if the URL path is set to bar/foo, the service functions can be called using http://<server>:<port>/bar/foo/<function-name> URL.

Options is a list of service options, see the description of default_service_options configuration entry below for details.


{foo_impl, foo_piqi_rpc, "bar/foo"}

{foo_impl, foo_piqi_rpc, "bar/debug_foo", [
    {pretty_print, true},
    {use_strict_parsing, true}

{foo_impl, foo_piqi_rpc, "bar/production_foo", [
    {pretty_print, false},
    {omit_internal_error_details, true}

2.5. Unregistering a Piqi-RPC service

Piqi-RPC service can be unregistered by calling


2.6. Configuring Piqi-RPC services in Erlang .config files

piqi_rpc Erlang application has the following main configuration entries:

  • rpc_services

    This entry specifies a list of statically configured Piqi-RPC services. Each entry is represented as <service-tuple> which format is described in the previous sections.

  • default_service_options

    This entry defines default options for Piqi-RPC services. Default options defined here can be overridden for a specific service.

    Service options control various aspects of service behavior, such as

    • pretty-print JSON and XML output (default = true)

    • omit fields with ‘null’ and ‘[]’ values from JSON output (default = true)

    • treat unknown and duplicate fields as errors when parsing JSON or XML input (default = false)

    • do not include Erlang stracktraces in HTTP 500 responses when request handler crashes (default = true)

Full documentation for piqi_rpc Erlang application configuration is available at piqi_rpc.app.src.

2.7. Getting details of HTTP request from service implementation code

Piqi-RPC provides a way to access original HTTP request parameters from the application code. This can be useful, for example, if some details of a request or session are passed using HTTP headers.

Before calling a user’s service implementation function, Piqi-RPC saves a Webmachine’s wrq record in the process dictionary so that it can be accessed later using erlang:get(wrq). Refer to the Webmachine documentation for information about how to extract details of HTTP request from the wrq object.

2.8. Modifying details of HTTP response from service implementation code

Similarly to getting details of HTTP request, the same mechanism is applicable to modifying HTTP responses.

Application code can manipulate Webmachine’s wrq data structure and save the modified version version of it by calling erlang:put(wrq).

This can be used, for example, to set custom HTTP response headers, return cookies, etc.

2.9. Making Piqi-PRC calls over HTTP

Piqi-RPC calls are performed as HTTP POST requests. Input parameters for a remote function are included in the post body and can be represented as JSON, XML or Protocol Buffers data structure. Output parameters are sent back in HTTP response body encoded in one of the above formats.

The Encodings section of the Piqi documentation describes the rules how data is represented in JSON, XML and Protocol Buffers format.

2.9.1. Content-Type and Accept headers

Input and output format is controlled by standard HTTP Content-Type and Accept headers.

Piqi-RPC supports the following media types: application/json, application/xml and application/x-protobuf.

If Accept header is not specified in the request, the output will be formatted as JSON.

2.9.2. Success response codes (2XX)

In case of successful function call, one of 200 "OK" or 204 "No Content" status codes is returned.

The 204 "No Content" status code is used only when the function doesn’t have output parameters.

2.9.3. Error response codes (4XX)

The following response codes can be generated by Piqi-RPC. In most cases, HTTP response will contain a body with a plain-text description of the problem that triggered the error response.

400 “Bad Request”

This status code is used when something is wrong with the request, for example, when input parameters do not conform to the interface definition.

404 “Not Found”

This status code is returned if a function is called what wasn’t defined by the Piqi interface definition.

405 "Method not allowed"

This status code is returned when other HTTP request method than POST or GET is called.

406 "Not Acceptable"

This status code is used when the request’s Accept header doesn’t allow to choose one of the supported media types for the response.

415 “Unsupported media type”

This status code is returned when Content-Type header of the request is not one of the supported media types.

411 “Length required”

This status code is used when the body of the request is empty and the called function is expected to have non-emtpy input.

2.9.4. Error response codes (5XX)

500 "Internal Server Error"

This status code is used in two cases:

  1. When an application error is returned by the implementation function. In such case, the error data structure will be encoded according to the Accept request header and the Content-Type header of the response will be set accordingly. This way the calling application will be able to interpret the returned application error and provide specific handling if necessary.

  2. When something goes wrong during the request execution. For example, when the implementation function crashes. In such case, the description of the error will be returned in plain text and the Content-Type header is set to text/plain.

503 "Service Unavailable"

This status code is currently not used. In future, it will mean that the Piqi-RPC service is paused.

502 "Bad Gateway"

This status code is used when the implementation function returns a value that doesn’t conform to the original Piqi interface definition.

2.10. Getting Piqi specification from a running Piqi-RPC service

Piqi-RPC provides a method to get the Piqi interface definition from a running service. This can be done by sending an HTTP GET request with any HTTP URL that starts with the service base URL (the base URL is defined during service registration).

For example, if a service is running at http://localhost:8888/foo/bar the Piqi service definition can be retrieved by running any of the following HTTP requests:

curl 'http://localhost:8888/foo/bar'

curl 'http://localhost:8888/foo/bar/have-no-idea-what-functions-are-there'

This functionality is used by Piqi-RPC command-line client described in the next section. The command-line client can fetch Piqi specifications of remote services and display them in several different formats.

2.11. piqi call — Piqi-RPC command-line client

piqi call interprets command-line arguments as input parameters for a remote function, converts them into a Protobuf-encoded data object and executes a Piqi-RPC function call over HTTP.

In addition to calling a remote function, piqi call can fetch Piqi specifications of a remote service and print them in several formats: Piqi (--piqi flag), Piqi-light (-p flag) and getopt-style help for remote functions (-h flag).

See some piqi call usage scenarios in the Examples section below.

piqi call command is documented here.

2.12. piqi_rpc Erlang application

By default, Piqi-RPC runs an HTTP server on port 8888 on all available network interfaces.

These parameters as well as some others specific to Mochiweb HTTP server can be customized through piqi_rpc application environment configuration (i.e. piqi_rpc.app or Erlang .config file).

2.13. Examples

There are several complete examples of Piqi-RPC servers and clients available in the "piqi-rpc/examples" sub-directory.

One of the examples contains a simple addressbook service that includes three functions:

  • add a person to the addressbook

  • get a person from the addressbook by identifier

  • list all addressbook entries

Below is complete definition of the service in Piqi language. This definition relies on another Piqi module named person. You can find its definition in the source code, or on the Examples page.

.include [ .module person ]

.erlang-type-prefix ""

.function [
    .name add-person
    .input person
    .error string

.function [
    .name get-person
    .input [
        .field [
            .name id
            .type int
    .output person
    .error string

.function [
    .name list-people
    .output addressbook

.list [
    .name addressbook
    .type person

These are Erlang types from the addressbook_piqi.hrl file generated from the above definitions:

-type(addressbook() :: [person()]).
-type(add_person_input() :: person()).
-type(add_person_error() :: string() | binary()).
-record(get_person_input, {
    id :: integer()
-type(get_person_output() :: person()).
-type(get_person_error() :: string() | binary()).
-type(list_people_output() :: addressbook()).

-type(person() :: #person{}).
-type(phone_number() :: #phone_number{}).
-type(get_person_input() :: #get_person_input{}).

Erlang specification for the service implementation functions (from the addressbook_piqi_impl.hrl file):

-spec add_person/1 :: (add_person_input()) ->
    ok | {error, add_person_error()}.

-spec get_person/1 :: (get_person_input()) ->
    {ok, get_person_output()} | {error, get_person_error()}.

-spec list_people/1 :: ('undefined') ->
    {ok, list_people_output()}.

The above functions is what actually Erlang developer needs to implement.

Once the functions are implemented, the code is loaded and the service is registered, this is how a typical client interaction may look like using the piqi call Piqi-RPC client. The first call is getting command-line help about the running service.

$ piqi call http://localhost:8888/addressbook -h
Piqi-RPC functions (use -p flag for more details):
  add-person -- <person>, which is a combination of:
    --name <string>
    --id <int>
    --email <string> (optional)
    --phone <phone-number> (repeated)
  get-person -- <input>, which is:
    --id <int>

$ piqi call -t json http://localhost:8888/addressbook/add-person -- \
    --name "J. Random Hacker" \
    --id 0 \
    --email "j.r.hacker@example.com" \
    --phone [ --number "(111) 123 45 67" ] \
    --phone [ \
        --number "(222) 123 45 67" \
        --mobile \

$ piqi call -t json http://localhost:8888/addressbook/get-person -- 0
  "name": "J. Random Hacker",
  "id": 0,
  "email": "j.r.hacker@example.com",
  "phone": [
    { "number": "(111) 123 45 67", "type": "home" },
    { "number": "(222) 123 45 67", "type": "mobile" },
    { "number": "(333) 123 45 67", "type": "work" }

2.14. Known limitations

Below is the list of known limitations in the existing implementation of Piqi-RPC.

  • No logging of user requests

    Currently, Piqi-RPC doesn’t support logging of requests or responses. Such facility would be useful for debugging Piqi-RPC services and user actions.

3. Implementation details

Piqi-RPC is build on top of Piqi data serialization for Erlang and Piqi data conversion functionality.

HTTP layer is implemented as a Webmachine behavior. Webmachine is an excellent HTTP library for writing well-behaving HTTP services. For instance, it handles general HTTP workflow, validates headers, performs automatic content-type negotiation, etc. Webmachine works on top of a popular Mochiweb HTTP server.

3.1. Piqi-RPC request execution

When a Piqi-RPC call comes in, the following sequence of actions is executed.

  1. The HTTP POST request is validated by Webmachine and Piq-RPC Webmachine behavior going through a series of checks: service availability, HTTP method, HTTP URL, Content-Type and Accept headers.

  2. If everything is fine, input parameter (i.e. the body of the HTTP POST request) is converted to Protocol Buffers format using Piqi data converter.

  3. Erlang deserialization code, generated from Piqi interface definition, is used to read the input parameter represented in Protobuf format into native Erlang term representation.

  4. The Erlang implementation callback (the actual users code) is called with the input parameter passed as a native Erlang data structure.

  5. The output of the implementation function is serialized to Protocol Buffers format using Erlang serialization code, generated from Piqi interface definition (symmetric operation to step 3).

  6. Output represented in Protobuf format is converted to the output format requested by the caller using Piqi data converter (symmetric operation to step 2).

  7. The HTTP response is generated by the Piqi-RPC HTTP layer and sent to the caller.

3.2. Piqi data converter

Data conversion between JSON, XML and Protocol Buffers format is performed by an external piqi server program running as Erlang port.

Several instances of piqi server can be started on multi-core or multi-CPU systems in order to increase throughput and decrease latencies. Currently, the number of workers is configured statically in piqi Erlang application.

3.3. Protocol Buffers serialization for Erlang

Protocol Buffers serialization for Erlang is a part of core Piqi functionality. It is covered in a separate section of the current documentation.

3.4. System portability

Around 80% of Piqi-RPC functionality is multi-format data conversion, which is implemented in OCaml. The rest of Piqi-RPC is written in Erlang.

The OCaml part runs as external program and communicates with Erlang part via Erlang port interface (i.e. Unix pipe).

The OCaml data converter — piqi server is packaged as a standalone natively compiled executable that depends only on libc. Everything else is statically linked in. This makes it easy to build the piqi binary once and then just copy it to any compatible system.

piqic erlang is another piece written in OCaml. It is a basic Piqi compiler that generates Protobuf serialization and deserialization code for Erlang. It is also packaged as standalone natively compiled binary executable. However, unlike the piqi binary, it doesn’t have to be included in the running Piqi-RPC system as it is only used at the build stage.

OCaml part is very straightforward to build (it doesn’t have any external build dependencies). It successfully builds and runs on 32- and 64-bit Linux, Mac OS X, Solaris and should work on any other Unix systems and major hardware architectures.

4. Additional resources

4.1. Erlang Factory SF 2011 presentation slides

Piqi and Piqi-RPC were presented at Erlang Factory conference in San Francisco in March 2011.

The slides from the presentation may serve as a good introduction to Piqi and Piqi-RPC.

Note that some slides are out of date and do not reflect the existing implementation. For instance, since then, the system has been improved to utilize multiple cores for running data conversion processes.

The slides are available on the conference website (direct link to PDF). Part of the presentation talking specifically about Piqi-RPC starts from slide 25.