Sequence Reset

8=FIX.4.2
9=99
34=1031
35=4
36=1034
43=Y
49=DWFIX01
52=20160201-22:36:16.853
56=TEST1
122=20160201-22:36:16
123=Y
10=098

How is a Sequence Reset <4> message used?

A Sequence Reset <4> is used by the sending application to reset the incoming sequence number on the opposing side. The Sequence Reset <4> consists of two (2) modes; Gap Fill and Gap Reset:
Mode 1: Gap Fill

• Sequence Reset <4>-Gap Fill when Y = GapFillFlag <123>

Mode 2: Gap Reset

• Sequence Reset <4>-Rest when N or Not Present = GapFillFlag <123>

Note: The Sequence Reset <4>-Reset mode should ONLY be used to recover from a disaster situation which cannot be otherwise recovered via "Gap Fill" mode.

When can Sequence Reset <4> message be used?

The Sequence Reset <4> message can be used in the following situations:

• Scenario 1: During normal resend processing, the sending application may choose not to
  send a message (e.g. an aged order). The Sequence Reset <4> - Gap Fill is used to mark the place of that message.

• Scenario 2: During normal resend processing, a number of administrative messages are
  not resent, the Sequence Reset <4> - Gap Fill message is used to fill the sequence gap created.

In the event of an application failure, it may be necessary to force synchronization of sequence numbers on the sending and receiving sides via the use of Sequence Reset <4> - Reset.

The sending application will initiate the sequence reset. The message in all situations specifies NewSeqNo <36> to reset as the value of the next sequence number immediately following the messages and/or sequence numbers being skipped.

Missing the GapFillFlag <123> message.

If the GapFillFlag <123> field is not present (or set to N), it can be assumed that the purpose of the Sequence Reset <4> message is to recover from an out-of-sequence condition.

The MsgSeqNum <34> in the header should be ignored (i.e. the receipt of a Sequence Reset <4> - Reset message with an out of sequence MsgSeqNum <34> should not generate resend requests).

The Sequence Reset <4> - Reset should NOT be used as a normal response to a Resend Request <2> (use Sequence Reset <4> - Gap Fill).

The Sequence Reset <4> - Reset should ONLY be used to recover from a disaster situation which cannot be recovered via the use of Sequence Reset <4> - Gap Fill.

Note: The use of Sequence Reset <4> - Reset may result in the possibility of lost messages.

Sequence Reset

The sequence reset can only increase the sequence number. If a sequence reset is received attempting to decrease the next expected sequence number, the message should be rejected and treated as a serious error.

Multiple ResendRequests

It is possible to have multiple ResendRequests issued in a row (i.e. 5 to 10 followed by 5 to 11).

If sequence number 8, 10, and 11 represent application messages while the 5-7 and 9 represent administrative messages, the series of messages as result of the Resend Request <2> may appear as the following:

• SeqReset-GapFill with NewSeqNo <36> of 8
• Message 8
• SeqReset-GapFill with NewSeqNo <36> of 10
• Message 10

This could then be followed by the following:

• SeqReset-GapFill with NewSeqNo <36> of 8
• Message 8
• SeqReset-GapFill with NewSeqNo <36> of 10
• Message 10
• Message 11

One must be careful to ignore the duplicate SeqReset-GapFill which is attempting to lower the next expected sequence number. This can be detected by checking to see if its MsgSeqNum <34> is less than expected. If so, the SeqReset-GapFill is a duplicate and should be discarded.