Concurrency Locking Versioning

Introduction: The Challenge of Concurrent Saves

In our increasingly collaborative world, multiple users often access the same document simultaneously. This situation presents a challenging problem for document servers: how can we handle concurrent saves without causing data loss or creating conflicts? When two users make changes to the same document and save their alterations, the document’s state can become unpredictable.

Understanding the Problem

Let’s say two users—Bob and Kate—open a document for editing. Bob makes some changes and saves them, but before Kate can save her own modifications, she does not have any notifications about Bob’s actions. If Kate saves her changes, the document’s final state depends entirely on who saved last, leading to undefined behavior. To ensure data integrity and optimize user experience, we need a robust solution to manage these concurrent modifications.

Proposed Solutions

Two main strategies can help mitigate this issue:

1. Document Locking Mechanism

One effective approach involves implementing a locking mechanism:

  • Lock Acquisition: When the first user (e.g., Bob) opens a document, a lock is placed on it. This means only Bob can make changes, and other users, like Kate, can only read the document until the lock is released.

  • Lock Release: The lock stays active as long as Bob is actively editing. Once Bob exits the document, the lock is released, allowing other users to edit the document.

Handling Unexpected Scenarios

However, there are risks involved:

  • Network Interruptions: If Bob experiences a sudden network disconnection, the document remains locked indefinitely. To counter this, the server can implement a pinging system whereby the client must send regular pings to indicate it is still active. If the server fails to receive a set number of pings within a predetermined time frame, it infers that the client is no longer responsive, thereby releasing the lock.

  • Defining Pings: The ping can include the document’s ID. The server tracks the last ping for each document and efficiently decides when to release locks.

2. Version Control System

Another potential solution is to employ a version control method:

  • Multiple Versions: Instead of locking documents, save multiple versions of the same document whenever a user makes changes. This allows the system to keep track of all modifications.

  • Merging or Selecting Versions: If changes are made in quick succession, the application provides users with options to merge changes or select a preferred version. This approach mimics behaviors seen in source control systems.

Storage Optimization

To conserve storage space:

  • Document Diffs: Instead of saving complete document copies, consider storing just the changes (diffs) made at each save, similar to how modern version control software operates.

Parameters for Implementation

For whichever strategy you choose (locking or versioning), consider the following parameters:

  • Ping Interval: Experiment with different intervals for sending pings. Too short may lead to unnecessary server traffic, while too long could prolong the lock state undesirably.

  • Rapid Succession Interval: For version control, determine how quickly users can save changes. Establish guidelines for consecutive saves so that the application can intelligently prompt users for merging changes.

Conclusion: Choose Wisely for Better Collaboration

Both methods offer viable pathways to handling concurrent saves effectively. If the primary concern is avoiding data conflicts and ensuring immediate editing capabilities, a locking mechanism supplemented by a pinging strategy is appropriate. If the aim is to preserve user autonomy without blocking edits, consider implementing a version control system. Ultimately, the choice will depend on your specific application’s needs and the importance of collaboration in your user experience.

By implementing an effective strategy for concurrent saves, you can enhance collaboration and ensure that no user’s contribution is lost amidst document edits.