FieldEnty order guarantees

dzmitry_shylovich
dzmitry_shylovich
in EMA


Is there any guarantees about field entries order?

Given response

    FieldList
    FieldEntry fid="25" name="ASK" dataType="Real" value="67.7"
    FieldEntry fid="31" name="ASKSIZE" dataType="Real" value="200.0"
    FieldEntry fid="3435" name="MMID" dataType="Rmtes" value="ARCA"
    FieldEntry fid="8464" name="ASK_TONE1" dataType="Rmtes" value="A"

Can I be 100% sure that 31 always comes after 25 or response can contain entries without any order?

   FieldList
   FieldEntry fid="8464" name="ASK_TONE1" dataType="Rmtes" value="A"
   FieldEntry fid="31" name="ASKSIZE" dataType="Real" value="200.0"
   FieldEntry fid="25" name="ASK" dataType="Real" value="67.7"
   FieldEntry fid="3435" name="MMID" dataType="Rmtes" value="ARCA"

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Best Answer

  • umer.nalla
    umer.nalla
    Answer ✓

    I have never an read or come across mention of a guarantee about field order entries.

    I suspect it may depend on the behaviour of the provider you are connected to (remember that if your consumer app connects to the ADS, then the ADS is provider as far as your consumer is concerned)

    I tried a quick test here with a Provider (connected to an ADH), using a simple record with 3 FIDs - publishing the initial image and then sending out an update.

    I intentionally added the FIDs to the msg Payload in the following order:

    • 22 (BID),
    • 7 (TRDPRC_2)
    • 8 (TRDPRC_3).

    The consumer connected to an ADS received the initial Image with the data sorted by FID :

    FieldList: standard data count = 3
            FieldEntry [     7] TRDPRC_2    120
            FieldEntry [     8] TRDPRC_3    310
            FieldEntry [    22] BID 250

    But the Update was received in the order as published 

    FieldList: standard data count = 3
            FieldEntry [    22] BID 260
            FieldEntry [     7] TRDPRC_2    130
            FieldEntry [     8] TRDPRC_3    320

    Therefore, I think it would be safer not to rely on a guaranteed order.

Answers

  • Steven McCoy
    Steven McCoy

    A payload caching device such as TREP or the RSSL Value Add Cache will re-order the Field List to its internal optimal ordering strategy. This means with an item stream you may even see different orders between the initial refresh and subsequent updates, for example:

    Received Source Directory Update

    MSFT.O
    DOMAIN: RSSL_DMT_MARKET_PRICE
    State: Open/Ok/None - text: ""
            RDNDISPLAY          100
            TRDPRC_1            11

    MSFT.O
    DOMAIN: RSSL_DMT_MARKET_PRICE
    State: Open/Ok/None - text: ""
            TRDPRC_1            12
            RDNDISPLAY          100


    MSFT.O
    DOMAIN: RSSL_DMT_MARKET_PRICE
    State: Open/Ok/None - text: ""
            TRDPRC_1            13
            RDNDISPLAY          100

    MSFT.O
    DOMAIN: RSSL_DMT_MARKET_PRICE
    State: Open/Ok/None - text: ""
            TRDPRC_1            14
            RDNDISPLAY          100
  • brian.sandri
    brian.sandri

    I agree with the above answers. Per OMM specifications, the order of field entries is not guaranteed and you should not expect it to come in the order of the field IDs. The only container type that supports strict ordering is the OMM Vector, and even then the application is responsible for ensuring the order; the actions on each vector entry provide the details needed to order.

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