FIX Market Data: Updates Change a Book Instead of Adding Orders

Establish whose book and which view the input represents

The named reference is FIX Trading Community's Recommended Practices for Book Management, version 2.0, revised January 2007. It targets FIX 5.0 and discusses applicability to earlier versions. These historical recommendations explain one book-management model; an actual feed needs its own version and vendor or venue conventions established.

The document calls the disseminator the Vendor and the recipient recreating the book the Client. Start by identifying the vendor's supplied view. Receiving its entries does not establish a consolidated market or the recipient's own orders.

Top-of-book represents the best bid and offer. Price-depth aggregates displayed quantity by price and side. Order-depth retains individual entries at a price level. The FIX 5.0 MDBookType(1021) definition distinguishes these three book types.

A price-depth row can combine several orders. An order-depth entry provides finer detail, but the supplied depth still limits the view. Match that representation to the intended AI input before treating a row count as an order count.

References: [FIX45][FIXBOOK]

A snapshot describes state; incremental instructions maintain it

A snapshot supplies the selected book at an observation point. Incremental Refresh supplies instructions that alter an existing book. Within this recommended model, an instruction can introduce an entry, change an entry or remove an entry. A message is an envelope that can contain several instructions.

The distinction matters at the input boundary. A historical instruction record can remain in a dataset after its effect has been applied. Its presence in that history does not make it another current entry. A snapshot and the later instructions therefore cannot be concatenated and labeled current orders.

MDEntryID(278) supplies market-data entry identity. In the recommendation's order-depth model, the vendor assigns that reference, and Change or Delete identifies the entry it concerns. It is distinct from ClOrdID, the private reference assigned when a client enters an order. The illustrated identifiers below locate entries in the selected vendor book; they identify no customer, account or private order owner.

References: [FIX45][FIXENTRY]

MD-45 begins with three bid entries and 60 displayed units

MD-45 is an original fictional case for one vendor, one instrument and the bid side of an order-depth view. The complete initial view contains E1 with quantity 10 at price 100, E2 with quantity 20 at price 99, and E3 with quantity 30 at price 98.

All quantities use the same abstract displayed unit. Prices are invented quoted-price units, with no currency, instrument, tick size or monetary value implied. The initial entry count is 3, and the displayed quantity is 10 + 20 + 30 = 60 units.

Exactly three subsequent instructions are supplied in order: change E1's quantity to 6; delete E2; introduce E4 with quantity 8 at price 97. They constitute the complete subsequent activity for this selected view. There is no price-changing cancel/replace, priority movement, missing or duplicate update, or depth truncation. These are conceptual descriptions of data, rather than valid FIX messages or actions to place or cancel orders.

The endpoint means immediately after the third instruction. No ask-side entries are supplied, so this case establishes neither a two-sided book nor a spread.

Three instructions leave E1, E3 and E4 totaling 44 units

The Change to E1 replaces its displayed quantity of 10 with 6. E1 remains the same entry at price 100. The selected view now contains E1, E2 and E3, with displayed quantity 6 + 20 + 30 = 56 units. Adding 6 to the old 10 would give E1 a quantity of 16 and misinterpret the supplied replacement value.

The Delete removes E2 and its 20 displayed units from the current view. E1 and E3 remain, totaling 6 + 30 = 36 units. The historical record describing E2 can still be retained, but E2 is no longer a current entry.

The New introduces E4 with 8 displayed units at price 97. The endpoint contains E1 at 100 with 6, E3 at 98 with 30, and E4 at 97 with 8. Its entry count is 3 and its displayed quantity is 6 + 30 + 8 = 44 units.

The original and final counts are equal even though membership and quantity changed. E2 left the view, E4 entered it, and E1's quantity fell. A count-only comparison would miss all three differences.

Six conceptual records are not six current orders

The supplied material contains three initial snapshot-entry records and three subsequent instruction records. Six is a count of those conceptual records. It is neither the endpoint entry count nor an established message count. The case does not specify how the instructions were packaged into messages.

Of the subsequent instructions, only the New introduces an additional entry into the selected view. The Change acts on E1, and the Delete removes E2. Counting every instruction as another active order would preserve obsolete state and add instructions to the population they are supposed to maintain.

The endpoint also cannot be obtained by keeping only rows labeled New: E1 and E3 were present in the initial snapshot and remain current. The useful population depends on the starting state and the subsequent meanings together.

For a state-based AI input, the correct endpoint in MD-45 is the annotated E1/E3/E4 population. For an instruction-history input, the three actions may be useful observations in their own right. Those tasks require different row meanings, even when they use the same underlying material.

The 16-unit decrease leaves execution unresolved

MD-45's displayed quantity falls from 60 to 44, a net decrease of 16 units. That difference reconciles as a 4-unit reduction at E1, removal of E2's 20 units and addition of E4's 8 units: 60 - 4 - 20 + 8 = 44.

Those changes describe the displayed book. The case supplies no cause for E1's reduction or E2's removal, and no trade observations. It supports neither a claim of three fills nor a claim that 16 units executed. An unchanged entry count likewise establishes no absence of activity.

In the named recommendation, trades are published through separate instructions from the book changes resulting from them. A trade instruction does not itself maintain the book in that model. This separation must not be generalized into a promise about every venue's feed.

References: [FIX45]

Compare offers by the population reaching the consumer

Suppose Offer A supplies MD-45's endpoint as E1/E3/E4 with their prices and current quantities, identifies the selected vendor, instrument, side and order-depth view, and explains the snapshot and update meanings behind that endpoint. Offer B supplies all six conceptual records and labels them six active orders, or labels the three updates three fills. These are fictional offers for the same complete case.

A supports the requested endpoint interpretation. B's labels fail it: the Change is not a new current entry, the Delete does not remain in the current population, and the supplied instructions establish no executions. Additional rows cannot repair those meanings.

Have the supplier explain the representation actually reaching the intended AI consumer. For this case, it should account for E1 at 6, E2 absent, E3 at 30 and E4 at 8, together with the 44-unit total. An annotation held upstream is useful only if the consumer receives it or can resolve the supplied reference. A generic quantity column leaves open whether its value is a current displayed size, a change amount or an execution amount.

Choose the observation requirement as well. A final snapshot can support the endpoint population without showing how it arose. An instruction history can retain the path without being a ready current-state input. If the intended task needs both, the offered representation must preserve both meanings and their relationship. MD-45 deliberately stipulates complete, ordered input; real delivery continuity and recovery require separate evidence and are outside this example.

Before accepting a real population, establish the actual instrument, price and quantity units, vendor or venue coverage, aggregation, available depth and observation point. The 44 abstract units describe only MD-45's selected visible bids. They supply no whole-market quantity, trading recommendation, predictive-return claim or measured model result.

The related execution-report guide addresses a participant order's lifecycle and executed quantity. The XBRL guide addresses financial facts' periods and units. Neither supplies the market-data book population examined here.

Connect a separate equipment requirement to physical deployment

If the wider project also needs customer-selected equipment received, integrated into racks, installed onsite or connected through IT cabling, Leviathan's Complete Physical Deployment service and project inquiry below provide a route to discuss those agreed physical stages. Share a nonconfidential equipment, site and connection outline to discuss scope, requirements and availability.

The physical inquiry concerns installation and handoff. Market-data supply, FIX integration and AI software remain with the appointed data and software providers.

References: [L1]