developers:fixapi:marketdataincrementalrefresh [Documentation]

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====== MARKET DATA INCREMENTAL REFRESH [35=X] ======

The Market Data Incremental Refresh message provides book updates as changes only, reducing bandwidth compared to full snapshots.

===== Update Actions (MDUpdateAction 279) =====
^ Action ^ Value ^ Description ^
| Add | 0 | Insert new price level, shift others down |
| Change | 1 | Update quantity at level (price unchanged) |
| Delete | 2 | Remove level, shift others up |

===== Message Flow =====
  1. Initial snapshot establishes book state
  2. Incremental updates modify specific levels
  3. Trades sent as separate snapshots
  4. Apply updates sequentially

===== Message Specification =====

**Message Direction:** T4 → Client

^ Tag ^ Field Name ^ Type ^ Req'd ^ Description ^
| | **Message Header** | | Y | MsgType = X |
| 262 | MDReqID | String | O | Original request ID |
| 268 | NoMDEntries | Int | Y | Number of updates |
| →279 | MDUpdateAction | Int | Y | 0=Add, 1=Change, 2=Delete |
| →269 | MDEntryType | Char | Y | 0=Bid, 1=Offer, 2=Implied bid, 3=Implied offer |
| →270 | MDEntryPx | Float | O | Price (for Add action) |
| →271 | MDEntrySize | Int | O | New quantity |
| →1023 | MDPriceLevel | Int | Y | Book level (1-10) |
| →48 | SecurityID | String | O | T4 Market ID (first entry only) |
| →326 | SecurityTradingStatus | Int | O | 2=Open, 5=Closed (first entry) |
| | **Message Trailer** | | Y | |

===== Sample Messages =====

**Change Quantity at Multiple Levels:**
<code>
8=FIX.4.2|9=200|35=X|49=T4|56=T4Example|52=20131125-17:35:57.944|268=4|
279=1|269=0|1023=1|271=287|48=CME_20131200_ESZ3|326=2|
279=1|269=0|1023=8|271=1407|
279=1|269=1|1023=1|271=783|
279=1|269=1|1023=5|271=1759|10=123|
</code>

**Delete Price Level:**
<code>
8=FIX.4.2|9=120|35=X|49=T4|56=T4Example|52=20131125-17:35:58.100|268=1|
279=2|269=0|1023=3|48=CME_20131200_ESZ3|326=2|10=234|
</code>

**Add New Price Level:**
<code>
8=FIX.4.2|9=140|35=X|49=T4|56=T4Example|52=20131125-17:35:58.200|268=1|
279=0|269=1|1023=2|270=180500|271=500|48=CME_20131200_ESZ3|326=2|10=234|
</code>

===== Notes =====
  * Requires SubscriptionRequestType=7 in Market Data Request
  * SecurityID appears only in first update entry
  * Trades always sent as full snapshots (35=W)
  * Delete beyond book depth when Add shifts levels
  * Natural refresh possible without initial snapshot