LOOK-BACK-WINDOW

Look-back window. The retention window during which received-message identifiers are kept to detect duplicates.

Definition

The look-back window is the duration during which an EDI system retains the identifiers of already-received messages (AS2 Message-ID, X12 Interchange Control Number, EDIFACT UNB+5, ebMS3 MessageId) in order to detect and reject duplicates. Typical values: 24h, 7 days, 30 days. Beyond the window, an identical identifier will be processed as a new message, with risk of business duplication.

Origin

The concept is inherent to AS2 and AS4 protocol design: RFC 4130 explicitly mentions the need for a duplicate-detection window to handle client retransmissions. PEPPOL standardised the practice by imposing a 7-day minimum window in its Access Points. The duration choice trades robustness to late retransmissions against identifier-storage cost.

Example in context

A PEPPOL AS4 Access Point typically configures a 14-day look-back window with persistence in an indexed database. If a partner resends a message whose MessageId is already in cache, the AP returns a positive acknowledgement without reprocessing the message: that is idempotency by replay. Beyond 14 days, the partner will need a new MessageId to get the message through — otherwise it will be processed as a new invoice, with a downstream business duplicate.

Related terms

• Idempotency — operational property ensured by the look-back window.
• MDN — AS2 acknowledgement that must be replayed while the window is open.
• Trailing window — related concept for late-arrival tolerance.