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• All    - MDEntryID and MDUpdateAction are always provided.
• Delete  - by definition the entry is no longer valid, so there is no need to redeliver the state of the outdated entry.
• New    - all other available fields will always be provided.
• Change - typically, only the fields that have changed will be provided.

MDEntryTypes

The following MDEntryTypes are used to manage the state of the Book:

• Bid, Offer
• ImpliedBid, ImpliedOffer - e.g. for CME - FXSpot+ via ebs_market_mdp3_sbe, ebs_market_mdp3_sbe_udp and FXLink via cme_mdp3_sbe_udp
• EmptyBook

All other MDEntryTypes represent monotonic attributes of the market data feed, for example Trade Ticker events, statistical measures (High, Low, Close etc).

Empty Book Handling

An empty book may occur in one of two ways:

• Explicit publication of an EmptyBook snapshot
• The implicit result of the incremental deletion of the last remaining entry(ies).

When clearing the book, only entries that form the book should be cleared. All other MDEntryTypes should be left alone.

Book Depth

Where clients subscribe for custom book depths against a given Venue, it is important to understand that entries that are pushed below the requested depth by more competitive orders will not necessarily result in an explicit Delete being delivered by the Venue for Whisperer to process.

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