Payment Safety

The core safety rule is simple: transport retry is allowed only while the protocol exchange itself is still controlled; business retry of a financial command is not automatic.

Never blindly retry a financial command

If a payment-like command may have reached the terminal, replaying it can duplicate money movement. Recover the result with sendLastResult().

Financial commands

Treat these as non-idempotent:

  • pay
  • payExtended
  • reverse
  • preAuth
  • incrementalAuth
  • preAuthClosure

Safer commands

Read-only or control commands such as status, totals, sendLastResult, and ECR-printing toggles are safer to run after reconnect, subject to your application flow.

Application policy

  1. Persist a local payment attempt before calling pay.
  2. Store the amount and sale identifier outside the terminal result.
  3. On success, attach stan, onlineId, authCode, and resultCode.
  4. On disconnect, call sendLastResult().
  5. Reconcile unresolved attempts before allowing another charge for the same sale.
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