Bug Fix: Cancel Button Fails Immediately After Run

by Aria Freeman 51 views

Hey guys! We've got a tricky bug on our hands in the y-scope/clp project, and it's all about our query cancel button. Let's dive into the details and figure out what's going on and how we can fix it. This article breaks down the bug, explores the possible root causes, provides steps to reproduce the issue, and discusses the implications and potential solutions. We will also touch on the importance of RESTful API design in handling such scenarios. So, buckle up, and let's get started!

Bug Description

The core of the issue is that the cancel button for Presto queries isn't working as expected when clicked immediately after hitting the Run button. Imagine you're running a query, and you realize you've made a mistake or want to change something. You quickly hit Cancel, but the query just keeps on chugging. This can lead to wasted resources, time, and a frustrating user experience. The unexpected behavior of the cancel button creates a significant usability issue. Users expect that clicking Cancel will immediately halt the query execution, preventing further resource consumption and providing a way to correct errors or refine their search. When the query continues to run despite clicking Cancel, it can lead to confusion and inefficiency, especially when dealing with large datasets or complex queries.

This bug particularly affects users who frequently iterate on their queries or need to make quick adjustments. For example, a data analyst might realize that they've included an incorrect filter or joined the wrong tables after starting the query. The ability to cancel a running query is crucial for these users to avoid waiting for potentially long and resource-intensive processes to complete unnecessarily. The reliability of the cancel button directly impacts the efficiency and productivity of users, making it essential to address this issue promptly.

Moreover, the inconsistent behavior can have implications for system performance. If users repeatedly attempt to cancel queries that continue to run, it can lead to a build-up of processes that consume system resources. This can degrade the overall performance of the system, affecting not only the user who initiated the query but also other users who are running queries concurrently. Therefore, resolving this bug is not only important for user experience but also for maintaining the stability and efficiency of the query processing infrastructure. We need to ensure our Presto queries behave as expected.

Root Cause Hypothesis

So, what's the culprit? Our prime suspect is that Presto's cancel route might be returning a success message even if the query hasn't actually been submitted to the Presto engine yet. Think of it like this: you press the elevator button, and the light comes on, but the elevator is still on another floor. The system acknowledges your request, but the action hasn't really happened yet. This discrepancy between the reported status and the actual state is the heart of the problem. This discrepancy can occur due to the asynchronous nature of query submission and execution. When a user clicks Run, the request is sent to the Presto engine, but the engine might not immediately start processing the query. If the user clicks Cancel during this brief window before the query is fully initiated, the cancel request might be acknowledged without actually stopping the query execution.

This behavior could stem from the design of the Presto API or the way our system interacts with it. The cancel route might simply check if a cancel request has been received for a query ID, without verifying whether the query has actually started running. This approach would result in a misleading success response, as the query continues to execute despite the user's attempt to cancel. The success message returned by the Presto cancel route gives a false sense of control to the user. They believe that the query has been stopped, leading to a mismatch between their expectations and the actual system behavior. This can erode user trust and make the system feel less reliable.

Furthermore, this issue highlights the importance of having a robust mechanism for tracking query status on both the client and server sides. The client needs to accurately reflect the state of the query, and the server needs to ensure that cancel requests are processed correctly, regardless of the query's execution stage. This requires careful synchronization between the client and server to avoid race conditions and ensure consistent behavior. Properly canceling queries is essential for a smooth user experience.

Reproduction Steps

Okay, let's see if we can recreate this bug ourselves. Here's how we can do it:

  1. Navigate to the search interface: Go to the page where you can enter and run queries. This is our starting point.
  2. Enter a query: Type in a query that you want to run. It doesn't have to be anything complex, just something that will execute.
  3. **Click