Nim library for implementing JSON-RPC clients and servers



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Json-Rpc is a library designed to provide an easier interface for working with remote procedure calls.


git clone https://github.com/status-im/nim-json-rpc


  • Nim 17.3 and up


Json-Rpc is a library for routing JSON 2.0 format remote procedure calls over different transports. It is designed to automatically generate marshalling and parameter checking code based on the RPC parameter types.


Remote procedure calls are created using the rpc macro on an instance of RpcRouter.

rpc allows you to provide a list of native Nim type parameters and a return type, generates marshalling to and from json for you, so you can concentrate on using native Nim types for your call.

Routing is then performed by the route procedure.

When an error occurs, the error is populated, otherwise result will be populated.

rpc Parameters

path: The string to match for the method.

body: The parameters and code to execute for this call.


Here's a simple example:

import json_rpc/rpcserver

var router = newRpcRouter()

  result = %"Hello"

As no return type was specified in this example, result defaults to the JsonNode type. A JSON string is returned by passing a string though the % operator, which converts simple types to JsonNode.

The body parameters can be defined by using do notation. This allows full Nim types to be used as RPC parameters.

Here we pass a string to an RPC and return a string.

router.rpc("hello") do(input: string) -> string:
  result = "Hello " & input

Json-Rpc will recursively parse the Nim types in order to produce marshalling code. This marshalling code uses the types to check the incoming JSON fields to ensure they exist and are of the correct kind.

The return type then performs the opposite process, converting Nim types to Json for transport.

Here is a more complex parameter example:

  HeaderKind = enum hkOne, hkTwo, hkThree

  Header = ref object
    kind: HeaderKind
    size: int64

  DataBlob = object
    items: seq[byte]
    headers: array[3, Header]

  MyObject = object
    data: DataBlob
    name: string

router.rpc("updateData") do(myObj: MyObject, newData: DataBlob) -> DataBlob:
  result = myObj.data
  myObj.data = newData

Behind the scenes, all RPC calls take a single json parameter param that must be of kind JArray. At runtime, the json is checked to ensure that it contains the correct number and type of your parameters to match the rpc definition.

Compiling with -d:nimDumpRpcs will show the output code for the RPC call. To see the output of the async generation, add -d:nimDumpAsync.

Special type : Option[T]

Option[T] is a special type indicating that parameter may have value or not.

  • If optional parameters located in the middle of parameters list, you set it to null to tell the server that it has no value.
  • If optional parameters located at the end of parameter list and there are no more mandatory parameters after that, those optional parameters can be omitted altogether.
# d can be omitted, b should use null to indicate it has no value
router.rpc("updateData") do(a: int, b: Option[int], c: string, d: Option[T]):
  if b.isSome:
    # do something
    # do something else
  • If Option[T] used as return type, it also denotes the returned value might not available.
router.rpc("getData") do(name: string) -> Option[int]:
  if name == "monkey":
    result = some(4)
  • If Option[T] used as field type of an object, it also tell us that field might be present or not, and the rpc mechanism will automatically set it to some value if it available.
  MyOptional = object
    maybeInt: Option[int]


Note that array parameters are explicitly checked for length, and will return an error node if the length differs from their declaration size.

If you wish to support custom types in a particular way, you can provide matching fromJson and % procedures.


This takes a Json type and returns the Nim type.


n: JsonNode: The current node being processed

argName: string: The name of the field in n

result: The type of this must be var X where X is the Nim type you wish to handle


proc fromJson[T](n: JsonNode, argName: string, result: var seq[T]) =
  n.kind.expect(JArray, argName)
  result = newSeq[T](n.len)
  for i in 0 ..< n.len:
    fromJson(n[i], argName, result[i])


This is the standard way to provide translations from a Nim type to a JsonNode.


n: The type you wish to convert


JsonNode: The newly encoded JsonNode type from the parameter type.


This is a simple procedure to state your expected type.

If the actual type doesn't match the expected type, an exception is thrown mentioning which field caused the failure.


actual: JsonNodeKind: The actual type of the JsonNode.

expected: JsonNodeKind: The desired type.

argName: string: The current field name.


myNode.kind.expect(JArray, argName)

JSON Format

The router expects either a string or JsonNode with the following structure:

  "id": JInt,
  "jsonrpc": "2.0",
  "method": JString,
  "params": JArray

Return values use the following node structure:

  "id": JInt,
  "jsonrpc": "2.0",
  "result": JsonNode,
  "error": JsonNode

Performing a route

To call and RPC through the router, use the route procedure.

There are three variants of route.

Note that once invoked all RPC calls are error trapped and any exceptions raised are passed back with the error message encoded as a JsonNode.

route by string

This route variant will handle all the conversion of string to JsonNode and check the format and type of the input data.


router: RpcRouter: The router object that contains the RPCs.

data: string: A string ready to be processed into a JsonNode.


Future[string]: This will be the stringified JSON response, which can be the JSON RPC result or a JSON wrapped error.

route by JsonNode

This variant allows simplified processing if you already have a JsonNode. However if the required fields are not present within node, exceptions will be raised.


router: RpcRouter: The router object that contains the RPCs.

node: JsonNode: A pre-processed JsonNode that matches the expected format as defined above.


Future[JsonNode]: The JSON RPC result or a JSON wrapped error.


This route variant allows you to invoke a call if possible, without raising an exception.


router: RpcRouter: The router object that contains the RPCs.

node: JsonNode: A pre-processed JsonNode that matches the expected format as defined above.

fut: var Future[JsonNode]: The JSON RPC result or a JSON wrapped error.


bool: true if the method field provided in node matches an available route. Returns false when the method cannot be found, or if method or params field cannot be found within node.

To see the result of a call, we need to provide Json in the expected format. Here's an example of how that looks by manually creating the JSON. Later we will see the helper utilities that make this easier.

let call = %*{
  "id": %1,
  "jsonrpc": %2.0,
  "method": %"hello",
  "params": %["Terry"]
# route the call we defined earlier
let localResult = waitFor router.route(call)

echo localResult
# We should see something like this
#   {"jsonrpc":"2.0","id":1,"result":"Hello Terry","error":null}


In order to make routing useful, RPCs must be invoked and transmitted over a transport.

The RpcServer type is given as a simple inheritable wrapper/container that simplifies designing your own transport layers using the router field.

Server Transports

Currently there are plans for the following transports to be implemented:

  • Sockets
  • HTTP
  • IPC
  • Websockets

Transport specific server need only call the route procedure using a string fetched from the transport in order to invoke the requested RPC.

Server example

This example uses the socket transport defined in socket.nim. Once executed, the "hello" RPC will be available to a socket based client.

import json_rpc/rpcserver

# Create a socket server for transport
var srv = newRpcSocketServer("localhost", Port(8585))

# srv.rpc is a shortcut for srv.router.rpc
srv.rpc("hello") do(input: string) -> string:
  result = "Hello " & input



Json-Rpc also comes with a client implementation, built to provide a framework for transports to work with.

To simplify demonstration, we will use the socket transport defined in socketclient.nim.

Below is the most basic way to use a remote call on the client. Here we manually supply the name and json parameters for the call.

The call procedure takes care of the basic format of the JSON to send to the server. However you still need to provide params as a JsonNode, which must exactly match the parameters defined in the equivalent rpc definition.

import json_rpc/[rpcclient, rpcserver], chronos, json

  server = newRpcSocketServer("localhost", Port(8545))
  client = newRpcSocketClient()


server.rpc("hello") do(input: string) -> string:
  result = "Hello " & input

waitFor client.connect("localhost", Port(8545))

let response = waitFor client.call("hello", %[%"Daisy"])

# the call returns a `Response` type which contains the result
echo response.result


To make things more readable and allow better static checking client side, Json-Rpc supports generating wrappers for client RPCs using createRpcSigs.

This macro takes a type name and the path of a file containing forward declarations of procedures that you wish to convert to client RPCs. The transformation generates procedures that match the forward declarations provided, plus a client parameter in the specified type.

Because the signatures are parsed at compile time, the file will be error checked and you can use import to share common types between your client and server.


clientType: This is the type you want to pass to your generated calls. Usually this would be a transport specific descendant from RpcClient.

path: The path to the Nim module that contains the RPC header signatures.


For example, to support this remote call:

server.rpc("bmi") do(height, weight: float) -> float:
  result = (height * height) / weight

You can have the following in your rpc signature file:

proc bmi(height, weight: float): float

When parsed through createRpcSigs, you can call the RPC as if it were a normal procedure. So instead of this:

let bmiIndex = await client.call("bmi", %[%120.5, %12.0])

You can use:

let bmiIndex = await client.bmi(120.5, 12.0)

This allows you to leverage Nim's static type checking whilst also aiding readability and providing a unified location to declare client side RPC definitions.

Working with client transports

Transport clients should provide a type that is inherited from RpcClient where they can store any transport related information.

Additionally, the following two procedures are useful:

  • Call

    self: a descendant of RpcClient name: string: the method to be called params: JsonNode: The parameters to the RPC call Returning Future[Response]: A wrapper for the result JsonNode and a flag to indicate if this contains an error.

Note: Although call isn't necessary for a client to function, it allows RPC signatures to be used by the createRpcSigs.

  • Connect

    client: a descendant of RpcClient Returning FutureBase: The base future returned when a procedure is annoted with {.async.}


To simplify and unify processing within the client, the processMessage procedure can be used to perform conversion and error checking from the received string originating from the transport to the JsonNode representation that is passed to the RPC.

After a RPC returns, this procedure then completes the futures set by call invocations using the id field of the processed JsonNode from line.


self: a client type descended from RpcClient

line: string: a string that contains the JSON to be processed


Pull requests are welcome. For major changes, please open an issue first to discuss what you would like to change.

Please make sure to update tests as appropriate.


Licensed and distributed under either of


at your option. This file may not be copied, modified, or distributed except according to those terms.

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