diff --git a/zebrad/src/components/mempool/downloads.rs b/zebrad/src/components/mempool/downloads.rs
index b284897a..e4289a65 100644
--- a/zebrad/src/components/mempool/downloads.rs
+++ b/zebrad/src/components/mempool/downloads.rs
@@ -1,3 +1,26 @@
+//! Transaction downloader and verifier.
+//!
+//! The main struct [`Downloads`] allows downloading and verifying transactions.
+//! It is used by the mempool to get transactions into it. It is also able to
+//! just verify transactions that were directly pushed.
+//!
+//! The verification itself is done by the [`zebra_consensus`] crate.
+//!
+//! Verified transactions are returned to the caller in [`Downloads::poll_next`].
+//! This is in contrast to the block downloader and verifiers which don't
+//! return anything and forward the verified blocks to the state themselves.
+//!
+//! # Correctness
+//!
+//! The mempool downloader doesn't send verified transactions to the [`Mempool`] service.
+//! So Zebra must spawn a task that regularly polls the downloader for ready transactions.
+//! (To ensure that transactions propagate across the entire network in each 75s block interval,
+//! the polling interval should be around 5-10 seconds.)
+//!
+//! Polling the downloader from [`Mempool::poll_ready`] is not sufficient.
+//! [`Service::poll_ready`] is only called when there is a service request.
+//! But we want to download and gossip transactions,
+//! even when there are no other service requests.
 use std::{
     collections::{HashMap, HashSet},
     pin::Pin,
@@ -204,8 +227,11 @@ where
     ZS: Service<zs::Request, Response = zs::Response, Error = BoxError> + Send + Clone + 'static,
     ZS::Future: Send,
 {
-    /// Initialize a new download stream with the provided `network` and
-    /// `verifier` services.
+    /// Initialize a new download stream with the provided services.
+    ///
+    /// `network` is used to download transactions.
+    /// `verifier` is used to verify transactions.
+    /// `state` is used to check if transactions are already in the state.
     ///
     /// The [`Downloads`] stream is agnostic to the network policy, so retry and
     /// timeout limits should be applied to the `network` service passed into