Merge 6eac21683d into 98d7d0b46b

Co-authored-by: Rob Ede <robjtede@icloud.com>

.cspell.yml

@@ -9,4 +9,5 @@ words:
   - rustls
   - rustup
   - serde
+  - uring
   - zstd

Cargo.lock

@@ -44,7 +44,7 @@ dependencies = [
 
 [[package]]
 name = "actix-files"
-version = "0.6.6"
+version = "0.6.7"
 dependencies = [
  "actix-http",
  "actix-rt",
@@ -891,18 +891,18 @@ dependencies = [
 
 [[package]]
 name = "clap"
-version = "4.5.45"
+version = "4.5.46"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1fc0e74a703892159f5ae7d3aac52c8e6c392f5ae5f359c70b5881d60aaac318"
+checksum = "2c5e4fcf9c21d2e544ca1ee9d8552de13019a42aa7dbf32747fa7aaf1df76e57"
 dependencies = [
  "clap_builder",
 ]
 
 [[package]]
 name = "clap_builder"
-version = "4.5.44"
+version = "4.5.46"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b3e7f4214277f3c7aa526a59dd3fbe306a370daee1f8b7b8c987069cd8e888a8"
+checksum = "fecb53a0e6fcfb055f686001bc2e2592fa527efaf38dbe81a6a9563562e57d41"
 dependencies = [
  "anstyle",
  "clap_lex",
@@ -1482,7 +1482,7 @@ dependencies = [
  "cfg-if",
  "libc",
  "r-efi",
- "wasi 0.14.2+wasi-0.2.4",
+ "wasi 0.14.3+wasi-0.2.4",
 ]
 
 [[package]]
@@ -2305,9 +2305,9 @@ dependencies = [
 
 [[package]]
 name = "potential_utf"
-version = "0.1.2"
+version = "0.1.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e5a7c30837279ca13e7c867e9e40053bc68740f988cb07f7ca6df43cc734b585"
+checksum = "84df19adbe5b5a0782edcab45899906947ab039ccf4573713735ee7de1e6b08a"
 dependencies = [
  "zerovec",
 ]
@@ -3461,11 +3461,11 @@ checksum = "ccf3ec651a847eb01de73ccad15eb7d99f80485de043efb2f370cd654f4ea44b"
 
 [[package]]
 name = "wasi"
-version = "0.14.2+wasi-0.2.4"
+version = "0.14.3+wasi-0.2.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9683f9a5a998d873c0d21fcbe3c083009670149a8fab228644b8bd36b2c48cb3"
+checksum = "6a51ae83037bdd272a9e28ce236db8c07016dd0d50c27038b3f407533c030c95"
 dependencies = [
- "wit-bindgen-rt",
+ "wit-bindgen",
 ]
 
 [[package]]
@@ -3873,13 +3873,10 @@ dependencies = [
 ]
 
 [[package]]
-name = "wit-bindgen-rt"
-version = "0.39.0"
+name = "wit-bindgen"
+version = "0.45.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6f42320e61fe2cfd34354ecb597f86f413484a798ba44a8ca1165c58d42da6c1"
-dependencies = [
- "bitflags 2.9.3",
-]
+checksum = "052283831dbae3d879dc7f51f3d92703a316ca49f91540417d38591826127814"
 
 [[package]]
 name = "writeable"

actix-files/CHANGES.md

@@ -2,6 +2,9 @@
 
 ## Unreleased
 
+## 0.6.7
+
+- Add `{Files, NamedFile}::read_mode_threshold()` methods to allow faster synchronous reads of small files.
 - Minimum supported Rust version (MSRV) is now 1.75.
 
 ## 0.6.6

actix-files/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "actix-files"
-version = "0.6.6"
+version = "0.6.7"
 authors = ["Nikolay Kim <fafhrd91@gmail.com>", "Rob Ede <robjtede@icloud.com>"]
 description = "Static file serving for Actix Web"
 keywords = ["actix", "http", "async", "futures"]

actix-files/README.md

@@ -3,11 +3,11 @@
 <!-- prettier-ignore-start -->
 
 [![crates.io](https://img.shields.io/crates/v/actix-files?label=latest)](https://crates.io/crates/actix-files)
-[![Documentation](https://docs.rs/actix-files/badge.svg?version=0.6.6)](https://docs.rs/actix-files/0.6.6)
+[![Documentation](https://docs.rs/actix-files/badge.svg?version=0.6.7)](https://docs.rs/actix-files/0.6.7)
 ![Version](https://img.shields.io/badge/rustc-1.72+-ab6000.svg)
 ![License](https://img.shields.io/crates/l/actix-files.svg)
 <br />
-[![dependency status](https://deps.rs/crate/actix-files/0.6.6/status.svg)](https://deps.rs/crate/actix-files/0.6.6)
+[![dependency status](https://deps.rs/crate/actix-files/0.6.7/status.svg)](https://deps.rs/crate/actix-files/0.6.7)
 [![Download](https://img.shields.io/crates/d/actix-files.svg)](https://crates.io/crates/actix-files)
 [![Chat on Discord](https://img.shields.io/discord/771444961383153695?label=chat&logo=discord)](https://discord.gg/NWpN5mmg3x)
 

actix-files/src/chunked.rs

@@ -14,6 +14,12 @@ use pin_project_lite::pin_project;
 
 use super::named::File;
 
+#[derive(Debug, Clone, Copy)]
+pub(crate) enum ReadMode {
+    Sync,
+    Async,
+}
+
 pin_project! {
     /// Adapter to read a `std::file::File` in chunks.
     #[doc(hidden)]
@@ -24,6 +30,7 @@ pin_project! {
         state: ChunkedReadFileState<Fut>,
         counter: u64,
         callback: F,
+        read_mode: ReadMode,
     }
 }
 
@@ -57,6 +64,7 @@ pub(crate) fn new_chunked_read(
     size: u64,
     offset: u64,
     file: File,
+    read_mode_threshold: u64,
 ) -> impl Stream<Item = Result<Bytes, Error>> {
     ChunkedReadFile {
         size,
@@ -69,31 +77,50 @@ pub(crate) fn new_chunked_read(
         },
         counter: 0,
         callback: chunked_read_file_callback,
+        read_mode: if size < read_mode_threshold {
+            ReadMode::Sync
+        } else {
+            ReadMode::Async
+        },
     }
 }
 
 #[cfg(not(feature = "experimental-io-uring"))]
-async fn chunked_read_file_callback(
+fn chunked_read_file_callback_sync(
     mut file: File,
     offset: u64,
     max_bytes: usize,
-) -> Result<(File, Bytes), Error> {
+) -> Result<(File, Bytes), io::Error> {
     use io::{Read as _, Seek as _};
 
-    let res = actix_web::web::block(move || {
-        let mut buf = Vec::with_capacity(max_bytes);
+    let mut buf = Vec::with_capacity(max_bytes);
 
-        file.seek(io::SeekFrom::Start(offset))?;
+    file.seek(io::SeekFrom::Start(offset))?;
 
-        let n_bytes = file.by_ref().take(max_bytes as u64).read_to_end(&mut buf)?;
+    let n_bytes = file.by_ref().take(max_bytes as u64).read_to_end(&mut buf)?;
 
-        if n_bytes == 0 {
-            Err(io::Error::from(io::ErrorKind::UnexpectedEof))
-        } else {
-            Ok((file, Bytes::from(buf)))
+    if n_bytes == 0 {
+        Err(io::Error::from(io::ErrorKind::UnexpectedEof))
+    } else {
+        Ok((file, Bytes::from(buf)))
+    }
+}
+
+#[cfg(not(feature = "experimental-io-uring"))]
+#[inline]
+async fn chunked_read_file_callback(
+    file: File,
+    offset: u64,
+    max_bytes: usize,
+    read_mode: ReadMode,
+) -> Result<(File, Bytes), Error> {
+    let res = match read_mode {
+        ReadMode::Sync => chunked_read_file_callback_sync(file, offset, max_bytes)?,
+        ReadMode::Async => {
+            actix_web::web::block(move || chunked_read_file_callback_sync(file, offset, max_bytes))
+                .await??
         }
-    })
-    .await??;
+    };
 
     Ok(res)
 }
@@ -171,7 +198,7 @@ where
 #[cfg(not(feature = "experimental-io-uring"))]
 impl<F, Fut> Stream for ChunkedReadFile<F, Fut>
 where
-    F: Fn(File, u64, usize) -> Fut,
+    F: Fn(File, u64, usize, ReadMode) -> Fut,
     Fut: Future<Output = Result<(File, Bytes), Error>>,
 {
     type Item = Result<Bytes, Error>;
@@ -193,7 +220,7 @@ where
                         .take()
                         .expect("ChunkedReadFile polled after completion");
 
-                    let fut = (this.callback)(file, offset, max_bytes);
+                    let fut = (this.callback)(file, offset, max_bytes, *this.read_mode);
 
                     this.state
                         .project_replace(ChunkedReadFileState::Future { fut });

actix-files/src/files.rs

@@ -49,6 +49,7 @@ pub struct Files {
     use_guards: Option<Rc<dyn Guard>>,
     guards: Vec<Rc<dyn Guard>>,
     hidden_files: bool,
+    read_mode_threshold: u64,
 }
 
 impl fmt::Debug for Files {
@@ -73,6 +74,7 @@ impl Clone for Files {
             use_guards: self.use_guards.clone(),
             guards: self.guards.clone(),
             hidden_files: self.hidden_files,
+            read_mode_threshold: self.read_mode_threshold,
         }
     }
 }
@@ -119,6 +121,7 @@ impl Files {
             use_guards: None,
             guards: Vec::new(),
             hidden_files: false,
+            read_mode_threshold: 0,
         }
     }
 
@@ -204,6 +207,23 @@ impl Files {
         self
     }
 
+    /// Sets the size threshold that determines file read mode (sync/async).
+    ///
+    /// When a file is smaller than the threshold (bytes), the reader will switch from synchronous
+    /// (blocking) file-reads to async reads to avoid blocking the main-thread when processing large
+    /// files.
+    ///
+    /// Tweaking this value according to your expected usage may lead to signifiant performance
+    /// gains (or losses in other handlers, if `size` is too high).
+    ///
+    /// When the `experimental-io-uring` crate feature is enabled, file reads are always async.
+    ///
+    /// Default is 0, meaning all files are read asynchronously.
+    pub fn read_mode_threshold(mut self, size: u64) -> Self {
+        self.read_mode_threshold = size;
+        self
+    }
+
     /// Specifies whether to use ETag or not.
     ///
     /// Default is true.
@@ -367,6 +387,7 @@ impl ServiceFactory<ServiceRequest> for Files {
             file_flags: self.file_flags,
             guards: self.use_guards.clone(),
             hidden_files: self.hidden_files,
+            size_threshold: self.read_mode_threshold,
         };
 
         if let Some(ref default) = *self.default.borrow() {

actix-files/src/named.rs

@@ -80,6 +80,7 @@ pub struct NamedFile {
     pub(crate) content_type: Mime,
     pub(crate) content_disposition: ContentDisposition,
     pub(crate) encoding: Option<ContentEncoding>,
+    pub(crate) read_mode_threshold: u64,
 }
 
 #[cfg(not(feature = "experimental-io-uring"))]
@@ -200,6 +201,7 @@ impl NamedFile {
             encoding,
             status_code: StatusCode::OK,
             flags: Flags::default(),
+            read_mode_threshold: 0,
         })
     }
 
@@ -353,6 +355,23 @@ impl NamedFile {
         self
     }
 
+    /// Sets the size threshold that determines file read mode (sync/async).
+    ///
+    /// When a file is smaller than the threshold (bytes), the reader will switch from synchronous
+    /// (blocking) file-reads to async reads to avoid blocking the main-thread when processing large
+    /// files.
+    ///
+    /// Tweaking this value according to your expected usage may lead to signifiant performance
+    /// gains (or losses in other handlers, if `size` is too high).
+    ///
+    /// When the `experimental-io-uring` crate feature is enabled, file reads are always async.
+    ///
+    /// Default is 0, meaning all files are read asynchronously.
+    pub fn read_mode_threshold(mut self, size: u64) -> Self {
+        self.read_mode_threshold = size;
+        self
+    }
+
     /// Specifies whether to return `ETag` header in response.
     ///
     /// Default is true.
@@ -440,7 +459,8 @@ impl NamedFile {
                 res.insert_header((header::CONTENT_ENCODING, current_encoding.as_str()));
             }
 
-            let reader = chunked::new_chunked_read(self.md.len(), 0, self.file);
+            let reader =
+                chunked::new_chunked_read(self.md.len(), 0, self.file, self.read_mode_threshold);
 
             return res.streaming(reader);
         }
@@ -577,7 +597,7 @@ impl NamedFile {
                 .map_into_boxed_body();
         }
 
-        let reader = chunked::new_chunked_read(length, offset, self.file);
+        let reader = chunked::new_chunked_read(length, offset, self.file, self.read_mode_threshold);
 
         if offset != 0 || length != self.md.len() {
             res.status(StatusCode::PARTIAL_CONTENT);

actix-files/src/service.rs

@@ -39,6 +39,7 @@ pub struct FilesServiceInner {
     pub(crate) file_flags: named::Flags,
     pub(crate) guards: Option<Rc<dyn Guard>>,
     pub(crate) hidden_files: bool,
+    pub(crate) size_threshold: u64,
 }
 
 impl fmt::Debug for FilesServiceInner {
@@ -70,7 +71,9 @@ impl FilesService {
         named_file.flags = self.file_flags;
 
         let (req, _) = req.into_parts();
-        let res = named_file.into_response(&req);
+        let res = named_file
+            .read_mode_threshold(self.size_threshold)
+            .into_response(&req);
         ServiceResponse::new(req, res)
     }
 
@@ -169,17 +172,7 @@ impl Service<ServiceRequest> for FilesService {
                 }
             } else {
                 match NamedFile::open_async(&path).await {
-                    Ok(mut named_file) => {
-                        if let Some(ref mime_override) = this.mime_override {
-                            let new_disposition = mime_override(&named_file.content_type.type_());
-                            named_file.content_disposition.disposition = new_disposition;
-                        }
-                        named_file.flags = this.file_flags;
-
-                        let (req, _) = req.into_parts();
-                        let res = named_file.into_response(&req);
-                        Ok(ServiceResponse::new(req, res))
-                    }
+                    Ok(named_file) => Ok(this.serve_named_file(req, named_file)),
                     Err(err) => this.handle_err(err, req).await,
                 }
             }

actix-http/CHANGES.md

@@ -2,6 +2,8 @@
 
 ## Unreleased
 
+- Properly wake Payload receivers when feeding errors or EOF
+
 ## 3.11.1
 
 - Prevent more hangs after client disconnects.

actix-http/Cargo.toml

@@ -156,7 +156,7 @@ serde_json = "1.0"
 static_assertions = "1"
 tls-openssl = { package = "openssl", version = "0.10.55" }
 tls-rustls_023 = { package = "rustls", version = "0.23" }
-tokio = { version = "1.38.2", features = ["net", "rt", "macros"] }
+tokio = { version = "1.38.2", features = ["net", "rt", "macros", "sync"] }
 
 [lints]
 workspace = true

actix-http/src/h1/payload.rs

@@ -200,11 +200,13 @@ impl Inner {
     #[inline]
     fn set_error(&mut self, err: PayloadError) {
         self.err = Some(err);
+        self.wake();
     }
 
     #[inline]
     fn feed_eof(&mut self) {
         self.eof = true;
+        self.wake();
     }
 
     #[inline]
@@ -253,8 +255,13 @@ impl Inner {
 
 #[cfg(test)]
 mod tests {
+    use std::{task::Poll, time::Duration};
+
+    use actix_rt::time::timeout;
     use actix_utils::future::poll_fn;
+    use futures_util::{FutureExt, StreamExt};
     use static_assertions::{assert_impl_all, assert_not_impl_any};
+    use tokio::sync::oneshot;
 
     use super::*;
 
@@ -263,6 +270,67 @@ mod tests {
 
     assert_impl_all!(Inner: Unpin, Send, Sync);
 
+    const WAKE_TIMEOUT: Duration = Duration::from_secs(2);
+
+    fn prepare_waking_test(
+        mut payload: Payload,
+        expected: Option<Result<(), ()>>,
+    ) -> (oneshot::Receiver<()>, actix_rt::task::JoinHandle<()>) {
+        let (tx, rx) = oneshot::channel();
+
+        let handle = actix_rt::spawn(async move {
+            // Make sure to poll once to set the waker
+            poll_fn(|cx| {
+                assert!(payload.poll_next_unpin(cx).is_pending());
+                Poll::Ready(())
+            })
+            .await;
+            tx.send(()).unwrap();
+
+            // actix-rt is single-threaded, so this won't race with `rx.await`
+            let mut pend_once = false;
+            poll_fn(|_| {
+                if pend_once {
+                    Poll::Ready(())
+                } else {
+                    // Return pending without storing wakers, we already did on the previous
+                    // `poll_fn`, now this task will only continue if the `sender` wakes us
+                    pend_once = true;
+                    Poll::Pending
+                }
+            })
+            .await;
+
+            let got = payload.next().now_or_never().unwrap();
+            match expected {
+                Some(Ok(_)) => assert!(got.unwrap().is_ok()),
+                Some(Err(_)) => assert!(got.unwrap().is_err()),
+                None => assert!(got.is_none()),
+            }
+        });
+        (rx, handle)
+    }
+
+    #[actix_rt::test]
+    async fn wake_on_error() {
+        let (mut sender, payload) = Payload::create(false);
+        let (rx, handle) = prepare_waking_test(payload, Some(Err(())));
+
+        rx.await.unwrap();
+        sender.set_error(PayloadError::Incomplete(None));
+        timeout(WAKE_TIMEOUT, handle).await.unwrap().unwrap();
+    }
+
+    #[actix_rt::test]
+    async fn wake_on_eof() {
+        let (mut sender, payload) = Payload::create(false);
+        let (rx, handle) = prepare_waking_test(payload, None);
+
+        rx.await.unwrap();
+        sender.feed_eof();
+        timeout(WAKE_TIMEOUT, handle).await.unwrap().unwrap();
+    }
+
     #[actix_rt::test]
     async fn test_unread_data() {
         let (_, mut payload) = Payload::create(false);

actix-multipart/src/form/mod.rs

@@ -187,6 +187,45 @@ where
     }
 }
 
+impl<'t, T> FieldGroupReader<'t> for Option<Vec<T>>
+where
+    T: FieldReader<'t>,
+{
+    type Future = LocalBoxFuture<'t, Result<(), MultipartError>>;
+
+    fn handle_field(
+        req: &'t HttpRequest,
+        field: Field,
+        limits: &'t mut Limits,
+        state: &'t mut State,
+        _duplicate_field: DuplicateField,
+    ) -> Self::Future {
+        let field_name = field.name().unwrap().to_string();
+
+        Box::pin(async move {
+            let vec = state
+                .entry(field_name)
+                .or_insert_with(|| Box::<Vec<T>>::default())
+                .downcast_mut::<Vec<T>>()
+                .unwrap();
+
+            let item = T::read_field(req, field, limits).await?;
+            vec.push(item);
+
+            Ok(())
+        })
+    }
+
+    fn from_state(name: &str, state: &'t mut State) -> Result<Self, MultipartError> {
+        if let Some(boxed_vec) = state.remove(name) {
+            let vec = *boxed_vec.downcast::<Vec<T>>().unwrap();
+            Ok(Some(vec))
+        } else {
+            Ok(None)
+        }
+    }
+}
+
 /// Trait that allows a type to be used in the [`struct@MultipartForm`] extractor.
 ///
 /// You should use the [`macro@MultipartForm`] macro to derive this for your struct.

actix-web-actors/Cargo.toml

@@ -24,7 +24,7 @@ allowed_external_types = [
 actix = { version = ">=0.12, <0.14", default-features = false }
 actix-codec = "0.5"
 actix-http = "3"
-actix-web = { version = "4", default-features = false }
+actix-web = { version = "4", default-features = false, features = ["ws"] }
 
 bytes = "1"
 bytestring = "1"

actix-web/CHANGES.md

@@ -4,6 +4,7 @@
 
 - `actix_web::response::builder::HttpResponseBuilder::streaming()` now sets `Content-Type` to `application/octet-stream` if `Content-Type` does not exist.
 - `actix_web::response::builder::HttpResponseBuilder::streaming()` now calls `actix_web::response::builder::HttpResponseBuilder::no_chunking()` if `Content-Length` is set by user.
+- Add `ws` crate feature (on-by-default) which forwards to `actix-http` and guards some of its `ResponseError` impls.
 
 ## 4.11.0
 

actix-web/Cargo.toml

@@ -67,6 +67,7 @@ default = [
   "http2",
   "unicode",
   "compat",
+  "ws",
 ]
 
 # Brotli algorithm content-encoding support
@@ -85,9 +86,12 @@ cookies = ["dep:cookie"]
 # Secure & signed cookies
 secure-cookies = ["cookies", "cookie/secure"]
 
-# HTTP/2 support (including h2c).
+# HTTP/2 support (including h2c)
 http2 = ["actix-http/http2"]
 
+# WebSocket support
+ws = ["actix-http/ws"]
+
 # TLS via OpenSSL
 openssl = ["__tls", "http2", "actix-http/openssl", "actix-tls/accept", "actix-tls/openssl"]
 
@@ -131,7 +135,7 @@ actix-service = "2"
 actix-tls = { version = "3.4", default-features = false, optional = true }
 actix-utils = "3"
 
-actix-http = { version = "3.11", features = ["ws"] }
+actix-http = "3.11"
 actix-router = { version = "0.5.3", default-features = false, features = ["http"] }
 actix-web-codegen = { version = "4.3", optional = true, default-features = false }
 

actix-web/src/error/response_error.rs

@@ -7,7 +7,6 @@ use std::{
     io::{self, Write as _},
 };
 
-use actix_http::Response;
 use bytes::BytesMut;
 
 use crate::{
@@ -126,20 +125,24 @@ impl ResponseError for actix_http::error::PayloadError {
     }
 }
 
-impl ResponseError for actix_http::ws::ProtocolError {}
-
 impl ResponseError for actix_http::error::ContentTypeError {
     fn status_code(&self) -> StatusCode {
         StatusCode::BAD_REQUEST
     }
 }
 
+#[cfg(feature = "ws")]
 impl ResponseError for actix_http::ws::HandshakeError {
     fn error_response(&self) -> HttpResponse<BoxBody> {
-        Response::from(self).map_into_boxed_body().into()
+        actix_http::Response::from(self)
+            .map_into_boxed_body()
+            .into()
     }
 }
 
+#[cfg(feature = "ws")]
+impl ResponseError for actix_http::ws::ProtocolError {}
+
 #[cfg(test)]
 mod tests {
     use super::*;

actix-web/src/middleware/authors-guide.md

@@ -2,16 +2,80 @@
 
 ## What Is A Middleware?
 
+Middleware in Actix Web is a powerful mechanism that allows you to add additional behavior to request/response processing. It enables you to:
+
+- Pre-process incoming requests (e.g., path normalization, authentication)
+- Post-process outgoing responses (e.g., logging, compression)
+- Modify application state through ServiceRequest
+- Access external services (e.g., sessions, caching)
+
+Middleware is registered for each App, Scope, or Resource and executed in the reverse order of registration. This means the last registered middleware is the first to process the request.
+
 ## Middleware Traits
 
+Actix Web's middleware system is built on two main traits:
+
+1. `Transform<S, Req>`: The builder trait that creates the actual Service. It's responsible for:
+
+   - Creating new middleware instances
+   - Assembling the middleware chain
+   - Handling initialization errors
+
+2. `Service<Req>`: The trait that represents the actual middleware functionality. It:
+   - Processes requests and responses
+   - Can modify both request and response
+   - Can short-circuit request processing
+   - Must be implemented for the middleware to work
+
 ## Understanding Body Types
 
+When working with middleware, it's important to understand body types:
+
+- Middleware can work with different body types for requests and responses
+- The `MessageBody` trait is used to handle different body types
+- You can use `EitherBody` when you need to handle multiple body types
+- Be careful with body consumption - once a body is consumed, it cannot be read again
+
 ## Best Practices
 
+1. Keep middleware focused and single-purpose
+2. Handle errors appropriately and propagate them correctly
+3. Be mindful of performance impact
+4. Use appropriate body types and handle them correctly
+5. Consider middleware ordering carefully
+6. Document your middleware's behavior and requirements
+7. Test your middleware thoroughly
+
 ## Error Propagation
 
+Proper error handling is crucial in middleware:
+
+1. Always propagate errors from the inner service
+2. Use appropriate error types
+3. Handle initialization errors
+4. Consider using custom error types for specific middleware errors
+5. Document error conditions and handling
+
 ## When To (Not) Use Middleware
 
+Use middleware when you need to:
+
+- Add cross-cutting concerns
+- Modify requests/responses globally
+- Add authentication/authorization
+- Add logging or monitoring
+- Handle compression or caching
+
+Avoid middleware when:
+
+- The functionality is specific to a single route
+- The operation is better handled by a service
+- The overhead would be too high
+- The functionality can be implemented more simply
+
 ## Author's References
 
 - `EitherBody` + when is middleware appropriate: https://discord.com/channels/771444961383153695/952016890723729428
+- Actix Web Documentation: https://docs.rs/actix-web
+- Service Trait Documentation: https://docs.rs/actix-service
+- MessageBody Trait Documentation: https://docs.rs/actix-web/latest/actix_web/body/trait.MessageBody.html