Integrate the blocktree proposal into the book (#2704)
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- [Anatomy of a Fullnode](fullnode.md)
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- [TPU](tpu.md)
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- [TVU](tvu.md)
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- [Blocktree](blocktree.md)
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- [Gossip Service](gossip.md)
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- [The Runtime](runtime.md)
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- [Secure Vote Signing](vote-signer.md)
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- [Staking Rewards](staking-rewards.md)
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- [Fork Selection](fork-selection.md)
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- [Blocktree](blocktree.md)
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- [Data Plane Fanout](data-plane-fanout.md)
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- [Reliable Vote Transmission](reliable-vote-transmission.md)
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- [Bank Forks](bank-forks.md)
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# Blocktree
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This document proposes a change to ledger and window to support Solana's [fork
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generation](fork-generation.md) behavior.
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After a block reaches finality, all blocks from that one on down
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to the genesis block form a linear chain with the familiar name
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blockchain. Until that point, however, the validator must maintain all
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potentially valid chains, called *forks*. The process by which forks
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naturally form as a result of leader rotation is described in
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[fork generation](fork-generation.md). The *blocktree* data structure
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described here is how a validator copes with those forks until blocks
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are finalized.
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## Current Design
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### Functionality of Window And Ledger
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The basic responsibilities of the window and the ledger in a Solana fullnode
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are:
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1. Window: serve as a temporary, RAM-backed store of blobs of the PoH chain
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for reordering and assembly into contiguous blocks to be sent to the bank
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for verification.
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2. Window: serve as a RAM-backed repair facility for other validator nodes,
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which may query the network for as-yet unreceived blobs.
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3. Ledger: provide disk-based storage of the PoH chain in case of node
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restart.
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4. Ledger: provide disk-backed repair facility for when the (smaller)
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RAM-backed window doesn't cover the repair request.
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The window is at the front of a validator node's processing pipeline, blobs are
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received, cached, re-ordered before being deserialized into Entries, passed to
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the bank for verification, and finally on to the ledger, which is at the back
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of a validator node's pipeline.
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The window holds blobs (the over-the-air format, serialized Entries,
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one-per-blob). The ledger holds serialized Entries without any blob
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information.
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### Limitations
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#### One-dimensional key space
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The window and the ledger are indexed by ledger height, which is number of
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Entries ever generated in the PoH chain until the current blob. This
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limitation prevents the window and the ledger from storing the overlapping
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histories possible in Solana's consensus protocol.
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#### Limited caching
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The window is a circular buffer. It cannot accept blobs that are farther in
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the future than the window is currently working. If a blob arrives that is too
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far ahead, it is dropped and will subsequently need to be repaired, incurring
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further delay for the node.
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#### Loss of blob signatures
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Because the blob signatures are stripped before being stored by the ledger,
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repair requests served from the ledger can't be verified to the original
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leader.
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#### Rollback and checkpoint, switching forks, separate functions
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The window and the ledger can't handle replay of alternate forks. Once a Blob
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has passed through the window, it's in the past. The replay stage of a
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validator will need to roll back to a previous checkpoint and decode an
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alternate set of Blobs to the Bank. The separated and one-way nature of window
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and ledger makes this hard.
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## New Design
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A unified window and ledger allows a validator to record every blob it observes
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on the network, in any order, as long as the blob is consistent with the
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network's leader schedule.
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The blocktree allows a validator to record every blob it observes
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on the network, in any order, as long as the blob is signed by the expected
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leader for a given slot.
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Blobs are moved to a fork-able key space the tuple of `leader slot` + `blob
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index` (within the slot). This permits the skip-list structure of the Solana
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