We are excited to announce the release of LND v0.19.0 beta! This new release focuses on making LND more secure, scalable, and reliable for powering global bitcoin and stablecoin transactions. The latest version of LND delivers improvements that give developers greater control over their on-chain interactions, dramatically enhance backend scalability for growing services, and provide a smoother, more reliable payment experience across the network. Some of the release highlights include:

• Replace-By-Fee Cooperative Close is a new cooperative channel close mechanism, which addresses an oft-requested protocol update enabling the channel closing peer to pay for the closing transaction, along with the flexibility for either peer to increase the transaction fee via RBF.
• Migration of Invoices to Native SQL helps to reduce LND startup time by allowing existing LND users to migrate their Invoice data from kvdb store to native SQL schema.
• Improved payment reliability, driven by optimized pathfinding, now powered by more accurate probability estimates from our bimodal model.
• Testnet4 support for LND, to provide a more stable environment for testing without the block storms that have affected testnet3.

As always, we are grateful for the ongoing support of our external contributors across development, review, and testing. A total of 51 developers contributed to this release. For the complete list of changes included in this release, check out the full release notes.

Cooperative Close Usability Improvements 🔄

In the past, Lightning developers have expressed concerns about how the channel initiator always has to pay the channel closure fee. This legacy flow can be frustrating for channel open initiators if their channel is closed by the remote party in a high-fee environment. If both peers opt-in to the Replace-By-Fee (RBF) cooperative closes, the peer who closes the channel will also be responsible for the on-chain channel close transaction fees. For added flexibility and convenience, it is also possible for the channel initiator to bump the transaction in order to receive funds in a more timely manner.

This improved cooperative close flow is enabled by the new option_simple_close protocol, which was merged earlier this year to the BOLT specification. This feature can be activated with a new config flag: --protocol.rbf-coop-close. With this new coop close type, either channel peer can issue multiple lncli closechannels commands with increasing fee rates, which uses RBF to bump an existing signed coop close transaction. This feature is not yet available for Taproot and Taproot Asset channels, but support will be added in a future release.

Lightning developers and node operators have also asked to be able to cooperatively close channels with active HTLCs. For node operators, it can be difficult to time a channel closure for exactly when there are no active HTLCs. So, with this release, we’ve added the ability to close the channel with active HTLCs via lncli. After the closechannel command has been executed, LND will disallow any new HTLCs from being added to the channel and wait until all existing HTLCs are resolved before proceeding with the cooperative negotiation process to close the channel.

These changes to the cooperative close flows directly address feedback from Lightning developers and node operators to make these flows more sensible, usable, and efficient.

LND Startup Time Reduction via SQL Migration 🛠️

For many years, one of the most requested improvements from node operators has been to reduce the startup time for LND nodes. We are making progress towards this goal by gradually enabling individual LND datastore migrations, with the Invoice datastore being the first one. The last LND release (v0.18.0) provided an option for users starting new nodes to initiate their node with a native SQL database for Invoices. Users with existing nodes, though, did not yet have the option to move their data to a native SQL database. This gap has now been addressed with an option to migrate the data from kvdb (SQLite/Postgres) node to a native SQL schema for invoices. With this change, new and legacy LND nodes can benefit from improvements to LND startup time along with better scaling and query capabilities. With future releases, we expect to continue down the path of greatly improving these critical flows for LND developers.

To migrate, for nodes with already configured SQLite/Postgres backends, users just need to set --db.use-native-sql flag in the config file. Migration will occur automatically on startup.

Users running nodes with bbolt db should note that the above migration process has a prerequisite where the nodes need to be migrated from the Bbolt backend to SQLite/Postgres backend first. This migration can be accomplished with lndinit, a tool to automate the LND wallet generation, including seed and password generation. While lndinit was initially built as a wallet initialization tool for LND, its functionality has now been extended to include the data migration feature for nodes with Bbolt DB backend. Detailed instructions to perform this migration can be found in lndinit’s data migration document.

Users planning to move to Postgres database should be aware that migrating from Bbolt might lead to slower performance with certain RPCs like listpayments and getnetworkinfo,especially for nodes with a larger data folder (greater than 3 GB). If a user is particularly concerned about the performance degradation, the migration can be postponed until the next release which will address the Postgres performance issues.

Transaction Sweeper Upgrades 🧹

The transaction sweeping budget was introduced in the last LND release to provide users more control over the cost of transaction sweeping, setting a ceiling on the amount of funds that can be spent as fee for sweeping HTLCs. With this functionality, node operators can more transparently and easily control their costs in Lightning node operations. To further improve the granularity in the sweeping budget, in this release, LND will now attempt to sweep funds as soon as a portion of the required budget is available. Additionally, there is now enhanced visibility into all the pending on-chain sweep transactions. As a result of this upgrade, node operators will now experience more cost-effective and predictable transaction sweeping.

Increasing Payment Reliability 🤝

One of the most critical aspects of any Lightning implementation is ensuring payments can reliably traverse the network to the intended destination, which ultimately relies on pathfinding throughout the network. Thus, we are constantly improving pathfinding on the LND to ensure developers and users have the highest possible rate of successful payments. To maintain and improve payment reliability, the models and algorithms underlying pathfinding need to be consistently updated and fine-tuned to keep up with network growth and liquidity distribution changes. The pathfinding updates included in this release address bugs and improve how outdated information is handled, resulting in probability estimates that are more accurate, especially for various payment amounts in changing network conditions. The overall improvements in pathfinding reliability contribute to a better user experience when sending payments on the Lightning Network. By employing a more accurate probability estimator based on the bimodal model, which assumes that a channel has liquidity either on the local or the remote side, and incorporating past payment knowledge, LND can now make smarter routing decisions, leading to a higher likelihood of successful payments.

Engineering LND for the future 🔮

As the Lightning Network continues to grow and scale, LND is focused on delivering a reliable, scalable developer experience for Lightning node operators and developers. With this release, we’ve included improvements to reduce startup time so that node operations such as upgrades are more efficient. Further, we’ve enhanced the cooperative channel close flows to allow for more flexibility and capital efficiency. And finally, at the core of the implementation, pathfinding has been further optimized to ensure high rates of payment success.

We’d like to thank the entire community for their feedback on and contributions to this release, and we look forward to hearing from you. You can find us on the LND Slack, reach out to us on Twitter, contribute a PR, or subscribe to our newsletter. Join us to make Lightning Usability Go Up! 📈