C++ server and client#

pw_rpc: Efficient, low-code-size RPC system for embedded devices

This page provides further guidance on how to use the C++ server and client libraries.

RPC server#

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

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

Delta

Server by itself

FLASH

+4

_ctype_

+4

quorem

+20

main

-2

fit::internal::unshared_target_type_id()

NEW

+330

pw::rpc::Server::ProcessPacket()

NEW

+188

pw::rpc::internal::Packet::Encode()

NEW

+186

pw::rpc::internal::Packet::FromBuffer()

NEW

+180

pw::protobuf::Decoder::GetFieldSize()

NEW

+152

pw::rpc::internal::Call::HandlePayload()

NEW

+124

pw::rpc::internal::Endpoint::ProcessPacket()

NEW

+120

pw::protobuf::StreamEncoder::CloseNestedMessage()

NEW

+120

pw::rpc::internal::ChannelBase::Send()

NEW

+116

pw::rpc::Server::HandleClientStreamPacket()

NEW

+112

pw::protobuf::Decoder::ConsumeKey()

NEW

+108

pw::protobuf::StreamEncoder::UpdateStatusForWrite()

NEW

+96

pw::protobuf::Decoder::ReadDelimited()

NEW

+86

pw_varint_Decode64

NEW

+82

pw::protobuf::StreamEncoder::WriteFixed()

NEW

+78

pw::protobuf::StreamEncoder::WriteVarintField()

NEW

+74

pw::protobuf::Decoder::ReadFixed()

NEW

+72

pw::rpc::internal::Endpoint::FindIteratorsForCall()

NEW

+70

pw::protobuf::StreamEncoder::WriteLengthDelimitedField()

NEW

+68

pw::rpc::Server::HandleCompletionRequest()

NEW

+66

pw::protobuf::Decoder::SkipField()

NEW

+66

pw::rpc::internal::Endpoint::RemoveAllCalls()

NEW

+66

pw::stream::MemoryWriter::DoWrite()

NEW

+64

_GLOBAL__sub_I_unoptimizable

NEW

+60

pw::protobuf::Decoder::FieldNumber()

NEW

+60

pw::protobuf::FieldKey::FieldKey()

NEW

+58

pw::protobuf::Decoder::ReadVarint()

NEW

+56

pw::rpc::internal::Call::CallOnError()

NEW

+54

pw::protobuf::Decoder::Next()

NEW

+54

pw::protobuf::FieldKey::IsValidKey()

NEW

+52

pw::protobuf::Decoder::ReadUint32()

NEW

+50

pw::rpc::internal::Endpoint::CleanUpCalls()

NEW

+48

pw::protobuf::StreamEncoder::CloseEncoder()

NEW

+48

pw::rpc::Server::FindMethodLocked()

NEW

+44

pw::rpc::Server::~Server()

NEW

+42

pw::protobuf::TagSizeBytes<>()

NEW

+42

pw::protobuf::WriteLengthDelimitedKeyAndLengthPrefix()

NEW

+42

pw::protobuf::WriteVarint()

NEW

+38

pw::rpc::internal::Call::UnregisterAndMarkClosed()

NEW

+36

pw::stream::MemoryWriter

NEW

+36

pw::stream::Stream::Write()

NEW

+36

pw::varint::EncodedSize<>()

NEW

+34

pw::rpc::Service::FindMethod()

NEW

+34

pw::stream::CalculateSeek()

NEW

+32

Output

NEW

+32

pw::protobuf::StreamEncoder::WriteVarint()

NEW

+32

pw::protobuf::ValidFieldNumber()

NEW

+32

pw::rpc::internal::ChannelList::Get()

NEW

+30

pw::intrusive_list_impl::List::remove()

NEW

+28

fit::internal::generic_function_base<>::move_target_from()

NEW

+28

pw::protobuf::StreamEncoder::WriteFixed32()

NEW

+26

Output::Send()

NEW

+20

fit::internal::null_target<>::ops

NEW

+20

pw::intrusive_list_impl::List::Item::unlist()

NEW

+20

pw::stream::MemoryWriter::DoSeek()

NEW

+16

Output::~Output()

NEW

+16

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

NEW

+16

pw::stream::MemoryWriter::~MemoryWriter()

NEW

+16

pw::stream::Writer::DoRead()

NEW

+14

pw::intrusive_list_impl::List::Item::previous()

NEW

+14

pw::stream::MemoryWriter::ConservativeLimit()

NEW

+12

pw::varint::Decode()

NEW

+8

pw::intrusive_list_impl::List::erase_after()

NEW

+8

pw::stream::Stream::ConservativeWriteLimit()

NEW

+6

fit::internal::generic_function_base<>::destroy_target()

NEW

+6

fit::internal::null_target<>::invoke()

NEW

+6

pw::protobuf::StreamEncoder::WriteUint32()

NEW

+6

pw::rpc::ChannelOutput::MaximumTransmissionUnit()

NEW

+4

fit::internal::null_target_get()

NEW

+4

pw::stream::MemoryWriter::DoTell()

NEW

+2

fit::internal::null_target_move()

+4,028

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#
flowchart LR packets[Packets] subgraph pw_rpc [pw_rpc Library] direction TB internalMethod[[internal::Method]] Server --> Service --> internalMethod end packets --> Server generatedServices{{generated services}} userDefinedRPCs(user-defined RPCs) generatedServices --> userDefinedRPCs internalMethod --> generatedServices
Responses#
flowchart LR generatedServices{{generated services}} userDefinedRPCs(user-defined RPCs) subgraph pw_rpc [pw_rpc Library] direction TB internalMethod[[internal::Method]] internalMethod --> Server --> Channel end packets[Packets] Channel --> packets userDefinedRPCs --> generatedServices generatedServices --> internalMethod

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);
}

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.

Tip

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

Example#

#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",
                 static_cast<int>(status.code()));
  }
}

}  // 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 (!echo_call.active()) {
    // The RPC call was not sent. This could occur due to, for example, an
    // invalid channel ID. Handle if necessary.
  }
}

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);
}

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.
client->CloseChannel(1);

Remapping channels#

Some pw_rpc deployments may find it helpful to remap channel IDs in RPC packets. This can remove the need for globally known channel IDs. Clients can use a generic channel ID. The server remaps the generic channel ID to an ID associated with the transport the client is using.

Result<uint32_t> ExtractChannelId(ConstByteSpan packet)#

Extracts the channel ID from a pw_rpc packet.

Returns:

Code

Description

OK

returns the channel ID in the packet

DATA_LOSS

the packet is corrupt and the channel ID could not be found.

template<uint32_t kNewChannelId>
Status ChangeEncodedChannelId(ByteSpan rpc_packet)#

Rewrites an encoded packet’s channel ID in place. Both channel IDs MUST be less than 128.

Returns:

Code

Description

OK

Successfully replaced the channel ID

DATA_LOSS

parsing the packet failed

OUT_OF_RANGE

the encoded packet’s channel ID was 128 or larger

inline Status ChangeEncodedChannelId(ByteSpan rpc_packet, uint32_t new_channel_id)#

Version of ChangeEncodedChannelId with a runtime variable channel ID. Prefer the template parameter version when possible to avoid a runtime check on the new channel ID.

A future revision of the pw_rpc protocol will remove the need for global channel IDs without requiring remapping.

Example deployment#

This section describes a hypothetical pw_rpc deployment that supports arbitrary pw_rpc clients with one pw_rpc server. Note that this assumes that the underlying transport provides some sort of addressing that the server-side can associate with a channel ID.

  • A pw_rpc server is running on one core. A variable number of pw_rpc clients need to call RPCs on the server from a different core.

  • The client core opens a socket (or similar feature) to connect to the server core.

  • The server core detects the inbound connection and allocates a new channel ID. It creates a new channel by calling pw::rpc::Server::OpenChannel() with the channel ID and a pw::rpc::ChannelOutput associated with the new connection.

  • The server maintains a mapping between channel IDs and pw_rpc client connections.

  • On the client core, pw_rpc clients all use the same channel ID (e.g. 1).

  • As packets arrive from pw_rpc client connections, the server-side code calls pw::rpc::ChangeEncodedChannelId() on the encoded packet to replace the generic channel ID (1) with the server-side channel ID allocated when the client connected. The encoded packet is then passed to pw::rpc::Server::ProcessPacket().

  • When the server sends pw_rpc packets, the pw::rpc::ChannelOutput calls pw::rpc::ChangeEncodedChannelId() to set the channel ID back to the generic 1.

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> {
 public:
  void List(ConstByteSpan request, RawServerWriter& writer);

 private:
  // 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

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 and whether it is being used by the server or client. See RPC call lifecycle.

The public call types are as follows:

RPC Type

Server call

Client call

Unary

(Raw|Nanopb|Pwpb)UnaryResponder

(Raw|Nanopb|Pwpb)UnaryReceiver

Server streaming

(Raw|Nanopb|Pwpb)ServerWriter

(Raw|Nanopb|Pwpb)ClientReader

Client streaming

(Raw|Nanopb|Pwpb)ServerReader

(Raw|Nanopb|Pwpb)ClientWriter

Bidirectional streaming

(Raw|Nanopb|Pwpb)ServerReaderWriter

(Raw|Nanopb|Pwpb)ClientReaderWriter

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 CloseAndWaitForCallbacks()#

Abandons this RPC and additionally blocks on completion of any running callbacks.

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.

Callbacks#

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

Name

Stream signature

Non-stream signature

Server

Client

on_error

void(pw::Status)

void(pw::Status)

on_next

n/a

void(const PayloadType&)

on_completed

void(pw::Status)

void(const PayloadType&, pw::Status)

on_client_requested_completion

void()

n/a

✅ (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.

Warning

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.

See https://pigweed.dev/pw_rpc#destructors-moves-wait-for-callbacks-to-complete
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.

Example#

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:

std::pair<some::package::MyMethodRequest::Message,
          some::package::MyMethodResponse::Message>;

Note

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#

pw_rpc provides wrappers that convert the asynchronous client API to a synchronous API. The SynchronousCall<RpcMethod> functions wrap the asynchronous client RPC call with a timed thread notification and returns once a result is known or a timeout has occurred. Only unary methods are supported.

The Nanopb and pwpb APIs return a SynchronousCallResult<Response> object, which can be queried to determine whether any error scenarios occurred and, if not, access the response. The raw API executes a function when the call completes or returns a pw::Status if it does not.

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

The following examples use the Nanopb API to make a call that blocks indefinitely. If you’d like to include a timeout for how long the call should block for, use the SynchronousCallFor() or SynchronousCallUntil() variants.

pw_rpc_EchoMessage request{.msg = "hello" };
pw::rpc::SynchronousCallResult<pw_rpc_EchoMessage> result =
  pw::rpc::SynchronousCall<EchoService::Echo>(rpc_client,
                                              channel_id,
                                              request);
if (result.ok()) {
  PW_LOG_INFO("%s", result.response().msg);
}

Additionally, the use of a generated Client object is supported:

pw_rpc::nanopb::EchoService::Client client(rpc_client, channel_id);
pw_rpc_EchoMessage request{.msg = "hello" };
pw::rpc::SynchronousCallResult<pw_rpc_EchoMessage> result =
  pw::rpc::SynchronousCall<EchoService::Echo>(client, request);

if (result.ok()) {
  PW_LOG_INFO("%s", result.response().msg);
}

SynchronousCall<RpcMethod> also supports using an optional custom response message class, SynchronousCall<RpcMethod, Response>. This enables the use of response messages with variable-length fields.

pw_rpc_MyMethodRequestMessage request{};
class CustomResponse : public pw_rpc_MyMethodResponseMessage {
 public:
  CustomResponse() {
    repeated_field.SetDecoder([this](
      MyMethodResponse::StreamDecoder& decoder) {
        return decoder.ReadRepeatedField(values);
      }
  }
  pw::Vector<uint32_t, 4> values();
 };
 pw::rpc::SynchronousCallResult<CustomResponse> result =
   pw::rpc::SynchronousCall<EchoService::Echo, CustomResponse>(rpc_client,
                                                               channel_id,
                                                               request);
 if (result.ok()) {
   PW_LOG_INFO("%d", result.response().values[0]);
 }
};

The raw API works similarly to the Nanopb API, but takes a pw::Function and returns a pw::Status . If the RPC completes, the pw::Function is called with the response and returned status, and the SynchronousCall invocation returns OK . If the RPC fails, SynchronousCall returns an error.

pw::Status rpc_status = pw::rpc::SynchronousCall<EchoService::Echo>(
    rpc_client, channel_id, encoded_request,
    [](pw::ConstByteSpan reply, pw::Status status) {
      PW_LOG_INFO("Received %zu bytes with status %s",
                  reply.size(),
                  status.str());
    });

Note

Use of the SynchronousCall wrappers requires a pw::sync::TimedThreadNotification backend.

Warning

These wrappers should not be used from any context that cannot be blocked! This method will block the calling thread until the RPC completes, and translate the response into a pw::rpc::SynchronousCallResult that contains the error type and status or the proto response.

Example#

#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, channel.id(), request);

  if (result.is_rpc_error()) {
    ShutdownClient(client);
  } else if (result.is_server_error()) {
    HandleServerError(result.status());
  } else if (result.is_timeout()) {
    // SynchronousCall will block indefinitely, so we should never get here.
    PW_UNREACHABLE();
  }
  HandleRoomInformation(std::move(result).response());
}

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()) {
    RetryRoomRequest();
  } 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));
  HandleRoomInformation(response);
  return pw::OkStatus();
}

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);
}

Testing#

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 synchronous 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 {
 public:
  // 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(), context.channel().id());

  // Execute the code that invokes the MyService.TheMethod RPC.
  things.DoSomethingThatInvokesAnRpc();

  // 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);

  VerifyThatTheExpectedMessageWasSent(msgs.back());

  // Send the response packet from the server and verify that the class reacts
  // accordingly.
  EXPECT_FALSE(thing.SetToTrueWhenRpcCompletes());

  context_.server().SendResponse<pw_rpc::raw::MyService::TheMethod>(
      final_message, OkStatus());

  EXPECT_TRUE(thing.SetToTrueWhenRpcCompletes());
}

To test client code that uses asynchronous responses, encapsulates multiple rpc calls to one or more services, or uses a custom service implementation, 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 asynchronously 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 {
 public:
  // 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> {
 public:
  Status TheMethod(const pw_rpc_test_TheMethod& request,
                      pw_rpc_test_TheMethod& response) {
    response.value = request.integer + 1;
    return pw::OkStatus();
  }
};

TEST(TestServiceTest, ReceivesUnaryRpcResponse) {
  NanopbClientServerTestContextThreaded<> ctx(pw::thread::stl::Options{});
  TestService service;
  ctx.server().RegisterService(service);
  ClientUnderTest client(ctx.client(), ctx.channel().id());

  // 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.response<MyService::TheMethod>(0);

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

Use the context’s response(uint32_t index, Response<kMethod>& response) to decode messages into a provided response object. You would use this version if decoder callbacks are needed to fully decode a message. For instance if it uses repeated fields.

TestResponse::Message response{};
response.repeated_field.SetDecoder(
    [&values](TestResponse::StreamDecoder& decoder) {
      return decoder.ReadRepeatedField(values);
    });
ctx.response<test::GeneratedService::TestAnotherUnaryRpc>(0, response);

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 implementations 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 implementation.

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

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

  Status SendRpcCall(uint32_t value);
};


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

TEST(TestServiceTest, ReceivesUnaryRpcResponse) {
  NanopbClientServerTestContext<> ctx();
  TestService service;
  ctx.server().RegisterService(service);
  ClientUnderTest client(ctx.client(), ctx.channel().id());

  // 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
  ctx.ForwardNewPackets();
  const auto request = ctx.request<MyService::TheMethod>(0);
  const auto response = ctx.response<MyService::TheMethod>(0);

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

Custom packet processing for ClientServerTestContext#

Optional constructor arguments for nanopb/pwpb *ClientServerTestContext and *ClientServerTestContextThreaded allow allow customized packet processing. By default the only thing is done is ProcessPacket() call on the ClientServer instance.

For cases when additional instrumentation or offloading to separate thread is needed, separate client and server processors can be passed to context constructors. A packet processor is a function that returns pw::Status and accepts two arguments: pw::rpc::ClientServer& and pw::ConstByteSpan. Default packet processing is equivalent to the next processor:

[](ClientServer& client_server, pw::ConstByteSpan packet) -> pw::Status {
  return client_server.ProcessPacket(packet);
};

The Server processor will be applied to all packets sent to the server (i.e. requests) and client processor will be applied to all packets sent to the client (i.e. responses).

Note

The packet processor MUST call ClientServer::ProcessPacket() method. Otherwise the packet won’t be processed.

Note

If the packet processor offloads processing to the separate thread, it MUST copy the packet. After the packet processor returns, the underlying array can go out of scope or be reused for other purposes.

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.client(), client_context.channel().id());

PW_PWPB_TEST_METHOD_CONTEXT(MyService, GetData)
context(other_service_client);
context.call({});

ASSERT_OK(pw::rpc::test::SendResponseIfCalled<
          other::pw_rpc::pwpb::OtherService::GetPart>(
    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.

Configuration options#

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

Defines

PW_RPC_COMPLETION_REQUEST_CALLBACK#

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.

PW_RPC_METHOD_STORES_TYPE#

pw_rpc Method’s can include their MethodType as a runtime accessible variable.

This isn’t needed for most applications so is disabled by default.

PW_RPC_NANOPB_STRUCT_MIN_BUFFER_SIZE#

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.

PW_RPC_USE_GLOBAL_MUTEX#

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_YIELD_MODE#

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:

PW_RPC_YIELD_MODE_BUSY_LOOP#
PW_RPC_YIELD_MODE_SLEEP#
PW_RPC_YIELD_MODE_YIELD#

Supported configuration values for PW_RPC_YIELD_MODE .

PW_RPC_YIELD_SLEEP_DURATION#

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_CALLBACK_TIMEOUT_TICKS#

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() .

PW_RPC_DYNAMIC_ALLOCATION#

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.

PW_RPC_DYNAMIC_CONTAINER(type)#

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())

PW_RPC_DYNAMIC_CONTAINER_INCLUDE#

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.

PW_RPC_MAKE_UNIQUE_PTR(type, ...)#

If PW_RPC_DYNAMIC_ALLOCATION is enabled, this macro must expand to a statement that creates a std::unique_ptr-like smart pointer.

Parameters:
  • type – The type of the object to construct (e.g. with new)

  • ... – Arguments to pass to the constructor, if any

PW_RPC_MAKE_UNIQUE_PTR_INCLUDE#

If PW_RPC_DYNAMIC_ALLOCATION is enabled, this header file is included in files that use PW_RPC_MAKE_UNIQUE_PTR . Defaults to <memory> for std::make_unique.

PW_RPC_ENCODING_BUFFER_SIZE_BYTES#

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.

PW_RPC_CONFIG_LOG_LEVEL#

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

PW_RPC_CONFIG_LOG_MODULE_NAME#

The log module name to use for this module.

PW_RPC_NANOPB_STRUCT_BUFFER_STACK_ALLOCATE#

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.

_PW_RPC_NANOPB_STRUCT_STORAGE_CLASS#

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&. Call as_writer() to get a Writer& of the client or server call object.

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.

Danger

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.