diff --git a/napi/src/env.rs b/napi/src/env.rs
index 2e46f0dc..2b4d2d98 100644
--- a/napi/src/env.rs
+++ b/napi/src/env.rs
@@ -5,10 +5,11 @@ use std::mem;
 use std::os::raw::{c_char, c_void};
 use std::ptr;
 
+use crate::async_work::AsyncWork;
 use crate::error::check_status;
 use crate::js_values::*;
 use crate::task::Task;
-use crate::{sys, AsyncWork, Error, NodeVersion, Result, Status};
+use crate::{sys, Error, NodeVersion, Result, Status};
 
 #[cfg(all(any(feature = "libuv", feature = "tokio_rt"), napi4))]
 use crate::promise;
diff --git a/napi/src/error.rs b/napi/src/error.rs
index 2dcabbf3..6c44603d 100644
--- a/napi/src/error.rs
+++ b/napi/src/error.rs
@@ -37,7 +37,7 @@ impl Error {
     }
   }
 
-  pub fn into_raw(self, env: sys::napi_env) -> sys::napi_value {
+  pub(crate) fn into_raw(self, env: sys::napi_env) -> sys::napi_value {
     let mut err = ptr::null_mut();
     let s = self.reason;
     unsafe {
diff --git a/napi/src/js_values/mod.rs b/napi/src/js_values/mod.rs
index 3d3472d0..37eb34ae 100644
--- a/napi/src/js_values/mod.rs
+++ b/napi/src/js_values/mod.rs
@@ -25,9 +25,9 @@ pub use function::JsFunction;
 pub use number::JsNumber;
 pub use object::JsObject;
 pub use string::JsString;
-pub use tagged_object::TaggedObject;
-pub use value::Value;
-pub use value_ref::Ref;
+pub(crate) use tagged_object::TaggedObject;
+pub(crate) use value::Value;
+pub(crate) use value_ref::Ref;
 pub use value_type::ValueType;
 
 // Value types
@@ -51,7 +51,7 @@ pub struct JsSymbol(pub(crate) Value);
 pub struct JsExternal(pub(crate) Value);
 
 #[inline]
-pub fn type_of(env: sys::napi_env, raw_value: sys::napi_value) -> Result<ValueType> {
+pub(crate) fn type_of(env: sys::napi_env, raw_value: sys::napi_value) -> Result<ValueType> {
   unsafe {
     let mut value_type = sys::napi_valuetype::napi_undefined;
     check_status(sys::napi_typeof(env, raw_value, &mut value_type))?;
diff --git a/napi/src/lib.rs b/napi/src/lib.rs
index 6abd9085..3a274512 100644
--- a/napi/src/lib.rs
+++ b/napi/src/lib.rs
@@ -1,3 +1,61 @@
+//! High level NodeJS [N-API](https://nodejs.org/api/n-api.html) binding
+//!
+//! **napi-rs** provides minimal overhead to write N-API modules in `Rust`.
+//! ## Feature flags
+//! ### libuv
+//! With `libuv` feature, you can execute a rust future in `libuv` in NodeJS, and return a `promise` object.
+//! ```
+//! use std::thread;
+//! use std::fs;
+//!
+//! use futures::prelude::*;
+//! use futures::channel::oneshot;
+//! use napi::{CallContext, Result, JsString, JsObject, Status, Error};
+//!
+//! #[js_function(1)]
+//! pub fn uv_read_file(ctx: CallContext) -> Result<JsObject> {
+//!   let path = ctx.get::<JsString>(0)?;
+//!   let (sender, receiver) = oneshot::channel();
+//!   let p = path.as_str()?.to_owned();
+//!   thread::spawn(|| {
+//!     let res = fs::read(p).map_err(|e| Error::new(Status::Unknown, format!("{}", e)));
+//!     sender.send(res).expect("Send data failed");
+//!   });
+//!   ctx.env.execute(receiver.map_err(|e| Error::new(Status::Unknown, format!("{}", e))).map(|x| x.and_then(|x| x)), |&mut env, data| {
+//!     env.create_buffer_with_data(data)
+//!   })
+//! }
+//! ```
+//! ### tokio_rt
+//! With `tokio_rt` feature, `napi-rs` provides a ***tokio runtime*** in an additional thread.
+//! And you can easily run tokio `future` in it and return `promise`.
+//!
+//! ```
+//! use futures::prelude::*;
+//! use napi::{CallContext, Error, JsObject, JsString, Result, Status};
+//! use tokio;
+//!
+//! #[js_function(1)]
+//! pub fn tokio_readfile(ctx: CallContext) -> Result<JsObject> {
+//!   let js_filepath = ctx.get::<JsString>(0)?;
+//!   let path_str = js_filepath.as_str()?;
+//!   ctx.env.execute_tokio_future(
+//!     tokio::fs::read(path_str.to_owned())
+//!       .map(|v| v.map_err(|e| Error::new(Status::Unknown, format!("failed to read file, {}", e)))),
+//!     |&mut env, data| env.create_buffer_with_data(data),
+//!   )
+//! }
+//! ```
+//!
+//! ***Tokio channel in `napi-rs` buffer size is default `100`.***
+//!
+//! ***You can adjust it via `NAPI_RS_TOKIO_CHANNEL_BUFFER_SIZE` environment variable***
+//!
+//! ```
+//! NAPI_RS_TOKIO_CHANNEL_BUFFER_SIZE=1000 node ./app.js
+//! ```
+//!
+
 mod async_work;
 mod call_context;
 mod env;
@@ -17,20 +75,28 @@ mod tokio_rt;
 mod uv;
 mod version;
 
-pub use async_work::AsyncWork;
 pub use call_context::CallContext;
 pub use env::*;
 pub use error::{Error, Result};
 pub use js_values::*;
 pub use module::Module;
 pub use status::Status;
-pub use sys::napi_valuetype;
 pub use task::Task;
 pub use version::NodeVersion;
 
 #[cfg(all(feature = "tokio_rt", napi4))]
 pub use tokio_rt::shutdown as shutdown_tokio_rt;
 
+/// register nodejs module
+///
+/// ## Example
+/// ```
+/// register_module!(test_module, init);
+///
+/// fn init(module: &mut Module) -> Result<()> {
+///   module.create_named_method("nativeFunction", native_function)?;
+/// }
+/// ```
 #[macro_export]
 macro_rules! register_module {
   ($module_name:ident, $init:ident) => {
diff --git a/napi/src/tokio_rt.rs b/napi/src/tokio_rt.rs
index e7432211..387cee6e 100644
--- a/napi/src/tokio_rt.rs
+++ b/napi/src/tokio_rt.rs
@@ -1,5 +1,5 @@
+use std::env::var;
 use std::ffi::c_void;
-use std::mem;
 use std::pin::Pin;
 use std::thread::spawn;
 use std::time::Duration;
@@ -20,7 +20,11 @@ pub(crate) enum Message {
 pub(crate) fn get_tokio_sender() -> &'static mpsc::Sender<Message> {
   static SENDER: OnceCell<mpsc::Sender<Message>> = OnceCell::new();
   SENDER.get_or_init(|| {
-    let (sender, mut receiver) = mpsc::channel(100);
+    let buffer_size = var("NAPI_RS_TOKIO_CHANNEL_BUFFER_SIZE")
+      .map_err(|_| ())
+      .and_then(|s| s.parse().map_err(|_| ()))
+      .unwrap_or(100);
+    let (sender, mut receiver) = mpsc::channel(buffer_size);
     spawn(move || {
       let mut rt = Runtime::new().expect("Failed to create tokio runtime");
       rt.block_on(async {
@@ -33,7 +37,6 @@ pub(crate) fn get_tokio_sender() -> &'static mpsc::Sender<Message> {
         }
       });
       rt.shutdown_timeout(Duration::from_secs(5));
-      mem::drop(receiver);
     });
 
     sender
diff --git a/test_module/__test__/napi4/tokio_rt-isolate.spec.js b/test_module/__test__/napi4/tokio_rt-isolate.spec.js
new file mode 100644
index 00000000..ea01323c
--- /dev/null
+++ b/test_module/__test__/napi4/tokio_rt-isolate.spec.js
@@ -0,0 +1,27 @@
+const test = require('ava')
+const { join } = require('path')
+
+const napiVersion = require('../napi-version')
+
+const filepath = join(__dirname, './example.txt')
+
+process.env.NAPI_RS_TOKIO_CHANNEL_BUFFER_SIZE = '1'
+
+const bindings = require('../../index.node')
+
+test('should be able adjust queue size via process.env', async (t) => {
+  if (napiVersion < 4) {
+    t.is(bindings.testExecuteTokioReadfile, undefined)
+    return
+  }
+  try {
+    await Promise.all(
+      Array.from({ length: 50 }).map((_) =>
+        bindings.testExecuteTokioReadfile(filepath),
+      ),
+    )
+    throw new TypeError('Unreachable')
+  } catch (e) {
+    t.is(e.message, 'QueueFull: Failed to run future: no available capacity')
+  }
+})