bitcoin/doc/design/assumeutxo.md

# assumeutxo

Assumeutxo is a feature that allows fast bootstrapping of a validating bitcoind
instance with a very similar security model to assumevalid.

The RPC commands `dumptxoutset` and `loadtxoutset` are used to respectively generate
and load UTXO snapshots. The utility script `./contrib/devtools/utxo_snapshot.sh` may
be of use.

## General background

- [assumeutxo proposal](https://github.com/jamesob/assumeutxo-docs/tree/2019-04-proposal/proposal)
- [Github issue](https://github.com/bitcoin/bitcoin/issues/15605)
- [draft PR](https://github.com/bitcoin/bitcoin/pull/15606)

## Design notes

- A new block index `nStatus` flag is introduced, `BLOCK_ASSUMED_VALID`, to mark block
  index entries that are required to be assumed-valid by a chainstate created
  from a UTXO snapshot. This flag is mostly used as a way to modify certain
  CheckBlockIndex() logic to account for index entries that are pending validation by a
  chainstate running asynchronously in the background. We also use this flag to control
  which index entries are added to setBlockIndexCandidates during LoadBlockIndex().

- Indexing implementations via BaseIndex can no longer assume that indexation happens
  sequentially, since background validation chainstates can submit BlockConnected
  events out of order with the active chain.

- The concept of UTXO snapshots is treated as an implementation detail that lives
  behind the ChainstateManager interface. The external presentation of the changes
  required to facilitate the use of UTXO snapshots is the understanding that there are
  now certain regions of the chain that can be temporarily assumed to be valid (using
  the nStatus flag mentioned above). In certain cases, e.g. wallet rescanning, this is
  very similar to dealing with a pruned chain.

  Logic outside ChainstateManager should try not to know about snapshots, instead
  preferring to work in terms of more general states like assumed-valid.


## Chainstate phases

Chainstate within the system goes through a number of phases when UTXO snapshots are
used, as managed by `ChainstateManager`. At various points there can be multiple
`Chainstate` objects in existence to facilitate both maintaining the network tip and
performing historical validation of the assumed-valid chain.

It is worth noting that though there are multiple separate chainstates, those
chainstates share use of a common block index (i.e. they hold the same `BlockManager`
reference).

The subheadings below outline the phases and the corresponding changes to chainstate
data.

### "Normal" operation via initial block download

`ChainstateManager` manages a single Chainstate object, for which
`m_snapshot_blockhash` is null. This chainstate is (maybe obviously)
considered active. This is the "traditional" mode of operation for bitcoind.

|    |    |
| ---------- | ----------- |
| number of chainstates | 1 |
| active chainstate | ibd |

### User loads a UTXO snapshot via `loadtxoutset` RPC

`ChainstateManager` initializes a new chainstate (see `ActivateSnapshot()`) to load the
snapshot contents into. During snapshot load and validation (see
`PopulateAndValidateSnapshot()`), the new chainstate is not considered active and the
original chainstate remains in use as active.

|    |    |
| ---------- | ----------- |
| number of chainstates | 2 |
| active chainstate | ibd |

Once the snapshot chainstate is loaded and validated, it is promoted to active
chainstate and a sync to tip begins. A new chainstate directory is created in the
datadir for the snapshot chainstate called
`chainstate_[SHA256 blockhash of snapshot base block]`.

|    |    |
| ---------- | ----------- |
| number of chainstates | 2 |
| active chainstate | snapshot |

The snapshot begins to sync to tip from its base block, technically in parallel with
the original chainstate, but it is given priority during block download and is
allocated most of the cache (see `MaybeRebalanceCaches()` and usages) as our chief
consideration is getting to network tip.

**Failure consideration:** if shutdown happens at any point during this phase, both
chainstates will be detected during the next init and the process will resume.

### Snapshot chainstate hits network tip

Once the snapshot chainstate leaves IBD, caches are rebalanced
(via `MaybeRebalanceCaches()` in `ActivateBestChain()`) and more cache is given
to the background chainstate, which is responsible for doing full validation of the
assumed-valid parts of the chain.

**Note:** at this point, ValidationInterface callbacks will be coming in from both
chainstates. Considerations here must be made for indexing, which may no longer be happening
sequentially.

### Background chainstate hits snapshot base block

Once the tip of the background chainstate hits the base block of the snapshot
chainstate, we stop use of the background chainstate by setting `m_stop_use` (not yet
committed - see #15606), in `CompleteSnapshotValidation()`, which is checked in
`ActivateBestChain()`). We hash the background chainstate's UTXO set contents and
ensure it matches the compiled value in `CMainParams::m_assumeutxo_data`.

The background chainstate data lingers on disk until shutdown, when in
`ChainstateManager::Reset()`, the background chainstate is cleaned up with
`ValidatedSnapshotShutdownCleanup()`, which renames the `chainstate_[hash]` datadir as
`chainstate`.

|    |    |
| ---------- | ----------- |
| number of chainstates | 2 (ibd has `m_stop_use=true`) |
| active chainstate | snapshot |

**Failure consideration:** if bitcoind unexpectedly halts after `m_stop_use` is set on
the background chainstate but before `CompleteSnapshotValidation()` can finish, the
need to complete snapshot validation will be detected on subsequent init by
`ChainstateManager::CheckForUncleanShutdown()`.

### Bitcoind restarts sometime after snapshot validation has completed

When bitcoind initializes again, what began as the snapshot chainstate is now
indistinguishable from a chainstate that has been built from the traditional IBD
process, and will be initialized as such.

|    |    |
| ---------- | ----------- |
| number of chainstates | 1 |
| active chainstate | ibd |