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Repair Service

docs/src/implemented-proposals/repair-service.md

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Repair Service

The RepairService is in charge of retrieving missing shreds that failed to be delivered by primary communication protocols like Turbine. It is in charge of managing the protocols described below in the Repair Protocols section below.

Challenges:

1) Validators can fail to receive particular shreds due to network failures

2) Consider a scenario where blockstore contains the set of slots {1, 3, 5}. Then Blockstore receives shreds for some slot 7, where for each of the shreds b, b.parent == 6, so then the parent-child relation 6 -> 7 is stored in blockstore. However, there is no way to chain these slots to any of the existing banks in Blockstore, and thus the Shred Repair protocol will not repair these slots. If these slots happen to be part of the main chain, this will halt replay progress on this node.

Epoch Slots: Each validator advertises separately on gossip the various parts of an Epoch Slots:

  • The stash: An epoch-long compressed set of all completed slots.
  • The cache: The Run-length Encoding (RLE) of the latest N completed slots starting from some some slot M, where N is the number of slots that will fit in an MTU-sized packet.

Epoch Slots in gossip are updated every time a validator receives a complete slot within the epoch. Completed slots are detected by blockstore and sent over a channel to RepairService. It is important to note that we know that by the time a slot X is complete, the epoch schedule must exist for the epoch that contains slot X because WindowService will reject shreds for unconfirmed epochs.

Every N/2 completed slots, the oldest N/2 slots are moved from the cache into the stash. The base value M for the RLE should also be updated.

Repair Request Protocols

The repair protocol makes best attempts to progress the forking structure of Blockstore.

The different protocol strategies to address the above challenges:

  1. Shred Repair (Addresses Challenge #1): This is the most basic repair protocol, with the purpose of detecting and filling "holes" in the ledger. Blockstore tracks the latest root slot. RepairService will then periodically iterate every fork in blockstore starting from the root slot, sending repair requests to validators for any missing shreds. It will send at most some N repair requests per iteration. Shred repair should prioritize repairing forks based on the leader's fork weight. Validators should only send repair requests to validators who have marked that slot as completed in their EpochSlots. Validators should prioritize repairing shreds in each slot that they are responsible for retransmitting through turbine. Validators can compute which shreds they are responsible for retransmitting because the seed for turbine is based on leader id, slot, and shred index.

    Note: Validators will only accept shreds within the current verifiable epoch (epoch the validator has a leader schedule for).

  2. Preemptive Slot Repair (Addresses Challenge #2): The goal of this protocol is to discover the chaining relationship of "orphan" slots that do not currently chain to any known fork. Shred repair should prioritize repairing orphan slots based on the leader's fork weight.

    • Blockstore will track the set of "orphan" slots in a separate column family.

    • RepairService will periodically make Orphan requests for each of the orphans in blockstore.

      Orphan(orphan) request - orphan is the orphan slot that the requestor wants to know the parents of Orphan(orphan) response - The highest shreds for each of the first N parents of the requested orphan

      On receiving the responses p, where p is some shred in a parent slot, validators will:

      • Insert an empty SlotMeta in blockstore for p.slot if it doesn't already exist.
      • If p.slot does exist, update the parent of p based on parents

      Note: that once these empty slots are added to blockstore, the Shred Repair protocol should attempt to fill those slots.

      Note: Validators will only accept responses containing shreds within the current verifiable epoch (epoch the validator has a leader schedule for).

Validators should try to send orphan requests to validators who have marked that orphan as completed in their EpochSlots. If no such validators exist, then randomly select a validator in a stake-weighted fashion.

Repair Response Protocol

When a validator receives a request for a shred S, they respond with the shred if they have it.

When a validator receives a shred through a repair response, they check EpochSlots to see if <= 1/3 of the network has marked this slot as completed. If so, they resubmit this shred through its associated turbine path, but only if this validator has not retransmitted this shred before.