diff --git a/app/src/api.rs b/app/src/api.rs
index a12ebec..bce8c0a 100644
--- a/app/src/api.rs
+++ b/app/src/api.rs
@@ -1,3 +1,5 @@
+//! Koreader Progress Sync API.
+
 use poem::{
     error::ResponseError,
     http::StatusCode,
@@ -19,17 +21,20 @@ use crate::{
     error_response,
 };
 
+/// Data for registering a new user.
 #[derive(Object, Deserialize)]
 struct RegisterRequest {
     username: String,
     password: String,
 }
 
+/// Answer after registering a new user.
 #[derive(Object, Serialize)]
 struct UserCreated {
     username: String,
 }
 
+/// Datafor pushing progress.
 #[derive(Debug, Clone, Object, Deserialize)]
 pub struct DocumentUpdate {
     pub device: String,
@@ -39,6 +44,7 @@ pub struct DocumentUpdate {
     pub progress: String,
 }
 
+/// Answer when pulling progress.
 #[derive(Debug, Clone, Object, Deserialize)]
 pub struct DocumentProgress {
     pub device: String,
@@ -70,6 +76,9 @@ pub struct Api;
 
 #[OpenApi]
 impl Api {
+    /// Register a new user.
+    ///
+    /// If a user of that id already exist, return a conflict.
     #[oai(path = "/users/create", method = "post")]
     async fn register(
         &self,
@@ -88,11 +97,13 @@ impl Api {
         }
     }
 
+    /// Return OK if a user authenticated successfully.
     #[oai(path = "/users/auth", method = "get", transform = "authorize")]
     async fn login(&self) -> Result<payload::Response<()>> {
         Ok(payload::Response::new(()).status(StatusCode::OK))
     }
 
+    /// Push new progress.
     #[oai(path = "/syncs/progress", method = "put", transform = "authorize")]
     async fn push_progress(
         &self,
@@ -108,6 +119,7 @@ impl Api {
         Ok(payload::Response::new(()).status(StatusCode::OK))
     }
 
+    /// Pull progress for document with id `doc_id`.
     #[oai(
         path = "/syncs/progress/:doc_id",
         method = "get",
diff --git a/app/src/app_state.rs b/app/src/app_state.rs
index 5c14d6d..c4543c3 100644
--- a/app/src/app_state.rs
+++ b/app/src/app_state.rs
@@ -1,6 +1,10 @@
+//! Global application state.
+
 use crate::{cli::Config, db::Db};
 
 pub struct AppState {
+    /// Application configuration.
     pub config: Config,
+    /// Database connection.
     pub db: Db,
 }
diff --git a/app/src/auth_middleware.rs b/app/src/auth_middleware.rs
index 9a59f12..ee0e149 100644
--- a/app/src/auth_middleware.rs
+++ b/app/src/auth_middleware.rs
@@ -1,3 +1,9 @@
+//! Poem middleware for koreader progress sync authentication.
+//!
+//! Authentication works with two headers:
+//! - x-auth-user: username
+//! - x-auth-key: md5 hashed password
+
 use poem::{
     error::ResponseError, http::StatusCode, Endpoint, EndpointExt, Error, Middleware, Request,
     Result,
diff --git a/app/src/db.rs b/app/src/db.rs
index 598183c..e87cdfe 100644
--- a/app/src/db.rs
+++ b/app/src/db.rs
@@ -1,3 +1,5 @@
+//! Data access and persistence.
+
 use ::entity::{document, user};
 use migration::{Migrator, MigratorTrait};
 use sea_orm::{
@@ -16,6 +18,7 @@ pub enum DataStoreError {
     DatabaseError(#[from] DbErr),
 }
 
+/// Document progress to insert/update.
 #[derive(Debug)]
 pub struct DocumentInsert {
     id: String,
@@ -44,6 +47,7 @@ pub struct Db {
 }
 
 impl Db {
+    /// Add a new user and return an error if one with `id` already exists.
     pub async fn add_user(&self, id: &str, key: &str) -> Result<user::Model, DataStoreError> {
         let user = user::ActiveModel {
             id: Set(id.to_owned()),
@@ -53,10 +57,13 @@ impl Db {
         Ok(user.insert(&self.connection).await?)
     }
 
+    /// Get user with id `id` or None if they do not exist.
     pub async fn get_user(&self, id: &str) -> Result<Option<user::Model>, DataStoreError> {
         Ok(user::Entity::find_by_id(id).one(&self.connection).await?)
     }
 
+    /// Get the progress for document with id `doc_id` from user with id `user_id` or None if that
+    /// combination does not exist.
     pub async fn get_position(
         &self,
         user_id: &str,
@@ -69,6 +76,9 @@ impl Db {
             .await?)
     }
 
+    /// Update progress for document with id `doc_id` from user with id `user_id`.
+    ///
+    /// Set the progress timestamp to now.
     pub async fn update_position(&self, doc: &DocumentInsert) -> Result<(), DataStoreError> {
         let now = OffsetDateTime::now_utc();
         let now = PrimitiveDateTime::new(now.date(), now.time());
@@ -93,6 +103,7 @@ impl Db {
     }
 }
 
+/// Connect to the database and return the opened connectoin pool.
 pub async fn connect(connection_string: &str) -> Result<Db, DataStoreError> {
     let connection: DatabaseConnection = Database::connect(connection_string).await?;
     Migrator::up(&connection, None).await?;
diff --git a/app/src/error_response.rs b/app/src/error_response.rs
index 2da937f..7127c91 100644
--- a/app/src/error_response.rs
+++ b/app/src/error_response.rs
@@ -1,3 +1,5 @@
+//! Error handling for the sync service.
+
 use poem::{http::StatusCode, Body, Response};
 use poem_openapi::Object;
 use serde::Serialize;
@@ -11,6 +13,8 @@ struct ErrorResponse {
     message: String,
 }
 
+/// Log `error` on the server side with a generated id and send back a sanitized error message with
+/// the same id.
 pub fn create_and_log<F>(error: impl Error, cb: F) -> Response
 where
     F: Fn() -> (String, StatusCode),
diff --git a/app/src/lib.rs b/app/src/lib.rs
index 352a3a5..712fe36 100644
--- a/app/src/lib.rs
+++ b/app/src/lib.rs
@@ -1,3 +1,5 @@
+//! Implementation of a koreader progress sync server.
+
 use anyhow::Result;
 use api::Api;
 use app_state::AppState;
@@ -18,6 +20,7 @@ pub mod cli;
 pub mod db;
 pub mod error_response;
 
+/// Run the progress sync server.
 pub async fn run(args: &Config, db_url: &str) -> Result<()> {
     let db = db::connect(db_url).await?;
     let app_state = Arc::new(AppState {