diff --git a/src/ext/hashers.rs b/src/ext/hashers.rs
index a64451e..615bd92 100644
--- a/src/ext/hashers.rs
+++ b/src/ext/hashers.rs
@@ -3,6 +3,17 @@ use std::hash::{BuildHasherDefault, Hasher};
 use smallvec::SmallVec;
 use cryptohelpers::sha256;
 use ::bytes::Buf;
+use cryptohelpers::sha2::{
+    Sha256,
+    Digest,
+};
+use std::borrow::BorrowMut;
+use tokio::io::AsyncWrite;
+use std::{
+    pin::Pin,
+    task::{Context, Poll},
+    io,
+};
 
 /// A hasher that takes the first 8 bytes from SHA256 hash as its output.
 ///
@@ -36,6 +47,107 @@ impl Hasher for Sha256TopHasher
     }
 }
 
+/// An `AsyncWrite` implementor that writes it's inputs to a sha256 digest.
+#[pin_project]
+#[derive(Debug)]
+pub struct Sha256Sink<H: BorrowMut<Sha256> = Sha256>
+{
+    digest: H
+}
+
+impl<H: BorrowMut<Sha256>> Sha256Sink<H>
+{
+    /// Create a new Sha256-computing `AsyncWrite` sink.
+    #[inline] pub fn new(digest: H) -> Self
+    {
+	Self{digest}
+    }
+    /// Consume into the inner digest
+    #[inline] pub fn into_inner(self) -> H
+    {
+	self.digest
+    }
+    /// The inner digest
+    #[inline] pub fn inner(&self) -> &H
+    {
+	&self.digest
+    }
+    /// The inner digest (mutable)
+    #[inline] pub fn inner_mut(&mut self) -> &mut H
+    {
+	&mut self.digest
+    }
+    
+    #[inline(always)] pub fn digest(&self) -> &Sha256
+    {
+	self.digest.borrow()
+    }
+    #[inline(always)] pub fn digest_mut(&mut self) -> &mut Sha256
+    {
+	self.digest.borrow_mut()
+    }
+}
+impl<H: BorrowMut<Sha256>> AsRef<Sha256> for Sha256Sink<H>
+{
+    fn as_ref(&self) -> &Sha256 {
+	self.digest.borrow()
+    }
+}
+impl<H: BorrowMut<Sha256>> AsMut<Sha256> for Sha256Sink<H>
+{
+    fn as_mut(&mut self) -> &mut Sha256 {
+	self.digest.borrow_mut()
+    }
+}
+/*
+impl<'a, H: BorrowMut<Sha256>> AsRef<Sha256> for &'a Sha256Sink<H>
+    //where H: 'a
+{
+    fn as_ref(&self) -> &Sha256 {
+	self.digest.borrow()
+    }
+}
+impl<'a, H: BorrowMut<Sha256>> AsMut<Sha256> for &'a mut Sha256Sink<H>
+   // where H: 'a
+{
+    fn as_mut(&mut self) -> &mut Sha256 {
+	self.digest.borrow_mut()
+    }
+}
+*/
+impl<H: BorrowMut<Sha256>> BorrowMut<Sha256> for Sha256Sink<H>
+{
+    #[inline] fn borrow_mut(&mut self) -> &mut Sha256 {
+	self.digest.borrow_mut()
+    }
+}
+
+impl<H: BorrowMut<Sha256>> Borrow<Sha256> for Sha256Sink<H>
+{
+    #[inline] fn borrow(&self) -> &Sha256 {
+	self.digest.borrow()
+    }
+}
+
+
+
+impl<H: BorrowMut<Sha256>> AsyncWrite for Sha256Sink<H>
+{
+    fn poll_write(self: Pin<&mut Self>, _: &mut Context<'_>, buf: &[u8]) -> Poll<Result<usize, io::Error>> {
+	let this = self.project();
+
+	this.digest.borrow_mut().update(buf);
+	Poll::Ready(Ok(buf.len()))
+    }
+    #[inline(always)] fn poll_flush(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
+	Poll::Ready(Ok(()))
+    }
+    #[inline(always)] fn poll_shutdown(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
+	Poll::Ready(Ok(()))
+
+    }
+}
+
 pub trait Sha256HashExt
 {
     fn compute_sha256_hash(&self) -> sha256::Sha256Hash;
@@ -61,3 +173,4 @@ impl<T: Buf> Sha256HashOwnedExt for T
 	sha256::compute_sync(self.reader()).unwrap()
     }
 }
+
diff --git a/src/ext/mod.rs b/src/ext/mod.rs
index 196b372..b671333 100644
--- a/src/ext/mod.rs
+++ b/src/ext/mod.rs
@@ -37,7 +37,7 @@ pub mod sync;
 pub mod plex;
 pub use plex::MultiplexStreamExt;
 
-// The extension traits are defined in this file, no need to re-export anything from here.
+// The extension traits are defined in this `mod.rs` file, no need to re-export anything from here.
 pub mod chunking;
 
 /// How many elements should `precollect` allocate on the stack before spilling to the heap.
diff --git a/src/service/cache/mod.rs b/src/service/cache/mod.rs
index ec46bd9..25d9368 100644
--- a/src/service/cache/mod.rs
+++ b/src/service/cache/mod.rs
@@ -18,6 +18,7 @@ use std::collections::HashMap;
 use chrono::DateTime;
 use uuid::Uuid;
 use crossbeam_utils::atomic::AtomicCell;
+use cryptohelpers::sha256;
 
 pub type Timezone = chrono::Utc;
 
@@ -84,6 +85,12 @@ pub struct CacheEntry //<K: Key> // `K` is the key in `entries`.
     tm_created: DateTime<Timezone>,
     accesses: AtomicUsize, // Can be mutated when read, hence the atomic.
 
+    /// Hash of the memcache
+    ///
+    /// Used to ensure integrity of written disk data on an explicit check
+    /// (implicitly, integrity is checked by comparing the length of the disk stream with the length of the memory stream, since they are write-only in partial entries.)
+    hash: sha256::Sha256Hash,
+    
     memory: Option<Memory>,
 
     // Pathname is computed from `id`.
diff --git a/src/service/cache/partial.rs b/src/service/cache/partial.rs
index 89b2e58..84f5bef 100644
--- a/src/service/cache/partial.rs
+++ b/src/service/cache/partial.rs
@@ -5,6 +5,7 @@ use std::path::Path;
 use tokio::fs::{self, File};
 use std::io;
 use tokio::io::AsyncWrite;
+use cryptohelpers::sha2::{Sha256, Digest};
 
 use mem::MemoryMut;
 
@@ -32,12 +33,16 @@ pub struct PartialCacheEntry<K>
     id: Uuid,
     key: K,
 
+    /// Written to with each write to any instance created by `writer()`.
+    /// Finalised only when freezing this to a completed entry.
+    hasher: Sha256,
+    
     file: Option<File>,
     memory: MemoryMut,
 }
 
 /// The writer type for writing to a `PartialCacheEntry`.
-pub type PartialCacheEntrySink<'a> = Box<dyn AsyncWrite + Send + Sync + Unpin + 'a>;
+pub type PartialCacheEntrySink<'a> = plex::MultiplexWrite<Box<dyn AsyncWrite + Send + Sync + Unpin + 'a>, Sha256Sink<&'a mut Sha256>>;
 
 impl<K: Key> PartialCacheEntry<K>
 {
@@ -49,6 +54,8 @@ impl<K: Key> PartialCacheEntry<K>
 
 	    file: None,
 	    memory: MemoryMut::new(),
+
+	    hasher: Sha256::new(),
 	    
 	    key,
 	}
@@ -67,12 +74,14 @@ impl<K: Key> PartialCacheEntry<K>
     /// Create a writer for this entry
     pub fn writer(&mut self) -> PartialCacheEntrySink<'_>
     {
-	if let Some(file) = self.file.as_mut()
+	let bx: Box<dyn AsyncWrite + Send + Sync + Unpin + '_> = if let Some(file) = self.file.as_mut()
 	{
 	    Box::new((&mut self.memory).multiplex_ruled::<_, CacheWriteRule>(file))
 	} else {
 	    Box::new(&mut self.memory)
-	}
+	};
+
+	bx.multiplex(Sha256Sink::new(&mut self.hasher))
     }
 }