Simple RPC Example

rpc-client.ts

export async function rpc(name: string, ...args: any[]): Promise<any> {
  const formData = new FormData();
  for (const [index, arg] of args.entries()) {
    const name = String(index);
    if (arg instanceof Blob) {
      formData.append(name, arg);
    } else if (arg instanceof ArrayBuffer) {
      formData.append(name, new Blob([arg]));
    } else {
      formData.append(name, JSON.stringify(arg));
    }
  }
  const res = await fetch(`/rpc/${name}`, {
    method: "POST",
    body: formData
  });
  const contentType = res.headers.get("content-type");
  if (contentType?.startsWith("application/octet-stream")) {
    return res.arrayBuffer();
  } else if (contentType?.startsWith("application/json")) {
    return res.json();
  }
  return res.text();
}

rpc-server.ts

import { Request, Router, Response } from "express";
import Busboy from "busboy";
import { v4 } from "uuid";
import * as path from "path";
import * as os from "os";
import * as fs from "fs";

function readFormData(tmpDir: string, req: Request): Promise<any[]> {
  return new Promise((resolve, reject) => {
    try {
      const busboy = new Busboy({ headers: req.headers });
      const data: any = [];
      busboy.on("file", (fieldname, file, filename, encoding, mimetype) => {
        const index = data.length++;
        const tmpFile = path.join(tmpDir, v4());
        data[index] = tmpFile;
        file.pipe(fs.createWriteStream(tmpFile));
      });
      busboy.on("field", (fieldname, val, fieldnameTruncated, valTruncated) => {
        const index = data.length++;
        data[index] = JSON.parse(val);
      });
      busboy.on("finish", () => {
        resolve(data);
      });
      req.pipe(busboy);
    } catch (e) {
      reject(e);
    }
  });
}

export function rpc(options: {
  functions: { [key: string]: Function };
  tmpDir?: string;
  onError?: (name: string, error: any, req: Request, res: Response) => void;
}): Router {
  const tmpDir = options.tmpDir ?? os.tmpdir();
  const onError =
    options.onError ??
    ((name, e, req, res) => {
      console.error(e.message);
      const message = `Unexpected error while executing RPC "${name}()"`;
      res.status(500).send({ message });
    });
  const router = Router();
  for (const name in options.functions) {
    const f = options.functions[name];
    router.post(`/rpc/${name}`, (req: Request, res: Response) => {
      (async () => {
        const args = await readFormData(tmpDir, req);
        const ret = await f.apply(null, args);
        res.send(ret);
      })().catch(e => {
        onError(name, e, req, res);
      });
    });
  }
  return router;
}