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Implement an observable execution policy
A-Book, B-Book and hybrid are descriptions of execution or exposure arrangements, not complete software specifications. Routing technology implements the policy selected by the business. Define which orders can go to external destinations, which exposures are retained and what information is needed to explain an individual routing decision.
Make rules explicit
Document the criteria used by the policy: account group, instrument, session availability, order attributes and operational overrides where relevant. Specify the priority of overlapping rules. A deterministic rule table is easier to review and test than a collection of undocumented exceptions spread across several systems.
Control changes to the policy
Routing changes may affect execution and risk. Keep a versioned baseline, review proposed changes and use a test environment before production activation. Record who approved a change and when it took effect. Maintain a rollback procedure that takes account of open orders and the current state of external sessions.
Handle outages deliberately
Decide what happens when a destination is unavailable, prices are stale or order state is uncertain. A fallback destination is only useful if instrument definitions and accepted workflows are compatible. Automatically resending an order without resolving its earlier state can create unintended execution; recovery must preserve order identity.
Report decisions and outcomes
Capture the selected destination, rule version, relevant session state and the external response. Monitoring should help operators distinguish a routing decision from a connectivity problem. Read the A-Book and B-Book guide for the conceptual distinction, and the liquidity bridge page for the interfaces that carry prices and orders.