Real-time balance engine

ID	MOD-003
System	SD01
Repo	bank-core
Build status	Deployed
Deployed	Yes
Last commit	35402a8a7d9c6f1e2b5c8d0e4f7a3b6c9d2e5f8a

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Customer and nostro balances updated synchronously on each ledger posting. Balance available via API within milliseconds of posting.

balance_updated event (v1.1.0)

MOD-003 consumes bank.core.posting_completed (from MOD-001) and re-emits bank.core.balance_updated v1.1.0 on every posting that affects a deposit account. The event carries the full before/after snapshot:

• previous_ledger_balance + ledger_balance — balance before and after the posting
• previous_available_balance + available_balance — available balance before and after
• posting_id — the causal posting reference, used as the idempotency sequence
• trace_id, correlation_id, jurisdiction, effective_at — context fields added in v1.1.0

No producer-side filtering is applied — all deposit account postings trigger an event. Consumers apply EventBridge detail-pattern filters for their specific use case (e.g. MOD-117 filters to accounts with an active overdraft facility).

The event is backwards-compatible with v1.0.0 consumers (MOD-070, MOD-077, MOD-032, MOD-042) — the new fields are additive. See event-catalogue.md §bank.core.balance_updated for the full field spec.

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Module dependencies

Depends on

Module	Title	Required?	Contract	Reason
MOD-001	Double-entry posting engine	Required	—	Derives account balances by aggregating posted ledger entries from the posting engine.
MOD-002	Immutable transaction log	Required	—	Reads the immutable transaction log to compute current balance positions.
MOD-104	AWS shared infrastructure bootstrap	Required	—	AWS shared infrastructure provisioned by MOD-104 (EventBridge buses, S3, KMS, Kinesis, Cognito) is required before this module can be deployed.
MOD-103	Neon database platform bootstrap	Required	—	Neon database and schema provisioned by MOD-103 must exist before this module can read or write Postgres.

Required by

Module	Title	As	Contract
MOD-005	Daily accrual calculator	Hard dependency	—
MOD-020	Pre-payment validation suite	Hard dependency	—
MOD-077	Account dashboard & insight feed	Hard dependency	—
MOD-082	Nostro & FX treasury management	Hard dependency	—
MOD-110	Fee engine	Hard dependency	—
MOD-113	Statement generation	Hard dependency	—
MOD-117	Overdraft management engine	Optional enhancement	—
MOD-120	PayID and Osko integration	Hard dependency	—
MOD-123	ATM network integration	Hard dependency	—
MOD-129	Teller operations and branch cash management	Hard dependency	—
MOD-137	Agency banking adapter	Hard dependency	—
MOD-141	Intra-bank transfer engine	Hard dependency	—
MOD-142	Deposit guarantee scheme disclosure	Hard dependency	—
MOD-159	Synthetic transaction engine	Hard dependency	—

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Policies satisfied

Policy	Title	Mode	How
CLQ-002	Liquidity Risk Management Policy	CALC	Liquidity monitoring uses live balances — no stale data risk in LCR calculation
PAY-001	Payment Operations Policy	GATE	Sufficient funds check uses real-time balance before authorising any payment
CON-005	Fee & Pricing Transparency Policy	AUTO	Customer-visible balance is always accurate and current — no lag between transaction and display
CLQ-004	Interest Rate Risk in the Banking Book (IRRBB) Policy	CALC	IRRBB repricing model uses live rate-sensitive balance positions

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Capabilities satisfied

Capability	Title	Mode	How
CAP-082	Interest offset calculation	CALC	Reads the live deposit balance and subtracts it from the linked loan principal before the accrual calculator runs daily interest.

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Part of SD01 — Core Banking Platform Compiled 2026-05-22 from source/entities/modules/MOD-003.yaml