add docs

docs/binance/binance-spot/testnet_websocket-api_data-sources.md

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+---
+title: "Data sources | Binance Open Platform"
+source: "https://developers.binance.com/docs/binance-spot-api-docs/testnet/websocket-api/data-sources"
+fetched_at: "2026-02-26T10:38:11.308Z"
+---
+-   The API system is asynchronous. Some delay in the response is normal and expected.
+    
+-   Each method has a data source indicating where the data is coming from, and thus how up-to-date it is.
+    
+
+Data Source
+
+Latency
+
+Description
+
+Matching Engine
+
+lowest
+
+The Matching Engine produces the response directly
+
+Memory
+
+low
+
+Data is fetched from API server's local or external memory cache
+
+Database
+
+moderate
+
+Data is retrieved from the database
+
+-   Some methods have more than one data source (e.g., Memory => Database).
+    
+    This means that the API will look for the latest data in that order: first in the cache, then in the database.