What data is stored in the log compaction snapshot of a Raft-based distributed file system? - Stack Overflow

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I'm working on a Raft implementation as part of my distributed file system and I've run into a problem with the log compaction process. Accurding to the official Raft paper, when a log reaches a certain length, it may be deleted after taking a 'snapshot' of the system. The snapshot contains the system's state at the time in which it was taken and replaces the logs committed up until then.

The problem is that the snapshot size will be about as big as all the data committed at the time of the snapshot. How can I efficiently handle log compaction and snapshotting without doubling the used system space?

I'm working on a Raft implementation as part of my distributed file system and I've run into a problem with the log compaction process. Accurding to the official Raft paper, when a log reaches a certain length, it may be deleted after taking a 'snapshot' of the system. The snapshot contains the system's state at the time in which it was taken and replaces the logs committed up until then.

The problem is that the snapshot size will be about as big as all the data committed at the time of the snapshot. How can I efficiently handle log compaction and snapshotting without doubling the used system space?

• snapshot
• distributed-system
• raft

Share Improve this question Follow asked Nov 21, 2024 at 15:46 Dror ChenDror Chen 1111 silver badge22 bronze badges

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Your reasoning is correct and points to an inherent limitation of Raft and other similar protocols involving logs with compaction: they aren't designed for situations where the size of the state they manage grows without limit. Their typical use case is to manage a state which is bounded in size regardless of how long it exists. A simple example is a bank account balance: it is a single number, so the size of the state - and the compacted log - is independent of the number of debit and credit operations it took to arrive at its current value. Other examples are configuration items such as Kubernetes objects stored in etcd. Kubernetes documentation advises not to store an unbounded amount of objects, say log entries, for precisely the reason you have discovered by yourself.

There are tricks one can use to mitigate the problem, such as handling old data that has not changed recently separately and including it in snapshots by reference, but such tricks rely on specific assumptions about operations and state to be managed and are apt to misbehave when these assumptions are violated. The general solution is to add a level of indirection and use consensus protocols like Raft to manage not your state directly, but the state of the database engine which contains your state.

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