add document for the backtest engine

pkg/backtest/exchange.go

@@ -1,3 +1,30 @@
+/*
+The backtest process
+
+The backtest engine loads the klines from the database into a kline-channel,
+there are multiple matching engine that matches the order sent from the strategy.
+
+for each kline, the backtest engine:
+
+1) load the kline, run matching logics to send out order update and trades to the user data stream.
+2) once the matching process for the kline is done, the kline will be pushed to the market data stream.
+3) go to 1 and load the next kline.
+
+There are 2 ways that a strategy could work with backtest engine:
+
+1. the strategy receives kline from the market data stream, and then it submits the order by the given market data to the backtest engine.
+   backtest engine receives the order and then pushes the trade and order updates to the user data stream.
+
+   the strategy receives the trade and update its position.
+
+2. the strategy places the orders when it starts. (like grid) the strategy then receives the order updates and then submit a new order
+   by its order update message.
+
+We need to ensure that:
+
+1. if the strategy submits the order from the market data stream, since it's a separate goroutine, the strategy should block the backtest engine
+   to process the trades before the next kline is published.
+*/
 package backtest
 
 import (

pkg/backtest/stream.go

@@ -93,6 +93,7 @@ func (s *Stream) Connect(ctx context.Context) error {
 						continue
 					}
 
+					// here we generate trades and order updates
 					matching.processKLine(k)
 					numKlines++
 				}