Chapter 1: Introduction to Multi-Leader Replication

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Section 1.1: Key Issues in Multi-Leader Replication

Multi-leader replication presents several challenges:

• Concurrent modifications of identical data across different data centers necessitate conflict resolution.
• Data loss can occur from a data center that suffers a permanent failure if the writes haven't been replicated to other leaders.
• In database contexts, managing auto-incrementing keys, triggers, and integrity constraints can be problematic.

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Subsection 1.1.1: Conflict Resolution Strategies

One of the critical issues with multi-leader replication is managing conflicting writes. Imagine editing a Google document simultaneously with someone else. If one user changes a heading from A to B while the other changes it from A to C, both changes are valid and recorded at the nearest leader. However, the conflict becomes apparent only later when the changes are replicated asynchronously between data centers.

In contrast, a single-leader architecture avoids these issues by serializing writes, allowing for blocking or aborting conflicting actions, prompting users to retry. To mitigate conflicts in a multi-leader setup, one approach is to replicate a write request to all leaders and followers before confirming success. However, this would hinder independent write acceptance among leaders.

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Chapter 2: Finalizing Data Consistency

To ensure that data copies across leaders converge to a unified snapshot, we can resolve conflicting writes through various methods. For example, if a record changes from A to B at leader #1 and from A to C at leader #2, we can handle the conflict in several ways:

1. Last Write Wins (LWW): The timestamp of the latest write determines the final value, although this can lead to data loss.
2. Unique ID Assignments: Assign unique IDs to each write and consider the highest ID as the final value.
3. Merge Conflicting Writes: Applications like Evernote merge conflicting writes, allowing users to resolve the outcome.
4. Custom Conflict Resolution: Store conflicting writes separately and use application code to determine the correct value.

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This video, titled "Google SWE teaches systems design | EP3: Multileader replication," dives deep into the nuances of multi-leader replication and its implications in system design.

In the second video, "Multi Leader Replication - chaos | Systems Design 0 to 1 with Ex-Google SWE," the discussion continues, shedding light on the chaotic aspects of managing multiple leaders in system design.