diff --git a/cas-core/src/cas.rs b/cas-core/src/cas.rs
index 089e7de..27750e4 100644
--- a/cas-core/src/cas.rs
+++ b/cas-core/src/cas.rs
@@ -14,10 +14,13 @@
 // along with cdb.  If not, see <https://www.gnu.org/licenses/>.
 
 
-use super::error::Result;
+use std::path::Path;
+
+use super::error::{Error, Result};
 use super::object_id::ObjectId;
 use super::object_type::ObjectType;
 use super::object_metadata::ObjectMetadata;
+use super::pipeline::{Reader, Writer};
 
 
 // pub struct ObjectIdIterator;
@@ -25,16 +28,41 @@ use super::object_metadata::ObjectMetadata;
 
 pub trait Cas {
     fn object_id_from_string(&self, hex: &str) -> Result<ObjectId>;
-    fn object_id_from_partial(&self, hex: &str) -> Result<ObjectId>;
 
     fn has_object_id(&self, oid: &ObjectId) -> Result<bool>;
-    // fn iter_object_id(&self) -> Result<ObjectIdIterator>;
+    fn iter_object_id<'s>(&'s self) -> Result<Box<dyn 's + Iterator<Item = Result<ObjectId>>>>;
+
+    fn open_object(&self, oid: &ObjectId) -> Result<(ObjectMetadata, Box<dyn Reader>)>;
+    fn new_writer(&mut self, otype: &ObjectType, size: u64) -> Result<Box<dyn Writer>>;
+
+    fn read_object(&self, oid: &ObjectId) -> Result<(ObjectMetadata, Vec<u8>)> {
+        let (metadata, mut reader) = self.open_object(oid)?;
+        let mut data = Vec::new();
+
+        reader.read_to_end(&mut data).or_else(|err|
+            Err(Error::error(format!("failed to read object {}: {}", oid, err))))?;
+
+        let result_oid = reader.finalize()?;
+        if &result_oid == oid {
+            Ok((metadata, data))
+        }
+        else {
+            Err(Error::error(format!("object id mismatch: requested {}, read {}", oid, result_oid)))
+        }
+    }
+
+    fn write_object(&mut self, otype: &ObjectType, data: &[u8]) -> Result<ObjectId> {
+        let mut writer = self.new_writer(otype, data.len() as u64)?;
+        writer.write_all(data).or_else(|err|
+            Err(Error::error(format!("failed to write object: {}", err))))?;
+        writer.finalize()
+    }
 
-    fn read_object(&self, oid: &ObjectId) -> Result<(ObjectMetadata, &dyn std::io::Read)>;
-    fn write_object(&mut self, otype: ObjectType, data: &mut dyn std::io::Read) -> Result<ObjectId>;
     fn remove_object(&mut self, oid: &ObjectId) -> Result<()>;
+}
 
-    fn read_ref(&self, key: &str) -> Result<ObjectId>;
-    fn write_ref(&mut self, key: &str, value: &ObjectId) -> Result<()>;
-    fn remove_ref(&mut self, key: &str) -> Result<()>;
+pub trait RefStore {
+    fn get_ref<P: AsRef<Path>>(&self, key: P) -> Result<ObjectId>;
+    fn set_ref<P: AsRef<Path>>(&mut self, key: P, value: &ObjectId) -> Result<()>;
+    fn remove_ref<P: AsRef<Path>>(&mut self, key: P) -> Result<()>;
 }
\ No newline at end of file
diff --git a/cas-core/src/error.rs b/cas-core/src/error.rs
index 6f36aa2..9903ecd 100644
--- a/cas-core/src/error.rs
+++ b/cas-core/src/error.rs
@@ -111,6 +111,10 @@ impl Error {
     pub fn error<M: Into<String>>(message: M) -> Box<Error> {
         Box::new(Error::Error(message.into()))
     }
+
+    pub fn err<M: Into<String>, T>(message: M) -> Result<T> {
+        Err(Self::error(message))
+    }
 }
 
 
diff --git a/cas-core/src/lib.rs b/cas-core/src/lib.rs
index 1f2e681..9d8bacd 100644
--- a/cas-core/src/lib.rs
+++ b/cas-core/src/lib.rs
@@ -38,7 +38,7 @@ pub use crate::{
     object_id::{ObjectId, hex, write_hex},
     object_type::{ObjectType},
     object_metadata::{ObjectMetadata},
-    pipeline::{Pipeline, DefaultPipeline},
+    pipeline::{Pipeline, DefaultPipeline, Reader, Writer, ReadWrapper, WriteWrapper},
     cas::{Cas},
 };
 
diff --git a/cas-core/src/object_metadata.rs b/cas-core/src/object_metadata.rs
index aeae1ea..13c741e 100644
--- a/cas-core/src/object_metadata.rs
+++ b/cas-core/src/object_metadata.rs
@@ -20,11 +20,11 @@ use super::object_type::{ObjectType};
 #[derive(Clone, Eq, PartialEq)]
 pub struct ObjectMetadata {
     otype: ObjectType,
-    size: usize,
+    size: u64,
 }
 
 impl ObjectMetadata {
-    pub fn new(otype: ObjectType, size: usize) -> Self {
+    pub fn new(otype: ObjectType, size: u64) -> Self {
         Self {
             otype,
             size,
@@ -35,7 +35,7 @@ impl ObjectMetadata {
         &self.otype
     }
 
-    pub fn size(&self) -> usize {
+    pub fn size(&self) -> u64 {
         self.size
     }
 }
diff --git a/cas-core/src/pipeline.rs b/cas-core/src/pipeline.rs
index 487720c..744a14f 100644
--- a/cas-core/src/pipeline.rs
+++ b/cas-core/src/pipeline.rs
@@ -19,29 +19,13 @@ use super::error::*;
 use super::object_id::ObjectId;
 
 
-pub trait Reader {
-    fn read(&mut self, buf: &mut [u8]) -> Result<usize>;
+pub trait Reader: std::io::Read {
     fn finalize(self: Box<Self>) -> Result<ObjectId>;
-
-    fn read_all(&mut self) -> Result<Vec<u8>> {
-        let mut buffer = [0u8; 1 << 13];
-        let mut data = Vec::new();
-        let mut count = usize::MAX;
-
-        while count != 0 {
-            count = self.read(&mut buffer)?;
-            if count != 0 {
-                data.extend_from_slice(&buffer[..count]);
-            }
-        }
-
-        Ok(data)
-    }
 }
 
-pub trait Writer {
-    fn write(&mut self, buf: &[u8]) -> Result<()>;
+pub trait Writer: std::io::Write {
     fn finalize(self: Box<Self>) -> Result<ObjectId>;
+    fn _finalize(self: Box<Self>, oid: ObjectId) -> Result<ObjectId>;
 }
 
 
@@ -51,29 +35,57 @@ pub trait Pipeline {
 }
 
 
-impl<R: std::io::Read> Reader for R {
-    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
-        <Self as std::io::Read>::read(self, buf).or_else(|err|
-            Err(Error::error(format!("Read error: {}", err)))
-        )
+pub struct ReadWrapper<R: std::io::Read> {
+    reader: R,
+}
+
+impl<R: std::io::Read> ReadWrapper<R> {
+    pub fn new(reader: R) -> Self {
+        Self { reader }
+    }
+}
+
+impl<R: std::io::Read> std::io::Read for ReadWrapper<R> {
+    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
+        self.reader.read(buf)
     }
+}
 
+impl<R: std::io::Read> Reader for ReadWrapper<R> {
     fn finalize(self: Box<Self>) -> Result<ObjectId> {
         Err(Error::error("reader pipline has no digest step"))
     }
 }
 
 
-impl<W: std::io::Write> Writer for W {
-    fn write(&mut self, buf: &[u8]) -> Result<()> {
-        self.write_all(buf).or_else(|err|
-            Err(Error::error(format!("Write error: {}", err)))
-        )
+pub struct WriteWrapper<W: std::io::Write> {
+    writer: W,
+}
+
+impl<W: std::io::Write> WriteWrapper<W> {
+    pub fn new(writer: W) -> Self {
+        Self { writer }
+    }
+}
+
+impl<W: std::io::Write> std::io::Write for WriteWrapper<W> {
+    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
+        self.writer.write(buf)
     }
 
+    fn flush(&mut self) -> std::io::Result<()> {
+        self.writer.flush()
+    }
+}
+
+impl<W: std::io::Write> Writer for WriteWrapper<W> {
     fn finalize(self: Box<Self>) -> Result<ObjectId> {
         Err(Error::error("reader pipline has no digest step"))
     }
+
+    fn _finalize(self: Box<Self>, oid: ObjectId) -> Result<ObjectId> {
+        Ok(oid)
+    }
 }
 
 
@@ -93,8 +105,8 @@ impl<'a> DigestReader<'a> {
     }
 }
 
-impl<'a> Reader for DigestReader<'a> {
-    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
+impl<'a> std::io::Read for DigestReader<'a> {
+    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
         let count = if let Some(dig) = &mut self.digest {
             let count = self.reader.read(buf)?;
             dig.update(&buf[..count]);
@@ -112,7 +124,9 @@ impl<'a> Reader for DigestReader<'a> {
 
         Ok(count)
     }
+}
 
+impl<'a> Reader for DigestReader<'a> {
     fn finalize(self: Box<Self>) -> Result<ObjectId> {
         self.oid.ok_or_else(|| Error::error("Reader not finalized"))
     }
@@ -133,15 +147,26 @@ impl<'a> DigestWriter<'a> {
     }
 }
 
-impl<'a> Writer for DigestWriter<'a> {
-    fn write(&mut self, buf: &[u8]) -> Result<()> {
-        self.digest.update(buf);
-        self.writer.write(buf)?;
-        Ok(())
+impl<'a> std::io::Write for DigestWriter<'a> {
+    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
+        let count = self.writer.write(buf)?;
+        self.digest.update(&buf[..count]);
+        Ok(count)
     }
 
+    fn flush(&mut self) -> std::io::Result<()> {
+        self.writer.flush()
+    }
+}
+
+impl<'a> Writer for DigestWriter<'a> {
     fn finalize(self: Box<Self>) -> Result<ObjectId> {
-        Ok(ObjectId::new(&self.digest.finalize()))
+        let oid = ObjectId::new(&self.digest.finalize());
+        self.writer._finalize(oid)
+    }
+
+    fn _finalize(self: Box<Self>, _oid: ObjectId) -> Result<ObjectId> {
+        Err(Error::error("writer pipeline has several digest steps"))
     }
 }
 
@@ -182,9 +207,10 @@ mod tests {
     #[test]
     fn std_read_as_reader() {
         let data = b"Hello World!".to_vec();
-        let mut reader: Box<dyn Reader> = Box::new(data.as_slice());
+        let mut reader: Box<dyn Reader> = Box::new(ReadWrapper::new(data.as_slice()));
 
-        let data2 = reader.read_all().expect("read failed");
+        let mut data2 = Vec::new();
+        reader.read_to_end(&mut data2).expect("read failed");
         let maybe_oid = reader.finalize();
 
         assert_eq!(data2, data);
@@ -197,7 +223,7 @@ mod tests {
         let mut buffer = [0u8; 32];
 
         let maybe_oid = {
-            let mut writer: Box<dyn Writer> = Box::new(&mut buffer[..]);
+            let mut writer: Box<dyn Writer> = Box::new(WriteWrapper::new(&mut buffer[..]));
             writer.write(&data).expect("write failed");
             writer.finalize()
         };
@@ -210,17 +236,20 @@ mod tests {
 
     #[test]
     fn digest_reader() {
+        use std::io::Read;
+
         let data = b"Hello World!".to_vec();
         let oid = ObjectId::from_str(
             "7f83b1657ff1fc53b92dc18148a1d65dfc2d4b1fa3d677284addd200126d9069"
         ).expect("invalid object id");
 
         let mut reader = Box::new(DigestReader::new(
-            Box::new(data.as_slice()),
+            Box::new(ReadWrapper::new(data.as_slice())),
             Box::new(Sha256::default()),
         ));
 
-        let data2 = reader.read_all().expect("read failed");
+        let mut data2 = Vec::new();
+        reader.read_to_end(&mut data2).expect("read failed");
         let oid2 = reader.finalize().expect("finalize failed");
 
         assert_eq!(data2, data);
@@ -229,6 +258,8 @@ mod tests {
 
     #[test]
     fn digest_writer() {
+        use std::io::Write;
+
         let data = b"Hello World!".to_vec();
         let oid = ObjectId::from_str(
             "7f83b1657ff1fc53b92dc18148a1d65dfc2d4b1fa3d677284addd200126d9069"
@@ -237,7 +268,7 @@ mod tests {
         let mut buffer = [0u8; 32];
         let oid2 = {
             let mut writer = Box::new(DigestWriter::new(
-                Box::new(&mut buffer[..]),
+                Box::new(WriteWrapper::new(&mut buffer[..])),
                 Box::new(Sha256::default()),
             ));
             writer.write(&data).expect("write failed");
diff --git a/cas-simple/src/cas.rs b/cas-simple/src/cas.rs
new file mode 100644
index 0000000..9d3714d
--- /dev/null
+++ b/cas-simple/src/cas.rs
@@ -0,0 +1,563 @@
+// This file is part of bsv.
+//
+// bsv is free software: you can redistribute it and/or modify it under the
+// terms of the GNU Affero General Public License as published by the Free
+// Software Foundation, either version 3 of the License, or (at your option)
+// any later version.
+//
+// cdb is distributed in the hope that it will be useful, but WITHOUT ANY
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+// FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for
+// more details.
+//
+// You should have received a copy of the Affero GNU General Public License
+// along with cdb.  If not, see <https://www.gnu.org/licenses/>.
+
+
+use std::str::FromStr;
+use std::path::{Path, PathBuf};
+
+use digest::DynDigest;
+use toml::Value;
+use cas_core::{
+    Cas, DefaultPipeline, Error, ObjectId, ObjectMetadata, ObjectType,
+    Pipeline, Reader, ReadWrapper, Result, Writer,
+};
+
+use crate::utils::{
+    obj_dir, obj_path, ref_dir, tmp_dir,
+    read_config, write_config,
+    read_metadata, write_metadata,
+    new_digest,
+};
+use crate::wfile::WFile;
+
+
+pub struct SimpleCas {
+    db_path: PathBuf,
+    digest: Box<dyn DynDigest>,
+    pipeline: DefaultPipeline,
+    config: Value,
+}
+
+
+impl SimpleCas {
+    pub fn create(db_path: PathBuf, config: Value) -> Result<Self> {
+        let digest_id = config["cas"]["digest"].as_str()
+            .ok_or_else(|| Error::error(
+                "mandatory cas.digest value is invalid or missing from config"
+            ))?;
+        let digest = new_digest(digest_id)?;
+        let pipeline = DefaultPipeline::new(digest.box_clone());
+
+        if db_path.exists() {
+            return Err(Error::error(format!(
+                "failed to create SimpleCas: target directory already exists ({})",
+                db_path.to_string_lossy(),
+            )));
+        }
+
+        for path in [
+            &db_path,
+            &obj_dir(&db_path),
+            &ref_dir(&db_path),
+            &tmp_dir(&db_path),
+        ] {
+            std::fs::create_dir(path).or_else(|e|
+                Err(Error::error(format!(
+                    "failed to create directory ({}): {}",
+                    path.to_string_lossy(), e,
+                )))
+            )?;
+        }
+
+        write_config(&config, &db_path)?;
+
+        Ok(SimpleCas {
+            db_path,
+            digest,
+            pipeline,
+            config,
+        })
+    }
+
+    pub fn open(db_path: PathBuf) -> Result<Self> {
+        let config = read_config(&db_path)?;
+
+        let digest_id = config["cas"]["digest"].as_str()
+            .ok_or_else(|| Error::error(
+                "mandatory cas.digest value is invalid or missing from config"
+            ))?;
+        let digest = new_digest(digest_id)?;
+        let pipeline = DefaultPipeline::new(digest.box_clone());
+
+        Ok(SimpleCas {
+            db_path,
+            digest,
+            pipeline,
+            config,
+        })
+    }
+
+    pub fn save_config(&self) -> Result<()> {
+        write_config(&self.config, &self.db_path)
+    }
+}
+
+
+impl Cas for SimpleCas {
+    fn object_id_from_string(&self, hex: &str) -> Result<ObjectId> {
+        if hex.len() == self.digest.output_size() * 2 {
+            ObjectId::from_str(hex)
+        }
+        else {
+            Err(Error::error(format!(
+                "invalid object id size: got {}, expected {}",
+                hex.len(), self.digest.output_size() * 2,
+            )))
+        }
+    }
+
+    fn has_object_id(&self, oid: &ObjectId) -> Result<bool> {
+        let opath = obj_path(&self.db_path, oid);
+        Ok(opath.is_file())
+    }
+
+    fn iter_object_id<'s>(&'s self) -> Result<Box<dyn 's + Iterator<Item = Result<ObjectId>>>> {
+        Ok(Box::new(ObjectIdIterator::new(self)?))
+    }
+
+
+    fn open_object(&self, oid: &ObjectId) -> Result<(ObjectMetadata, Box<dyn Reader>)> {
+        let opath = obj_path(&self.db_path, oid);
+        if !opath.is_file() {
+            return Err(Error::error(format!("object not found: {}", oid)));
+        }
+
+        let file = std::fs::File::open(opath).or_else(|err|
+            Err(Error::error(format!("failed to open object {}: {}", oid, err))))?;
+        let wrapper = Box::new(ReadWrapper::new(file));
+        let mut reader = self.pipeline.new_reader(wrapper);
+        let metadata = read_metadata(&mut reader)?;
+
+        Ok((metadata, reader))
+    }
+
+    fn new_writer(&mut self, otype: &ObjectType, size: u64) -> Result<Box<dyn Writer>> {
+        let file = Box::new(WFile::new(self.db_path.clone())?);
+        let mut writer = self.pipeline.new_writer(file);
+        write_metadata(&mut writer, otype, size)?;
+
+        Ok(writer)
+    }
+
+    fn remove_object(&mut self, oid: &ObjectId) -> Result<()> {
+        let opath = obj_path(&self.db_path, oid);
+        if !opath.is_file() {
+            return Err(Error::error(format!("object not found: {}", oid)));
+        }
+        std::fs::remove_file(opath).or_else(|err|
+            Err(Error::error(format!("failed to remove object {}: {}", oid, err))))?;
+        Ok(())
+    }
+}
+
+
+pub struct ObjectIdIterator {
+    root_dirs: Vec<PathBuf>,
+    inner_dirs: Vec<PathBuf>,
+    objects: Vec<PathBuf>,
+    root_index: usize,
+    inner_index: usize,
+    object_index: usize,
+}
+
+impl ObjectIdIterator {
+    fn new(cas: &SimpleCas) -> Result<Self> {
+        let root_dirs = read_dir(&obj_dir(&cas.db_path))?;
+        let mut it = Self {
+            root_dirs,
+            inner_dirs: Vec::new(),
+            objects: Vec::new(),
+            root_index: 0,
+            inner_index: 0,
+            object_index: 0,
+        };
+
+        it.fetch_inner_dirs()?;
+        if !it.is_item_valid() {
+            it.next_valid_item()?;
+        }
+
+        Ok(it)
+    }
+
+    fn get_object_id(&mut self) -> Result<ObjectId> {
+        let path = &self.objects[self.object_index];
+
+        if !path.is_file() {
+            return Error::err(format!("item in object database is not a file: {:?}", path));
+        }
+
+        let id = path.ancestors()
+            .take(3)
+            .collect::<Vec<_>>()
+            .iter()
+            .rev()
+            .map(|p| p.file_name()?.to_str())
+            .collect::<Option<String>>()
+            .ok_or_else(|| Error::error(format!("invalid object in object database: {:?}", path)))?;
+        ObjectId::from_str(&id)
+    }
+
+    fn is_done(&self) -> bool {
+        self.root_index == self.root_dirs.len()
+    }
+
+    fn is_item_valid(&self) -> bool {
+        self.root_index < self.root_dirs.len() &&
+        self.inner_index < self.inner_dirs.len() &&
+        self.object_index < self.objects.len()
+    }
+
+    fn next_item(&mut self) -> Result<()> {
+        if self.object_index < self.objects.len() {
+            self.object_index += 1;
+        }
+        else if self.inner_index < self.inner_dirs.len() {
+            self.inner_index += 1;
+            self.fetch_objects()?;
+        }
+        else if self.root_index < self.root_dirs.len() {
+            self.root_index += 1;
+            self.fetch_inner_dirs()?;
+        }
+        Ok(())
+    }
+
+    fn next_valid_item(&mut self) -> Result<bool> {
+        if !self.is_done() {
+            self.next_item()?;
+        }
+        while !self.is_done() && !self.is_item_valid() {
+            self.next_item()?;
+        }
+        Ok(self.is_done())
+    }
+
+    fn fetch_objects(&mut self) -> Result<()> {
+        self.object_index = 0;
+        if self.inner_index < self.inner_dirs.len() {
+            let path = &self.inner_dirs[self.inner_index];
+            self.objects = read_dir(path)?;
+        }
+        else {
+            self.objects.clear();
+        }
+        Ok(())
+    }
+
+    fn fetch_inner_dirs(&mut self) -> Result<()> {
+        self.inner_index = 0;
+        if self.root_index < self.root_dirs.len() {
+            let path = &self.root_dirs[self.root_index];
+            self.inner_dirs = read_dir(path)?;
+            self.fetch_objects()?;
+        }
+        else {
+            self.inner_dirs.clear();
+        }
+        Ok(())
+    }
+}
+
+impl Iterator for ObjectIdIterator {
+    type Item = Result<ObjectId>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if !self.is_done() {
+            debug_assert!(self.is_item_valid());
+            let item = self.get_object_id();
+            if item.is_ok() {
+                if let Err(err) = self.next_valid_item() {
+                    return Some(Err(err));
+                }
+            }
+            Some(item)
+        }
+        else {
+            None
+        }
+    }
+}
+
+fn read_dir(path: &Path) -> Result<Vec<PathBuf>> {
+    let mut paths = std::fs::read_dir(path)
+        .or_else(|err| Err(Error::error(format!(
+            "failed to read directory {:?}: {}", path, err))))?
+        .map(|dir| dir.map(|dir| dir.path()))
+        .collect::<std::io::Result<Vec<_>>>()
+        .or_else(|err| Err(Error::error(format!(
+            "error while reading directory {:?}: {}", path, err))))?;
+    paths.sort();
+    Ok(paths)
+}
+
+
+#[cfg(test)]
+mod tests {
+    use std::path::Path;
+
+    use super::*;
+
+    fn get_config() -> Value {
+         toml::toml!(
+            [cas]
+            digest = "sha256"
+        )
+    }
+
+    fn get_cas_path(dir: &Path) -> PathBuf {
+        let mut cas_path = dir.to_path_buf();
+        cas_path.push(".bsv");
+        cas_path
+    }
+
+    #[test]
+    fn test_create_simple_cas() {
+        let dir = tempfile::tempdir().expect("failed to create temp test dir");
+        let cas_path = get_cas_path(dir.path());
+        let config = get_config();
+
+        let cas = SimpleCas::create(cas_path, config)
+            .expect("failed to create SimpleCas object");
+        let oid = cas.object_id_from_string("f731f6bc6a6a73bad170e56452473ef6930b7a0ab33cc54be44221a89b49d786")
+            .expect("failed to create object id");
+
+        assert!(!cas.has_object_id(&oid).expect("has_object_id failed"));
+    }
+
+    #[test]
+    fn test_write_object() {
+        let dir = tempfile::tempdir().expect("failed to create temp test dir");
+        let cas_path = get_cas_path(dir.path());
+        let config = get_config();
+
+        let otype = ObjectType::new(b"blob").expect("failed to create object type");
+        let payload = b"Hello World!";
+
+        let mut cas = SimpleCas::create(cas_path.clone(), config)
+            .expect("failed to create SimpleCas object");
+        let oid = cas.object_id_from_string("f731f6bc6a6a73bad170e56452473ef6930b7a0ab33cc54be44221a89b49d786")
+            .expect("failed to create object id");
+
+        assert!(!cas.has_object_id(&oid).expect("has_object_id failed"));
+
+        {
+            let mut writer = cas.new_writer(&otype, payload.len() as u64)
+                .expect("failed to create object writer");
+            writer.write_all(payload).expect("failed to write object");
+            let oid2 = writer.finalize().expect("failed to finalize writer");
+
+            assert_eq!(oid2, oid);
+        }
+
+        assert!(cas.has_object_id(&oid).expect("has_object_id failed"));
+    }
+
+    #[test]
+    fn test_read_object() {
+        use std::io::Write;
+
+        let dir = tempfile::tempdir().expect("failed to create temp test dir");
+        let cas_path = get_cas_path(dir.path());
+        let config = get_config();
+
+        let otype = ObjectType::new(b"blob").expect("failed to create object type");
+        let payload = b"Hello World!";
+
+        let cas = SimpleCas::create(cas_path.clone(), config)
+            .expect("failed to create SimpleCas object");
+        let oid = cas.object_id_from_string("f731f6bc6a6a73bad170e56452473ef6930b7a0ab33cc54be44221a89b49d786")
+            .expect("failed to create object id");
+
+        assert!(!cas.has_object_id(&oid).expect("has_object_id failed"));
+
+        {
+            let opath = obj_path(&cas_path, &oid);
+            std::fs::create_dir_all(opath.parent().unwrap()).expect("failed to create object dir");
+            let mut file = std::fs::File::create(opath)
+                .expect("failed to open object file for write");
+            file.write_all(b"blob\x00\x00\x00\x00\x00\x00\x00\x0c")
+                .expect("failed to write object header");
+            file.write_all(payload).expect("failed to write object payload");
+        }
+
+        assert!(cas.has_object_id(&oid).expect("has_object_id failed"));
+
+        let (metadata, mut reader) = cas.open_object(&oid).expect("failed to open object");
+        let mut buf = Vec::new();
+        reader.read_to_end(&mut buf).expect("failed to read object");
+        let oid2 = reader.finalize().expect("failed to finalize object reader");
+
+        assert_eq!(oid2, oid);
+        assert_eq!(metadata.otype(), &otype);
+        assert_eq!(metadata.size(), payload.len() as u64);
+        assert_eq!(buf, payload);
+    }
+
+    #[test]
+    fn test_read_write_object() {
+        let dir = tempfile::tempdir().expect("failed to create temp test dir");
+        let cas_path = get_cas_path(dir.path());
+        let config = get_config();
+
+        let otype = ObjectType::new(b"blob").expect("failed to create object type");
+        let payload = b"This is a test.";
+
+        let mut cas = SimpleCas::create(cas_path.clone(), config)
+            .expect("failed to create SimpleCas object");
+
+        let oid = cas.write_object(&otype, payload).expect("failed to write object");
+        let (metadata, data) = cas.read_object(&oid).expect("failed to read object");
+
+        assert_eq!(metadata.otype(), &otype);
+        assert_eq!(metadata.size(), payload.len() as u64);
+        assert_eq!(data, payload);
+    }
+
+    #[test]
+    fn test_remove_object() {
+        let dir = tempfile::tempdir().expect("failed to create temp test dir");
+        let cas_path = get_cas_path(dir.path());
+        let config = get_config();
+
+        let otype = ObjectType::new(b"blob").expect("failed to create object type");
+        let payload = b"This is a test.";
+
+        let mut cas = SimpleCas::create(cas_path.clone(), config)
+            .expect("failed to create SimpleCas object");
+
+        let oid = cas.write_object(&otype, payload).expect("failed to write object");
+
+        assert!(cas.has_object_id(&oid).unwrap());
+
+        cas.remove_object(&oid).expect("failed to remove object");
+
+        assert!(!cas.has_object_id(&oid).unwrap());
+    }
+
+    #[test]
+    fn test_object_id_iterator() {
+        let dir = tempfile::tempdir().expect("failed to create temp test dir");
+        let cas_path = get_cas_path(dir.path());
+        let config = get_config();
+
+        let mut cas = SimpleCas::create(cas_path.clone(), config)
+            .expect("failed to create SimpleCas object");
+
+        let add_fake_object = |oid: &ObjectId| {
+            use std::io::Write;
+
+            let path = obj_path(&cas.db_path, oid);
+            std::fs::create_dir_all(path.parent().unwrap())
+                .expect("failed to create object directory");
+            let mut file = std::fs::File::create(path).unwrap();
+            file.write(b"Test").unwrap();
+        };
+
+        let oid_0_0_0 = ObjectId::from_str("0000000000000000000000000000000000000000000000000000000000000000").unwrap();
+        let oid_0_0_1 = ObjectId::from_str("0000000000000000000000000000000000000000000000000000000000000001").unwrap();
+        let oid_0_0_2 = ObjectId::from_str("0000000000000000000000000000000000000000000000000000000000000002").unwrap();
+
+        let oid_0_1_0 = ObjectId::from_str("0001000000000000000000000000000000000000000000000000000000000000").unwrap();
+        let oid_0_2_0 = ObjectId::from_str("0002000000000000000000000000000000000000000000000000000000000000").unwrap();
+
+        let oid_1_0_0 = ObjectId::from_str("0100000000000000000000000000000000000000000000000000000000000000").unwrap();
+        let oid_2_0_0 = ObjectId::from_str("0200000000000000000000000000000000000000000000000000000000000000").unwrap();
+
+        let objects = cas.iter_object_id().unwrap().collect::<Result<Vec<_>>>().unwrap();
+        assert_eq!(objects, []);
+
+        let all_oids = [
+            oid_0_0_0.clone(), oid_0_0_1.clone(), oid_0_0_2.clone(),
+            oid_0_1_0.clone(), oid_0_2_0.clone(),
+            oid_1_0_0.clone(), oid_2_0_0.clone(),
+        ];
+        for oid in all_oids.iter() {
+            add_fake_object(&oid);
+        }
+
+        let objects = cas.iter_object_id().unwrap()
+            .collect::<Result<Vec<_>>>().unwrap();
+        assert_eq!(objects, all_oids);
+
+        cas.remove_object(&oid_0_0_0).unwrap();
+
+        let oids = [
+            oid_0_0_1.clone(), oid_0_0_2.clone(),
+            oid_0_1_0.clone(), oid_0_2_0.clone(),
+            oid_1_0_0.clone(), oid_2_0_0.clone(),
+        ];
+        let objects = cas.iter_object_id().unwrap()
+            .collect::<Result<Vec<_>>>().unwrap();
+        assert_eq!(objects, oids);
+
+        cas.remove_object(&oid_0_0_2).unwrap();
+
+        let oids = [
+            oid_0_0_1.clone(),
+            oid_0_1_0.clone(), oid_0_2_0.clone(),
+            oid_1_0_0.clone(), oid_2_0_0.clone(),
+        ];
+        let objects = cas.iter_object_id().unwrap()
+            .collect::<Result<Vec<_>>>().unwrap();
+        assert_eq!(objects, oids);
+
+        cas.remove_object(&oid_0_0_1).unwrap();
+
+        let oids = [
+            oid_0_1_0.clone(), oid_0_2_0.clone(),
+            oid_1_0_0.clone(), oid_2_0_0.clone(),
+        ];
+        let objects = cas.iter_object_id().unwrap()
+            .collect::<Result<Vec<_>>>().unwrap();
+        assert_eq!(objects, oids);
+
+        cas.remove_object(&oid_2_0_0).unwrap();
+
+        let oids = [
+            oid_0_1_0.clone(), oid_0_2_0.clone(),
+            oid_1_0_0.clone(),
+        ];
+        let objects = cas.iter_object_id().unwrap()
+            .collect::<Result<Vec<_>>>().unwrap();
+        assert_eq!(objects, oids);
+
+        cas.remove_object(&oid_1_0_0).unwrap();
+
+        let oids = [
+            oid_0_1_0.clone(), oid_0_2_0.clone(),
+        ];
+        let objects = cas.iter_object_id().unwrap()
+            .collect::<Result<Vec<_>>>().unwrap();
+        assert_eq!(objects, oids);
+
+        cas.remove_object(&oid_0_1_0).unwrap();
+
+        let oids = [
+            oid_0_2_0.clone(),
+        ];
+        let objects = cas.iter_object_id().unwrap()
+            .collect::<Result<Vec<_>>>().unwrap();
+        assert_eq!(objects, oids);
+
+        cas.remove_object(&oid_0_2_0).unwrap();
+
+        let oids = [
+        ];
+        let objects = cas.iter_object_id().unwrap()
+            .collect::<Result<Vec<_>>>().unwrap();
+        assert_eq!(objects, oids);
+    }
+}
diff --git a/cas-simple/src/lib.rs b/cas-simple/src/lib.rs
index d65b5d0..57cfba3 100644
--- a/cas-simple/src/lib.rs
+++ b/cas-simple/src/lib.rs
@@ -23,223 +23,12 @@ extern crate digest;
 extern crate sha2;
 extern crate toml;
 extern crate cas_core;
+extern crate tempfile;
 
 
-use std::str::FromStr;
-use std::path::{Path, PathBuf};
+mod utils;
+mod wfile;
+mod cas;
 
-use digest::DynDigest;
-use toml::Value;
-use cas_core::{Cas, Error, hex, ObjectId, ObjectMetadata, ObjectType, Result};
 
-
-fn new_digest(id: &str) -> Result<Box<dyn DynDigest>> {
-    match id {
-        "sha256" => { Ok(Box::new(sha2::Sha256::default())) },
-        _ => { Err(Error::error(format!("unknown digest '{}'", id))) }
-    }
-}
-
-
-pub struct SimpleCas {
-    db_path: PathBuf,
-    digest: Box<dyn DynDigest>,
-    config: Value,
-}
-
-
-impl SimpleCas {
-    pub fn create(db_path: PathBuf, config: Value) -> Result<Self> {
-        let digest_id = config["cas"]["digest"].as_str()
-            .ok_or_else(|| Error::error(
-                "mandatory cas.digest value is invalid or missing from config"
-            ))?;
-        let digest = new_digest(digest_id)?;
-
-        if db_path.exists() {
-            return Err(Error::error(format!(
-                "failed to create SimpleCas: target directory already exists ({})",
-                db_path.to_string_lossy(),
-            )));
-        }
-
-        for path in [
-            &db_path,
-            &obj_dir(&db_path),
-            &ref_dir(&db_path),
-            &tmp_dir(&db_path),
-        ] {
-            std::fs::create_dir(path).or_else(|e|
-                Err(Error::error(format!(
-                    "failed to create directory ({}): {}",
-                    path.to_string_lossy(), e,
-                )))
-            )?;
-        }
-
-        write_config(&config, &db_path)?;
-
-        Ok(SimpleCas {
-            db_path,
-            digest,
-            config,
-        })
-    }
-
-    pub fn open(db_path: PathBuf) -> Result<Self> {
-        let config = read_config(&db_path)?;
-
-        let digest_id = config["cas"]["digest"].as_str()
-            .ok_or_else(|| Error::error(
-                "mandatory cas.digest value is invalid or missing from config"
-            ))?;
-        let digest = new_digest(digest_id)?;
-
-        Ok(SimpleCas {
-            db_path,
-            digest,
-            config,
-        })
-    }
-
-    pub fn save_config(&self) -> Result<()> {
-        write_config(&self.config, &self.db_path)
-    }
-}
-
-
-impl Cas for SimpleCas {
-    fn object_id_from_string(&self, hex: &str) -> Result<ObjectId> {
-        if hex.len() == self.digest.output_size() * 2 {
-            ObjectId::from_str(hex)
-        }
-        else {
-            Err(Error::error(format!(
-                "invalid object id size: got {}, expected {}",
-                hex.len(), self.digest.output_size() * 2,
-            )))
-        }
-    }
-
-    fn object_id_from_partial(&self, _hex: &str) -> Result<ObjectId> {
-        Err(Error::error("Not implemented"))
-    }
-
-    fn has_object_id(&self, oid: &ObjectId) -> Result<bool> {
-        let opath = obj_path(&self.db_path, oid);
-        Ok(opath.is_file())
-    }
-
-    fn read_object(&self, _oid: &ObjectId) -> Result<(ObjectMetadata, &dyn std::io::Read)> {
-        Err(Error::error("Not implemented"))
-    }
-
-    fn write_object(&mut self, _otype: ObjectType, _data: &mut dyn std::io::Read) -> Result<ObjectId> {
-        Err(Error::error("Not implemented"))
-    }
-
-    fn remove_object(&mut self, _oid: &ObjectId) -> Result<()> {
-        Err(Error::error("Not implemented"))
-    }
-
-    fn read_ref(&self, _key: &str) -> Result<ObjectId> {
-        Err(Error::error("Not implemented"))
-    }
-
-    fn write_ref(&mut self, _key: &str, _value: &ObjectId) -> Result<()> {
-        Err(Error::error("Not implemented"))
-    }
-
-    fn remove_ref(&mut self, _key: &str) -> Result<()> {
-        Err(Error::error("Not implemented"))
-    }
-}
-
-
-fn obj_dir(cas_path: &Path) -> PathBuf {
-    cas_path.join("obj")
-}
-
-fn obj_path(cas_path: &Path, oid: &ObjectId) -> PathBuf {
-    let mut path = cas_path.to_path_buf();
-    path.push(hex(&oid.id()[0..1]));
-    path.push(hex(&oid.id()[1..2]));
-    path.push(hex(&oid.id()[2..]));
-    path
-}
-
-fn ref_dir(cas_path: &Path) -> PathBuf {
-    cas_path.join("ref")
-}
-
-fn tmp_dir(cas_path: &Path) -> PathBuf {
-    cas_path.join("tmp")
-}
-
-fn config_path(cas_path: &Path) -> PathBuf {
-    cas_path.join("config")
-}
-
-fn read_config(db_path: &Path) -> Result<Value> {
-    use std::io::Read;
-
-    let mut file = std::fs::File::open(config_path(db_path)).or_else(|err|
-        Err(Error::error(format!("invalid repository: no config file: {}", err)))
-    )?;
-    let mut config_str = String::new();
-    file.read_to_string(&mut config_str).or_else(|err|
-        Err(Error::error(format!("cannot read config file: {}", err)))
-    )?;
-    let config = toml::from_str(&config_str).or_else(|err|
-        Err(Error::error(format!("error while reading config file: {}", err)))
-    )?;
-    Ok(config)
-}
-
-fn write_config(config: &Value, db_path: &Path) -> Result<()> {
-    use std::io::Write;
-
-    let config_str = toml::to_string_pretty(config).or_else(|err|
-        Err(Error::error(format!("failed to serialize config: {}", err)))
-    )?;
-    let mut file = tempfile::NamedTempFile::new_in(tmp_dir(db_path)).or_else(|err|
-        Err(Error::error(format!("cannot create temp config file: {}", err)))
-    )?;
-    file.write_all(config_str.as_bytes()).or_else(|err|
-        Err(Error::error(format!("failed to write to temp config: {}", err)))
-    )?;
-    file.persist(config_path(db_path)).or_else(|err|
-        Err(Error::error(format!("failed to (over)write config: {}", err)))
-    )?;
-    Ok(())
-}
-
-
-#[cfg(test)]
-mod tests {
-    extern crate tempfile;
-
-    use super::*;
-
-    #[test]
-    fn create_simple_cas() {
-        let dir = tempfile::tempdir()
-            .expect("failed to create temp test dir");
-        let cas_path = {
-            let mut cas_path = dir.path().to_path_buf();
-            cas_path.push(".bsv");
-            cas_path
-        };
-        let config = toml::toml!(
-            [cas]
-            digest = "sha256"
-        );
-
-        let cas = SimpleCas::create(cas_path, config)
-            .expect("failed to create SimpleCas object");
-        let oid = cas.object_id_from_string("0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef")
-            .expect("failed to create object id");
-
-        assert!(!cas.has_object_id(&oid).expect("has_object_id failed"));
-    }
-}
+pub use cas::SimpleCas;
diff --git a/cas-simple/src/utils.rs b/cas-simple/src/utils.rs
new file mode 100644
index 0000000..5cbe802
--- /dev/null
+++ b/cas-simple/src/utils.rs
@@ -0,0 +1,210 @@
+
+// This file is part of bsv.
+//
+// bsv is free software: you can redistribute it and/or modify it under the
+// terms of the GNU Affero General Public License as published by the Free
+// Software Foundation, either version 3 of the License, or (at your option)
+// any later version.
+//
+// cdb is distributed in the hope that it will be useful, but WITHOUT ANY
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+// FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for
+// more details.
+//
+// You should have received a copy of the Affero GNU General Public License
+// along with cdb.  If not, see <https://www.gnu.org/licenses/>.
+
+
+use std::path::{Path, PathBuf};
+
+use digest::DynDigest;
+use toml::Value;
+
+use cas_core::{
+    Error, hex, ObjectId, ObjectMetadata, ObjectType, Result,
+};
+
+
+pub fn obj_dir(cas_path: &Path) -> PathBuf {
+    cas_path.join("obj")
+}
+
+pub fn obj_path(cas_path: &Path, oid: &ObjectId) -> PathBuf {
+    let mut path = obj_dir(cas_path);
+    path.push(hex(&oid.id()[0..1]));
+    path.push(hex(&oid.id()[1..2]));
+    path.push(hex(&oid.id()[2..]));
+    path
+}
+
+pub fn ref_dir(cas_path: &Path) -> PathBuf {
+    cas_path.join("ref")
+}
+
+pub fn tmp_dir(cas_path: &Path) -> PathBuf {
+    cas_path.join("tmp")
+}
+
+pub fn config_path(cas_path: &Path) -> PathBuf {
+    cas_path.join("config")
+}
+
+
+pub fn read_config(db_path: &Path) -> Result<Value> {
+    use std::io::Read;
+
+    let mut file = std::fs::File::open(config_path(db_path)).or_else(|err|
+        Err(Error::error(format!("invalid repository: no config file: {}", err)))
+    )?;
+    let mut config_str = String::new();
+    file.read_to_string(&mut config_str).or_else(|err|
+        Err(Error::error(format!("cannot read config file: {}", err)))
+    )?;
+    let config = toml::from_str(&config_str).or_else(|err|
+        Err(Error::error(format!("error while reading config file: {}", err)))
+    )?;
+    Ok(config)
+}
+
+pub fn write_config(config: &Value, db_path: &Path) -> Result<()> {
+    use std::io::Write;
+
+    let config_str = toml::to_string_pretty(config).or_else(|err|
+        Err(Error::error(format!("failed to serialize config: {}", err)))
+    )?;
+    let mut file = tempfile::NamedTempFile::new_in(tmp_dir(db_path)).or_else(|err|
+        Err(Error::error(format!("cannot create temp config file: {}", err)))
+    )?;
+    file.write_all(config_str.as_bytes()).or_else(|err|
+        Err(Error::error(format!("failed to write to temp config: {}", err)))
+    )?;
+    file.persist(config_path(db_path)).or_else(|err|
+        Err(Error::error(format!("failed to (over)write config: {}", err)))
+    )?;
+    Ok(())
+}
+
+
+pub fn read_metadata(read: &mut dyn std::io::Read) -> Result<ObjectMetadata> {
+    let otype = ObjectType::new(&{
+        let mut buf = [0; 4];
+        read.read_exact(&mut buf).or_else(|err|
+            Err(Error::error(format!("failed to read object type: {}", err))))?;
+        buf
+    })?;
+    let size = u64::from_be_bytes({
+        let mut buf = [0; 8];
+        read.read_exact(&mut buf).or_else(|err|
+            Err(Error::error(format!("failed to read object size: {}", err))))?;
+        buf
+    });
+    Ok(ObjectMetadata::new(otype, size))
+}
+
+pub fn write_metadata(write: &mut dyn std::io::Write, otype: &ObjectType, size: u64) -> Result<()> {
+    write.write_all(otype.id()).or_else(|err|
+        Err(Error::error(format!("failed to write object type: {}", err))))?;
+    write.write_all(&size.to_be_bytes()).or_else(|err|
+        Err(Error::error(format!("failed to write object size: {}", err))))?;
+    Ok(())
+}
+
+
+pub fn new_digest(id: &str) -> Result<Box<dyn DynDigest>> {
+    match id {
+        "sha256" => { Ok(Box::new(sha2::Sha256::default())) },
+        _ => { Err(Error::error(format!("unknown digest '{}'", id))) }
+    }
+}
+
+
+#[cfg(test)]
+mod tests {
+    use std::str::FromStr;
+
+    use super::*;
+
+    #[test]
+    fn test_dirs() {
+        let base_path = PathBuf::from("/foo/bar");
+        let oid = ObjectId::from_str("0123456789abcdef")
+            .expect("failed to create object id");
+
+        assert_eq!(
+            config_path(&base_path),
+            PathBuf::from("/foo/bar/config")
+        );
+        assert_eq!(
+            obj_dir(&base_path),
+            PathBuf::from("/foo/bar/obj")
+        );
+        assert_eq!(
+            obj_path(&base_path, &oid),
+            PathBuf::from("/foo/bar/obj/01/23/456789abcdef")
+        );
+        assert_eq!(
+            ref_dir(&base_path),
+            PathBuf::from("/foo/bar/ref")
+        );
+        assert_eq!(
+            tmp_dir(&base_path),
+            PathBuf::from("/foo/bar/tmp")
+        );
+    }
+
+    #[test]
+    fn test_read_write_config() {
+        let config = toml::toml!{
+            [cas]
+            path = "/foo/bar"
+            digest = "sha1"
+
+            [extra]
+            test = 42
+        };
+
+        let dir = tempfile::TempDir::new()
+            .expect("failed to create tmp dir");
+        std::fs::create_dir(tmp_dir(dir.path())).expect("failed to create db/tmp dir");
+
+        write_config(&config, dir.path()).expect("failed to write config");
+        let config2 = read_config(dir.path()).expect("failed to read config");
+
+        assert_eq!(config2, config);
+
+        dir.close().expect("failed to close tmp dir")
+    }
+
+    #[test]
+    fn test_read_write_metadata() {
+        use std::io::Write;
+
+        let data = b"test\x00\x00\x00\x00\x00\x00\x00\x0cHello World!";
+        let reader: &mut dyn std::io::Read = &mut &data[..];
+        let mut payload = Vec::new();
+
+        let metadata = read_metadata(reader).expect("failed to read metadata");
+        reader.read_to_end(&mut payload).expect("failed to read payload");
+
+        let otype = ObjectType::new(b"test").expect("failed to create ObjectType");
+        assert_eq!(metadata, ObjectMetadata::new(otype.clone(), 12));
+        assert_eq!(payload, b"Hello World!");
+
+        let mut data2 = Vec::new();
+
+        write_metadata(&mut data2, &otype, payload.len() as u64).expect("failed to write metadata");
+        data2.write_all(&payload).expect("failed to write payload");
+
+        assert_eq!(data2, data);
+    }
+
+    #[test]
+    fn test_new_digest() {
+        let mut sha256 = new_digest("sha256").expect("failed to create sha256 digest");
+
+        sha256.update(b"Hello World!");
+        assert_eq!(hex(&sha256.finalize()), "7f83b1657ff1fc53b92dc18148a1d65dfc2d4b1fa3d677284addd200126d9069");
+
+        assert!(new_digest("do_not_exists").is_err());
+    }
+}
diff --git a/cas-simple/src/wfile.rs b/cas-simple/src/wfile.rs
new file mode 100644
index 0000000..af03e58
--- /dev/null
+++ b/cas-simple/src/wfile.rs
@@ -0,0 +1,106 @@
+// This file is part of bsv.
+//
+// bsv is free software: you can redistribute it and/or modify it under the
+// terms of the GNU Affero General Public License as published by the Free
+// Software Foundation, either version 3 of the License, or (at your option)
+// any later version.
+//
+// cdb is distributed in the hope that it will be useful, but WITHOUT ANY
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+// FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for
+// more details.
+//
+// You should have received a copy of the Affero GNU General Public License
+// along with cdb.  If not, see <https://www.gnu.org/licenses/>.
+
+
+use std::path::{PathBuf};
+
+use cas_core::{
+    Error, ObjectId, Result, Writer,
+};
+
+use crate::utils::{obj_path, tmp_dir};
+
+
+pub struct WFile {
+    file: tempfile::NamedTempFile,
+    db_path: PathBuf,
+}
+
+impl WFile {
+    pub fn new(db_path: PathBuf) -> Result<Self> {
+        let file = tempfile::NamedTempFile::new_in(tmp_dir(&db_path)).or_else(|err|
+            Err(Error::error(format!("failed to create_file: {}", err))))?;
+        Ok(WFile {
+            file,
+            db_path,
+        })
+    }
+}
+
+impl std::io::Write for WFile {
+    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
+        self.file.write(buf)
+    }
+
+    fn flush(&mut self) -> std::io::Result<()> {
+        self.file.flush()
+    }
+}
+
+impl Writer for WFile {
+    fn finalize(self: Box<Self>) -> Result<ObjectId> {
+        Err(Error::error("reader pipline has no digest step"))
+    }
+
+    fn _finalize(self: Box<Self>, oid: ObjectId) -> Result<ObjectId> {
+        let path = obj_path(&self.db_path, &oid);
+        std::fs::create_dir_all(
+            path.parent().expect("cannot access to object parent directory")
+        ).or_else(|err|
+            Err(Error::error(format!("failed to create object dir: {}", err))))?;
+        self.file.persist(path).or_else(|err|
+            Err(Error::error(format!("failed to persist file: {}", err))))?;
+        Ok(oid)
+    }
+}
+
+
+#[cfg(test)]
+mod tests {
+    use std::str::FromStr;
+
+    use crate::utils::{obj_dir, obj_path};
+    use super::*;
+
+    #[test]
+    fn test_wfile() {
+        use std::io::{Read, Write};
+
+        let data = b"Hello World!";
+        let oid = ObjectId::from_str(
+            "7f83b1657ff1fc53b92dc18148a1d65dfc2d4b1fa3d677284addd200126d9069"
+        ).expect("invalid object id");
+
+        let dir = tempfile::TempDir::new()
+            .expect("failed to create tmp dir");
+
+        std::fs::create_dir(tmp_dir(dir.path())).expect("failed to create db/obj dir");
+        std::fs::create_dir(obj_dir(dir.path())).expect("failed to create db/tmp dir");
+
+        {
+            let mut writer = Box::new(WFile::new(dir.path().to_path_buf()).expect("failed to create WFile"));
+            writer.write(data).expect("failed to write to WFile");
+            let oid2 = writer._finalize(oid.clone()).expect("failed to finalize WFile");
+
+            assert_eq!(oid2, oid);
+        }
+
+        let mut file = std::fs::File::open(obj_path(dir.path(), &oid)).expect("failed to open object file");
+        let mut buf = Vec::new();
+        file.read_to_end(&mut buf).expect("failed to read object file");
+
+        assert_eq!(buf, data);
+    }
+}