diff --git a/lib/src/async_db.rs b/lib/src/async_db.rs
new file mode 100644
index 0000000..3883160
--- /dev/null
+++ b/lib/src/async_db.rs
@@ -0,0 +1,188 @@
+use std::{marker::PhantomData, path::Path, sync::Arc};
+
+use async_stream::stream;
+use tokio::pin;
+
+use async_compression::tokio::bufread::ZstdDecoder;
+use async_compression::tokio::write::ZstdEncoder;
+use async_compression::Level;
+use futures::stream::{self, StreamExt};
+use futures_core::stream::Stream;
+use futures_util::pin_mut;
+
+use tokio::{
+    fs,
+    io::{self, AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt},
+};
+
+use fmmap::tokio::{AsyncMmapFile, AsyncOptions};
+
+type LSize = u32;
+const LEN_SIZE: usize = std::mem::size_of::<LSize>();
+const BINCODE_CFG: bincode::config::Configuration = bincode::config::standard();
+
+trait ErrorToString {
+    type Output;
+    fn str_err(self) -> std::result::Result<Self::Output, String>;
+}
+
+impl<T, E> ErrorToString for std::result::Result<T, E>
+where
+    E: std::error::Error,
+{
+    type Output = T;
+    fn str_err(self) -> std::result::Result<Self::Output, String> {
+        self.map_err(|e| e.to_string())
+    }
+}
+
+pub struct WriterOpts {
+    pub compress_lvl: Level,
+    pub data_buf_size: usize,
+    pub out_buf_size: usize,
+}
+
+impl Default for WriterOpts {
+    fn default() -> Self {
+        Self {
+            compress_lvl: Level::Default,
+            data_buf_size: 500 * 1024 * 1024,
+            out_buf_size: 200 * 1024 * 1024,
+        }
+    }
+}
+
+pub struct Writer<T>
+where
+    T: bincode::Encode,
+{
+    out: io::BufWriter<fs::File>,
+    data_buf: Vec<u8>,
+    table: Vec<LSize>,
+    compress_lvl: Level,
+    _t: PhantomData<*const T>,
+}
+
+impl<T> Writer<T>
+where
+    T: bincode::Encode,
+{
+    pub async fn new<P: AsRef<Path>>(path: P, opts: WriterOpts) -> Result<Self, String> {
+        let out = fs::File::create(path).await.str_err()?;
+        let out = io::BufWriter::with_capacity(opts.out_buf_size, out);
+        let data_buf: Vec<u8> = Vec::with_capacity(opts.data_buf_size);
+
+        let compress_lvl = opts.compress_lvl;
+
+        let table: Vec<LSize> = vec![];
+
+        Ok(Self {
+            out,
+            data_buf,
+            table,
+            compress_lvl,
+            _t: PhantomData,
+        })
+    }
+
+    pub async fn push(&mut self, item: T) -> Result<(), String> {
+        let pos: LSize = self.data_buf.len() as LSize;
+
+        let item_data = bincode::encode_to_vec(item, BINCODE_CFG).str_err()?;
+
+        let mut zencoder = ZstdEncoder::with_quality(&mut self.data_buf, self.compress_lvl);
+        zencoder.write_all(&item_data).await.str_err()?;
+        zencoder.flush().await.str_err()?;
+
+        self.table.push(pos);
+
+        Ok(())
+    }
+
+    pub async fn load<S>(&mut self, source: S) -> Result<(), String>
+    where
+        S: Stream<Item = T> + std::marker::Unpin,
+    {
+        let hint = source.size_hint();
+        let hint = std::cmp::max(hint.0, hint.1.unwrap_or(0));
+        if hint > 0 {
+            self.table.reserve(hint);
+        }
+
+        pin_mut!(source);
+        while let Some(item) = source.next().await {
+            self.push(item).await?;
+        }
+
+        Ok(())
+    }
+
+    pub async fn finish(mut self) -> Result<(), String> {
+        // finish tab
+        let pos: LSize = self.data_buf.len() as LSize;
+        self.table.push(pos);
+
+        // write tab
+        let tab_size = (self.table.len() * LEN_SIZE) as LSize;
+        for pos in self.table {
+            let pos_data = (pos + tab_size).to_le_bytes();
+            self.out.write_all(&pos_data).await.str_err()?;
+        }
+
+        // copy data
+        self.out.write_all(&self.data_buf[..]).await.str_err()?;
+
+        self.out.flush().await.str_err()?;
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use tempfile::tempdir;
+
+    #[derive(bincode::Encode, bincode::Decode, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+    struct TestData {
+        num: u64,
+        test: String,
+    }
+
+    fn gen_data(count: usize) -> impl Iterator<Item = TestData> {
+        (0..count).into_iter().map(|i| TestData {
+            num: i as u64,
+            test: "test".repeat(i),
+        })
+    }
+
+    fn stream_iter<I, T>(it: I) -> impl Stream<Item = T>
+    where
+        I: Iterator<Item = T>,
+    {
+        stream! {
+            for i in it {
+                yield i;
+            }
+        }
+    }
+
+    #[tokio::test]
+    async fn test_write() {
+        let dir = tempdir().expect("tempdir");
+        let tmpfile = dir.path().join("test.tmp");
+        let opts = WriterOpts {
+            compress_lvl: Level::Default,
+            data_buf_size: 10 * 1024 * 1024,
+            out_buf_size: 10 * 1024 * 1024,
+        };
+        let mut writer: Writer<TestData> = Writer::new(&tmpfile, opts).await.expect("new writer");
+
+        let items_iter = gen_data(5);
+        let items: Vec<TestData> = items_iter.collect();
+
+        let src = stream_iter(items.clone().into_iter());
+        pin_mut!(src);
+        writer.load(src).await.expect("load");
+        writer.finish().await.expect("finish write");
+    }
+}
diff --git a/lib/src/lib.rs b/lib/src/lib.rs
index b1ff844..5987921 100644
--- a/lib/src/lib.rs
+++ b/lib/src/lib.rs
@@ -1,3 +1,6 @@
+#[cfg(feature = "async")]
+pub mod async_db;
+#[cfg(feature = "sync")]
 pub mod db;
 pub mod questions;
 pub mod source;