tendermint/docs/specification/new-spec/blockchain_reactor.md

# Blockchain Reactor

The Blockchain Reactor's high level responsibility is to request blocks from peers or provide them
with blocks, validate and persist the blocks to disk and play blocks to the ABCI app.

## Message Types

```go
const (
    msgTypeBlockRequest    = byte(0x10)
    msgTypeBlockResponse   = byte(0x11)
    msgTypeNoBlockResponse = byte(0x12)
    msgTypeStatusResponse  = byte(0x20)
    msgTypeStatusRequest   = byte(0x21)
)
```

```go
type bcBlockRequestMessage struct {
    Height int64
}

type bcNoBlockResponseMessage struct {
    Height int64
}

type bcBlockResponseMessage struct {
    Block *types.Block
}

type bcStatusRequestMessage struct {
    Height int64

type bcStatusResponseMessage struct {
    Height int64
}
```

## Block Reactor

* coordinates the pool for syncing
* coordinates the store for persistence
* coordinates the playing of blocks towards the app using a sm.BlockExecutor
* handles switching between fastsync and consensus
* it is a p2p.BaseReactor
* starts the pool.Start() and its poolRoutine()
* registers all the concrete types and interfaces for serialisation

### poolRoutine

* listens to these channels:
  * pool requests blocks from a specific peer by posting to requestsCh, block reactor then sends
    a &bcBlockRequestMessage for a specific height
  * pool signals timeout of a specific peer by posting to timeoutsCh
  * switchToConsensusTicker to periodically try and switch to consensus
  * trySyncTicker to periodically check if we have fallen behind and then catch-up sync
    * if there aren't any new blocks available on the pool it skips syncing
* tries to sync the app by taking downloaded blocks from the pool, gives them to the app and stores
  them on disk
* implements Receive which is called by the switch/peer
  * calls AddBlock on the pool when it receives a new block from a peer

## Block Pool

* responsible for downloading blocks from peers
* makeRequestersRoutine()
  * removes timeout peers
  * starts new requesters by calling makeNextRequester()
* requestRoutine():
  * picks a peer and sends the request, then blocks until:
    * pool is stopped by listening to pool.Quit
    * requester is stopped by listening to Quit
    * request is redone
    * we receive a block
      * gotBlockCh is strange

## Block Store

* persists blocks to disk

# TODO

* How does the switch from bcR to conR happen? Does conR persist blocks to disk too?
* What is the interaction between the consensus and blockchain reactors?