New answers tagged market-microstructure
As you've mentioned, it depends on the trading venue and the exact market data product that you're subscribed to. Unless otherwise stated, the data is usually updated at every occurrence of an event (explains the irregualr intervals), and often, the data is not disseminated immediately and multiple events may be batched in a single message informing you of ...
The adopting release of Reg NMS http://www.sec.gov/rules/final/34-51808.pdf discusses the problem(s) they were looking to solve. That will provide the SEC's thought process.
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