Wyatt Hepler | f9fb90f | 2020-09-30 18:59:33 -0700 | [diff] [blame] | 1 | .. _module-pw_rpc: |
Alexei Frolov | 26e3ae6 | 2020-05-04 17:06:17 -0700 | [diff] [blame] | 2 | |
| 3 | ------ |
| 4 | pw_rpc |
| 5 | ------ |
| 6 | The ``pw_rpc`` module provides a system for defining and invoking remote |
| 7 | procedure calls (RPCs) on a device. |
| 8 | |
Wyatt Hepler | 830d26d | 2021-02-17 09:07:43 -0800 | [diff] [blame] | 9 | This document discusses the ``pw_rpc`` protocol and its C++ implementation. |
| 10 | ``pw_rpc`` implementations for other languages are described in their own |
| 11 | documents: |
| 12 | |
| 13 | .. toctree:: |
| 14 | :maxdepth: 1 |
| 15 | |
| 16 | py/docs |
| 17 | |
Wyatt Hepler | 455b492 | 2020-09-18 00:19:21 -0700 | [diff] [blame] | 18 | .. admonition:: Try it out! |
| 19 | |
Wyatt Hepler | f9fb90f | 2020-09-30 18:59:33 -0700 | [diff] [blame] | 20 | For a quick intro to ``pw_rpc``, see the |
Alexei Frolov | d3e5cb7 | 2021-01-08 13:08:45 -0800 | [diff] [blame] | 21 | :ref:`module-pw_hdlc-rpc-example` in the :ref:`module-pw_hdlc` module. |
Wyatt Hepler | 455b492 | 2020-09-18 00:19:21 -0700 | [diff] [blame] | 22 | |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 23 | .. attention:: |
Alexei Frolov | 26e3ae6 | 2020-05-04 17:06:17 -0700 | [diff] [blame] | 24 | |
Wyatt Hepler | 455b492 | 2020-09-18 00:19:21 -0700 | [diff] [blame] | 25 | This documentation is under construction. |
| 26 | |
| 27 | Creating an RPC |
| 28 | =============== |
| 29 | |
| 30 | 1. RPC service declaration |
| 31 | -------------------------- |
| 32 | Pigweed RPCs are declared in a protocol buffer service definition. |
| 33 | |
| 34 | * `Protocol Buffer service documentation |
| 35 | <https://developers.google.com/protocol-buffers/docs/proto3#services>`_ |
| 36 | * `gRPC service definition documentation |
| 37 | <https://grpc.io/docs/what-is-grpc/core-concepts/#service-definition>`_ |
| 38 | |
| 39 | .. code-block:: protobuf |
| 40 | |
| 41 | syntax = "proto3"; |
| 42 | |
| 43 | package foo.bar; |
| 44 | |
| 45 | message Request {} |
| 46 | |
| 47 | message Response { |
| 48 | int32 number = 1; |
| 49 | } |
| 50 | |
| 51 | service TheService { |
| 52 | rpc MethodOne(Request) returns (Response) {} |
| 53 | rpc MethodTwo(Request) returns (stream Response) {} |
| 54 | } |
| 55 | |
| 56 | This protocol buffer is declared in a ``BUILD.gn`` file as follows: |
| 57 | |
| 58 | .. code-block:: python |
| 59 | |
| 60 | import("//build_overrides/pigweed.gni") |
| 61 | import("$dir_pw_protobuf_compiler/proto.gni") |
| 62 | |
| 63 | pw_proto_library("the_service_proto") { |
| 64 | sources = [ "foo_bar/the_service.proto" ] |
| 65 | } |
| 66 | |
Wyatt Hepler | 830d26d | 2021-02-17 09:07:43 -0800 | [diff] [blame] | 67 | .. admonition:: proto2 or proto3 syntax? |
| 68 | |
| 69 | Always use proto3 syntax rather than proto2 for new protocol buffers. Proto2 |
| 70 | protobufs can be compiled for ``pw_rpc``, but they are not as well supported |
| 71 | as proto3. Specifically, ``pw_rpc`` lacks support for non-zero default values |
| 72 | in proto2. When using Nanopb with ``pw_rpc``, proto2 response protobufs with |
| 73 | non-zero field defaults should be manually initialized to the default struct. |
| 74 | |
| 75 | In the past, proto3 was sometimes avoided because it lacked support for field |
| 76 | presence detection. Fortunately, this has been fixed: proto3 now supports |
| 77 | ``optional`` fields, which are equivalent to proto2 ``optional`` fields. |
| 78 | |
| 79 | If you need to distinguish between a default-valued field and a missing field, |
| 80 | mark the field as ``optional``. The presence of the field can be detected |
| 81 | with a ``HasField(name)`` or ``has_<field>`` member, depending on the library. |
| 82 | |
| 83 | Optional fields have some overhead --- default-valued fields are included in |
| 84 | the encoded proto, and, if using Nanopb, the proto structs have a |
| 85 | ``has_<field>`` flag for each optional field. Use plain fields if field |
| 86 | presence detection is not needed. |
| 87 | |
| 88 | .. code-block:: protobuf |
| 89 | |
| 90 | syntax = "proto3"; |
| 91 | |
| 92 | message MyMessage { |
| 93 | // Leaving this field unset is equivalent to setting it to 0. |
| 94 | int32 number = 1; |
| 95 | |
| 96 | // Setting this field to 0 is different from leaving it unset. |
| 97 | optional int32 other_number = 2; |
| 98 | } |
| 99 | |
Wyatt Hepler | 8779bcd | 2020-11-25 07:25:16 -0800 | [diff] [blame] | 100 | 2. RPC code generation |
| 101 | ---------------------- |
| 102 | ``pw_rpc`` generates a C++ header file for each ``.proto`` file. This header is |
| 103 | generated in the build output directory. Its exact location varies by build |
| 104 | system and toolchain, but the C++ include path always matches the sources |
| 105 | declaration in the ``pw_proto_library``. The ``.proto`` extension is replaced |
| 106 | with an extension corresponding to the protobuf library in use. |
Wyatt Hepler | 455b492 | 2020-09-18 00:19:21 -0700 | [diff] [blame] | 107 | |
Wyatt Hepler | 8779bcd | 2020-11-25 07:25:16 -0800 | [diff] [blame] | 108 | ================== =============== =============== ============= |
| 109 | Protobuf libraries Build subtarget Protobuf header pw_rpc header |
| 110 | ================== =============== =============== ============= |
| 111 | Raw only .raw_rpc (none) .raw_rpc.pb.h |
| 112 | Nanopb or raw .nanopb_rpc .pb.h .rpc.pb.h |
| 113 | pw_protobuf or raw .pwpb_rpc .pwpb.h .rpc.pwpb.h |
| 114 | ================== =============== =============== ============= |
| 115 | |
| 116 | For example, the generated RPC header for ``"foo_bar/the_service.proto"`` is |
| 117 | ``"foo_bar/the_service.rpc.pb.h"`` for Nanopb or |
| 118 | ``"foo_bar/the_service.raw_rpc.pb.h"`` for raw RPCs. |
| 119 | |
| 120 | The generated header defines a base class for each RPC service declared in the |
| 121 | ``.proto`` file. A service named ``TheService`` in package ``foo.bar`` would |
| 122 | generate the following base class: |
Wyatt Hepler | 455b492 | 2020-09-18 00:19:21 -0700 | [diff] [blame] | 123 | |
| 124 | .. cpp:class:: template <typename Implementation> foo::bar::generated::TheService |
| 125 | |
Wyatt Hepler | 8779bcd | 2020-11-25 07:25:16 -0800 | [diff] [blame] | 126 | 3. RPC service definition |
| 127 | ------------------------- |
| 128 | The serivce class is implemented by inheriting from the generated RPC service |
| 129 | base class and defining a method for each RPC. The methods must match the name |
| 130 | and function signature for one of the supported protobuf implementations. |
| 131 | Services may mix and match protobuf implementations within one service. |
| 132 | |
| 133 | .. tip:: |
| 134 | |
| 135 | The generated code includes RPC service implementation stubs. You can |
| 136 | reference or copy and paste these to get started with implementing a service. |
| 137 | These stub classes are generated at the bottom of the pw_rpc proto header. |
| 138 | |
Wyatt Hepler | df38ed1 | 2021-03-24 08:42:48 -0700 | [diff] [blame] | 139 | To use the stubs, do the following: |
| 140 | |
| 141 | #. Locate the generated RPC header in the build directory. For example: |
| 142 | |
| 143 | .. code-block:: sh |
| 144 | |
| 145 | find out/ -name <proto_name>.rpc.pb.h |
| 146 | |
| 147 | #. Scroll to the bottom of the generated RPC header. |
| 148 | #. Copy the stub class declaration to a header file. |
| 149 | #. Copy the member function definitions to a source file. |
| 150 | #. Rename the class or change the namespace, if desired. |
| 151 | #. List these files in a build target with a dependency on the |
| 152 | ``pw_proto_library``. |
| 153 | |
Wyatt Hepler | 455b492 | 2020-09-18 00:19:21 -0700 | [diff] [blame] | 154 | A Nanopb implementation of this service would be as follows: |
| 155 | |
| 156 | .. code-block:: cpp |
| 157 | |
Wyatt Hepler | 8779bcd | 2020-11-25 07:25:16 -0800 | [diff] [blame] | 158 | #include "foo_bar/the_service.rpc.pb.h" |
| 159 | |
Wyatt Hepler | 455b492 | 2020-09-18 00:19:21 -0700 | [diff] [blame] | 160 | namespace foo::bar { |
| 161 | |
| 162 | class TheService : public generated::TheService<TheService> { |
| 163 | public: |
| 164 | pw::Status MethodOne(ServerContext& ctx, |
| 165 | const foo_bar_Request& request, |
| 166 | foo_bar_Response& response) { |
| 167 | // implementation |
Wyatt Hepler | 1b3da3a | 2021-01-07 13:26:57 -0800 | [diff] [blame] | 168 | return pw::OkStatus(); |
Wyatt Hepler | 455b492 | 2020-09-18 00:19:21 -0700 | [diff] [blame] | 169 | } |
| 170 | |
| 171 | void MethodTwo(ServerContext& ctx, |
| 172 | const foo_bar_Request& request, |
| 173 | ServerWriter<foo_bar_Response>& response) { |
| 174 | // implementation |
| 175 | response.Write(foo_bar_Response{.number = 123}); |
| 176 | } |
| 177 | }; |
| 178 | |
| 179 | } // namespace foo::bar |
| 180 | |
| 181 | The Nanopb implementation would be declared in a ``BUILD.gn``: |
| 182 | |
| 183 | .. code-block:: python |
| 184 | |
| 185 | import("//build_overrides/pigweed.gni") |
| 186 | |
| 187 | import("$dir_pw_build/target_types.gni") |
| 188 | |
| 189 | pw_source_set("the_service") { |
| 190 | public_configs = [ ":public" ] |
| 191 | public = [ "public/foo_bar/service.h" ] |
Wyatt Hepler | 8779bcd | 2020-11-25 07:25:16 -0800 | [diff] [blame] | 192 | public_deps = [ ":the_service_proto.nanopb_rpc" ] |
Wyatt Hepler | 455b492 | 2020-09-18 00:19:21 -0700 | [diff] [blame] | 193 | } |
| 194 | |
| 195 | .. attention:: |
| 196 | |
| 197 | pw_rpc's generated classes will support using ``pw_protobuf`` or raw buffers |
| 198 | (no protobuf library) in the future. |
| 199 | |
Wyatt Hepler | 8779bcd | 2020-11-25 07:25:16 -0800 | [diff] [blame] | 200 | 4. Register the service with a server |
Wyatt Hepler | 455b492 | 2020-09-18 00:19:21 -0700 | [diff] [blame] | 201 | ------------------------------------- |
Alexei Frolov | d3e5cb7 | 2021-01-08 13:08:45 -0800 | [diff] [blame] | 202 | This example code sets up an RPC server with an :ref:`HDLC<module-pw_hdlc>` |
Wyatt Hepler | f9fb90f | 2020-09-30 18:59:33 -0700 | [diff] [blame] | 203 | channel output and the example service. |
Wyatt Hepler | 455b492 | 2020-09-18 00:19:21 -0700 | [diff] [blame] | 204 | |
| 205 | .. code-block:: cpp |
| 206 | |
| 207 | // Set up the output channel for the pw_rpc server to use. This configures the |
| 208 | // pw_rpc server to use HDLC over UART; projects not using UART and HDLC must |
| 209 | // adapt this as necessary. |
| 210 | pw::stream::SysIoWriter writer; |
| 211 | pw::rpc::RpcChannelOutput<kMaxTransmissionUnit> hdlc_channel_output( |
Alexei Frolov | d3e5cb7 | 2021-01-08 13:08:45 -0800 | [diff] [blame] | 212 | writer, pw::hdlc::kDefaultRpcAddress, "HDLC output"); |
Wyatt Hepler | 455b492 | 2020-09-18 00:19:21 -0700 | [diff] [blame] | 213 | |
| 214 | pw::rpc::Channel channels[] = { |
| 215 | pw::rpc::Channel::Create<1>(&hdlc_channel_output)}; |
| 216 | |
| 217 | // Declare the pw_rpc server with the HDLC channel. |
| 218 | pw::rpc::Server server(channels); |
| 219 | |
| 220 | pw::rpc::TheService the_service; |
| 221 | |
| 222 | void RegisterServices() { |
| 223 | // Register the foo.bar.TheService example service. |
| 224 | server.Register(the_service); |
| 225 | |
| 226 | // Register other services |
| 227 | } |
| 228 | |
| 229 | int main() { |
| 230 | // Set up the server. |
| 231 | RegisterServices(); |
| 232 | |
| 233 | // Declare a buffer for decoding incoming HDLC frames. |
| 234 | std::array<std::byte, kMaxTransmissionUnit> input_buffer; |
| 235 | |
| 236 | PW_LOG_INFO("Starting pw_rpc server"); |
Alexei Frolov | d3e5cb7 | 2021-01-08 13:08:45 -0800 | [diff] [blame] | 237 | pw::hdlc::ReadAndProcessPackets( |
Wyatt Hepler | 455b492 | 2020-09-18 00:19:21 -0700 | [diff] [blame] | 238 | server, hdlc_channel_output, input_buffer); |
| 239 | } |
Wyatt Hepler | 948f547 | 2020-06-02 16:52:28 -0700 | [diff] [blame] | 240 | |
Alexei Frolov | 1c670a2 | 2021-04-09 10:18:17 -0700 | [diff] [blame^] | 241 | Channels |
| 242 | ======== |
| 243 | ``pw_rpc`` sends all of its packets over channels. These are logical, |
| 244 | application-layer routes used to tell the RPC system where a packet should go. |
| 245 | |
| 246 | Channels over a client-server connection must all have a unique ID, which can be |
| 247 | assigned statically at compile time or dynamically. |
| 248 | |
| 249 | .. code-block:: cpp |
| 250 | |
| 251 | // Creating a channel with the static ID 3. |
| 252 | pw::rpc::Channel static_channel = pw::rpc::Channel::Create<3>(&output); |
| 253 | |
| 254 | // Grouping channel IDs within an enum can lead to clearer code. |
| 255 | enum ChannelId { |
| 256 | kUartChannel = 1, |
| 257 | kSpiChannel = 2, |
| 258 | }; |
| 259 | |
| 260 | // Creating a channel with a static ID defined within an enum. |
| 261 | pw::rpc::Channel another_static_channel = |
| 262 | pw::rpc::Channel::Create<ChannelId::kUartChannel>(&output); |
| 263 | |
| 264 | // Creating a channel with a dynamic ID (note that no output is provided; it |
| 265 | // will be set when the channel is used. |
| 266 | pw::rpc::Channel dynamic_channel; |
| 267 | |
Alexei Frolov | 7c7a386 | 2020-07-16 15:36:02 -0700 | [diff] [blame] | 268 | Services |
| 269 | ======== |
| 270 | A service is a logical grouping of RPCs defined within a .proto file. ``pw_rpc`` |
| 271 | uses these .proto definitions to generate code for a base service, from which |
| 272 | user-defined RPCs are implemented. |
| 273 | |
| 274 | ``pw_rpc`` supports multiple protobuf libraries, and the generated code API |
| 275 | depends on which is used. |
| 276 | |
Wyatt Hepler | f9fb90f | 2020-09-30 18:59:33 -0700 | [diff] [blame] | 277 | .. _module-pw_rpc-protobuf-library-apis: |
Alexei Frolov | 4d2adde | 2020-08-04 10:19:24 -0700 | [diff] [blame] | 278 | |
| 279 | Protobuf library APIs |
| 280 | ===================== |
| 281 | |
Alexei Frolov | 7c7a386 | 2020-07-16 15:36:02 -0700 | [diff] [blame] | 282 | .. toctree:: |
| 283 | :maxdepth: 1 |
| 284 | |
| 285 | nanopb/docs |
| 286 | |
| 287 | Testing a pw_rpc integration |
| 288 | ============================ |
| 289 | After setting up a ``pw_rpc`` server in your project, you can test that it is |
| 290 | working as intended by registering the provided ``EchoService``, defined in |
Wyatt Hepler | 752d7d3 | 2021-03-02 09:02:23 -0800 | [diff] [blame] | 291 | ``echo.proto``, which echoes back a message that it receives. |
Alexei Frolov | 7c7a386 | 2020-07-16 15:36:02 -0700 | [diff] [blame] | 292 | |
Wyatt Hepler | 752d7d3 | 2021-03-02 09:02:23 -0800 | [diff] [blame] | 293 | .. literalinclude:: echo.proto |
Alexei Frolov | 7c7a386 | 2020-07-16 15:36:02 -0700 | [diff] [blame] | 294 | :language: protobuf |
| 295 | :lines: 14- |
| 296 | |
| 297 | For example, in C++ with nanopb: |
| 298 | |
| 299 | .. code:: c++ |
| 300 | |
| 301 | #include "pw_rpc/server.h" |
| 302 | |
| 303 | // Include the apporpriate header for your protobuf library. |
| 304 | #include "pw_rpc/echo_service_nanopb.h" |
| 305 | |
| 306 | constexpr pw::rpc::Channel kChannels[] = { /* ... */ }; |
| 307 | static pw::rpc::Server server(kChannels); |
| 308 | |
| 309 | static pw::rpc::EchoService echo_service; |
| 310 | |
| 311 | void Init() { |
| 312 | server.RegisterService(&echo_service); |
| 313 | } |
| 314 | |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 315 | Protocol description |
| 316 | ==================== |
| 317 | Pigweed RPC servers and clients communicate using ``pw_rpc`` packets. These |
| 318 | packets are used to send requests and responses, control streams, cancel ongoing |
| 319 | RPCs, and report errors. |
| 320 | |
| 321 | Packet format |
| 322 | ------------- |
| 323 | Pigweed RPC packets consist of a type and a set of fields. The packets are |
| 324 | encoded as protocol buffers. The full packet format is described in |
Wyatt Hepler | ba325e4 | 2021-03-08 14:23:34 -0800 | [diff] [blame] | 325 | ``pw_rpc/pw_rpc/internal/packet.proto``. |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 326 | |
Wyatt Hepler | 752d7d3 | 2021-03-02 09:02:23 -0800 | [diff] [blame] | 327 | .. literalinclude:: internal/packet.proto |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 328 | :language: protobuf |
| 329 | :lines: 14- |
| 330 | |
| 331 | The packet type and RPC type determine which fields are present in a Pigweed RPC |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 332 | packet. Each packet type is only sent by either the client or the server. |
| 333 | These tables describe the meaning of and fields included with each packet type. |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 334 | |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 335 | Client-to-server packets |
| 336 | ^^^^^^^^^^^^^^^^^^^^^^^^ |
| 337 | +---------------------------+----------------------------------+ |
| 338 | | packet type | description | |
| 339 | +===========================+==================================+ |
| 340 | | REQUEST | RPC request | |
| 341 | | | | |
| 342 | | | .. code-block:: text | |
| 343 | | | | |
| 344 | | | - channel_id | |
| 345 | | | - service_id | |
| 346 | | | - method_id | |
| 347 | | | - payload | |
| 348 | | | (unless first client stream) | |
| 349 | | | | |
| 350 | +---------------------------+----------------------------------+ |
| 351 | | CLIENT_STREAM_END | Client stream finished | |
| 352 | | | | |
| 353 | | | .. code-block:: text | |
| 354 | | | | |
| 355 | | | - channel_id | |
| 356 | | | - service_id | |
| 357 | | | - method_id | |
| 358 | | | | |
| 359 | | | | |
| 360 | +---------------------------+----------------------------------+ |
| 361 | | CLIENT_ERROR | Received unexpected packet | |
| 362 | | | | |
| 363 | | | .. code-block:: text | |
| 364 | | | | |
| 365 | | | - channel_id | |
| 366 | | | - service_id | |
| 367 | | | - method_id | |
| 368 | | | - status | |
| 369 | +---------------------------+----------------------------------+ |
| 370 | | CANCEL_SERVER_STREAM | Cancel a server stream | |
| 371 | | | | |
| 372 | | | .. code-block:: text | |
| 373 | | | | |
| 374 | | | - channel_id | |
| 375 | | | - service_id | |
| 376 | | | - method_id | |
| 377 | | | | |
| 378 | +---------------------------+----------------------------------+ |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 379 | |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 380 | **Errors** |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 381 | |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 382 | The client sends ``CLIENT_ERROR`` packets to a server when it receives a packet |
| 383 | it did not request. If the RPC is a streaming RPC, the server should abort it. |
| 384 | |
| 385 | The status code indicates the type of error. If the client does not distinguish |
| 386 | between the error types, it can send whichever status is most relevant. The |
| 387 | status code is logged, but all status codes result in the same action by the |
| 388 | server: aborting the RPC. |
| 389 | |
| 390 | * ``NOT_FOUND`` -- Received a packet for a service method the client does not |
| 391 | recognize. |
| 392 | * ``FAILED_PRECONDITION`` -- Received a packet for a service method that the |
| 393 | client did not invoke. |
| 394 | |
| 395 | Server-to-client packets |
| 396 | ^^^^^^^^^^^^^^^^^^^^^^^^ |
| 397 | +-------------------+--------------------------------+ |
| 398 | | packet type | description | |
| 399 | +===================+================================+ |
| 400 | | RESPONSE | RPC response | |
| 401 | | | | |
| 402 | | | .. code-block:: text | |
| 403 | | | | |
| 404 | | | - channel_id | |
| 405 | | | - service_id | |
| 406 | | | - method_id | |
| 407 | | | - payload | |
| 408 | | | - status | |
| 409 | | | (unless in server stream) | |
| 410 | +-------------------+--------------------------------+ |
| 411 | | SERVER_STREAM_END | Server stream and RPC finished | |
| 412 | | | | |
| 413 | | | .. code-block:: text | |
| 414 | | | | |
| 415 | | | - channel_id | |
| 416 | | | - service_id | |
| 417 | | | - method_id | |
| 418 | | | - status | |
| 419 | +-------------------+--------------------------------+ |
| 420 | | SERVER_ERROR | Received unexpected packet | |
| 421 | | | | |
| 422 | | | .. code-block:: text | |
| 423 | | | | |
| 424 | | | - channel_id | |
| 425 | | | - service_id (if relevant) | |
| 426 | | | - method_id (if relevant) | |
| 427 | | | - status | |
| 428 | +-------------------+--------------------------------+ |
| 429 | |
| 430 | **Errors** |
| 431 | |
| 432 | The server sends ``SERVER_ERROR`` packets when it receives a packet it cannot |
| 433 | process. The client should abort any RPC for which it receives an error. The |
| 434 | status field indicates the type of error. |
| 435 | |
| 436 | * ``NOT_FOUND`` -- The requested service or method does not exist. |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 437 | * ``FAILED_PRECONDITION`` -- Attempted to cancel an RPC that is not pending. |
| 438 | * ``RESOURCE_EXHAUSTED`` -- The request came on a new channel, but a channel |
| 439 | could not be allocated for it. |
Wyatt Hepler | 712d367 | 2020-07-13 15:52:11 -0700 | [diff] [blame] | 440 | * ``INTERNAL`` -- The server was unable to respond to an RPC due to an |
| 441 | unrecoverable internal error. |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 442 | |
| 443 | Inovking a service method |
| 444 | ------------------------- |
| 445 | Calling an RPC requires a specific sequence of packets. This section describes |
| 446 | the protocol for calling service methods of each type: unary, server streaming, |
| 447 | client streaming, and bidirectional streaming. |
| 448 | |
| 449 | Unary RPC |
| 450 | ^^^^^^^^^ |
| 451 | In a unary RPC, the client sends a single request and the server sends a single |
| 452 | response. |
| 453 | |
| 454 | .. seqdiag:: |
| 455 | :scale: 110 |
| 456 | |
| 457 | seqdiag { |
| 458 | default_note_color = aliceblue; |
| 459 | |
| 460 | client -> server [ |
| 461 | label = "request", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 462 | leftnote = "PacketType.REQUEST\nchannel ID\nservice ID\nmethod ID\npayload" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 463 | ]; |
| 464 | |
| 465 | client <- server [ |
| 466 | label = "response", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 467 | rightnote = "PacketType.RESPONSE\nchannel ID\nservice ID\nmethod ID\npayload\nstatus" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 468 | ]; |
| 469 | } |
| 470 | |
| 471 | Server streaming RPC |
| 472 | ^^^^^^^^^^^^^^^^^^^^ |
| 473 | In a server streaming RPC, the client sends a single request and the server |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 474 | sends any number of responses followed by a ``SERVER_STREAM_END`` packet. |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 475 | |
| 476 | .. seqdiag:: |
| 477 | :scale: 110 |
| 478 | |
| 479 | seqdiag { |
| 480 | default_note_color = aliceblue; |
| 481 | |
| 482 | client -> server [ |
| 483 | label = "request", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 484 | leftnote = "PacketType.REQUEST\nchannel ID\nservice ID\nmethod ID\npayload" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 485 | ]; |
| 486 | |
| 487 | client <-- server [ |
| 488 | noactivate, |
| 489 | label = "responses (zero or more)", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 490 | rightnote = "PacketType.RESPONSE\nchannel ID\nservice ID\nmethod ID\npayload" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 491 | ]; |
| 492 | |
| 493 | client <- server [ |
| 494 | label = "done", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 495 | rightnote = "PacketType.SERVER_STREAM_END\nchannel ID\nservice ID\nmethod ID\nstatus" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 496 | ]; |
| 497 | } |
| 498 | |
| 499 | Server streaming RPCs may be cancelled by the client. The client sends a |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 500 | ``CANCEL_SERVER_STREAM`` packet to terminate the RPC. |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 501 | |
| 502 | .. seqdiag:: |
| 503 | :scale: 110 |
| 504 | |
| 505 | seqdiag { |
| 506 | default_note_color = aliceblue; |
| 507 | |
| 508 | client -> server [ |
| 509 | label = "request", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 510 | leftnote = "PacketType.REQUEST\nchannel ID\nservice ID\nmethod ID\npayload" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 511 | ]; |
| 512 | |
| 513 | client <-- server [ |
| 514 | noactivate, |
| 515 | label = "responses (zero or more)", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 516 | rightnote = "PacketType.RESPONSE\nchannel ID\nservice ID\nmethod ID\npayload" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 517 | ]; |
| 518 | |
| 519 | client -> server [ |
| 520 | noactivate, |
| 521 | label = "cancel", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 522 | leftnote = "PacketType.CANCEL_SERVER_STREAM\nchannel ID\nservice ID\nmethod ID" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 523 | ]; |
| 524 | |
| 525 | client <- server [ |
| 526 | label = "done", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 527 | rightnote = "PacketType.SERVER_STREAM_END\nchannel ID\nservice ID\nmethod ID\nstatus" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 528 | ]; |
| 529 | } |
| 530 | |
| 531 | Client streaming RPC |
| 532 | ^^^^^^^^^^^^^^^^^^^^ |
| 533 | In a client streaming RPC, the client sends any number of RPC requests followed |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 534 | by a ``CLIENT_STREAM_END`` packet. The server then sends a single response. |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 535 | |
| 536 | The first client-to-server RPC packet does not include a payload. |
| 537 | |
| 538 | .. attention:: |
| 539 | |
| 540 | ``pw_rpc`` does not yet support client streaming RPCs. |
| 541 | |
| 542 | .. seqdiag:: |
| 543 | :scale: 110 |
| 544 | |
| 545 | seqdiag { |
| 546 | default_note_color = aliceblue; |
| 547 | |
| 548 | client -> server [ |
| 549 | label = "start", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 550 | leftnote = "PacketType.REQUEST\nchannel ID\nservice ID\nmethod ID" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 551 | ]; |
| 552 | |
| 553 | client --> server [ |
| 554 | noactivate, |
| 555 | label = "requests (zero or more)", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 556 | leftnote = "PacketType.REQUEST\nchannel ID\nservice ID\nmethod ID\npayload" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 557 | ]; |
| 558 | |
| 559 | client -> server [ |
| 560 | noactivate, |
| 561 | label = "done", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 562 | leftnote = "PacketType.CLIENT_STREAM_END\nchannel ID\nservice ID\nmethod ID" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 563 | ]; |
| 564 | |
| 565 | client <- server [ |
| 566 | label = "response", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 567 | rightnote = "PacketType.RESPONSE\nchannel ID\nservice ID\nmethod ID\npayload\nstatus" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 568 | ]; |
| 569 | } |
| 570 | |
| 571 | The server may terminate a client streaming RPC at any time by sending its |
| 572 | response packet. |
| 573 | |
| 574 | .. seqdiag:: |
| 575 | :scale: 110 |
| 576 | |
| 577 | seqdiag { |
| 578 | default_note_color = aliceblue; |
| 579 | |
| 580 | client -> server [ |
| 581 | label = "start", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 582 | leftnote = "PacketType.REQUEST\nchannel ID\nservice ID\nmethod ID" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 583 | ]; |
| 584 | |
| 585 | client --> server [ |
| 586 | noactivate, |
| 587 | label = "requests (zero or more)", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 588 | leftnote = "PacketType.REQUEST\nchannel ID\nservice ID\nmethod ID\npayload" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 589 | ]; |
| 590 | |
| 591 | client <- server [ |
| 592 | label = "response", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 593 | rightnote = "PacketType.RESPONSE\nchannel ID\nservice ID\nmethod ID\npayload\nstatus" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 594 | ]; |
| 595 | } |
| 596 | |
| 597 | Bidirectional streaming RPC |
| 598 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| 599 | In a bidirectional streaming RPC, the client sends any number of requests and |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 600 | the server sends any number of responses. The client sends a |
| 601 | ``CLIENT_STREAM_END`` packet when it has finished sending requests. The server |
| 602 | sends a ``SERVER_STREAM_END`` packet after it receives the client's |
| 603 | ``CLIENT_STREAM_END`` and finished sending its responses. |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 604 | |
| 605 | The first client-to-server RPC packet does not include a payload. |
| 606 | |
| 607 | .. attention:: |
| 608 | |
| 609 | ``pw_rpc`` does not yet support bidirectional streaming RPCs. |
| 610 | |
| 611 | .. seqdiag:: |
| 612 | :scale: 110 |
| 613 | |
| 614 | seqdiag { |
| 615 | default_note_color = aliceblue; |
| 616 | |
| 617 | client -> server [ |
| 618 | label = "start", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 619 | leftnote = "PacketType.REQUEST\nchannel ID\nservice ID\nmethod ID" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 620 | ]; |
| 621 | |
| 622 | client --> server [ |
| 623 | noactivate, |
| 624 | label = "requests (zero or more)", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 625 | leftnote = "PacketType.REQUEST\nchannel ID\nservice ID\nmethod ID\npayload" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 626 | ]; |
| 627 | |
| 628 | ... (messages in any order) ... |
| 629 | |
| 630 | client <-- server [ |
| 631 | noactivate, |
| 632 | label = "responses (zero or more)", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 633 | rightnote = "PacketType.RESPONSE\nchannel ID\nservice ID\nmethod ID\npayload" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 634 | ]; |
| 635 | |
| 636 | client -> server [ |
| 637 | noactivate, |
| 638 | label = "done", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 639 | leftnote = "PacketType.CLIENT_STREAM_END\nchannel ID\nservice ID\nmethod ID" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 640 | ]; |
| 641 | |
| 642 | client <-- server [ |
| 643 | noactivate, |
| 644 | label = "responses (zero or more)", |
| 645 | rightnote = "PacketType.RPC\nchannel ID\nservice ID\nmethod ID\npayload" |
| 646 | ]; |
| 647 | |
| 648 | client <- server [ |
| 649 | label = "done", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 650 | rightnote = "PacketType.SERVER_STREAM_END\nchannel ID\nservice ID\nmethod ID\nstatus" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 651 | ]; |
| 652 | } |
| 653 | |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 654 | The server may terminate the RPC at any time by sending a ``SERVER_STREAM_END`` |
| 655 | packet with the status, even if the client has not sent its ``STREAM_END``. The |
| 656 | client may cancel the RPC at any time by sending a ``CANCEL_SERVER_STREAM`` |
| 657 | packet. |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 658 | |
| 659 | .. seqdiag:: |
| 660 | :scale: 110 |
| 661 | |
| 662 | seqdiag { |
| 663 | default_note_color = aliceblue; |
| 664 | |
| 665 | client -> server [ |
| 666 | label = "start", |
| 667 | leftnote = "PacketType.RPC\nchannel ID\nservice ID\nmethod ID" |
| 668 | ]; |
| 669 | |
| 670 | client --> server [ |
| 671 | noactivate, |
| 672 | label = "requests (zero or more)", |
| 673 | leftnote = "PacketType.RPC\nchannel ID\nservice ID\nmethod ID\npayload" |
| 674 | ]; |
| 675 | |
| 676 | client <-- server [ |
| 677 | noactivate, |
| 678 | label = "responses (zero or more)", |
| 679 | rightnote = "PacketType.RPC\nchannel ID\nservice ID\nmethod ID\npayload" |
| 680 | ]; |
| 681 | |
| 682 | client -> server [ |
| 683 | noactivate, |
| 684 | label = "cancel", |
Wyatt Hepler | 0f26235 | 2020-07-29 09:51:27 -0700 | [diff] [blame] | 685 | leftnote = "PacketType.CANCEL_SERVER_STREAM\nchannel ID\nservice ID\nmethod ID" |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 686 | ]; |
| 687 | |
| 688 | client <- server [ |
| 689 | label = "done", |
| 690 | rightnote = "PacketType.STREAM_END\nchannel ID\nservice ID\nmethod ID\nstatus" |
| 691 | ]; |
| 692 | } |
| 693 | |
Wyatt Hepler | 948f547 | 2020-06-02 16:52:28 -0700 | [diff] [blame] | 694 | RPC server |
| 695 | ========== |
| 696 | Declare an instance of ``rpc::Server`` and register services with it. |
| 697 | |
| 698 | .. admonition:: TODO |
| 699 | |
| 700 | Document the public interface |
| 701 | |
Alexei Frolov | bf33d21 | 2020-09-15 17:13:45 -0700 | [diff] [blame] | 702 | Size report |
| 703 | ----------- |
| 704 | The following size report showcases the memory usage of the core RPC server. It |
| 705 | is configured with a single channel using a basic transport interface that |
| 706 | directly reads from and writes to ``pw_sys_io``. The transport has a 128-byte |
| 707 | packet buffer, which comprises the plurality of the example's RAM usage. This is |
| 708 | not a suitable transport for an actual product; a real implementation would have |
| 709 | additional overhead proportional to the complexity of the transport. |
| 710 | |
| 711 | .. include:: server_size |
| 712 | |
Wyatt Hepler | 948f547 | 2020-06-02 16:52:28 -0700 | [diff] [blame] | 713 | RPC server implementation |
| 714 | ------------------------- |
| 715 | |
| 716 | The Method class |
| 717 | ^^^^^^^^^^^^^^^^ |
| 718 | The RPC Server depends on the ``pw::rpc::internal::Method`` class. ``Method`` |
| 719 | serves as the bridge between the ``pw_rpc`` server library and the user-defined |
Wyatt Hepler | e95bd72 | 2020-11-23 07:49:47 -0800 | [diff] [blame] | 720 | RPC functions. Each supported protobuf implementation extends ``Method`` to |
| 721 | implement its request and response proto handling. The ``pw_rpc`` server |
| 722 | calls into the ``Method`` implementation through the base class's ``Invoke`` |
| 723 | function. |
Wyatt Hepler | 948f547 | 2020-06-02 16:52:28 -0700 | [diff] [blame] | 724 | |
Wyatt Hepler | e95bd72 | 2020-11-23 07:49:47 -0800 | [diff] [blame] | 725 | ``Method`` implementations store metadata about each method, including a |
| 726 | function pointer to the user-defined method implementation. They also provide |
| 727 | ``static constexpr`` functions for creating each type of method. ``Method`` |
| 728 | implementations must satisfy the ``MethodImplTester`` test class in |
| 729 | ``pw_rpc_private/method_impl_tester.h``. |
Wyatt Hepler | 948f547 | 2020-06-02 16:52:28 -0700 | [diff] [blame] | 730 | |
Wyatt Hepler | e95bd72 | 2020-11-23 07:49:47 -0800 | [diff] [blame] | 731 | See ``pw_rpc/internal/method.h`` for more details about ``Method``. |
Wyatt Hepler | 948f547 | 2020-06-02 16:52:28 -0700 | [diff] [blame] | 732 | |
| 733 | Packet flow |
| 734 | ^^^^^^^^^^^ |
| 735 | |
| 736 | Requests |
| 737 | ~~~~~~~~ |
| 738 | |
| 739 | .. blockdiag:: |
| 740 | |
| 741 | blockdiag { |
| 742 | packets [shape = beginpoint]; |
| 743 | |
| 744 | group { |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 745 | label = "pw_rpc library"; |
Wyatt Hepler | 948f547 | 2020-06-02 16:52:28 -0700 | [diff] [blame] | 746 | |
| 747 | server [label = "Server"]; |
Alexei Frolov | 9a4d6bf | 2020-08-04 10:33:26 -0700 | [diff] [blame] | 748 | service [label = "Service"]; |
Wyatt Hepler | 948f547 | 2020-06-02 16:52:28 -0700 | [diff] [blame] | 749 | method [label = "internal::Method"]; |
| 750 | } |
| 751 | |
| 752 | stubs [label = "generated services", shape = ellipse]; |
| 753 | user [label = "user-defined RPCs", shape = roundedbox]; |
| 754 | |
| 755 | packets -> server -> service -> method -> stubs -> user; |
| 756 | packets -> server [folded]; |
| 757 | method -> stubs [folded]; |
| 758 | } |
| 759 | |
| 760 | Responses |
| 761 | ~~~~~~~~~ |
| 762 | |
| 763 | .. blockdiag:: |
| 764 | |
| 765 | blockdiag { |
| 766 | user -> stubs [folded]; |
| 767 | |
| 768 | group { |
Wyatt Hepler | 067dd7e | 2020-07-14 19:34:32 -0700 | [diff] [blame] | 769 | label = "pw_rpc library"; |
Wyatt Hepler | 948f547 | 2020-06-02 16:52:28 -0700 | [diff] [blame] | 770 | |
| 771 | server [label = "Server"]; |
| 772 | method [label = "internal::Method"]; |
| 773 | channel [label = "Channel"]; |
| 774 | } |
| 775 | |
| 776 | stubs [label = "generated services", shape = ellipse]; |
| 777 | user [label = "user-defined RPCs", shape = roundedbox]; |
| 778 | packets [shape = beginpoint]; |
| 779 | |
| 780 | user -> stubs -> method [folded]; |
| 781 | method -> server -> channel; |
| 782 | channel -> packets [folded]; |
| 783 | } |
Alexei Frolov | 4d2adde | 2020-08-04 10:19:24 -0700 | [diff] [blame] | 784 | |
| 785 | RPC client |
| 786 | ========== |
| 787 | The RPC client is used to send requests to a server and manages the contexts of |
| 788 | ongoing RPCs. |
| 789 | |
| 790 | Setting up a client |
| 791 | ------------------- |
| 792 | The ``pw::rpc::Client`` class is instantiated with a list of channels that it |
| 793 | uses to communicate. These channels can be shared with a server, but multiple |
| 794 | clients cannot use the same channels. |
| 795 | |
| 796 | To send incoming RPC packets from the transport layer to be processed by a |
| 797 | client, the client's ``ProcessPacket`` function is called with the packet data. |
| 798 | |
| 799 | .. code:: c++ |
| 800 | |
| 801 | #include "pw_rpc/client.h" |
| 802 | |
| 803 | namespace { |
| 804 | |
| 805 | pw::rpc::Channel my_channels[] = { |
| 806 | pw::rpc::Channel::Create<1>(&my_channel_output)}; |
| 807 | pw::rpc::Client my_client(my_channels); |
| 808 | |
| 809 | } // namespace |
| 810 | |
| 811 | // Called when the transport layer receives an RPC packet. |
| 812 | void ProcessRpcPacket(ConstByteSpan packet) { |
| 813 | my_client.ProcessPacket(packet); |
| 814 | } |
| 815 | |
Alexei Frolov | 5a3a61c | 2020-10-01 18:51:41 -0700 | [diff] [blame] | 816 | .. _module-pw_rpc-making-calls: |
| 817 | |
Alexei Frolov | 4d2adde | 2020-08-04 10:19:24 -0700 | [diff] [blame] | 818 | Making RPC calls |
| 819 | ---------------- |
| 820 | RPC calls are not made directly through the client, but using one of its |
| 821 | registered channels instead. A service client class is generated from a .proto |
| 822 | file for each selected protobuf library, which is then used to send RPC requests |
| 823 | through a given channel. The API for this depends on the protobuf library; |
Wyatt Hepler | f9fb90f | 2020-09-30 18:59:33 -0700 | [diff] [blame] | 824 | please refer to the |
| 825 | :ref:`appropriate documentation<module-pw_rpc-protobuf-library-apis>`. Multiple |
| 826 | service client implementations can exist simulatenously and share the same |
| 827 | ``Client`` class. |
Alexei Frolov | 4d2adde | 2020-08-04 10:19:24 -0700 | [diff] [blame] | 828 | |
| 829 | When a call is made, a ``pw::rpc::ClientCall`` object is returned to the caller. |
| 830 | This object tracks the ongoing RPC call, and can be used to manage it. An RPC |
| 831 | call is only active as long as its ``ClientCall`` object is alive. |
| 832 | |
| 833 | .. tip:: |
| 834 | Use ``std::move`` when passing around ``ClientCall`` objects to keep RPCs |
| 835 | alive. |
| 836 | |
| 837 | Client implementation details |
| 838 | ----------------------------- |
| 839 | |
| 840 | The ClientCall class |
| 841 | ^^^^^^^^^^^^^^^^^^^^ |
| 842 | ``ClientCall`` stores the context of an active RPC, and serves as the user's |
| 843 | interface to the RPC client. The core RPC library provides a base ``ClientCall`` |
| 844 | class with common functionality, which is then extended for RPC client |
| 845 | implementations tied to different protobuf libraries to provide convenient |
| 846 | interfaces for working with RPCs. |
| 847 | |
| 848 | The RPC server stores a list of all of active ``ClientCall`` objects. When an |
| 849 | incoming packet is recieved, it dispatches to one of its active calls, which |
| 850 | then decodes the payload and presents it to the user. |