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In this commit, we add a model for the linear fee function we use in lnd for fee bumping. This models can be used to reproduce the issue reported in https://github.com/lightningnetwork/lnd/issues/8741, and can also be shown that that bug fix resolves a counter example found by the model checker.
50 lines
2.8 KiB
Markdown
50 lines
2.8 KiB
Markdown
# Alloy Models
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This folder houses "lightweight formal models" written using the
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[Alloy](https://alloytools.org/about.html) model checker and language.
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Compared to typical formal methods, Alloy is a _bounded_ model checker,
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considered a [lightweight
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method](https://en.wikipedia.org/wiki/Formal_methods#:~:text=As%20an%20alternative,Tools.%5B31)
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to formal analysis. Lightweight formal methods are easier to use than fully
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fledged formal methods as rather than attempting to prove the that a model is
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_always_ correct (for all instances), they instead operate on an input of a set
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of bounded parameters and iterations. These models can then be used to specify
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a model, then use the model checker to find counter examples of a given
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assertion. If a counter example can't be found, then the model _may_ be valid.
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Alloy in particular is an expressive, human readable language for formal
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modeling. It also has a nice visualizer which can show counter examples, aiding
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in development and understanding.
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Alloy is useful as when used during upfront software design (or even after the
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fact), one can create a formal model of a software system to gain better
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confidence in the _correctness_ of a system. The model can then be translated
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to code. Many times, writing the model (either before or after the code) can
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help one to better understand a given software system. Models can also be used
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to specify protocols in p2p systems such as Lightning (as it [supports temporal
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logic](https://alloytools.org/alloy6.html#:~:text=Meaning%20of%20temporal%20connectives),
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which enables creation of state machines and other interactive transcripts),
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serving as a complement to a normal prosed based specification.
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## How To Learn Alloy?
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We recommend the following places to learn Alloy:
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* [The online tutorial for Alloy 4.0](https://alloytools.org/about.html):
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* Even though this is written with Alloy 4.0 (which doesn't support
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temporal logic), the tutorial is very useful, as it introduces
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fundamental concept of Alloy, using an accessible example based on a file
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system.
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* [Alloy Docs](https://alloy.readthedocs.io/en/latest/index.html):
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* This serves as a one stop shop for reference to the Alloy language. It
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explains all the major syntax, tooling, and also how to model time in
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Alloy.
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* [Formal Software Design with Alloy 6](https://haslab.github.io/formal-software-design/index.html):
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* A more in depth tutorial which uses Alloy 6. This tutorial covers more
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advanced topics such as [event
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reification](https://alloytools.discourse.group/t/modelling-a-state-machine-in-electrum-towards-alloy-6/88).
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This tutorial is also very useful, as it includes examples for how to
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model interactions in a p2p network, such as one peer sending a message
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to another.
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