Metadata

The webrpc module lets transports pass optional metadata into RPC handlers.

Metadata is not part of the RPC request envelope. It belongs to the layer that received the call. An HTTP adapter may attach headers, a WebSocket adapter may attach a peer id, a P2P adapter may attach node information, and a local adapter may attach tracing data. WebRPC keeps this information separate from params so the method input stays clean while the handler can still inspect useful execution context.

The flow is:

txt

transport adapter
  -> metadata map
  -> Dispatcher or Router
  -> Context
  -> handler

Use the public WebRPC header:

cpp

#include <vix/webrpc/webrpc.hpp>

WebRPC metadata often appears together with JSON responses:

cpp

#include <vix/json/Simple.hpp>

For examples that print output:

cpp

#include <vix/print.hpp>

Metadata map

Metadata is represented by Context::MetaMap.

cpp

using MetaMap =
    std::unordered_map<std::string, std::string>;

The map is optional. A dispatcher or router call can pass a pointer to metadata:

cpp

Context::MetaMap meta{
    {"peer", "client-1"},
    {"trace_id", "abc-123"},
};

auto response = dispatcher.handle(payload, "websocket", &meta);

The context stores only a pointer to this map. The caller owns the metadata and must keep it alive during dispatch.

Basic metadata usage

A handler reads metadata through Context.

cpp

#include <string>
#include <vix/webrpc/webrpc.hpp>
#include <vix/json/Simple.hpp>
#include <vix/print.hpp>

int main()
{
  using namespace vix::json;
  using namespace vix::webrpc;

  Router router;

  router.add("whoami", [](const Context &ctx) -> RpcResult
  {
    return obj({
        "peer", std::string(ctx.meta_value("peer")),
        "trace_id", std::string(ctx.meta_value("trace_id")),
    });
  });

  Context::MetaMap meta{
      {"peer", "client-1"},
      {"trace_id", "abc-123"},
  };

  token request = obj({
      "id", 1LL,
      "method", "whoami",
  });

  Dispatcher dispatcher(router);

  auto response = dispatcher.handle(request, "websocket", &meta);

  if (response.has_value())
  {
    vix::print("metadata response created");
  }

  return 0;
}

The metadata values are available only during the handler call.

Read a metadata value

Use meta_value(...) to read one metadata value.

cpp

std::string_view value = ctx.meta_value("peer");

If the metadata map is missing or the key does not exist, the function returns an empty view.

cpp

#include <string>
#include <vix/webrpc/webrpc.hpp>
#include <vix/json/Simple.hpp>
#include <vix/print.hpp>

int main()
{
  using namespace vix::json;
  using namespace vix::webrpc;

  Router router;

  router.add("peer.name", [](const Context &ctx) -> RpcResult
  {
    std::string_view peer = ctx.meta_value("peer");

    if (peer.empty())
    {
      return RpcError::invalid_params("missing peer metadata");
    }

    return obj({
        "peer", std::string(peer),
    });
  });

  Context::MetaMap meta{
      {"peer", "client-1"},
  };

  token request = obj({
      "id", 1LL,
      "method", "peer.name",
  });

  Dispatcher dispatcher(router);

  auto response = dispatcher.handle(request, "p2p", &meta);

  if (response.has_value())
  {
    vix::print("peer metadata handled");
  }

  return 0;
}

This is useful when metadata is required by a handler.

Check whether metadata exists

Use has_meta(...) when the difference between a missing key and an empty value matters.

cpp

bool exists = ctx.has_meta("trace_id");

Example:

cpp

#include <string>
#include <vix/webrpc/webrpc.hpp>
#include <vix/json/Simple.hpp>
#include <vix/print.hpp>

int main()
{
  using namespace vix::json;
  using namespace vix::webrpc;

  Router router;

  router.add("trace.check", [](const Context &ctx) -> RpcResult
  {
    return obj({
        "has_trace", ctx.has_meta("trace_id"),
    });
  });

  Context::MetaMap meta{
      {"trace_id", "abc-123"},
  };

  token request = obj({
      "id", 1LL,
      "method", "trace.check",
  });

  Dispatcher dispatcher(router);

  auto response = dispatcher.handle(request, "http", &meta);

  if (response.has_value())
  {
    vix::print("trace metadata checked");
  }

  return 0;
}

This keeps optional metadata handling explicit.

Metadata with Dispatcher

Dispatcher::handle(...) accepts metadata and passes it to every handler involved in the payload.

cpp

std::optional<vix::json::token> handle(
    const vix::json::token &payload,
    std::string_view transport = {},
    const Context::MetaMap *meta = nullptr) const;

Example:

cpp

#include <string>
#include <vix/webrpc/webrpc.hpp>
#include <vix/json/Simple.hpp>
#include <vix/print.hpp>

int main()
{
  using namespace vix::json;
  using namespace vix::webrpc;

  Router router;

  router.add("request.info", [](const Context &ctx) -> RpcResult
  {
    return obj({
        "transport", std::string(ctx.transport),
        "peer", std::string(ctx.meta_value("peer")),
    });
  });

  Context::MetaMap meta{
      {"peer", "client-1"},
  };

  token request = obj({
      "id", 1LL,
      "method", "request.info",
  });

  Dispatcher dispatcher(router);

  auto response = dispatcher.handle(request, "websocket", &meta);

  if (response.has_value())
  {
    vix::print("request info response created");
  }

  return 0;
}

The dispatcher does not interpret metadata. It only carries the pointer into the handler context.

Metadata with Router

Router::dispatch(...) also accepts metadata.

cpp

RpcResult dispatch(
    const RpcRequest &req,
    std::string_view transport = {},
    const Context::MetaMap *meta = nullptr) const;

Example:

cpp

#include <variant>
#include <string>
#include <vix/webrpc/webrpc.hpp>
#include <vix/json/Simple.hpp>
#include <vix/print.hpp>

int main()
{
  using namespace vix::json;
  using namespace vix::webrpc;

  Router router;

  router.add("whoami", [](const Context &ctx) -> RpcResult
  {
    return obj({
        "peer", std::string(ctx.meta_value("peer")),
        "transport", std::string(ctx.transport),
    });
  });

  Context::MetaMap meta{
      {"peer", "client-1"},
  };

  RpcRequest request{
      token(1LL),
      "whoami",
      token(nullptr),
  };

  RpcResult result = router.dispatch(request, "local", &meta);

  if (std::holds_alternative<RpcError>(result))
  {
    const auto &error = std::get<RpcError>(result);
    vix::print("error:", error.code);
    return 1;
  }

  vix::print("metadata dispatch succeeded");

  return 0;
}

This is useful in tests or internal calls where the request is already parsed.

Metadata in batch calls

When a batch is dispatched, the same metadata pointer is passed to every item in the batch.

cpp

#include <string>
#include <vix/webrpc/webrpc.hpp>
#include <vix/json/Simple.hpp>
#include <vix/print.hpp>

int main()
{
  using namespace vix::json;
  using namespace vix::webrpc;

  Router router;

  router.add("whoami", [](const Context &ctx) -> RpcResult
  {
    return obj({
        "peer", std::string(ctx.meta_value("peer")),
        "trace_id", std::string(ctx.meta_value("trace_id")),
    });
  });

  Context::MetaMap meta{
      {"peer", "client-1"},
      {"trace_id", "abc-123"},
  };

  token batch = array_t{
      {
          obj({
              "id", 1LL,
              "method", "whoami",
          }),
          obj({
              "id", 2LL,
              "method", "whoami",
          }),
      },
  };

  Dispatcher dispatcher(router);

  auto response = dispatcher.handle(batch, "websocket", &meta);

  if (response.has_value())
  {
    vix::print("batch metadata response created");
  }

  return 0;
}

This is useful when all calls in one payload came from the same connection, peer, request, or transport event.

Metadata and notifications

Notifications also receive metadata.

cpp

#include <string>
#include <vix/webrpc/webrpc.hpp>
#include <vix/json/Simple.hpp>
#include <vix/print.hpp>

int main()
{
  using namespace vix::json;
  using namespace vix::webrpc;

  Router router;

  router.add("events.track", [](const Context &ctx) -> RpcResult
  {
    vix::print("peer:", ctx.meta_value("peer"));
    vix::print("event received");

    return obj({
        "ok", true,
    });
  });

  Context::MetaMap meta{
      {"peer", "client-1"},
  };

  token notification = obj({
      "method", "events.track",
      "params", obj({
          "name", "page.open",
      }),
  });

  Dispatcher dispatcher(router);

  auto response = dispatcher.handle(notification, "websocket", &meta);

  if (!response.has_value())
  {
    vix::print("notification handled without response");
  }

  return 0;
}

The handler can use metadata during execution. The dispatcher still returns no response because the request has no id.

Missing metadata

Metadata is optional. A handler must be able to handle a missing metadata map when the metadata is not required.

cpp

#include <string>
#include <vix/webrpc/webrpc.hpp>
#include <vix/json/Simple.hpp>
#include <vix/print.hpp>

int main()
{
  using namespace vix::json;
  using namespace vix::webrpc;

  Router router;

  router.add("optional.peer", [](const Context &ctx) -> RpcResult
  {
    std::string_view peer = ctx.meta_value("peer");

    return obj({
        "peer", std::string(peer),
        "has_peer", ctx.has_meta("peer"),
    });
  });

  token request = obj({
      "id", 1LL,
      "method", "optional.peer",
  });

  Dispatcher dispatcher(router);

  auto response = dispatcher.handle(request, "local");

  if (response.has_value())
  {
    vix::print("optional metadata response created");
  }

  return 0;
}

When no map is passed, meta_value(...) returns an empty view and has_meta(...) returns false.

Required metadata

Some handlers may require metadata. In that case, return a structured error when it is missing.

cpp

#include <string>
#include <vix/webrpc/webrpc.hpp>
#include <vix/json/Simple.hpp>
#include <vix/print.hpp>

int main()
{
  using namespace vix::json;
  using namespace vix::webrpc;

  Router router;

  router.add("secure.action", [](const Context &ctx) -> RpcResult
  {
    if (!ctx.has_meta("user_id"))
    {
      return RpcError::invalid_params("missing user_id metadata");
    }

    return obj({
        "ok", true,
        "user_id", std::string(ctx.meta_value("user_id")),
    });
  });

  token request = obj({
      "id", 1LL,
      "method", "secure.action",
  });

  Dispatcher dispatcher(router);

  auto response = dispatcher.handle(request, "http");

  if (response.has_value())
  {
    vix::print("secure action response created");
  }

  return 0;
}

This keeps metadata validation explicit and visible in the handler.

Metadata and params

Metadata should not replace params.

Use params for method input. Use metadata for execution context around the call.

txt

params:
  user id requested by the method
  product id
  command arguments
  form data
  method payload

metadata:
  peer id
  trace id
  transport name
  authenticated user id
  headers converted by the adapter
  source address or connection tag

Keeping these separate makes handlers easier to reason about. The method payload remains the method payload, while metadata describes the environment in which the call arrived.

Metadata and transport

The transport label and metadata often work together.

cpp

return obj({
    "transport", std::string(ctx.transport),
    "peer", std::string(ctx.meta_value("peer")),
});

The transport label is a simple name such as http, websocket, p2p, or local. Metadata carries additional values that are specific to that transport event.

Lifetime rules

Context does not own metadata. It stores a pointer.

This means the metadata map must outlive the dispatch call.

cpp

Context::MetaMap meta{
    {"trace_id", "abc-123"},
};

auto response = dispatcher.handle(payload, "http", &meta);

This is safe because meta remains alive while handle(...) is executing.

Do not store ctx.meta, metadata string views, or Context itself beyond the handler call.

String view lifetime

meta_value(...) returns std::string_view.

The returned view points into the std::string stored in the metadata map. It is valid only while the map is alive and unchanged.

cpp

router.add("example", [](const vix::webrpc::Context &ctx)
    -> vix::webrpc::RpcResult
{
  std::string_view peer = ctx.meta_value("peer");

  return vix::json::obj({
      "peer", std::string(peer),
  });
});

Convert the value to std::string when it must be stored in a JSON result or kept beyond immediate inspection.

Building metadata in an adapter

A transport adapter can build metadata before calling the dispatcher.

cpp

vix::webrpc::Context::MetaMap meta;
meta["trace_id"] = "abc-123";
meta["peer"] = "client-1";

auto response = dispatcher.handle(payload, "http", &meta);

For an HTTP adapter, metadata may come from headers. For a WebSocket adapter, it may come from connection state. For a P2P adapter, it may come from peer identity. WebRPC does not require a fixed metadata schema.

Complete example

cpp

#include <string>
#include <variant>
#include <vix/webrpc/webrpc.hpp>
#include <vix/json/Simple.hpp>
#include <vix/print.hpp>

int main()
{
  using namespace vix::json;
  using namespace vix::webrpc;

  Router router;

  router.add("session.info", [](const Context &ctx) -> RpcResult
  {
    if (!ctx.has_meta("session_id"))
    {
      return RpcError::invalid_params("missing session_id metadata");
    }

    const auto *params = ctx.params_object_ptr();
    if (!params)
    {
      return RpcError::invalid_params("params must be an object");
    }

    const token *verbose = params->get_ptr("verbose");

    return obj({
        "transport", std::string(ctx.transport),
        "session_id", std::string(ctx.meta_value("session_id")),
        "trace_id", std::string(ctx.meta_value("trace_id")),
        "verbose", verbose && verbose->is_bool()
            ? verbose->as_bool_or(false)
            : false,
    });
  });

  Context::MetaMap meta{
      {"session_id", "sess-123"},
      {"trace_id", "trace-456"},
  };

  token request = obj({
      "id", 1LL,
      "method", "session.info",
      "params", obj({
          "verbose", true,
      }),
  });

  Dispatcher dispatcher(router);

  auto maybe_response = dispatcher.handle(request, "http", &meta);

  if (!maybe_response.has_value())
  {
    vix::print("no response");
    return 0;
  }

  auto parsed = RpcResponse::parse(*maybe_response);

  if (std::holds_alternative<RpcError>(parsed))
  {
    const auto &error = std::get<RpcError>(parsed);
    vix::print("invalid response:", error.code);
    return 1;
  }

  const RpcResponse &response = std::get<RpcResponse>(parsed);

  if (!response.ok())
  {
    vix::print("rpc error:", response.error.code);
    vix::print("message:", response.error.message);
    return 1;
  }

  const auto *result = response.result.as_object_ptr().get();
  if (!result)
  {
    vix::print("result must be an object");
    return 1;
  }

  vix::print("session:", result->get_string_or("session_id", ""));
  vix::print("trace:", result->get_string_or("trace_id", ""));

  return 0;
}

This example shows a handler that uses both method parameters and metadata. The verbose value belongs to params, while session_id and trace_id belong to metadata.

Common metadata keys

WebRPC does not enforce metadata names, but these names are practical in adapters:

Key	Meaning
`trace_id`	Request or message trace id.
`peer`	Peer or client identifier.
`session_id`	Session identifier.
`user_id`	Authenticated user id.
`request_id`	Transport-level request id.
`remote_addr`	Remote address or endpoint label.
`auth_scheme`	Authentication scheme used by the adapter.

Keep metadata names stable inside one application so handlers and tests can rely on them.

API overview

API	Purpose
`Context::MetaMap`	Metadata map type.
`Context::meta`	Optional metadata map pointer.
`Context::meta_value(...)`	Read a metadata value or return an empty view.
`Context::has_meta(...)`	Check whether a metadata key exists.
`Dispatcher::handle(..., meta)`	Pass metadata through the dispatcher.
`Dispatcher::handle_one(..., meta)`	Pass metadata for a single request.
`Router::dispatch(..., meta)`	Pass metadata through direct router dispatch.
`Context::transport`	Optional transport label related to the metadata source.

Next step

Continue with the API reference for a compact list of the public types and functions exposed by the WebRPC module.

API Reference

Application

Backend

Web

Vue.js

Game

Library

Execution

Tasks

Scheduling

Cancellation and Time

Structured Work

Parallel Algorithms

Metadata

Header

Metadata map

Basic metadata usage

Read a metadata value

Check whether metadata exists

Metadata with Dispatcher

Metadata with Router

Metadata in batch calls

Metadata and notifications

Missing metadata

Required metadata

Metadata and params

Metadata and transport

Lifetime rules

String view lifetime

Building metadata in an adapter

Complete example

Common metadata keys

API overview

Next step

Metadata ​

Header ​

Metadata map ​

Basic metadata usage ​

Read a metadata value ​

Check whether metadata exists ​

Metadata with Dispatcher ​

Metadata with Router ​

Metadata in batch calls ​

Metadata and notifications ​

Missing metadata ​

Required metadata ​

Metadata and params ​

Metadata and transport ​

Lifetime rules ​

String view lifetime ​

Building metadata in an adapter ​

Complete example ​

Common metadata keys ​

API overview ​

Next step ​

Metadata

Header

Metadata map

Basic metadata usage

Read a metadata value

Check whether metadata exists

Metadata with Dispatcher

Metadata with Router

Metadata in batch calls

Metadata and notifications

Missing metadata

Required metadata

Metadata and params

Metadata and transport

Lifetime rules

String view lifetime

Building metadata in an adapter

Complete example

Common metadata keys

API overview

Next step