docs(book): tracking state in ExExes (#8804)

Co-authored-by: Dan Cline <6798349+Rjected@users.noreply.github.com>
Co-authored-by: Oliver <onbjerg@users.noreply.github.com>

book/developers/exex/exex.md

@@ -27,3 +27,4 @@ and run it on the Holesky testnet.
 
 1. [How do ExExes work?](./how-it-works.md)
 1. [Hello World](./hello-world.md)
+1. [Tracking State](./tracking-state.md)

book/developers/exex/hello-world.md

@@ -160,3 +160,7 @@ and it's safe to prune the associated data.
 </div>
 
 What we've arrived at is the [minimal ExEx example](https://github.com/paradigmxyz/reth/blob/b8cd7be6c92a71aea5341cdeba685f124c6de540/examples/exex/minimal/src/main.rs) that we provide in the Reth repository.
+
+## What's next?
+
+Let's do something a bit more interesting, and see how you can [keep track of some state](./tracking-state.md) inside your ExEx.

book/developers/exex/tracking-state.md

@@ -0,0 +1,193 @@
+# Tracking State
+
+In this chapter, we'll learn how to keep track of some state inside our ExEx.
+
+Let's continue with our Hello World example from the [previous chapter](./hello-world.md).
+
+### Turning ExEx into a struct
+
+First, we need to turn our ExEx into a stateful struct.
+
+Before, we had just an async function, but now we'll need to implement
+the [`Future`](https://doc.rust-lang.org/std/future/trait.Future.html) trait manually.
+
+<div class="warning">
+
+Having a stateful async function is also possible, but it makes testing harder,
+because you can't access variables inside the function to assert the state of your ExEx.
+
+</div>
+
+```rust,norun,noplayground,ignore
+use std::{
+    future::Future,
+    pin::Pin,
+    task::{ready, Context, Poll},
+};
+
+use reth::api::FullNodeComponents;
+use reth_exex::{ExExContext, ExExEvent, ExExNotification};
+use reth_node_ethereum::EthereumNode;
+use reth_tracing::tracing::info;
+
+struct MyExEx<Node: FullNodeComponents> {
+    ctx: ExExContext<Node>,
+}
+
+impl<Node: FullNodeComponents> Future for MyExEx<Node> {
+    type Output = eyre::Result<()>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let this = self.get_mut();
+
+        while let Some(notification) = ready!(this.ctx.notifications.poll_recv(cx)) {
+            match &notification {
+                ExExNotification::ChainCommitted { new } => {
+                    info!(committed_chain = ?new.range(), "Received commit");
+                }
+                ExExNotification::ChainReorged { old, new } => {
+                    info!(from_chain = ?old.range(), to_chain = ?new.range(), "Received reorg");
+                }
+                ExExNotification::ChainReverted { old } => {
+                    info!(reverted_chain = ?old.range(), "Received revert");
+                }
+            };
+
+            if let Some(committed_chain) = notification.committed_chain() {
+                this.ctx
+                    .events
+                    .send(ExExEvent::FinishedHeight(committed_chain.tip().number))?;
+            }
+        }
+
+        Poll::Ready(Ok(()))
+    }
+}
+
+fn main() -> eyre::Result<()> {
+    reth::cli::Cli::parse_args().run(|builder, _| async move {
+        let handle = builder
+            .node(EthereumNode::default())
+            .install_exex("my-exex", |ctx| async move { Ok(MyExEx { ctx }) })
+            .launch()
+            .await?;
+
+        handle.wait_for_node_exit().await
+    })
+}
+```
+
+For those who are not familiar with how async Rust works on a lower level, that may seem scary,
+but let's unpack what's going on here:
+
+1. Our ExEx is now a `struct` that contains the context and implements the `Future` trait. It's now pollable (hence `await`-able).
+1. We can't use `self` directly inside our `poll` method, and instead need to acquire a mutable reference to the data inside of the `Pin`.
+   Read more about pinning in [the book](https://rust-lang.github.io/async-book/04_pinning/01_chapter.html).
+1. We also can't use `await` directly inside `poll`, and instead need to poll futures manually.
+   We wrap the call to `poll_recv(cx)` into a [`ready!`](https://doc.rust-lang.org/std/task/macro.ready.html) macro,
+   so that if the channel of notifications has no value ready, we will instantly return `Poll::Pending` from our Future.
+1. We initialize and return the `MyExEx` struct directly in the `install_exex` method, because it's a Future.
+
+With all that done, we're now free to add more fields to our `MyExEx` struct, and track some state in them.
+
+### Adding state
+
+Our ExEx will count the number of transactions in each block and log it to the console.
+
+```rust,norun,noplayground,ignore
+use std::{
+    future::Future,
+    pin::Pin,
+    task::{ready, Context, Poll},
+};
+
+use reth::{api::FullNodeComponents, primitives::BlockNumber};
+use reth_exex::{ExExContext, ExExEvent};
+use reth_node_ethereum::EthereumNode;
+use reth_tracing::tracing::info;
+
+struct MyExEx<Node: FullNodeComponents> {
+    ctx: ExExContext<Node>,
+    /// First block that was committed since the start of the ExEx.
+    first_block: Option<BlockNumber>,
+    /// Total number of transactions committed.
+    transactions: u64,
+}
+
+impl<Node: FullNodeComponents> MyExEx<Node> {
+    fn new(ctx: ExExContext<Node>) -> Self {
+        Self {
+            ctx,
+            first_block: None,
+            transactions: 0,
+        }
+    }
+}
+
+impl<Node: FullNodeComponents> Future for MyExEx<Node> {
+    type Output = eyre::Result<()>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let this = self.get_mut();
+
+        while let Some(notification) = ready!(this.ctx.notifications.poll_recv(cx)) {
+            if let Some(reverted_chain) = notification.reverted_chain() {
+                this.transactions = this.transactions.saturating_sub(
+                    reverted_chain
+                        .blocks_iter()
+                        .map(|b| b.body.len() as u64)
+                        .sum(),
+                );
+            }
+
+            if let Some(committed_chain) = notification.committed_chain() {
+                this.first_block.get_or_insert(committed_chain.first().number);
+
+                this.transactions += committed_chain
+                    .blocks_iter()
+                    .map(|b| b.body.len() as u64)
+                    .sum::<u64>();
+
+                this.ctx
+                    .events
+                    .send(ExExEvent::FinishedHeight(committed_chain.tip().number))?;
+            }
+
+            if let Some(first_block) = this.first_block {
+                info!(%first_block, transactions = %this.transactions, "Total number of transactions");
+            }
+        }
+
+        Poll::Ready(Ok(()))
+    }
+}
+
+fn main() -> eyre::Result<()> {
+    reth::cli::Cli::parse_args().run(|builder, _| async move {
+        let handle = builder
+            .node(EthereumNode::default())
+            .install_exex("my-exex", |ctx| async move { Ok(MyExEx::new(ctx)) })
+            .launch()
+            .await?;
+
+        handle.wait_for_node_exit().await
+    })
+}
+```
+
+As you can see, we added two fields to our ExEx struct:
+- `first_block` to keep track of the first block that was committed since the start of the ExEx.
+- `transactions` to keep track of the total number of transactions committed, accounting for reorgs and reverts.
+
+We also changed our `match` block to two `if` clauses:
+- First one checks if there's a reverted chain using `notification.reverted_chain()`. If there is:
+    - We subtract the number of transactions in the reverted chain from the total number of transactions.
+    - It's important to do the `saturating_sub` here, because if we just started our node and
+      instantly received a reorg, our `transactions` field will still be zero.
+- Second one checks if there's a committed chain using `notification.committed_chain()`. If there is:
+    - We update the `first_block` field to the first block of the committed chain.
+    - We add the number of transactions in the committed chain to the total number of transactions.
+    - We send a `FinishedHeight` event back to the main node.
+
+Finally, on every notification, we log the total number of transactions and
+the first block that was committed since the start of the ExEx.