ADR: No Transaction Gossiping and Mempool for Light Clients (cosmos#532)

* No Transaction Gossiping and Mempool for Light Clients

Ref: cosmos#526

docs/lazy-adr/adr-008-mempool-optimint-light-client.md

@@ -0,0 +1,56 @@
+# ADR 008: Light Client Transaction Gossip & Mempool
+
+## Changelog
+
+- 20.09.2022: Initial Draft
+
+## Context
+
+Rollmint Light Clients cannot validate transactions without a state. Therefore Light Clients should not gossip incoming transactions, and the mempool can be disabled.
+
+### Explanation
+
+There is a wish for the light clients to participate in the P2P Layer. One of the ways a full node participates in the network is to gossip valid transactions throughout the network. Each full node that receives a transaction sends an ABCI message, `CheckTx`, to the application layer to check for validity and receives an `abci.ResponseCheckTx` .
+There are 2 Types of checks.
+Current stateless checks:
+
+- Check that the size is less than the configured maximum transaction size.
+- Call any Pre-Check hooks if defined
+- Check if the proxy connection has an error
+- Check if the transaction is already in cache / mempool
+
+Stateful:
+
+- Checks if transactions and messages are valid based on a committed state
+
+Light clients cannot do stateful checks because they don't have access to the state.
+Light clients can do stateless checks. However, creating invalid transactions that pass the current stateless checks is easy. Light clients could therefore support a DOS attack of the network when they gossip invalid transactions.
+If light clients do not check transactions, they do not need the mempool.
+
+### Libp2p pubsub
+
+If the transaction originates from the light client i.e. submitting a new transaction, then this transaction must be gossiped to the network.
+
+A light client will use the [fan-out](https://docs.libp2p.io/concepts/publish-subscribe/#fan-out) functionality of pubsub. It will send its transaction to the network but will not subscribe to receiving and propagating other transactions.
+
+## Alternative Approaches
+
+- We create more rigorous stateless checks on the transactions that would reduce or prevent the DOS attack and enable transaction gossiping
+
+## Status
+
+Proposed
+
+## Consequences
+
+### Positive
+
+- Reduction of complexity and keeping the light client lightweight
+
+### Negative
+
+- Light clients do not participate in gossiping transactions
+
+## References
+
+Issue #100 [References](https://github.com/celestiaorg/oollmint/issues/100#issuecomment-921848268)