pepijnve commented on code in PR #75:
URL: https://github.com/apache/datafusion-site/pull/75#discussion_r2152976180
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content/blog/2025-06-15-cancellation.md:
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+---
+layout: post
+title: Query Cancellation
+date: 2025-06-27
+author: Pepijn Van Eeckhoudt
+categories: [features]
+---
+<!--
+{% comment %}
+Licensed to the Apache Software Foundation (ASF) under one or more
+contributor license agreements. See the NOTICE file distributed with
+this work for additional information regarding copyright ownership.
+The ASF licenses this file to you under the Apache License, Version 2.0
+(the "License"); you may not use this file except in compliance with
+the License. You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+{% endcomment %}
+-->
+
+
+## The Challenge of Cancelling Long-Running Queries
+
+Have you ever tried to cancel a query that just wouldn't stop?
+In this post, we'll take a look at why that can happen in DataFusion and what
the community did to resolve the problem in depth.
+
+### Understanding Rust's Async Model
+
+To really understand the cancellation problem you need to be somewhat familiar
with Rust's asynchronous programming model.
+This is a bit different than what you might be used to from other ecosystems.
+Let's go over the basics again as a refresher.
+If you're familiar with the ins and outs of `Future` and `async` you can skip
this section.
+
+#### Futures Are Inert
+
+Rust's asynchronous programming model is built around the `Future<T>` trait.
+In contrast to, for instance, Javascript's `Promise` or Java's `Future` a Rust
`Future` does not necessarily represent an actively running asynchronous job.
+Instead, a `Future<T>` represents a lazy calculation that only makes progress
when explicitly polled.
+If nothing tells a `Future` to try and make progress explicitly, it is [an
inert
object](https://doc.rust-lang.org/std/future/trait.Future.html#runtime-characteristics).
+
+You ask a `Future<T>`to advance its calculation as much as possible by calling
the `poll` method.
+The `Future` responds with either:
+- `Poll::Pending` if it needs to wait for something (like I/O) before it can
continue
+- `Poll::Ready<T>` when it has completed and produced a value
+
+When a `Future` returns `Pending`, it saves its internal state so it can pick
up where it left off the next time you poll it.
+This state management is what makes Rust's `Future`s memory-efficient and
composable.
+It also needs to set up the necessary signaling so that the caller gets
notified when it should try to call `poll` again.
+This avoids having to call `poll` in a busy-waiting loop.
+
+Rust's `async` keyword provides syntactic sugar over this model.
+When you write an `async` function or block, the compiler transforms it into
an implementation of the `Future` trait for you.
+Since all the state management is compiler generated and hidden from sight,
async code tends to be more readable while maintaining the same underlying
mechanics.
+
+The `await` keyword complements this by letting you pause execution until a
`Future` completes.
+When you `.await` a `Future`, you're essentially telling the compiler to poll
that `Future` until it's ready before program execution continues with the
statement after the await.
Review Comment:
`.await` is sort of equivalent to `ready!(future.poll)` along with a state
transition on ready, right?
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