What Happens to a Tick Before a Strategy Sees It

What happens to a tick between the exchange and the moment a strategy is allowed to read it? This note lists those steps, using an E-mini S&P 500 futures update as the public example, and marks which steps are allowed to change the message.

The message is not born on the desk

A tick, in ordinary speech, is one update from a market: a new bid, a new offer, a trade, or a change in size. On the wire it is a message in a feed the exchange defines. The strategy does not read the wire. It reads a record some internal path has decoded, ordered, and sometimes normalized. Between those two objects there is a sequence of steps. If the sequence is invisible, people argue about the market when they are really arguing about the path.

The first steps are mechanical. Bytes are received, framed into messages, and decoded according to the feed's specification. A decoder that guesses, instead of following the specification, will invent fields. Later steps may attach a receipt time, drop a heartbeat, or translate an exchange symbol into an internal symbol. Each of those is a decision. A decision that can change what the strategy sees should be named. A decision that only copies bytes should be boring and tested.

A tick, by the time a strategy sees it, is a decoded and sequenced record, sometimes reduced from the exchange message, with its feed, sequence, and receipt time still attached.

One futures update, followed through

Take a single book update in the E-mini S&P 500 future on CME as the example. The exchange publishes a message that changes the visible price or size at some level of the book. The path receives a packet, pulls out the message, and checks that the sequence number is the next one expected. A gap in sequence numbers is not a quiet moment. It is a hole. The strategy should see a hole, or the path should refuse to present a smooth book that skipped a message. Filling the hole with the previous book hides the failure.

Normalization is the step most likely to change meaning. The exchange may publish implied prices, multiple channels, or a combination of incremental updates and snapshots. An internal record that stores only a top-of-book quote has thrown away depth. That can be the right record for a given consumer. It is a different tick from the message that arrived. The E-mini example keeps the question concrete: if the strategy's record does not state whether it is a raw update or a top-of-book summary, two readers will disagree about what the strategy saw and both will be looking at something real.

The strategy is last, and it is not entitled to every intermediate form. A research process may be allowed to see a normalized record. A component that checks risk may need the raw message and the normalized record both. The path should make that split on purpose. What the strategy sees is a published interface. Changing the interface is a change to the strategy's inputs, even when nobody edited the strategy.

What the path owes the reader

A readable path can answer a short list of questions about the E-mini update. Which feed and which channel did it come from? What sequence number did it carry? What time was stamped at receipt, and by which clock? Was the record a raw message or a summary? Were any messages missing before it? If the path cannot answer, the strategy is reading an anonymous number. Anonymous numbers are how quiet mistakes survive.

None of these steps is a reason to describe the path as quick or slow. Speed is a separate question, and it is not answered by naming the steps. The point of the list is narrower. Before a strategy sees a tick, the tick has been decoded, sequenced, and possibly reduced. Those verbs are the design. The E-mini message is only a way to talk about them without pretending the wire and the strategy are the same place.

This piece does not claim that any feed is complete, that a normalized quote is the exchange's book, or that the path confers an advantage. It lists the steps that sit between a public futures update and the record a strategy is allowed to read.