External Host Interface Integration Guide
3. Client System Expectations
Your system should be prepared to receive and process the full EHI payload, including transaction, merchant, balance, fee, network, tokenization, authentication, matching, and lifecycle fields included in the message.
3.1 Client implementation expectations
Your system should process EHI messages using the field names and transaction logic.
This means your system should:
| Requirement | Description |
|---|---|
| Preserve field sensitivity | Treat EHI field names and casing as significant. |
| Handle full payloads | Expect large messages with many fields, including empty elements. |
| Ignore unexpected fields safely | Do not fail only because an additional field is present. |
| Use original transaction identifiers | Use EHI identifiers such as Txn_ID, TXn_ID, traceid_lifecycle, Traceid_Message, Trans_link, Ret_Ref_No_DE37, and Auth_Code_DE38 as applicable. |
| Apply EHI matching logic | Match related messages using the criteria defined for each transaction type. |
| Return the required response | Return the correct authorization response, acknowledgement, or cut-off result based on the message type. |
| Meet the response timeout requirement | Return the required response within the 500 ms timeout window. |
Your system should ignore unexpected fields safely instead of treating them as errors, because additional fields or values may be introduced across EHI versions.
3.2 Client maintains balances
In this model, your system maintains the cardholder balance and makes authorization decisions for applicable transactions.
Your system is responsible for determining whether a transaction should be approved, declined, or partially approved based on your balance logic, account status, transaction rules, fraud controls, and reconciliation requirements.
Your system should calculate the total transaction impact, including billing amount, fees, pads, reversals, and any applicable adjustments, before returning an authorization response.
EHI Model
Client maintains balances
Processor
Payblr
Client
Card Balance Database
Next steps
Continue to How Integration Works.