Propose ADR on Direct chain

docs/adr/0010-use-direct-chain.md

@@ -0,0 +1,28 @@
+# 10. Use direct connection to cardano-node
+
+Date: 2021-10-23
+
+## Status
+
+:hammer_and_wrench:
+
+## Context
+
+* On-Chain Validation is a critical part of the Hydra protocol, it requires both the ability to _submit_ transactions to the _Layer 1_ chain advancing the state of a Head, and observing those transactions as the [Plutus](https://github.com/input-output-hk/plutus) contracts are validated
+* The [Plutus Application Framework](https://github.com/input-output-hk/plutus-apps) is expected to provide the necessary machinery to allow "Smart Contracts" applications to interact with the chain but it's still under active development and not ready for deployment on-chain
+* We want to gather feedback as early as possible and deliver a fully functional Hydra Head node for early adopters to test on a "real" chain (testnet)
+* Our experiment connecting directly to a Cardano node has been conclusive. We can:
+  * Connect to a node using local protocols,
+  * Build and submit Head transactions triggering smart contracts validation, and
+  * Observe transactions using chain-sync protocol.
+
+## Decision
+
+_Therefore_
+
+For the time being, until _Plutus Application Framework_ is released, we will implement on-chain interaction of Hydra nodes _directly_, connecting to a Cardano node through a _local socket_.
+
+## Consequences
+
+* Limit Hydra dependencies to [plutus](https://github.com/input-output-hk/plutus) repository
+* Remove all PAB-related code as well as off-chain `Contract`s code and related dependencies