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Summary

This RFC describes a modification to the PBFT implementation to allow new nodes or nodes that have fallen behind to quickly catch up with the rest of the network. It builds on the consensus seal concept described in an earlier RFC.

Motivation

A major flaw in the current PBFT implementation is that a node that has fallen behind or a node that has just joined the network has no way to catch up with the rest of the network. This is because, in order to commit a block, a node must receive 2f+1 messages from other nodes agreeing that the block should be committed. However, nodes do not have a way to request historic messages from each other and nodes only send out commit messages once when they are ready to commit. If a node is not running when this happens, it will never receive the messages and can never commit a block.

Guide-level explanation

In order for new nodes and nodes that have fallen behind to quickly catch up with the rest of the network, Sawtooth PBFT provides a mechanism by which the normal consensus process can be short-circuited without weakening the consensus protocol's guarantees.

Under normal operation, when a new block is received by a PBFT node, it does an initial validation and, if the validation is successful, broadcasts a COMMIT message to its peers indicating that it is ready to commit the new block. Prior to committing the block, the PBFT node waits to receive 2f+1 COMMIT messages from other nodes. After receiving these COMMIT messages, the node knows that the block is safe to commit and will not need to be reverted later.

If, while waiting to receive 2f+1 COMMIT messages from its peers, a node receives the next block in the chain, the node has an opportunity to short-circuit consensus. This is done by validating the consensus seal in the new block and confirming that it is a valid proof for the block that the node is currently trying to commit. If it is, then the node copies the COMMIT messages from the consensus seal into its own message log and commits the block.

Reference-level explanation

When a new block is received by a node that has not yet committed the previous block, the new block's consensus seal will be inspected to determine whether it contains a valid seal to commit the previous block. This will be done by adding the messages included in the seal to the node's message log and committing the previous block.

Drawbacks

There are no known drawbacks to this change.

Rationale and alternatives

The first alternative that was considered was to allow nodes to receive historic messages from other nodes' message logs, either by requesting messages or requesting that a node catch another node up. However, this solution does not work if a node is behind far enough that the other nodes have already cleaned their logs and no longer have the relevant messages.

Another alternative that was considered is putting nodes into a special "catchup" mode when they startup up if they are behind. The problem with this alternative is there is no clear point where the node should transition from "catchup" to regular mode and it does not solve the problem where a node misses a bunch of messages because of a networking problem.

Prior art

The original PBFT paper describes this problem briefly. From Section 4.3:

For the safety condition to hold, messages must be kept in a replica’s log until it knows that the requests they concern have been executed by at least 1 non-faulty replicas and it can prove this to others in view changes. In addition, if some replica misses messages that were discarded by all non-faulty replicas, it will need to be brought up to date by transferring all or a portion of the service state. Therefore, replicas also need some proof that the state is correct.

The "proof" that the paper states is required is provided by the consensus seal, which is signed and contains the necessary messages (state).

Unresolved questions

No unresolved questions

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