diff --git a/docs/index.html b/docs/index.html
index 0468b95..9a60a59 100644
--- a/docs/index.html
+++ b/docs/index.html
@@ -20,22 +20,7 @@
     <label for="name">Name : </label><input id="name" value="World">
     <button onclick="hello()">Hello</button>
     <script>
-      const startWasm = async (wasm, { scope, base = '' } = {}) => {
-        try {
-          const options = {}
-          if (scope) options.scope = scope
-          const registration = await navigator.serviceWorker.register('sw.js', options)
-          await navigator.serviceWorker.ready
-          registration.active.postMessage({
-            type: 'startWasm',
-            wasm,
-            base,
-          })
-        } catch (e) {
-          console.error(e)
-        }
-      }
-      startWasm('api.wasm')
+      wasmhttp.register('api.wasm', { base: 'api' })
     </script>
   </body>
 </html>
\ No newline at end of file
diff --git a/docs/sw.js b/docs/sw.js
deleted file mode 100644
index 5d3690d..0000000
--- a/docs/sw.js
+++ /dev/null
@@ -1,23 +0,0 @@
-importScripts('https://cdn.jsdelivr.net/gh/golang/go@go1.13.4/misc/wasm/wasm_exec.js')
-
-addEventListener('install', (event) => {
-  console.log('install!')
-  // wasmhttp.serve({
-  //   wasm: 'api.wasm',
-  //   base: '/api',
-  // })
-  event.waitUntil(skipWaiting())
-})
-
-addEventListener('activate', event => {
-  console.log('activate!')
-  event.waitUntil(clients.claim())
-})
-
-addEventListener('fetch', () => {
-  console.log('fetch!')
-})
-
-addEventListener('message', ({ data }) => {
-  console.log('message', data)
-})
diff --git a/index.js b/index.js
new file mode 100644
index 0000000..f35a13a
--- /dev/null
+++ b/index.js
@@ -0,0 +1,14 @@
+window.wasmhttp = {
+  register: async (wasm, { scope, base = '' } = {}) => {
+    const options = {}
+    if (scope) options.scope = scope
+    //FIXME register once
+    const registration = await navigator.serviceWorker.register('sw.js', options)
+    await navigator.serviceWorker.ready
+    registration.active.postMessage({
+      type: 'wasmhttp.register',
+      wasm,
+      base,
+    })
+  }
+}
diff --git a/sw.js b/sw.js
index 7460c39..0442478 100644
--- a/sw.js
+++ b/sw.js
@@ -1,5 +1,4 @@
-let nextHandlerId = 1
-const handlerResolvers = {}
+importScripts('https://cdn.jsdelivr.net/gh/golang/go@go1.13.4/misc/wasm/wasm_exec.js')
 
 const startWasm = async (wasm, WASMHTTP_HANDLER_ID, WASMHTTP_PATH) => {
   const go = new Go()
@@ -23,33 +22,50 @@ const trimEnd = (s, c) => {
   return r
 }
 
+// addEventListener('install', (event) => {
+//   event.waitUntil(skipWaiting())
+// })
+
+const running = new Set()
+let nextHandlerId = 1
+const handlerResolvers = {}
+const handlers = []
+
 self.wasmhttp = {
-  serve: ({ wasm, base } = {}) => {
-    try {
-      if (!wasm) throw TypeError('options.wasm must be defined')
-
-      const handlerId = `${nextHandlerId++}`
-      const handler = new Promise(resolve => handlerResolvers[handlerId] = resolve)
-
-      let path = new URL(registration.scope).pathname
-      if (base && base !== '') path = `${trimEnd(path, '/')}/${trimStart(base, '/')}`
-
-      startWasm(wasm, handlerId, path)
-
-      addEventListener('fetch', async e => {
-        if (!new URL(e.request.url).pathname.startsWith(path)) return
-
-        // FIXME try catch
-        e.respondWith((await handler)(e.request))
-      })
-    } catch (e) {
-      console.error('wasmhttp: error:', e)
-      throw e
-    }
-  },
-
   registerHandler: (handlerId, handler) => {
     handlerResolvers[handlerId](handler)
     delete handlerResolvers[handlerId]
   },
 }
+
+addEventListener('activate', event => event.waitUntil(clients.claim()))
+
+addEventListener('message', async ({ data }) => {
+  if (data.type !== 'wasmhttp.register') return
+
+  const { wasm, base } = data
+
+  let path = new URL(registration.scope).pathname
+  if (base && base !== '') path = `${trimEnd(path, '/')}/${trimStart(base, '/')}`
+
+  const key = `${wasm}:${path}`
+
+  if (!running.has(key)) {
+    const handlerId = `${nextHandlerId++}`
+    const handler = new Promise(resolve => handlerResolvers[handlerId] = resolve)
+
+    startWasm(wasm, handlerId, path)
+    running.add(key)
+
+  // FIXME try catch
+    handlers.push([path, await handler])
+  }
+})
+
+addEventListener('fetch', e => {
+  const { pathname } = new URL(e.request.url)
+  const [, handler] = handlers.find(([path]) => pathname.startsWith(path)) || []
+  if (!handler) return
+
+  e.respondWith((handler)(e.request))
+})