Store money as integer paise, store rates as integer basis points, round exactly once with a written policy, and record every payment as an append-only ledger row that PostgreSQL itself refuses to update or delete. That combination removes the most common money bugs in a parking app. Those bugs are drifting totals, mismatched receipts and “fixed” history nobody can explain. This guide shows how to build it and where the protection stops. It is an implementation guide, not tax or legal advice. Rounding rules, GST treatment and tariffs depend on your jurisdiction and charge type, so confirm them before you hard-code anything.
Why not floating point
Floating-point values are inexact, and the PostgreSQL manual recommends numeric when you need exact storage and calculation, which includes monetary amounts. Source: PostgreSQL 18, Numeric Types. Never use real, double precision, or a JavaScript or Python float for a balance or a posted charge.
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Integer paise or numeric: choosing the storage type
Both options are exact. They differ in what they can represent and what they cost.
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numeric |
Floating point |
|---|---|---|---|
| Exactness | Exact in whole paise | Exact where possible | Approximate |
| Fractional-paisa intermediates | Cannot represent them without a separate rule | Can represent decimal intermediates | Represents them inexactly |
| Tie rounding in PostgreSQL | Not applicable until you divide | Ties round away from zero (2.5 becomes 3) | Ties typically round to nearest even on most machines (2.5 becomes 2) |
| Speed | Fast | Documented as slower than integer or floating-point arithmetic | Fast |
All rows come from the PostgreSQL numeric types documentation. The manual says only that numeric is slower, with no figure, so measure your own workload before treating that as a reason to avoid it.
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- Essential toll booth reach extender --- struggling to reach the card machine? this telescopic parking rod acts as an extended arm, bridging the gap between your auto window and the payment terminal. it allows your parking pass or credit card swiftly without unbuckling your stretching dangerously, ensuring safe and contactless payment experience every time
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A practical rule: if every posted charge is a whole number of paise, keep posted amounts as bigint paise. They are easy to sum, compare and index. Use numeric for intermediate values that may carry fractions of a paisa. Remember that NUMERIC(p,s) rounds input to its declared scale, so a scale that is too small rounds silently.
Rupees are a display concern. Convert at the edge, at 100 paise per rupee, and never let a formatted string re-enter a calculation.
CREATE TABLE ledger_entry (
entry_id uuid PRIMARY KEY,
parking_session uuid NOT NULL,
payment_attempt uuid,
provider_txn_id text,
account text NOT NULL,
currency char(3) NOT NULL DEFAULT 'INR',
amount_paise bigint NOT NULL,
rate_bps integer,
rounding_policy text,
reverses uuid REFERENCES ledger_entry(entry_id),
created_at timestamptz NOT NULL DEFAULT now()
);The rate_bps and rounding_policy columns keep the inputs and the rule next to the result, so any amount can be recomputed later.
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- Card Holder Stick Clip Opening: Insert the parking card into the clip at the rod end before presenting it to the reader. The clipped card sits away from the hand holding the stick
- Telescoping Card Stick for Reach: Extend the telescoping rod to bring a held card toward a parking terminal. Adjust the rod before positioning the card at the beside the vehicle
- Plastic Card Grabber for Parking Terminals: Direct the held card toward the parking reader with the plastic rod. The card-holding end it away from the hand while the vehicle remains stopped
- Black White Card Holder Stick Pair: One black reacher and one white reacher are included. Keep the two separate when assigning a card-handling stick to each of the vehicles in a household
- Card Stick for Parking Access: Use at a parking entrance or toll-payment point that accepts the held card. The rod presents the card during the stopped vehicle's access transaction
Rates in basis points
A basis point is one hundredth of one percent. Store rates as integers and write the convention down: a stored 100 means 1.00%, and 1800 means 18.00%. The 18 here is only an example of the scale. Nothing in this article asserts which rate applies to any parking fee or tax.
For an amount of A paise and a rate of B basis points, the unrounded result is A × B / 10,000 paise. This is an illustration of the arithmetic, not a mandated tax formula.
A worked example
Take a 4,750 paise fee at 1,850 bps (18.50%, chosen only to show a fraction). The product is 4,750 × 1,850 = 8,787,500. Dividing by 10,000 gives 878.75 paise, so the unrounded charge is a fraction of a paisa. Round-half-up gives 879. Floor gives 878. Pick one, because the answer differs by a paisa and across millions of rows that adds up.
Rank #3
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With integers, do the multiplication first and divide last. In SQL, bigint division truncates, so rounding half-up needs an explicit step:
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SELECT (amount_paise * rate_bps + 5000) / 10000 AS charge_paise;
-- or via numeric, rounding ties away from zero
SELECT round(amount_paise::numeric * rate_bps / 10000) AS charge_paise;Negative values (refunds, reversals) behave differently under truncating integer division, so define the policy for them explicitly, or compute the magnitude and apply the sign afterward.
One rounding boundary
Define exactly one place where fractional paise become whole paise, and keep it out of the display layer. Rules to follow:
Rank #4
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- ☆Thick products, thick base and no serious damage
- ☆The elbow can be formed once without welding, which greatly improves the bearing pressure and impact resistance, ideal for parking and driveways
- ☆Mark your parking spot by leaving it locked to prevent someone else from driving your car and improve property security.
- ☆This wheel locator is suitable for garages, large parking lots or outdoor parking lots to provide adequate protection for your vehicle and your personal safety.
- Compute with exact arithmetic (integers, or
numericfor fractions) all the way to that point. - Round once, per a named policy such as half-up, half-even or floor. Your legal or finance owner should approve it. This article does not establish a required rule.
- Post the rounded result, together with the inputs and the policy name or version.
- Never round, then re-derive and round again, for example when a total is computed from line items and again from the subtotal.
Do not let the application and the database round differently. PostgreSQL’s numeric rounds ties away from zero, while floating-point types typically round ties to nearest even (documented example: 2.5 gives 3 as numeric, 2 as double precision). Some language runtimes default to half-even as well. If two services round the same value, test them against the same fixtures that include exact ties like 0.5, 1.5 and 2.5 paise.
An append-only ledger
Model each accepted event as new rows, never as edits. Each row carries a stable event ID and references the parking session, payment attempt, provider transaction, currency, amount, account and timestamps. For a double-entry design, each transaction posts debits and credits that sum to zero, and you check this inside the same database transaction. A mistake is corrected by posting a reversal (the reverses column above) and then the correct entry. History stays intact, so the correction is itself visible. These are design practices, not legal requirements.
Two further details help in a payments setting:
- Put a unique constraint on the provider transaction ID, scoped appropriately, so a retried webhook cannot post twice.
- Keep current balances as derived values (a view, or a summary table fed by inserts), never as a hand-edited column.
Making PostgreSQL refuse edits
Triggers can fire on INSERT, UPDATE, DELETE and TRUNCATE. They run in the same transaction as the triggering statement, so if the trigger raises an error, the statement’s effects roll back. See Overview of Trigger Behavior and CREATE TRIGGER.
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CREATE FUNCTION ledger_reject_change() RETURNS trigger
LANGUAGE plpgsql AS $$
BEGIN
RAISE EXCEPTION 'ledger_entry is append-only (% blocked)', TG_OP;
END;
$$;
CREATE TRIGGER ledger_no_update_delete
BEFORE UPDATE OR DELETE ON ledger_entry
FOR EACH ROW EXECUTE FUNCTION ledger_reject_change();
CREATE TRIGGER ledger_no_truncate
BEFORE TRUNCATE ON ledger_entry
FOR EACH STATEMENT EXECUTE FUNCTION ledger_reject_change();Row-level triggers do not fire for TRUNCATE, which is why it gets its own statement-level trigger. Test both: attempt an UPDATE, a DELETE and a TRUNCATE as the application role and confirm that each fails and leaves the data unchanged.
Where the guarantee ends
A trigger rejects unauthorized changes. It is not a defense against someone who can drop or disable it, or who owns the table. The following is an architectural inference from how triggers work, not a statement from the PostgreSQL manual. Layer the controls, since each one covers a different failure:
- Privileges: the application role gets
INSERTandSELECTonly, with noUPDATE,DELETEorTRUNCATE. Prefer exposing a narrow insert function over raw table access. - Ownership: a separate migration or owner role holds schema rights, and the application never connects as it.
- Triggers: catch mistakes and bugs from roles that do have write rights, and make intent explicit in the schema.
- Reversals: handle legitimate corrections without rewriting history.
- Audited operational access, backups and reconciliation: detect or recover from privileged mistakes. Reconcile your ledger regularly against payment provider statements.
What to confirm before launch
This design does not settle your operator, payment provider, tariff, tax, jurisdiction or settlement flow. Confirm with the relevant authorities and your provider the applicable tax treatment, the required rounding rule, settlement timing, and where payment data must reside.
Quick Recap
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