eclair/docs/release-notes/eclair-vnext.md

# Eclair vnext

<insert here a high-level description of the release>

## Major changes

### Dual funding

After many years of work and refining the protocol, [dual funding](https://github.com/lightning/bolts/pull/851) has been added to the BOLTs.

This release of eclair activates dual funding, and it will automatically be used with [cln](https://github.com/ElementsProject/lightning/) nodes. When opening channels to nodes that don't support dual funding, the older funding protocol will be used automatically.

One of the immediate benefits of dual funding is that the funding transaction can now be RBF-ed, using the `rbfopen` RPC.

There is currently no way to automatically add funds to channels that are being opened to your node, as deciding whether to do so or not really depends on each node operator's peering strategy.
We have however created a [sample plugin](https://github.com/ACINQ/eclair-plugins/tree/master/channel-funding) that node operators can fork to implement their own strategy for contributing to inbound channels.

### Update minimal version of Bitcoin Core

With this release, eclair requires using Bitcoin Core 24.1.
Newer versions of Bitcoin Core may be used, but haven't been extensively tested.

### Use priority instead of block target for feerates

Eclair now uses a `slow`/`medium`/`fast` notation for feerates (in the style of mempool.space),
instead of block targets. Only the funding and closing priorities can be configured, the feerate
for commitment transactions is managed by eclair, so is the fee bumping for htlcs in force close
scenarii. Note that even in a force close scenario, when an output is only spendable by eclair, then
the normal closing priority is used.

Default setting is `medium` for both funding and closing. Node operators may configure their values like so:

```eclair.conf
eclair.on-chain-fees.confirmation-priority {
    funding = fast
    closing = slow
}
```

This configuration section replaces the previous `eclair.on-chain-fees.target-blocks` section.

### Add configurable maximum anchor fee

Whenever an anchor outputs channel force-closes, we regularly bump the fees of the commitment transaction to get it to confirm.
We previously ensured that the fees paid couldn't exceed 5% of our channel balance, but that may already be extremely high for large channels.
Without package relay, our anchor transactions may not propagate to miners, and eclair may end up bumping the fees more than the actual feerate, because it cannot know why the transaction isn't confirming.

We introduced a new parameter to control the maximum fee that will be paid during fee-bumping, that node operators may configure:

```eclair.conf
// maximum amount of fees we will pay to bump an anchor output when we have no HTLC at risk
eclair.on-chain-fees.anchor-without-htlcs-max-fee-satoshis = 10000
```

We also limit the feerate used to be at most the same order of magnitude of `fatest` feerate provided by our fee estimator.

### Managing Bitcoin Core wallet keys

You can now use Eclair to manage the private keys for on-chain funds monitored by a Bitcoin Core watch-only wallet.

See `docs/BitcoinCoreKeys.md` for more details.

### Advertise low balance with htlc_maximum_msat

Eclair used to disable a channel when there was no liquidity on our side so that other nodes stop trying to use it.
However, other implementations use disabled channels as a sign that the other peer is offline.
To be consistent with other implementations, we now only disable channels when our peer is offline and signal that a channel has very low balance by setting htlc_maximum_msat to a low value.
The balance thresholds at which to update htlc_maximum_msat are configurable like this:

```eclair.conf
eclair.channel.channel-update {
    balance-thresholds = [{
        available-sat = 1000  // If our balance goes below this,
        max-htlc-sat =  0     // set the maximum HTLC amount to this (or htlc-minimum-msat if it's higher).
    },{
        available-sat = 10000
        max-htlc-sat =  1000
    }]

    min-time-between-updates = 1 hour // minimum time between channel updates because the balance changed
}
```

This feature leaks a bit of information about the balance when the channel is almost empty, if you do not wish to use it, set `eclair.channel.channel-update.balance-thresholds = []`.

### API changes

- `bumpforceclose` can be used to make a force-close confirm faster, by spending the anchor output (#2743)
- `open` now takes an optional parameter `--fundingFeeBudgetSatoshis` to define the maximum acceptable value for the mining fee of the funding transaction. This mining fee can sometimes be unexpectedly high depending on available UTXOs in the wallet. Default value is 0.1% of the funding amount (#2808)
- `rbfopen` now takes a mandatory parameter `--fundingFeeBudgetSatoshis`, with the same semantics as for `open` (#2808)

### Miscellaneous improvements and bug fixes

#### Use bitcoinheaders.net v2

Eclair uses <https://bitcoinheaders.net/> as one of its sources of blockchain data to detect when our node is being eclipsed.
The format of this service is changing, and the older format will be deprecated soon.
We thus encourage eclair nodes to update to ensure that they still have access to this blockchain watchdog.

#### Force-closing anchor channels fee management

Various improvements have been made to force-closing channels that need fees to be attached using CPFP or RBF.
Those changes ensure that eclair nodes don't end up paying unnecessarily high fees to force-close channels, even when the mempool is full.

#### Improve DB usage when closing channels

When channels that have relayed a lot of HTLCs are closed, we can forget the revocation data for all of those HTLCs and free up space in our DB. We previously did that synchronously, which meant deleting potentially millions of rows synchronously. This isn't a high priority task, so we're now asynchronously deleting that data in smaller batches.

Node operators can control the rate at which that data is deleted by updating the following values in `eclair.conf`:

```conf
// During normal channel operation, we need to store information about past HTLCs to be able to punish our peer if
// they publish a revoked commitment. Once a channel closes or a splice transaction confirms, we can clean up past
// data (which reduces the size of our DB). Since there may be millions of rows to delete and we don't want to slow
// down the node, we delete those rows in batches at regular intervals.
eclair.db.revoked-htlc-info-cleaner {
  // Number of rows to delete per batch: a higher value will clean up the DB faster, but may have a higher impact on performance.
  batch-size = 50000
  // Frequency at which batches of rows are deleted: a lower value will clean up the DB faster, but may have a higher impact on performance.
  interval = 15 minutes
}
```

See <https://github.com/ACINQ/eclair/pull/2705> for more details.

#### Correctly unlock wallet inputs during transaction eviction

When the mempool is full and transactions are evicted, and potentially double-spent, the Bitcoin Core wallet cannot always safely unlock inputs.

Eclair is now automatically detecting such cases and telling Bitcoin Core to unlock inputs that are safe to use. This ensures that node operators don't end up with unavailable liquidity.

See <https://github.com/ACINQ/eclair/pull/2817> and <https://github.com/ACINQ/eclair/pull/2818> for more details.

## Verifying signatures

You will need `gpg` and our release signing key 7A73FE77DE2C4027. Note that you can get it:

- from our website: https://acinq.co/pgp/drouinf.asc
- from github user @sstone, a committer on eclair: https://api.github.com/users/sstone/gpg_keys

To import our signing key:

```sh
$ gpg --import drouinf.asc
```

To verify the release file checksums and signatures:

```sh
$ gpg -d SHA256SUMS.asc > SHA256SUMS.stripped
$ sha256sum -c SHA256SUMS.stripped
```

## Building

Eclair builds are deterministic. To reproduce our builds, please use the following environment (*):

- Ubuntu 22.04
- AdoptOpenJDK 11.0.6
- Maven 3.9.2

Use the following command to generate the eclair-node package:

```sh
mvn clean install -DskipTests
```

That should generate `eclair-node/target/eclair-node-<version>-XXXXXXX-bin.zip` with sha256 checksums that match the one we provide and sign in `SHA256SUMS.asc`

(*) You may be able to build the exact same artefacts with other operating systems or versions of JDK 11, we have not tried everything.

## Upgrading

This release is fully compatible with previous eclair versions. You don't need to close your channels, just stop eclair, upgrade and restart.

## Changelog

<fill this section when publishing the release with `git log v0.9.0... --format=oneline --reverse`>