Atomic Deletion In TiKV: Implementing CompareAndDelete

by Kenji Nakamura 55 views

Introduction

Guys, in this article, we'll dive into a crucial feature request for TiKV's RawKV API: the implementation of a CompareAndDelete operation. This enhancement aims to address a significant challenge in atomically deleting keys, especially in scenarios where concurrent operations might lead to data inconsistencies. We'll explore the problem, the proposed solutions, and the potential impact on users of the RawKV API. Let's get started!

The Problem: Atomic Deletion in RawKV

The RawKV API in TiKV offers a CompareAndSwap (CAS) operation, a powerful tool for atomically reading, modifying, and writing keys. This is incredibly useful for users who want to avoid the overhead of the full transactional API (Txn API). However, a glaring gap exists: the lack of an atomic "compare-and-delete" operation. Without it, safely deleting a key becomes surprisingly tricky.

Imagine this scenario: you have a key=foo already stored in your TiKV cluster. Now, consider two threads attempting to interact with this key concurrently:

  1. Thread 1: Reads key=foo, evaluates a condition, and decides the key needs to be deleted. It initiates a Delete operation.
  2. Thread 1: The Delete operation is taking its sweet time (perhaps due to network hiccups or a slow server).
  3. Thread 2: Performs a CompareAndSwap operation to update the value associated with key=foo.
  4. Thread 2: The CompareAndSwap succeeds, and the new value is happily stored.
  5. Thread 1: The slow Delete operation finally completes, removing the key entirely.

Disaster! The update performed by Thread 2 in step 4 is now gone, wiped out by the delayed Delete from Thread 1. This race condition can lead to data loss and application inconsistencies. An atomic CompareAndDelete operation would be the superhero in this situation, ensuring that the key is deleted only if its value hasn't changed in the meantime, preventing the accidental deletion of updated data.

Why CompareAndDelete Matters

To emphasize the importance, consider a real-world use case. Imagine a distributed counter where you want to atomically decrement the counter and delete it when it reaches zero. Using just CompareAndSwap and Delete, you're vulnerable to the race condition described above. A CompareAndDelete operation guarantees that the key is deleted only if the decrement operation resulted in zero and no other thread modified the value concurrently. This ensures data integrity and prevents unintended consequences.

In essence, the need for atomic operations extends beyond simple updates. Deletion, especially in concurrent environments, requires the same level of atomicity to prevent data corruption. The CompareAndDelete operation fills this critical gap in the RawKV API, providing a robust mechanism for safely managing key deletions.

Proposed Solutions: Two Paths to Atomicity

To address this challenge, two primary solutions have been proposed, each with its own merits and potential implementation complexities. Let's explore these options in detail:

1. Extending CompareAndSwap

The first approach involves enhancing the existing CompareAndSwap (CAS) API to incorporate key deletion functionality. This can be achieved by modifying the RawCASRequest protobuf message, the data structure used to represent CAS requests. A new flag could be added to this message, specifically indicating whether the operation should delete the key if the expected value matches.

How it would work:

  • The client sends a RawCASRequest with the new "delete" flag set.
  • If the current value of the key matches the expected value provided in the request,
    • TiKV deletes the key if the “delete” flag is set.
    • Otherwise, TiKV updates the key with the new value.
  • If the current value does not match the expected value, the operation fails, and no changes are made.

Advantages:

  • Leverages existing infrastructure: This approach reuses the established CAS code path, potentially simplifying implementation and reducing code duplication.
  • Minimal API changes: The impact on the API surface is relatively small, as it only involves adding a flag to the existing request message.

Disadvantages:

  • Increased complexity of CAS: The CAS operation becomes more complex, handling both update and delete scenarios.
  • Potential for confusion: Users might find it less intuitive to use CAS for deletion, as the primary purpose of CAS is typically for updates.

2. Introducing a New CompareAndDelete API

The second option involves introducing a completely new API specifically for the CompareAndDelete operation. This would entail defining a new request message and implementing a separate code path within TiKV to handle these requests.

How it would work:

  • The client sends a RawCompareAndDeleteRequest (or similar) with the key and expected value.
  • If the current value of the key matches the expected value,
    • TiKV deletes the key.
  • If the current value does not match the expected value, the operation fails.

Advantages:

  • Clear separation of concerns: A dedicated API for deletion makes the intent clear and reduces the cognitive load for users.
  • Simplified CAS operation: The CAS operation remains focused on updates, leading to a cleaner and more maintainable codebase.

Disadvantages:

  • Larger API impact: Introducing a new API requires more significant changes to the API surface and client libraries.
  • Increased code complexity: A separate code path needs to be implemented and maintained for the CompareAndDelete operation.

Choosing the Right Path

The decision between these two approaches involves weighing the trade-offs between API clarity, implementation complexity, and code maintainability. Extending CAS might be quicker to implement initially, but a dedicated CompareAndDelete API could offer a more robust and user-friendly solution in the long run. Ultimately, the choice depends on the specific design goals and priorities of the TiKV project.

A Proof-of-Concept Implementation

As mentioned in the feature request, a preliminary Proof-of-Concept (PoC) implementation was attempted. This PoC, while not production-ready, provides valuable insights into the potential implementation challenges and the core logic required for CompareAndDelete.

The PoC essentially "hijacks" the existing CompareAndSwap code path. It treats an empty new_value in the RawCASRequest as a signal to delete the key. While this approach allowed for a quick initial implementation, it has a critical flaw: TiKV does allow writing empty keys. This means the PoC's logic would incorrectly delete keys if a user intentionally set their value to empty.

Key Takeaways from the PoC:

  • The core logic of comparing the expected value and deleting the key is relatively straightforward.
  • A naive approach of using empty values as a deletion signal is problematic.
  • A robust implementation requires a dedicated mechanism to indicate deletion intent.

The question remains: aside from this "empty value hack," how much of the existing CAS code path can be reused for a proper CompareAndDelete implementation? This is a crucial consideration for minimizing code duplication and ensuring consistency.

Alternatives Considered: The Tombstone Approach

Before proposing CompareAndDelete, the feature request authors explored an alternative solution: using a "tombstone" value. This approach involves using CompareAndSwap to set a specific application-defined value (the "tombstone") to indicate that a key is logically deleted. The application then interprets this tombstone value as a signal to ignore the key.

How it works:

  1. A thread wants to delete a key.
  2. It uses CompareAndSwap to set the key's value to a predefined tombstone value.
  3. Subsequent reads of the key return the tombstone value.
  4. The application logic interprets the tombstone value as a deleted key.

The Problem with Tombstones:

While tombstones can work as a workaround, they introduce a new problem: how do you safely remove the tombstone itself? If you're absolutely certain the key will never be reused, a simple Delete operation might suffice. However, if there's any possibility of the key being reused (e.g., a subsequent Put operation), an atomic Delete is insufficient. It can still race with a concurrent CompareAndSwap operation, leading to inconsistencies.

The Race Condition:

  1. Thread 1 wants to delete the key (which currently holds a tombstone value).
  2. Thread 1 issues a Delete operation.
  3. Thread 2 wants to reuse the key and performs a CompareAndSwap to set a new value.
  4. The CompareAndSwap succeeds.
  5. The Delete operation from Thread 1 completes, removing the key and the new value set by Thread 2.

This is precisely the problem CompareAndDelete is designed to solve. Tombstones, while a clever workaround, ultimately highlight the need for a true atomic deletion mechanism.

Teachability, Documentation, Adoption, and Migration Strategy

Implementing CompareAndDelete will undoubtedly enhance the usability and robustness of the RawKV API. To ensure successful adoption, careful consideration must be given to the following aspects:

Teachability

The concept of atomic operations and the potential race conditions they address needs to be clearly explained. The documentation should provide concrete examples illustrating the benefits of CompareAndDelete over using Delete alone in concurrent scenarios. Highlighting the use cases, such as the atomic counter example mentioned earlier, will make the feature more relatable and understandable.

Documentation

The API documentation must be comprehensive and easy to navigate. It should clearly define the semantics of the CompareAndDelete operation, including its failure modes and potential error conditions. Examples in multiple programming languages would be highly beneficial, demonstrating how to use the API effectively in different contexts.

Adoption

To encourage adoption, the benefits of CompareAndDelete should be actively promoted. Blog posts, tutorials, and conference talks can help raise awareness and educate users about the feature. Providing clear migration paths from existing workarounds (like tombstones) will also be crucial.

Migration Strategy

For users currently relying on tombstones or other techniques to achieve atomic deletion, a smooth migration path is essential. This might involve providing helper functions or code examples that demonstrate how to replace existing code with CompareAndDelete. Backward compatibility should be carefully considered to minimize disruption to existing applications.

Use Case Scenarios

To further illustrate the value of CompareAndDelete, let's consider some specific scenarios:

  • Distributed Locks: Imagine a distributed lock implemented using RawKV. CompareAndDelete can be used to atomically release the lock, ensuring that only the lock holder can release it and preventing accidental release by other threads.
  • Session Management: In a session management system, CompareAndDelete can be used to atomically delete expired sessions, preventing race conditions that could lead to session hijacking or data corruption.
  • Metadata Management: Systems that store metadata in RawKV can use CompareAndDelete to ensure that metadata updates and deletions are atomic, preventing inconsistencies and data loss.

Conclusion

The implementation of CompareAndDelete in TiKV's RawKV API is a significant step towards providing a more robust and user-friendly experience. By addressing the critical gap in atomic deletion, this feature will empower users to build more reliable and consistent distributed applications. Whether through extending CompareAndSwap or introducing a new API, the addition of CompareAndDelete will undoubtedly enhance the capabilities of TiKV and solidify its position as a leading distributed key-value store.

Guys, this feature request highlights the importance of carefully considering all aspects of data manipulation, including deletion, in concurrent environments. The discussion around CompareAndDelete serves as a valuable reminder of the challenges and complexities involved in building distributed systems and the importance of providing developers with the right tools to tackle those challenges effectively.