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.


This documentation is under construction. Many sections are outdated or incomplete. The content needs to be reorgnanized.


Pigweed provides several client and server implementations of pw_rpc.




C++ (raw)

C++ (Nanopb)

C++ (pw_protobuf)




in development

RPC semantics#

The semantics of pw_rpc are similar to gRPC.

RPC call lifecycle#

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

Depending the type of RPC, the client and server exchange zero or more protobuf request or response payloads. There are four RPC types:

  • Unary. The client sends one request and the server sends one response with a status.

  • Server streaming. The client sends one request and the server sends zero or more responses followed by a status.

  • Client streaming. The client sends zero or more requests and the server sends one response with a status.

  • Bidirectional streaming. The client sends zero or more requests and the server sends zero or more responses followed by a status.


The key events in the RPC lifecycle are:

  • Start. The client initiates the RPC. The server’s RPC body executes.

  • Finish. The server sends a status and completes the RPC. The client calls a callback.

  • Request. The client sends a request protobuf. The server calls a callback when it receives it. In unary and server streaming RPCs, there is only one request and it is handled when the RPC starts.

  • Response. The server sends a response protobuf. The client calls a callback when it receives it. In unary and client streaming RPCs, there is only one response and it is handled when the RPC completes.

  • Error. The server or client terminates the RPC abnormally with a status. The receiving endpoint calls a callback.

  • Request Completion. The client sends a message that it would like to request call completion. The server calls a callback when it receives it. Some servers may ignore the request completion message. In client and bidirectional streaming RPCs, this also indicates that client has finished sending requests.

Status codes#

pw_rpc call objects (ClientReaderWriter, ServerReaderWriter, etc.) use certain status codes to indicate what occurred. These codes are returned from functions like Write() or Finish().

  • OK – The operation succeeded.

  • UNAVAILABLE – The channel is not currently registered with the server or client.

  • UNKNOWN – Sending a packet failed due to an unrecoverable pw::rpc::ChannelOutput::Send() error.

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.

// Finish the RPC.

Creating an RPC#

1. RPC service declaration#

Pigweed RPCs are declared in a protocol buffer service definition.

syntax = "proto3";


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 file as follows:


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 std::optional, a HasField(name), or has_<field> member, depending on the library.

Optional fields have some overhead — if using Nanopb, default-valued fields are included in the encoded proto, and 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




Nanopb or raw




pw_protobuf or raw




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 would generate the following base class for pw_protobuf:

template<typename Implementation>
class foo::bar::pw_rpc::pwpb::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.


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.pwpb.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 pw_protobuf implementation of this service would be as follows:

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

namespace foo::bar {

class TheService : public pw_rpc::pwpb::TheService::Service<TheService> {
  pw::Status MethodOne(const Request::Message& request,
                       Response::Message& response) {
    // implementation
    response.number = 123;
    return pw::OkStatus();

  void MethodTwo(const Request::Message& request,
                 ServerWriter<Response::Message>& response) {
    // implementation
    response.Write({.number = 123});

}  // namespace foo::bar

The pw_protobuf implementation would be declared in a



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

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::FixedMtuChannelOutput<kMaxTransmissionUnit> hdlc_channel_output(
    writer, pw::hdlc::kDefaultRpcAddress, "HDLC output");

// Allocate an array of channels for the server to use. If dynamic allocation
// is enabled (PW_RPC_DYNAMIC_ALLOCATION=1), the server can be initialized
// without any channels, and they can be added later.
pw::rpc::Channel channels[] = {

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

foo::bar::TheService the_service;
pw::rpc::SomeOtherService some_other_service;

void RegisterServices() {
  // Register the example service and another service.
  server.RegisterService(the_service, some_other_service);

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

  // 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, input_buffer);


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 =

// 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);

Adding and removing channels#

New channels may be registered with the OpenChannel function. If dynamic allocation is enabled (PW_RPC_DYNAMIC_ALLOCATION is 1), any number of channels may be registered. If dynamic allocation is disabled, new channels may only be registered if there are availale channel slots in the span provided to the RPC endpoint at construction.

A channel may be closed and unregistered with an endpoint by calling ChannelClose on the endpoint with the corresponding channel ID. This will terminate any pending calls and call their on_error callback with the ABORTED status.

// When a channel is closed, any pending calls will receive
// on_error callbacks with ABORTED status.


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.

Services must be registered with a server in order to call their methods. Services may later be unregistered, which aborts calls for methods in that service and prevents future calls to them, until the service is re-registered.

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;

option java_package = "dev.pigweed.pw_rpc.proto";

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

message EchoMessage {
  string msg = 1;

For example, in C++ with pw_protobuf:

#include "pw_rpc/server.h"

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

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

static pw::rpc::EchoService echo_service;

void Init() {

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.


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(ConstByteSpan request, RawServerWriter& writer);

}  // namespace pw::file

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.

C++ payload sizing limitations#

The individual size of each sent RPC request or response is limited by pw_rpc’s PW_RPC_ENCODING_BUFFER_SIZE_BYTES configuration option when using Pigweed’s C++ implementation. While multiple RPC messages can be enqueued (as permitted by the underlying transport), if a single individual sent message exceeds the limitations of the statically allocated encode buffer, the packet will fail to encode and be dropped.

This applies to all C++ RPC service implementations (nanopb, raw, and pwpb), so it’s important to ensure request and response message sizes do not exceed this limitation.

As pw_rpc has some additional encoding overhead, a helper, pw::rpc::MaxSafePayloadSize() is provided to expose the practical max RPC message payload size.

#include "pw_file/file.raw_rpc.pb.h"
#include "pw_rpc/channel.h"

namespace pw::file {

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

  // Allocate a buffer for building proto responses.
  static constexpr size_t kEncodeBufferSize = pw::rpc::MaxSafePayloadSize();
  std::array<std::byte, kEncodeBufferSize> encode_buffer_;

}  // namespace pw::file

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

  // The client received a packet for an RPC it did not request.

  // Client has requested for call completion. In client streaming and
  // bi-directional streaming RPCs, this also indicates that the client is done
  // with sending requests.

  // Server-to-client packets

  // The RPC has finished.

  // The server was unable to process a request.

  // A message in a server stream.

  // Reserve field numbers for deprecated PacketTypes.
  reserved 3;  // SERVER_STREAM_END (equivalent to RESPONSE now)
  reserved 6;  // CANCEL (replaced by CLIENT_ERROR with status CANCELLED)

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;

  // Unique identifier for the call that initiated this RPC. Optionally set by
  // the client in the initial request and sent in all subsequent client
  // packets; echoed by the server.
  uint32 call_id = 7;

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



Invoke an RPC

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


Message in a client stream

- channel_id
- service_id
- method_id
- payload
- call_id (if set in REQUEST)


Client requested stream completion

- channel_id
- service_id
- method_id
- call_id (if set in REQUEST)


Abort an ongoing RPC

- channel_id
- service_id
- method_id
- status
- call_id (if set in REQUEST)

Client errors

The client sends CLIENT_ERROR packets to a server when it receives a packet it did not request. If possible, 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.

  • CANCELLED – The client requested that the RPC be cancelled.

  • ABORTED – The RPC was aborted due its channel being closed.

  • 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.

  • INVALID_ARGUMENT – The server sent a packet type to an RPC that does not support it (a SERVER_STREAM was sent to an RPC with no server stream).

  • UNAVAILABLE – Received a packet for an unknown channel.

Server-to-client packets#

packet type



The RPC is complete

- channel_id
- service_id
- method_id
- status
- payload
  (unary & client streaming only)
- call_id (if set in REQUEST)


Message in a server stream

- channel_id
- service_id
- method_id
- payload
- call_id (if set in REQUEST)


Received unexpected packet

- channel_id
- service_id (if relevant)
- method_id (if relevant)
- status
- call_id (if set in REQUEST)

All server packets contain the same call_id that was set in the initial request made by the client, if any.

Server 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 (a CLIENT_STREAM was sent to an RPC with no client stream).

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

  • ABORTED – The RPC was aborted due its channel being closed.

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

  • UNAVAILABLE – Received a packet for an unknown channel.

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 CLIENT_ERROR packet with status CANCELLED. 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.


The client may attempt to cancel a unary RPC by sending a CLIENT_ERROR packet with status CANCELLED. 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.


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.


The client may terminate a server streaming RPC by sending a CLIENT_STREAM packet with status CANCELLED. The server sends no response.


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_REQUEST_COMPLETION. The server sends a single RESPONSE to finish the RPC.


The server may finish the RPC at any time by sending its RESPONSE packet, even if it has not yet received the CLIENT_REQUEST_COMPLETION packet. The client may terminate the RPC at any time by sending a CLIENT_ERROR packet with status CANCELLED.


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_REQUEST_COMPLETION packet when it has finished sending requests. The server sends a RESPONSE packet to finish the RPC.


The server may finish the RPC at any time by sending the RESPONSE packet, even if it has not received the CLIENT_REQUEST_COMPLETION packet. The client may terminate the RPC at any time by sending a CLIENT_ERROR packet with status CANCELLED.


C++ API#

RPC server#

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


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.




Server by itself























































































































































































fit::internal::generic_function_base<>::operator bool()





















operator delete()



































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#

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::Client my_client(my_channels);

}  // namespace

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

Note that client processing such as callbacks will be invoked within the body of ProcessPacket.

If certain packets need to be filtered out, or if certain client processing needs to be invoked from a specific thread or context, the PacketMeta class can be used to determine which service or channel a packet is targeting. After filtering, ProcessPacket can be called from the appropriate environment.

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 call 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 call object is alive.


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

#include "pw_rpc/echo_service_nanopb.h"

namespace {
// Generated clients are namespaced with their proto library.
using EchoClient = pw_rpc::nanopb::EchoService::Client;

// RPC channel ID on which to make client calls. RPC calls cannot be made on
// channel 0 (Channel::kUnassignedChannelId).
constexpr uint32_t kDefaultChannelId = 1;

pw::rpc::NanopbUnaryReceiver<pw_rpc_EchoMessage> 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",

}  // namespace

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

  pw_rpc_EchoMessage request{};
  pw::string::Copy(message, request.msg);

  // By assigning the returned call 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 (! {
    // The RPC call was not sent. This could occur due to, for example, an
    // invalid channel ID. Handle if necessary.

Call objects#

An RPC call is represented by a call object. Server and client calls use the same base call class in C++, but the public API is different depending on the type of call (see RPC call lifecycle) and whether it is being used by the server or client.

The public call types are as follows:

RPC Type

Server call

Client call




Server streaming



Client streaming



Bidirectional streaming



Client call API#

Client call objects provide a few common methods.

class pw::rpc::ClientCallType#

The ClientCallType will be one of the following types:

  • (Raw|Nanopb|Pwpb)UnaryReceiver for unary

  • (Raw|Nanopb|Pwpb)ClientReader for server streaming

  • (Raw|Nanopb|Pwpb)ClientWriter for client streaming

  • (Raw|Nanopb|Pwpb)ClientReaderWriter for bidirectional streaming

bool active() const#

Returns true if the call is active.

uint32_t channel_id() const#

Returns the channel ID of this call, which is 0 if the call is inactive.

uint32_t id() const#

Returns the call ID, a unique identifier for this call.

void Write(RequestType)#

Only available on client and bidirectional streaming calls. Sends a stream request. Returns:

  • OK - the request was successfully sent

  • FAILED_PRECONDITION - the writer is closed

  • INTERNAL - pw_rpc was unable to encode message; does not apply to raw calls

  • other errors - the ChannelOutput failed to send the packet; the error codes are determined by the ChannelOutput implementation

pw::Status RequestCompletion()#

Notifies the server that client has requested for call completion. On client and bidirectional streaming calls no further client stream messages will be sent.

pw::Status Cancel()#

Cancels this RPC. Closes the call and sends a CANCELLED error to the server. Return statuses are the same as Write().

void Abandon()#

Closes this RPC locally. Sends a CLIENT_REQUEST_COMPLETION, but no cancellation packet. Future packets for this RPC are dropped, and the client sends a FAILED_PRECONDITION error in response because the call is not active.

void set_on_completed(pw::Function<void(ResponseTypeIfUnaryOnly, pw::Status)>)#

Sets the callback that is called when the RPC completes normally. The signature depends on whether the call has a unary or stream response.

void set_on_error(pw::Function<void(pw::Status)>)#

Sets the callback that is called when the RPC is terminated due to an error.

void set_on_next(pw::Function<void(ResponseType)>)#

Only available on server and bidirectional streaming calls. Sets the callback that is called for each stream response.


The C++ call objects allow users to set callbacks that are invoked when RPC events occur.


Stream signature

Non-stream signature








void(const PayloadType&)



void(const PayloadType&, pw::Status)




✅ (optional)

Limitations and restrictions#

RPC callbacks are free to perform most actions, including invoking new RPCs or cancelling pending calls. However, the C++ implementation imposes some limitations and restrictions that must be observed.

Destructors & moves wait for callbacks to complete#
  • Callbacks must not destroy their call object. Attempting to do so will result in deadlock.

  • Other threads may destroy a call while its callback is running, but that thread will block until all callbacks complete.

  • Callbacks must not move their call object if it the call is still active. They may move their call object after it has terminated. Callbacks may move a different call into their call object, since moving closes the destination call.

  • Other threads may move a call object while it has a callback running, but they will block until the callback completes if the call is still active.


Deadlocks or crashes occur if a callback:

  • attempts to destroy its call object

  • attempts to move its call object while the call is still active

  • never returns

If pw_rpc a callback violates these restrictions, a crash may occur, depending on the value of PW_RPC_CALLBACK_TIMEOUT_TICKS. These crashes have a message like the following:

A callback for RPC 1:cc0f6de0/31e616ce has not finished after 10000 ticks.
This may indicate that an RPC callback attempted to destroy or move its own
call object, which is not permitted. Fix this condition or change the value of
PW_RPC_CALLBACK_TIMEOUT_TICKS to avoid this crash.

for details.
Only one thread at a time may execute on_next#

Only one thread may execute the on_next callback for a specific service method at a time. If a second thread calls ProcessPacket() with a stream packet before the on_next callback for the previous packet completes, the second packet will be dropped. The RPC endpoint logs a warning when this occurs.

Example warning for a dropped stream packet:

WRN  Received stream packet for 1:cc0f6de0/31e616ce before the callback for
     a previous packet completed! This packet will be dropped. This can be
     avoided by handling packets for a particular RPC on only one thread.

RPC calls introspection#

pw_rpc provides pw_rpc/method_info.h header that allows to obtain information about the generated RPC method in compile time.

For now it provides only two types: MethodRequestType<RpcMethod> and MethodResponseType<RpcMethod>. They are aliases to the types that are used as a request and response respectively for the given RpcMethod.


We have an RPC service SpecialService with MyMethod method:

package some.package;
service SpecialService {
  rpc MyMethod(MyMethodRequest) returns (MyMethodResponse) {}

We also have a templated Storage type alias:

template <auto kMethod>
using Storage =
   std::pair<MethodRequestType<kMethod>, MethodResponseType<kMethod>>;

Storage<some::package::pw_rpc::pwpb::SpecialService::MyMethod> will instantiate as:



Only nanopb and pw_protobuf have real types as MethodRequestType<RpcMethod>/MethodResponseType<RpcMethod>. Raw has them both set as void. In reality, they are pw::ConstByteSpan. Any helper/trait that wants to use this types for raw methods should do a custom implementation that copies the bytes under the span instead of copying just the span.

Client Synchronous Call wrappers#

If synchronous behavior is desired when making client calls, users can use one of the SynchronousCall<RpcMethod> wrapper functions to make their RPC call. These wrappers effectively wrap the asynchronous Client RPC call with a timed thread notification and return once a result is known or a timeout has occurred. These return a SynchronousCallResult<Response> object, which can be queried to determine whether any error scenarios occurred and, if not, access the response.

SynchronousCall<RpcMethod> will block indefinitely, whereas SynchronousCallFor<RpcMethod> and SynchronousCallUntil<RpcMethod> will block for a given timeout or until a deadline, respectively. All wrappers work with both the standalone static RPC functions and the generated Client member methods.


Use of the SynchronousCall wrappers requires a TimedThreadNotification backend.


Only nanopb and pw_protobuf Unary RPC methods are supported.


#include "pw_rpc/synchronous_call.h"

void InvokeUnaryRpc() {
  pw::rpc::Client client;
  pw::rpc::Channel channel;

  RoomInfoRequest request;
  SynchronousCallResult<RoomInfoResponse> result =
    SynchronousCall<Chat::GetRoomInformation>(client,, request);

  if (result.is_rpc_error()) {
  } else if (result.is_server_error()) {
  } else if (result.is_timeout()) {
    // SynchronousCall will block indefinitely, so we should never get here.

void AnotherExample() {
  pw_rpc::nanopb::Chat::Client chat_client(client, channel);
  constexpr auto kTimeout = pw::chrono::SystemClock::for_at_least(500ms);

  RoomInfoRequest request;
  auto result = SynchronousCallFor<Chat::GetRoomInformation>(
      chat_client, request, kTimeout);

  if (result.is_timeout()) {
  } else {

The SynchronousCallResult<Response> is also compatible with the PW_TRY family of macros, but users should be aware that their use will lose information about the type of error. This should only be used if the caller will handle all error scenarios the same.

pw::Status SyncRpc() {
  const RoomInfoRequest request;
  PW_TRY_ASSIGN(const RoomInfoResponse& response,
                SynchronousCall<Chat::GetRoomInformation>(client, request));
  return pw::OkStatus();


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[] = {

// 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.


pw_rpc provides utilities for unit testing RPC services and client calls.

Client unit testing in C++#

pw_rpc supports invoking RPCs, simulating server responses, and checking what packets are sent by an RPC client in tests. Raw, Nanopb and Pwpb interfaces are supported. Code that uses the raw API may be tested with the raw test helpers, and vice versa. The Nanopb and Pwpb APIs also provides a test helper with a real client-server pair that supports testing of asynchronous messaging.

To test sychronous code that invokes RPCs, declare a RawClientTestContext, PwpbClientTestContext, or NanopbClientTestContext. These test context objects provide a preconfigured RPC client, channel, server fake, and buffer for encoding packets.

These test classes are defined in pw_rpc/raw/client_testing.h, pw_rpc/pwpb/client_testing.h, or pw_rpc/nanopb/client_testing.h.

Use the context’s client() and channel() to invoke RPCs. Use the context’s server() to simulate responses. To verify that the client sent the expected data, use the context’s output(), which is a FakeChannelOutput.

For example, the following tests a class that invokes an RPC. It checks that the expected data was sent and then simulates a response from the server.

#include "pw_rpc/raw/client_testing.h"

class ClientUnderTest {
  // To support injecting an RPC client for testing, classes that make RPC
  // calls should take an RPC client and channel ID or an RPC service client
  // (e.g. pw_rpc::raw::MyService::Client).
  ClientUnderTest(pw::rpc::Client& client, uint32_t channel_id);

  void DoSomethingThatInvokesAnRpc();

  bool SetToTrueWhenRpcCompletes();

TEST(TestAThing, InvokesRpcAndHandlesResponse) {
  RawClientTestContext context;
  ClientUnderTest thing(context.client(),;

  // Execute the code that invokes the MyService.TheMethod RPC.

  // Find and verify the payloads sent for the MyService.TheMethod RPC.
  auto msgs = context.output().payloads<pw_rpc::raw::MyService::TheMethod>();
  ASSERT_EQ(msgs.size(), 1u);


  // Send the response packet from the server and verify that the class reacts
  // accordingly.

      final_message, OkStatus());


To test client code that uses asynchronous responses, encapsulates multiple rpc calls to one or more services, or uses a custom service implemenation, declare a NanopbClientServerTestContextThreaded or PwpbClientServerTestContextThreaded. These test object are defined in pw_rpc/nanopb/client_server_testing_threaded.h and pw_rpc/pwpb/client_server_testing_threaded.h.

Use the context’s server() to register a Service implementation, and client() and channel() to invoke RPCs. Create a Thread using the context as a ThreadCore to have it asycronously forward request/responses or call ForwardNewPackets to synchronously process all messages. To verify that the client/server sent the expected data, use the context’s request(uint32_t index) and response(uint32_t index) to retrieve the ordered messages.

For example, the following tests a class that invokes an RPC and blocks till a response is received. It verifies that expected data was both sent and received.

#include "my_library_protos/my_service.rpc.pb.h"
#include "pw_rpc/nanopb/client_server_testing_threaded.h"
#include "pw_thread_stl/options.h"

class ClientUnderTest {
  // To support injecting an RPC client for testing, classes that make RPC
  // calls should take an RPC client and channel ID or an RPC service client
  // (e.g. pw_rpc::raw::MyService::Client).
  ClientUnderTest(pw::rpc::Client& client, uint32_t channel_id);

  Status BlockOnResponse(uint32_t value);

class TestService final : public MyService<TestService> {
  Status TheMethod(const pw_rpc_test_TheMethod& request,
                      pw_rpc_test_TheMethod& response) {
    response.value = request.integer + 1;
    return pw::OkStatus();

TEST(TestServiceTest, ReceivesUnaryRpcReponse) {
  NanopbClientServerTestContextThreaded<> ctx(pw::thread::stl::Options{});
  TestService service;
  ClientUnderTest client(ctx.client(),;

  // Execute the code that invokes the MyService.TheMethod RPC.
  constexpr uint32_t value = 1;
  const auto result = client.BlockOnResponse(value);
  const auto request = ctx.request<MyService::TheMethod>(0);
  const auto response = ctx.resonse<MyService::TheMethod>(0);

  // Verify content of messages
  EXPECT_EQ(result, pw::OkStatus());
  EXPECT_EQ(request.value, value);
  EXPECT_EQ(response.value, value + 1);

Synchronous versions of these test contexts also exist that may be used on non-threaded systems NanopbClientServerTestContext and PwpbClientServerTestContext. While these do not allow for asynchronous messaging they support the use of service implemenations and use a similar syntax. When these are used .ForwardNewPackets() should be called after each rpc call to trigger sending of queued messages.

For example, the following tests a class that invokes an RPC that is responded to with a test service implemenation.

#include "my_library_protos/my_service.rpc.pb.h"
#include "pw_rpc/nanopb/client_server_testing.h"

class ClientUnderTest {
  ClientUnderTest(pw::rpc::Client& client, uint32_t channel_id);

  Status SendRpcCall(uint32_t value);

class TestService final : public MyService<TestService> {
  Status TheMethod(const pw_rpc_test_TheMethod& request,
                      pw_rpc_test_TheMethod& response) {
    response.value = request.integer + 1;
    return pw::OkStatus();

TEST(TestServiceTest, ReceivesUnaryRpcReponse) {
  NanopbClientServerTestContext<> ctx();
  TestService service;
  ClientUnderTest client(ctx.client(),;

  // Execute the code that invokes the MyService.TheMethod RPC.
  constexpr uint32_t value = 1;
  const auto result = client.SendRpcCall(value);
  // Needed after ever RPC call to trigger forward of packets
  const auto request = ctx.request<MyService::TheMethod>(0);
  const auto response = ctx.resonse<MyService::TheMethod>(0);

  // Verify content of messages
  EXPECT_EQ(result, pw::OkStatus());
  EXPECT_EQ(request.value, value);
  EXPECT_EQ(response.value, value + 1);

SendResponseIfCalled() helper#

SendResponseIfCalled() function waits on *ClientTestContext* output to have a call for the specified method and then responses to it. It supports timeout for the waiting part (default timeout is 100ms).

#include "pw_rpc/test_helpers.h"

pw::rpc::PwpbClientTestContext client_context;
other::pw_rpc::pwpb::OtherService::Client other_service_client(


    client_context, {.value = 42}));

// At this point MyService::GetData handler received the GetPartResponse.

Integration testing with pw_rpc#

pw_rpc provides utilities to simplify writing integration tests for systems that communicate with pw_rpc. The integration test utitilies set up a socket to use for IPC between an RPC server and client process.

The server binary uses the system RPC server facade defined pw_rpc_system_server/rpc_server.h. The client binary uses the functions defined in pw_rpc/integration_testing.h:

constexpr uint32_t kChannelId#

The RPC channel for integration test RPCs.

pw::rpc::Client &pw::rpc::integration_test::Client()#

Returns the global RPC client for integration test use.

pw::Status pw::rpc::integration_test::InitializeClient(int argc, char *argv[], const char *usage_args = "PORT")#

Initializes logging and the global RPC client for integration testing. Starts a background thread that processes incoming.

Module Configuration Options#

The following configurations can be adjusted via compile-time configuration of this module, see the module documentation for more details.



pw_rpc clients may request call completion by sending CLIENT_REQUEST_COMPLETION packet. For client streaming or bi-direction RPCs, this also indicates that the client is done sending requests. While this can be useful in some circumstances, it is often not necessary.

This option controls whether or not include a callback that is called when the client stream requests for completion. The callback is included in all ServerReader/Writer objects as a pw::Function , so may have a significant cost.

This is disabled by default.


The Nanopb-based pw_rpc implementation allocates memory to use for Nanopb structs for the request and response protobufs. The template function that allocates these structs rounds struct sizes up to this value so that different structs can be allocated with the same function. Structs with sizes larger than this value cause an extra function to be created, which slightly increases code size.

Ideally, this value will be set to the size of the largest Nanopb struct used as an RPC request or response. The buffer can be stack or globally allocated (see PW_RPC_NANOPB_STRUCT_BUFFER_STACK_ALLOCATE ).

This defaults to 64 bytes.


Enable global synchronization for RPC calls. If this is set, a backend must be configured for pw_sync:mutex.

This is enabled by default.


pw_rpc must yield the current thread when waiting for a callback to complete in a different thread. PW_RPC_YIELD_MODE determines how to yield. There are three supported settings:


Supported configuration values for PW_RPC_YIELD_MODE .


If PW_RPC_YIELD_MODE == PW_RPC_YIELD_MODE_SLEEP, PW_RPC_YIELD_SLEEP_DURATION sets how long to sleep during each iteration of the yield loop. The value must be a constant expression that converts to a pw::chrono::SystemClock::duration .


pw_rpc call objects wait for their callbacks to complete before they are moved or destoyed. Deadlocks occur if a callback:

  • attempts to destroy its call object,

  • attempts to move its call object while the call is still active, or

  • never returns.

If PW_RPC_CALLBACK_TIMEOUT_TICKS is greater than 0, then PW_CRASH is invoked if a thread waits for an RPC callback to complete for more than the specified tick count.

A “tick” in this context is one iteration of a loop that yields releases the RPC lock and yields the thread according to PW_RPC_YIELD_MODE . By default, the thread yields with a 1-tick call to pw::this_thread::sleep_for() .


Whether pw_rpc should use dynamic memory allocation internally. If enabled, pw_rpc dynamically allocates channels and its encoding buffer. RPC users may use dynamic allocation independently of this option (e.g. to allocate pw_rpc call objects).

The semantics for allocating and initializing channels change depending on this option. If dynamic allocation is disabled, pw_rpc endpoints (servers or clients) use an externally-allocated, fixed-size array of channels. That array must include unassigned channels or existing channels must be closed to add new channels.

If dynamic allocation is enabled, an span of channels may be passed to the endpoint at construction, but these channels are only used to initialize its internal channels container. External channel objects are NOT used by the endpoint and cannot be updated if dynamic allocation is enabled. No unassigned channels should be passed to the endpoint; they will be ignored. Any number of channels may be added to the endpoint, without closing existing channels, but adding channels will use more memory.


If PW_RPC_DYNAMIC_ALLOCATION is enabled, this macro must expand to a container capable of storing objects of the provided type. This container will be used internally by pw_rpc to allocate the channels list and encoding buffer. Defaults to std::vector<type>, but may be set to any type that supports the following std::vector operations:

  • Default construction

  • emplace_back()

  • pop_back()

  • back()

  • resize()

  • clear()

  • Range-based for loop iteration (begin(), end())


If PW_RPC_DYNAMIC_ALLOCATION is enabled, this header file is included in files that use PW_RPC_DYNAMIC_CONTAINER . Defaults to <vector>, but may be set in conjunction with PW_RPC_DYNAMIC_CONTAINER to use a different container type for dynamic allocations in pw_rpc.


Size of the global RPC packet encoding buffer in bytes. If dynamic allocation is enabled, this value is only used for test helpers that allocate RPC encoding buffers.


The log level to use for this module. Logs below this level are omitted.


The log module name to use for this module.


This option determines whether to allocate the Nanopb structs on the stack or in a global variable. Globally allocated structs are NOT thread safe, but work fine when the RPC server’s ProcessPacket function is only called from one thread.


Internal macro for declaring the Nanopb struct; do not use.

Sharing server and client code#

Streaming RPCs support writing multiple requests or responses. To facilitate sharing code between servers and clients, pw_rpc provides the pw::rpc::Writer interface. On the client side, a client or bidirectional streaming RPC call object (ClientWriter or ClientReaderWriter) can be used as a pw::rpc::Writer&. On the server side, a server or bidirectional streaming RPC call object (ServerWriter or ServerReaderWriter) can be used as a pw::rpc::Writer&.


To enable pw_rpc.* for Zephyr add CONFIG_PIGWEED_RPC=y to the project’s configuration. This will enable the Kconfig menu for the following:

  • pw_rpc.server which can be enabled via CONFIG_PIGWEED_RPC_SERVER=y.

  • pw_rpc.client which can be enabled via CONFIG_PIGWEED_RPC_CLIENT=y.

  • pw_rpc.client_server which can be enabled via CONFIG_PIGWEED_RPC_CLIENT_SERVER=y.

  • pw_rpc.common` which can be enabled via ``CONFIG_PIGWEED_RPC_COMMON=y.

Encoding and sending packets#

pw_rpc has to manage interactions among multiple RPC clients, servers, client calls, and server calls. To safely synchronize these interactions with minimal overhead, pw_rpc uses a single, global mutex (when PW_RPC_USE_GLOBAL_MUTEX is enabled).

Because pw_rpc uses a global mutex, it also uses a global buffer to encode outgoing packets. The size of the buffer is set with PW_RPC_ENCODING_BUFFER_SIZE_BYTES, which defaults to 512 B. If dynamic allocation is enabled, this size does not affect how large RPC messages can be, but it is still used for sizing buffers in test utilities.

Users of pw_rpc must implement the pw::rpc::ChannelOutput interface.

class pw::rpc::ChannelOutput#

pw_rpc endpoints use ChannelOutput instances to send packets. Systems that integrate pw_rpc must use one or more ChannelOutput instances.

static constexpr size_t kUnlimited = std::numeric_limits<size_t>::max()#

Value returned from MaximumTransmissionUnit() to indicate an unlimited MTU.

virtual size_t MaximumTransmissionUnit()#

Returns the size of the largest packet the ChannelOutput can send. ChannelOutput implementations should only override this function if they impose a limit on the MTU. The default implementation returns kUnlimited, which indicates that there is no MTU limit.

virtual pw::Status Send(span<std::byte> packet)#

Sends an encoded RPC packet. Returns OK if further packets may be sent, even if the current packet could not be sent. Returns any other status if the Channel is no longer able to send packets.

The RPC system’s internal lock is held while this function is called. Avoid long-running operations, since these will delay any other users of the RPC system.


No pw_rpc APIs may be accessed in this function! Implementations MUST NOT access any RPC endpoints (pw::rpc::Client, pw::rpc::Server) or call objects (pw::rpc::ServerReaderWriter, pw::rpc::ClientReaderWriter, etc.) inside the Send() function or any descendent calls. Doing so will result in deadlock! RPC APIs may be used by other threads, just not within Send().

The buffer provided in packet must NOT be accessed outside of this function. It must be sent immediately or copied elsewhere before the function returns.