adr: ADR-052: Tendermint Mode (#4302)

* Separate ADR Tendermint Mode from ADR-051

* Update docs/architecture/adr-052-tendermint-mode.md

Co-Authored-By: Anton Kaliaev <anton.kalyaev@gmail.com>

* Apply suggestions from code review

Co-Authored-By: Marko <marbar3778@yahoo.com>

* Apply suggestions from code review

Co-Authored-By: Marko <marbar3778@yahoo.com>

* remove line of mode info of rpc

* Add link to ADR table of contents

Co-authored-by: Anton Kaliaev <anton.kalyaev@gmail.com>
Co-authored-by: Marko <marbar3778@yahoo.com>


    Fullnode mode : fullnode mode does not have any capability to participate on consensus
    Validator mode : this mode is exactly same as existing state machine behavior. sync without voting on consensus, and participate consensus when fully synced
    Seed mode : lightweight seed mode only for maintain an address book, p2p like TenderSeed

Separate ADR Tendermint Mode from ADR-051 #4262

docs/architecture/README.md

@@ -64,3 +64,4 @@ Note the context/background should be written in the present tense.
 - [ADR-039-Peer-Behaviour](./adr-039-peer-behaviour.md)
 - [ADR-041-Proposer-Selection-via-ABCI](./adr-041-proposer-selection-via-abci.md)
 - [ADR-043-Blockchain-RiRi-Org](./adr-043-blockchain-riri-org.md)
+- [ADR-052-Tendermint-Mode](./adr-052-tendermint-mode.md)

docs/architecture/adr-052-tendermint-mode.md

@@ -0,0 +1,81 @@
+# ADR 052: Tendermint Mode
+
+## Changelog
+
+* 27-11-2019: Initial draft from ADR-051
+* 13-01-2020: Separate ADR Tendermint Mode from ADR-051
+
+## Context
+
+- Fullnode mode: fullnode mode does not have the capability to become a validator. 
+- Validator mode : this mode is exactly same as existing state machine behavior. sync without voting on consensus, and participate consensus when fully synced
+- Seed mode : lightweight seed mode maintaining an address book, p2p like [TenderSeed](https://gitlab.com/polychainlabs/tenderseed)
+
+## Decision
+
+We would like to suggest a simple Tendermint mode abstraction. These modes will live under one binary, and when initializing a node the user will be able to specify which node they would like to create.
+
+- Which reactor, component to include for each node
+    - fullnode *(default)*
+        - switch, transport
+        - reactors
+          - mempool
+          - consensus
+          - evidence
+          - blockchain
+          - p2p/pex
+        - rpc (safe connections only)
+        - *~~no privValidator(priv_validator_key.json, priv_validator_state.json)~~*
+    - validator
+        - switch, transport
+        - reactors
+          - mempool
+          - consensus
+          - evidence
+          - blockchain
+          - p2p/pex
+        - rpc (safe connections only)
+        - with privValidator(priv_validator_key.json, priv_validator_state.json)
+    - seed
+        - switch, transport
+        - reactor
+           - p2p/pex
+- Configuration, cli command
+    - We would like to suggest by introducing `mode` parameter in `config.toml` and cli
+    - `mode = "{{ .BaseConfig.Mode }}"` in `config.toml`
+    - `tendermint node --mode validator`  in cli
+    - fullnode | validator | seed (default: "fullnode")
+- RPC modification
+    - `host:26657/status`
+        - return empty `validator_info` when fullnode mode
+    - no rpc server in seed mode
+- Where to modify in codebase
+    - Add  switch for `config.Mode` on `node/node.go:DefaultNewNode`
+    - If `config.Mode==validator`, call default `NewNode` (current logic)
+    - If `config.Mode==fullnode`, call `NewNode` with `nil` `privValidator` (do not load or generation)
+        - Need to add exception routine for `nil` `privValidator` to related functions
+    - If `config.Mode==seed`, call `NewSeedNode` (seed version of `node/node.go:NewNode`)
+        - Need to add exception routine for `nil` `reactor`, `component` to related functions
+
+## Status
+
+Proposed
+
+## Consequences
+
+### Positive
+
+- Node operators can choose mode when they run state machine according to the purpose of the node.
+- Mode can prevent mistakes because users have to specify which mode they want to run via flag. (eg. If a user want to run a validator node, she/he should explicitly write down validator as mode)
+- Different mode needs different reactors, resulting in efficient resource usage.
+
+### Negative
+
+- Users need to study how each mode operate and which capability it has.
+
+### Neutral
+
+## References
+
+- Issue [#2237](https://github.com/tendermint/tendermint/issues/2237) : Tendermint "mode"
+- [TenderSeed](https://gitlab.com/polychainlabs/tenderseed) : A lightweight Tendermint Seed Node.