pw_rpc

The pw_rpc module provides a system for defining and invoking remote procedure calls (RPCs) on a device.

This document discusses the pw_rpc protocol and its C++ implementation. pw_rpc implementations for other languages are described in their own documents:

Try it out!

For a quick intro to pw_rpc, see the RPC over HDLC example project in the pw_hdlc module.

Attention

This documentation is under construction.

RPC semantics

The semantics of pw_rpc are similar to gRPC.

RPC call lifecycle

In pw_rpc, an RPC begins when the client sends a request packet. The server receives the request, looks up the relevant service method, then calls into the RPC function. The RPC is considered active until the server sends a response packet with the RPC’s status. The client may terminate an ongoing RPC by cancelling it.

pw_rpc supports only one RPC invocation per service/method/channel. If a client calls an ongoing RPC on the same channel, the server cancels the ongoing call and reinvokes the RPC with the new request. This applies to unary and streaming RPCs, though the server may not have an opportunity to cancel a synchronously handled unary RPC before it completes. The same RPC may be invoked multiple times simultaneously if the invocations are on different channels.

Unrequested responses

pw_rpc supports sending responses to RPCs that have not yet been invoked by a client. This is useful in testing and in situations like an RPC that triggers reboot. After the reboot, the device opens the writer object and sends its response to the client.

The C++ API for opening a server reader/writer takes the generated RPC function as a template parameter. The server to use, channel ID, and service instance are passed as arguments. The API is the same for all RPC types, except the appropriate reader/writer class must be used.

// Open a ServerWriter for a server streaming RPC.
auto writer = RawServerWriter::Open<pw_rpc::raw::ServiceName::MethodName>(
    server, channel_id, service_instance);

// Send some responses, even though the client has not yet called this RPC.
CHECK_OK(writer.Write(encoded_response_1));
CHECK_OK(writer.Write(encoded_response_2));

// Finish the RPC.
CHECK_OK(writer.Finish(OkStatus()));

Creating an RPC

1. RPC service declaration

Pigweed RPCs are declared in a protocol buffer service definition.

syntax = "proto3";

package foo.bar;

message Request {}

message Response {
  int32 number = 1;
}

service TheService {
  rpc MethodOne(Request) returns (Response) {}
  rpc MethodTwo(Request) returns (stream Response) {}
}

This protocol buffer is declared in a BUILD.gn file as follows:

import("//build_overrides/pigweed.gni")
import("$dir_pw_protobuf_compiler/proto.gni")

pw_proto_library("the_service_proto") {
  sources = [ "foo_bar/the_service.proto" ]
}

proto2 or proto3 syntax?

Always use proto3 syntax rather than proto2 for new protocol buffers. Proto2 protobufs can be compiled for pw_rpc, but they are not as well supported as proto3. Specifically, pw_rpc lacks support for non-zero default values in proto2. When using Nanopb with pw_rpc, proto2 response protobufs with non-zero field defaults should be manually initialized to the default struct.

In the past, proto3 was sometimes avoided because it lacked support for field presence detection. Fortunately, this has been fixed: proto3 now supports optional fields, which are equivalent to proto2 optional fields.

If you need to distinguish between a default-valued field and a missing field, mark the field as optional. The presence of the field can be detected with a HasField(name) or has_<field> member, depending on the library.

Optional fields have some overhead — default-valued fields are included in the encoded proto, and, if using Nanopb, the proto structs have a has_<field> flag for each optional field. Use plain fields if field presence detection is not needed.

syntax = "proto3";

message MyMessage {
  // Leaving this field unset is equivalent to setting it to 0.
  int32 number = 1;

  // Setting this field to 0 is different from leaving it unset.
  optional int32 other_number = 2;
}

2. RPC code generation

pw_rpc generates a C++ header file for each .proto file. This header is generated in the build output directory. Its exact location varies by build system and toolchain, but the C++ include path always matches the sources declaration in the pw_proto_library. The .proto extension is replaced with an extension corresponding to the protobuf library in use.

Protobuf libraries

Build subtarget

Protobuf header

pw_rpc header

Raw only

.raw_rpc

(none)

.raw_rpc.pb.h

Nanopb or raw

.nanopb_rpc

.pb.h

.rpc.pb.h

pw_protobuf or raw

.pwpb_rpc

.pwpb.h

.rpc.pwpb.h

For example, the generated RPC header for "foo_bar/the_service.proto" is "foo_bar/the_service.rpc.pb.h" for Nanopb or "foo_bar/the_service.raw_rpc.pb.h" for raw RPCs.

The generated header defines a base class for each RPC service declared in the .proto file. A service named TheService in package foo.bar would generate the following base class for Nanopb:

template<typename Implementation>
class foo::bar::pw_rpc::nanopb::TheService::Service

3. RPC service definition

The serivce class is implemented by inheriting from the generated RPC service base class and defining a method for each RPC. The methods must match the name and function signature for one of the supported protobuf implementations. Services may mix and match protobuf implementations within one service.

Tip

The generated code includes RPC service implementation stubs. You can reference or copy and paste these to get started with implementing a service. These stub classes are generated at the bottom of the pw_rpc proto header.

To use the stubs, do the following:

  1. Locate the generated RPC header in the build directory. For example:

    find out/ -name <proto_name>.rpc.pb.h
    
  2. Scroll to the bottom of the generated RPC header.

  3. Copy the stub class declaration to a header file.

  4. Copy the member function definitions to a source file.

  5. Rename the class or change the namespace, if desired.

  6. List these files in a build target with a dependency on the pw_proto_library.

A Nanopb implementation of this service would be as follows:

#include "foo_bar/the_service.rpc.pb.h"

namespace foo::bar {

class TheService : public pw_rpc::nanopb::TheService::Service<TheService> {
 public:
  pw::Status MethodOne(ServerContext&,
                       const foo_bar_Request& request,
                       foo_bar_Response& response) {
    // implementation
    return pw::OkStatus();
  }

  void MethodTwo(ServerContext&,
                 const foo_bar_Request& request,
                 ServerWriter<foo_bar_Response>& response) {
    // implementation
    response.Write(foo_bar_Response{.number = 123});
  }
};

}  // namespace foo::bar

The Nanopb implementation would be declared in a BUILD.gn:

import("//build_overrides/pigweed.gni")

import("$dir_pw_build/target_types.gni")

pw_source_set("the_service") {
  public_configs = [ ":public" ]
  public = [ "public/foo_bar/service.h" ]
  public_deps = [ ":the_service_proto.nanopb_rpc" ]
}

Attention

pw_rpc’s generated classes will support using pw_protobuf or raw buffers (no protobuf library) in the future.

4. Register the service with a server

This example code sets up an RPC server with an HDLC channel output and the example service.

// Set up the output channel for the pw_rpc server to use. This configures the
// pw_rpc server to use HDLC over UART; projects not using UART and HDLC must
// adapt this as necessary.
pw::stream::SysIoWriter writer;
pw::rpc::RpcChannelOutput<kMaxTransmissionUnit> hdlc_channel_output(
    writer, pw::hdlc::kDefaultRpcAddress, "HDLC output");

pw::rpc::Channel channels[] = {
    pw::rpc::Channel::Create<1>(&hdlc_channel_output)};

// Declare the pw_rpc server with the HDLC channel.
pw::rpc::Server server(channels);

pw::rpc::TheService the_service;

void RegisterServices() {
  // Register the foo.bar.TheService example service.
  server.Register(the_service);

  // Register other services
}

int main() {
  // Set up the server.
  RegisterServices();

  // Declare a buffer for decoding incoming HDLC frames.
  std::array<std::byte, kMaxTransmissionUnit> input_buffer;

  PW_LOG_INFO("Starting pw_rpc server");
  pw::hdlc::ReadAndProcessPackets(
      server, hdlc_channel_output, input_buffer);
}

Channels

pw_rpc sends all of its packets over channels. These are logical, application-layer routes used to tell the RPC system where a packet should go.

Channels over a client-server connection must all have a unique ID, which can be assigned statically at compile time or dynamically.

// Creating a channel with the static ID 3.
pw::rpc::Channel static_channel = pw::rpc::Channel::Create<3>(&output);

// Grouping channel IDs within an enum can lead to clearer code.
enum ChannelId {
  kUartChannel = 1,
  kSpiChannel = 2,
};

// Creating a channel with a static ID defined within an enum.
pw::rpc::Channel another_static_channel =
    pw::rpc::Channel::Create<ChannelId::kUartChannel>(&output);

// Creating a channel with a dynamic ID (note that no output is provided; it
// will be set when the channel is used.
pw::rpc::Channel dynamic_channel;

Sometimes, the ID and output of a channel are not known at compile time as they depend on information stored on the physical device. To support this use case, a dynamically-assignable channel can be configured once at runtime with an ID and output.

// Create a dynamic channel without a compile-time ID or output.
pw::rpc::Channel dynamic_channel;

void Init() {
  // Called during boot to pull the channel configuration from the system.
  dynamic_channel.Configure(GetChannelId(), some_output);
}

Services

A service is a logical grouping of RPCs defined within a .proto file. pw_rpc uses these .proto definitions to generate code for a base service, from which user-defined RPCs are implemented.

pw_rpc supports multiple protobuf libraries, and the generated code API depends on which is used.

Protobuf library APIs

Testing a pw_rpc integration

After setting up a pw_rpc server in your project, you can test that it is working as intended by registering the provided EchoService, defined in echo.proto, which echoes back a message that it receives.

syntax = "proto3";

package pw.rpc;

service EchoService {
  rpc Echo(EchoMessage) returns (EchoMessage) {}
}

message EchoMessage {
  string msg = 1;
}

For example, in C++ with nanopb:

#include "pw_rpc/server.h"

// Include the apporpriate header for your protobuf library.
#include "pw_rpc/echo_service_nanopb.h"

constexpr pw::rpc::Channel kChannels[] = { /* ... */ };
static pw::rpc::Server server(kChannels);

static pw::rpc::EchoService echo_service;

void Init() {
  server.RegisterService(echo_service);
}

Benchmarking and stress testing

pw_rpc provides an RPC service and Python module for stress testing and benchmarking a pw_rpc deployment. See pw_rpc Benchmarking.

Naming

Reserved names

pw_rpc reserves a few service method names so they can be used for generated classes. The following names cannnot be used for service methods:

  • Client

  • Service

  • Any reserved words in the languages pw_rpc supports (e.g. class).

pw_rpc does not reserve any service names, but the restriction of avoiding reserved words in supported languages applies.

Service naming style

pw_rpc service names should use capitalized camel case and should not use the term “Service”. Appending “Service” to a service name is redundant, similar to appending “Class” or “Function” to a class or function name. The C++ implementation class may use “Service” in its name, however.

For example, a service for accessing a file system should simply be named service FileSystem, rather than service FileSystemService, in the .proto file.

// file.proto
package pw.file;

service FileSystem {
    rpc List(ListRequest) returns (stream ListResponse);
}

The C++ service implementation class may append “Service” to the name.

// file_system_service.h
#include "pw_file/file.raw_rpc.pb.h"

namespace pw::file {

class FileSystemService : public pw_rpc::raw::FileSystem::Service<FileSystemService> {
  void List(ServerContext&, ConstByteSpan request, RawServerWriter& writer);
};

}

For upstream Pigweed services, this naming style is a requirement. Note that some services created before this was established may use non-compliant names. For Pigweed users, this naming style is a suggestion.

Protocol description

Pigweed RPC servers and clients communicate using pw_rpc packets. These packets are used to send requests and responses, control streams, cancel ongoing RPCs, and report errors.

Packet format

Pigweed RPC packets consist of a type and a set of fields. The packets are encoded as protocol buffers. The full packet format is described in pw_rpc/pw_rpc/internal/packet.proto.

syntax = "proto3";

package pw.rpc.internal;

option java_package = "dev.pigweed.pw.rpc.internal";

enum PacketType {
  // To simplify identifying the origin of a packet, client-to-server packets
  // use even numbers and server-to-client packets use odd numbers.

  // Client-to-server packets

  // The client invokes an RPC. Always the first packet.
  REQUEST = 0;

  // A message in a client stream. Always sent after a REQUEST and before a
  // CLIENT_STREAM_END.
  CLIENT_STREAM = 2;

  // The client received a packet for an RPC it did not request.
  CLIENT_ERROR = 4;

  // The client requests cancellation of an ongoing RPC.
  CANCEL = 6;

  // A client stream has completed.
  CLIENT_STREAM_END = 8;

  // Server-to-client packets

  // The RPC has finished.
  RESPONSE = 1;

  // Deprecated, do not use. Formerly was used as the last packet in a server
  // stream.
  DEPRECATED_SERVER_STREAM_END = 3;

  // The server was unable to process a request.
  SERVER_ERROR = 5;

  // A message in a server stream.
  SERVER_STREAM = 7;
}

message RpcPacket {
  // The type of packet. Determines which other fields are used.
  PacketType type = 1;

  // Channel through which the packet is sent.
  uint32 channel_id = 2;

  // Hash of the fully-qualified name of the service with which this packet is
  // associated. For RPC packets, this is the service that processes the packet.
  fixed32 service_id = 3;

  // Hash of the name of the method which should process this packet.
  fixed32 method_id = 4;

  // The packet's payload, which is an encoded protobuf.
  bytes payload = 5;

  // Status code for the RPC response or error.
  uint32 status = 6;
}

The packet type and RPC type determine which fields are present in a Pigweed RPC packet. Each packet type is only sent by either the client or the server. These tables describe the meaning of and fields included with each packet type.

Client-to-server packets

packet type

description

REQUEST

Invoke an RPC

- channel_id
- service_id
- method_id
- payload
  (unary & server streaming only)

CLIENT_STREAM

Message in a client stream

- channel_id
- service_id
- method_id
- payload

CLIENT_STREAM_END

Client stream is complete

- channel_id
- service_id
- method_id

CLIENT_ERROR

Received unexpected packet

- channel_id
- service_id
- method_id
- status

CANCEL

Cancel an ongoing RPC

- channel_id
- service_id
- method_id

Errors

The client sends CLIENT_ERROR packets to a server when it receives a packet it did not request. If the RPC is a streaming RPC, the server should abort it.

The status code indicates the type of error. The status code is logged, but all status codes result in the same action by the server: aborting the RPC.

  • NOT_FOUND – Received a packet for a service method the client does not recognize.

  • FAILED_PRECONDITION – Received a packet for a service method that the client did not invoke.

  • DATA_LOSS – Received a corrupt packet for a pending service method.

Server-to-client packets

packet type

description

RESPONSE

The RPC is complete

- channel_id
- service_id
- method_id
- status
- payload
  (unary & client streaming only)

SERVER_STREAM

Message in a server stream

- channel_id
- service_id
- method_id
- payload

SERVER_ERROR

Received unexpected packet

- channel_id
- service_id (if relevant)
- method_id (if relevant)
- status

Errors

The server sends SERVER_ERROR packets when it receives a packet it cannot process. The client should abort any RPC for which it receives an error. The status field indicates the type of error.

  • NOT_FOUND – The requested service or method does not exist.

  • FAILED_PRECONDITION – A client stream or cancel packet was sent for an RPC that is not pending.

  • INVALID_ARGUMENT – The client sent a packet type to an RPC that does not support it (e.g. a CLIENT_STREAM was sent to a server streaming RPC).

  • RESOURCE_EXHAUSTED – The request came on a new channel, but a channel could not be allocated for it.

  • INTERNAL – The server was unable to respond to an RPC due to an unrecoverable internal error.

Inovking a service method

Calling an RPC requires a specific sequence of packets. This section describes the protocol for calling service methods of each type: unary, server streaming, client streaming, and bidirectional streaming.

The basic flow for all RPC invocations is as follows:

  • Client sends a REQUEST packet. Includes a payload for unary & server streaming RPCs.

  • For client and bidirectional streaming RPCs, the client may send any number of CLIENT_STREAM packets with payloads.

  • For server and bidirectional streaming RPCs, the server may send any number of SERVER_STREAM packets.

  • The server sends a RESPONSE packet. Includes a payload for unary & client streaming RPCs. The RPC is complete.

The client may cancel an ongoing RPC at any time by sending a CANCEL packet. The server may finish an ongoing RPC at any time by sending the RESPONSE packet.

Unary RPC

In a unary RPC, the client sends a single request and the server sends a single response.

../_images/unary_rpc.svg

The client may attempt to cancel a unary RPC by sending a CANCEL packet. The server sends no response to a cancelled RPC. If the server processes the unary RPC synchronously (the handling thread sends the response), it may not be possible to cancel the RPC.

../_images/unary_rpc_cancelled.svg

Server streaming RPC

In a server streaming RPC, the client sends a single request and the server sends any number of SERVER_STREAM packets followed by a RESPONSE packet.

../_images/server_streaming_rpc.svg

The client may terminate a server streaming RPC by sending a CANCEL packet. The server sends no response.

../_images/server_streaming_rpc_cancelled.svg

Client streaming RPC

In a client streaming RPC, the client starts the RPC by sending a REQUEST packet with no payload. It then sends any number of messages in CLIENT_STREAM packets, followed by a CLIENT_STREAM_END. The server sends a single RESPONSE to finish the RPC.

../_images/client_streaming_rpc.svg

The server may finish the RPC at any time by sending its RESPONSE packet, even if it has not yet received the CLIENT_STREAM_END packet. The client may terminate the RPC at any time by sending a CANCEL packet.

../_images/client_streaming_rpc_cancelled.svg

Bidirectional streaming RPC

In a bidirectional streaming RPC, the client sends any number of requests and the server sends any number of responses. The client invokes the RPC by sending a REQUEST with no payload. It sends a CLIENT_STREAM_END packet when it has finished sending requests. The server sends a RESPONSE packet to finish the RPC.

../_images/bidirectional_streaming_rpc.svg

The server may finish the RPC at any time by sending the RESPONSE packet, even if it has not received the CLIENT_STREAM_END packet. The client may terminate the RPC at any time by sending a CANCEL packet.

../_images/bidirectional_streaming_rpc_cancelled.svg

RPC server

Declare an instance of rpc::Server and register services with it.

TODO

Document the public interface

Size report

The following size report showcases the memory usage of the core RPC server. It is configured with a single channel using a basic transport interface that directly reads from and writes to pw_sys_io. The transport has a 128-byte packet buffer, which comprises the plurality of the example’s RAM usage. This is not a suitable transport for an actual product; a real implementation would have additional overhead proportional to the complexity of the transport.

Label

Segment

Before

Delta

After

Server by itself

FLASH
RAM
20,880
672
+5,132
+168
26,012
840

RPC server implementation

The Method class

The RPC Server depends on the pw::rpc::internal::Method class. Method serves as the bridge between the pw_rpc server library and the user-defined RPC functions. Each supported protobuf implementation extends Method to implement its request and response proto handling. The pw_rpc server calls into the Method implementation through the base class’s Invoke function.

Method implementations store metadata about each method, including a function pointer to the user-defined method implementation. They also provide static constexpr functions for creating each type of method. Method implementations must satisfy the MethodImplTester test class in pw_rpc/internal/method_impl_tester.h.

See pw_rpc/internal/method.h for more details about Method.

Packet flow

Requests
../_images/request_packets.svg
Responses
../_images/response_packets.svg

RPC client

The RPC client is used to send requests to a server and manages the contexts of ongoing RPCs.

Setting up a client

The pw::rpc::Client class is instantiated with a list of channels that it uses to communicate. These channels can be shared with a server, but multiple clients cannot use the same channels.

To send incoming RPC packets from the transport layer to be processed by a client, the client’s ProcessPacket function is called with the packet data.

#include "pw_rpc/client.h"

namespace {

pw::rpc::Channel my_channels[] = {
    pw::rpc::Channel::Create<1>(&my_channel_output)};
pw::rpc::Client my_client(my_channels);

}  // namespace

// Called when the transport layer receives an RPC packet.
void ProcessRpcPacket(ConstByteSpan packet) {
  my_client.ProcessPacket(packet);
}

Making RPC calls

RPC calls are not made directly through the client, but using one of its registered channels instead. A service client class is generated from a .proto file for each selected protobuf library, which is then used to send RPC requests through a given channel. The API for this depends on the protobuf library; please refer to the appropriate documentation. Multiple service client implementations can exist simulatenously and share the same Client class.

When a call is made, a pw::rpc::ClientCall object is returned to the caller. This object tracks the ongoing RPC call, and can be used to manage it. An RPC call is only active as long as its ClientCall object is alive.

Tip

Use std::move when passing around ClientCall objects to keep RPCs alive.

Example

#include "pw_rpc/echo_service_nanopb.h"

namespace {
// Generated clients are namespaced with their proto library.
using pw::rpc::nanopb::EchoServiceClient;

// RPC channel ID on which to make client calls.
constexpr uint32_t kDefaultChannelId = 1;

EchoServiceClient::EchoCall echo_call;

// Callback invoked when a response is received. This is called synchronously
// from Client::ProcessPacket.
void EchoResponse(const pw_rpc_EchoMessage& response,
                  pw::Status status) {
  if (status.ok()) {
    PW_LOG_INFO("Received echo response: %s", response.msg);
  } else {
    PW_LOG_ERROR("Echo failed with status %d",
                 static_cast<int>(status.code()));
  }
}

}  // namespace

void CallEcho(const char* message) {
  // Create a client to call the EchoService.
  EchoServiceClient echo_client(my_rpc_client, kDefaultChannelId);

  pw_rpc_EchoMessage request = pw_rpc_EchoMessage_init_default;
  pw::string::Copy(message, request.msg);

  // By assigning the returned ClientCall to the global echo_call, the RPC
  // call is kept alive until it completes. When a response is received, it
  // will be logged by the handler function and the call will complete.
  echo_call = echo_client.Echo(request, EchoResponse);
  if (!echo_call.active()) {
    // The RPC call was not sent. This could occur due to, for example, an
    // invalid channel ID. Handle if necessary.
  }
}

Client implementation details

The ClientCall class

ClientCall stores the context of an active RPC, and serves as the user’s interface to the RPC client. The core RPC library provides a base ClientCall class with common functionality, which is then extended for RPC client implementations tied to different protobuf libraries to provide convenient interfaces for working with RPCs.

The RPC server stores a list of all of active ClientCall objects. When an incoming packet is recieved, it dispatches to one of its active calls, which then decodes the payload and presents it to the user.

ClientServer

Sometimes, a device needs to both process RPCs as a server, as well as making calls to another device as a client. To do this, both a client and server must be set up, and incoming packets must be sent to both of them.

Pigweed simplifies this setup by providing a ClientServer class which wraps an RPC client and server with the same set of channels.

pw::rpc::Channel channels[] = {
    pw::rpc::Channel::Create<1>(&channel_output)};

// Creates both a client and a server.
pw::rpc::ClientServer client_server(channels);

void ProcessRpcData(pw::ConstByteSpan packet) {
  // Calls into both the client and the server, sending the packet to the
  // appropriate one.
  client_server.ProcessPacket(packet, output);
}