LEARN. PRACTICE. GROW.Concept and mechanism
The FIX session layer orders messages between peers and supports communication recovery after interruptions. A session can span multiple sequential connections; a new TCP connection does not imply erasing history. If the receiver expects 811 and receives 814, a gap requires treatment under the protocol and session agreement. Changing the counter to remove an alarm does not demonstrate recovery. It also does not prove earlier orders failed. Concrete behavior depends on the engine, version, and agreed configuration. The inspected recovery reference is the June 2020 baseline; the publisher page lists November 2020 errata that must be considered for a real implementation.
Guided application
At the application layer, distinguish the reported event from current state. ExecType identifies the report reason and OrdStatus identifies order state. PendingCancel means the request is pending, not that all executions have stopped. A valid event may arrive during that interval and change the position. For quantities, CumQty accumulates executions and LastQty represents only the latest one; do not subtract both from the total as independent amounts. For an active order without amendments or corrections, 750 minus 280 executed leaves 470. During failover, recover messages and reconcile identifiers and effects with the counterparty before deciding resubmission. These controls are related but answer different questions.
Example: 250 units executed plus 100 during PendingCancel require reconciliation to 350. Support should not create an opposite trade to hide a dashboard error.
Common pitfalls
Reconnection treated as new session; reset treated as reconciliation; pending cancellation treated as terminal; LastQty counted twice.
Related topics: Trade lifecycle and responsibilities · Reference and market data · Post-trade and settlement
Recover sequence and independently establish business state.
Reference: FIX Session Layer overview · DR financial markets technology professional assessment2026.10