pw_rpc_transport#

The pw_rpc_transport provides a transport layer for pw_rpc.

Warning

This is an experimental module currently under development. APIs and functionality may change at any time.

pw_rpc provides a system for defining and invoking remote procedure calls (RPCs) on a device. It does not include any transports for sending these RPC calls. On a real device there could be multiple ways of inter-process and/or inter-core communication: hardware mailboxes, shared memory, network sockets, Unix domain sockets. pw_rpc_transport provides means to implement various transports and integrate them with pw_rpc services.

pw_rpc_transport relies on the assumption that a pw_rpc channel ID uniquely identifies both sides of an RPC conversation. It allows developers to define transports, egresses and ingresses for various channel IDs and choose what framing will be used to send RPC packets over those transports.

RpcFrame#

Framed RPC data ready to be sent via RpcFrameSender. Consists of a header and a payload. Some RPC transport encodings may not require a header and put all of the framed data into the payload (in which case the header can be an empty span).

A single RPC packet can be split into multiple RpcFrame’s depending on the MTU of the transport.

All frames for an RPC packet are expected to be sent and received in order without being interleaved by other packets’ frames.

RpcFrameSender#

Sends RPC frames over some communication channel (e.g. a hardware mailbox, shared memory, or a socket). It exposes its MTU size and generally only knows how to send an RpcFrame of a size that doesn’t exceed that MTU.

RpcPacketEncoder / RpcPacketDecoder#

RpcPacketEncoder is used to split and frame an RPC packet. RpcPacketDecoder then does the opposite e.g. stitches together received frames and removes any framing added by the encoder.

RpcEgressHandler#

Provides means of sending an RPC packet to its destination. Typically it ties together an RpcPacketEncoder and RpcFrameSender.

RpcIngressHandler#

Provides means of receiving RPC packets over some transport. Typically it has logic for reading RPC frames from some transport (a network connection, shared memory, or a hardware mailbox), stitching and decoding them with RpcPacketDecoder and passing full RPC packets to their intended processor via RpcPacketProcessor.

RpcPacketProcessor#

Used by RpcIngressHandler to send the received RPC packet to its intended handler (e.g. a pw_rpc Service).

Creating a transport#

RPC transports implement pw::rpc::RpcFrameSender. The transport exposes its maximum transmission unit (MTU) and only knows how to send packets of up to the size of that MTU.

class MyRpcTransport : public RpcFrameSender {
public:
  size_t mtu() const override { return 128; }

  Status Send(RpcFrame frame) override {
    // Send the frame via mailbox, shared memory or some other mechanism...
  }
};

Integration with pw_stream#

An RpcFrameSender implementaion that wraps a pw::stream::Writer is provided by pw::rpc::StreamRpcFrameSender. As the stream interface doesn’t know about MTU’s, it’s up to the user to select one.

stream::SysIoWriter writer;
StreamRpcFrameSender<kMtu> sender(writer);

A thread to feed data to a pw::rpc::RpcIngressHandler from a pw::stream::Reader is provided by pw::rpc::StreamRpcDispatcher.

rpc::HdlcRpcIngress<kMaxRpcPacketSize> hdlc_ingress(...);
stream::SysIoReader reader;

// Feed Hdlc ingress with bytes from sysio.
rpc::StreamRpcDispatcher<kMaxSysioRead> sysio_dispatcher(reader,
                                                         hdlc_ingress);

thread::DetachedThread(SysioDispatcherThreadOptions(),
                       sysio_dispatcher);

Using transports: a sample three-node setup#

A transport must be properly registered in order for pw_rpc to correctly route its packets. Below is an example of using a SocketRpcTransport and a (hypothetical) SharedMemoryRpcTransport to set up RPC connectivity between three endpoints.

Node A runs pw_rpc clients who want to talk to nodes B and C using kChannelAB and kChannelAC respectively. However there is no direct connectivity from A to C: only B can talk to C over shared memory while A can talk to B over a socket connection. Also, some services on A are self-hosted and accessed from the same process on kChannelAA:

// Set up A->B transport over a network socket where B is a server
// and A is a client.
SocketRpcTransport<kSocketReadBufferSize> a_to_b_transport(
  SocketRpcTransport<kSocketReadBufferSize>::kAsClient, "localhost",
  kNodeBPortNumber);

// LocalRpcEgress handles RPC packets received from other nodes and destined
// to this node.
LocalRpcEgress<kLocalEgressQueueSize, kMaxPacketSize> local_egress;
// HdlcRpcEgress applies HDLC framing to all packets outgoing over the A->B
// transport.
HdlcRpcEgress<kMaxPacketSize> a_to_b_egress("a->b", a_to_b_transport);

// List of channels for all packets originated locally at A.
std::array tx_channels = {
  // Self-destined packets go directly to local egress.
  Channel::Create<kChannelAA>(&local_egress),
  // Packets to B and C go over A->B transport.
  Channel::Create<kChannelAB>(&a_to_b_egress),
  Channel::Create<kChannelAC>(&a_to_b_egress),
};

// Here we list all egresses for the packets _incoming_ from B.
std::array b_rx_channels = {
  // Packets on both AB and AC channels are destined locally; hence sending
  // to the local egress.
  ChannelEgress{kChannelAB, local_egress},
  ChannelEgress{kChannelAC, local_egress},
};

// HdlcRpcIngress complements HdlcRpcEgress: all packets received on
// `b_rx_channels` are assumed to have HDLC framing.
HdlcRpcIngress<kMaxPacketSize> b_ingress(b_rx_channels);

// Local egress needs to know how to send received packets to their target
// pw_rpc service.
ServiceRegistry registry(tx_channels);
local_egress.set_packet_processor(registry);
// Socket transport needs to be aware of what ingress it's handling.
a_to_b_transport.set_ingress(b_ingress);

// Both RpcSocketTransport and LocalRpcEgress are ThreadCore's and
// need to be started in order for packet processing to start.
DetachedThread(/*...*/, a_to_b_transport);
DetachedThread(/*...*/, local_egress);

Node B setup is the most complicated since it needs to deal with egress and ingress from both A and B and needs to support two kinds of transports. Note that A is unaware of which transport and framing B is using when talking to C:

// This is the server counterpart to A's client socket.
SocketRpcTransport<kSocketReadBufferSize> b_to_a_transport(
  SocketRpcTransport<kSocketReadBufferSize>::kAsServer, "localhost",
  kNodeBPortNumber);

SharedMemoryRpcTransport b_to_c_transport(/*...*/);

LocalRpcEgress<kLocalEgressQueueSize, kMaxPacketSize> local_egress;
HdlcRpcEgress<kMaxPacketSize> b_to_a_egress("b->a", b_to_a_transport);
// SimpleRpcEgress applies a very simple length-prefixed framing to B->C
// traffic (because HDLC adds unnecessary overhead over shared memory).
SimpleRpcEgress<kMaxPacketSize> b_to_c_egress("b->c", b_to_c_transport);

// List of channels for all packets originated locally at B (note that in
// this example B doesn't need to talk to C directly; it only proxies for A).
std::array tx_channels = {
  Channel::Create<kChannelAB>(&b_to_a_egress),
};

// Here we list all egresses for the packets _incoming_ from A.
std::array a_rx_channels = {
  ChannelEgress{kChannelAB, local_egress},
  ChannelEgress{kChannelAC, b_to_c_egress},
};

// Here we list all egresses for the packets _incoming_ from C.
std::array c_rx_channels = {
  ChannelEgress{kChannelAC, b_to_a_egress},
};

HdlcRpcIngress<kMaxPacketSize> b_ingress(b_rx_channels);
SimpleRpcIngress<kMaxPacketSize> c_ingress(c_rx_channels);

ServiceRegistry registry(tx_channels);
local_egress.set_packet_processor(registry);

b_to_a_transport.set_ingress(a_ingress);
b_to_c_transport.set_ingress(c_ingress);

DetachedThread({}, b_to_a_transport);
DetachedThread({}, b_to_c_transport);
DetachedThread({}, local_egress);

Node C setup is straightforward since it only needs to handle ingress from B:

SharedMemoryRpcTransport c_to_b_transport(/*...*/);
LocalRpcEgress<kLocalEgressQueueSize, kMaxPacketSize> local_egress;
SimpleRpcEgress<kMaxPacketSize> c_to_b_egress("c->b", c_to_b_transport);

std::array tx_channels = {
  Channel::Create<kChannelAC>(&c_to_b_egress),
};

// Here we list all egresses for the packets _incoming_ from B.
std::array b_rx_channels = {
  ChannelEgress{kChannelAC, local_egress},
};

SimpleRpcIngress<kMaxPacketSize> b_ingress(b_rx_channels);

ServiceRegistry registry(tx_channels);
local_egress.set_packet_processor(registry);

c_to_b_transport.set_ingress(b_ingress);

DetachedThread(/*...*/, c_to_b_transport);
DetachedThread(/*...*/, local_egress);
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