Non-working attempt to enable shared cache in deno-sqlite. See https://github.com/dyedgreen/deno-sqlite/issues/187

import_map.json

{
  "imports": {
    "https://deno.land/x/sqlite@v3.3.0/build/sqlite.js": "./sqlite.js"
  }
}

multi.ts

// multi.ts
// wait for the worker to run first.
const worker: Worker = await new Promise((resolve, reject) => {
  const worker = new Worker(new URL("./multi.worker.ts", import.meta.url), {
    type: "module",
  });
  worker.addEventListener("error", reject);
  worker.addEventListener(
    "message",
    (msg) => {
      worker.removeEventListener("error", reject);
      msg.data === "ready" ? resolve(worker) : reject(msg);
    },
    { once: true }
  );
});
console.log("main");
import { DB } from "https://deno.land/x/sqlite@v3.3.0/mod.ts";
const db = new DB("file:memdb1?mode=memory&cache=shared", { uri: true });
db.execute(`INSERT INTO kv (key, value) VALUES ('main', 'hello')`);
const result = db.query(`select * from kv`);
console.log("main", { result });
worker.terminate();

multi.worker.ts

// multi.worker.ts
/// <reference lib="deno.worker" />
console.log("worker");
import { DB } from "https://deno.land/x/sqlite@v3.3.0/mod.ts";
const db = new DB("file:memdb1?mode=memory&cache=shared", { uri: true });
console.log("sqlite_version", db.query(`select sqlite_version()`));
db.execute(`CREATE TABLE kv (key TEXT PRIMARY KEY, value TEXT)`);
db.execute(`INSERT INTO kv (key, value) VALUES ('worker', 'hello')`);
const result = db.query(`select * from kv`);
console.log("worker", { result });
self.postMessage("ready");

run.txt

% deno run --no-check --unstable --allow-read --allow-net --import-map import_map.json multi.ts
worker
sqlite_version [ [ "3.36.0" ] ]
worker { result: [ [ "worker", "hello" ] ] }
main
error: Uncaught (in promise) SqliteError: no such table: kv
      throw new SqliteError(this._wasm, status);
            ^
    at DB.execute (https://deno.land/x/sqlite@v3.3.0/src/db.ts:287:13)
    at file:///.../multi.ts:20:4

sqlite.js

/// <reference types="https://deno.land/x/sqlite@v3.3.0/build/sqlite.d.ts" />

/* This file is automatically generated. Do not edit directly. */
import env from "https://deno.land/x/sqlite@v3.3.0/build/vfs.js";

const moduleOrInstance = {
  memory: null,
  module: null,
  instances: [],
};

export async function compile() {
  moduleOrInstance.memory = new WebAssembly.Memory({
    initial: 10,
    maximum: 10000,
    shared: true,
  });
  moduleOrInstance.module = await WebAssembly.compileStreaming(
    fetch("https://deno.land/x/sqlite@v3.3.0/build/sqlite.wasm")
  );
}

export function instantiate() {
  if (moduleOrInstance.instances.length) {
    return moduleOrInstance.instances.pop();
  } else {
    const placeholder = { exports: null };
    const importObject = env(placeholder);
    importObject.env.memory = moduleOrInstance.memory;
    //console.log({ importObject });
    const instance = new WebAssembly.Instance(
      moduleOrInstance.module,
      importObject
    );
    placeholder.exports = instance.exports;
    instance.exports.seed_rng(Date.now());
    return instance;
  }
}