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spec/blocksync/data_structures.md

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+### Data structures
+
+These are the core data structures necessary to provide the Blocksync Reactor logic.
+
+The requester data structure is used to track the assignment of a request for a `block` at position `height` to a peer whose id equals to `peerID`.
+
+```go
+type bpRequester {
+  mtx          Mutex
+  block        *types.Block
+  height       int64
+  peerID       types.nodeID
+  redoChannel  chan type.nodeID //redo may send multi-time; peerId is used to identify repeat
+  goBlockCh chan struct{}{}
+}
+```
+Pool is a core data structure that stores last executed block (`height`), assignment of requests to peers (`requesters`), current height for each peer and number of pending requests for each peer (`peers`), maximum peer height, etc.
+
+```go
+type BlockPool {
+  mtx                Mutex
+  requesters         map[int64]*bpRequester
+  height             int64
+  peers              map[p2p.ID]*bpPeer
+  maxPeerHeight      int64
+  numPending         int32
+  store              BlockStore
+  requestsChannel    chan<- BlockRequest
+  errorsChannel      chan<- peerError
+}
+```
+
+The `Peer` data structure stores for each peer its current `height` and number of pending requests sent to the peer (`numPending`), etc.
+
+```go
+type bpPeer struct {
+  id           p2p.ID
+  height       int64
+  base         int64
+  numPending   int32
+  timeout      *time.Timer
+  didTimeout   bool
+  pool          *blockPool
+}
+```
+
+BlockRequest is an internal data structure used to denote current mapping of request for a block at some `height` to a peer (`PeerID`).
+
+```go
+type BlockRequest {
+  Height int64
+  PeerID p2p.ID
+}
+```
+