Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsUse a stable idempotency key for each intended donation operation, store it in PostgreSQL with a NOT NULL unique constraint, and insert with ON CONFLICT. When a request times out or is retried, reuse the same key and return the existing donation rather than creating another. Treat the payment provider’s request retries and webhook deliveries as separate deduplication boundaries; a database constraint alone cannot make those external operations idempotent.
First decide what counts as the same donation
An idempotency key identifies one intended operation, not one donor or one amount. A donor may give more than once, including the same amount to the same campaign, so donor identity, campaign, amount, or a time window alone will usually collapse valid gifts as well as duplicates.
Generate a fresh, unpredictable key when a donor starts a new intended donation. Reuse it only when retrying that operation. If a key should be unique only within a tenant or account, make the scope explicit with a composite constraint. PostgreSQL’s unique constraints can enforce uniqueness over one or more columns and automatically create a unique B-tree index.
Illustrative table
CREATE TABLE donations (
id bigint GENERATED ALWAYS AS IDENTITY PRIMARY KEY,
account_id bigint NOT NULL,
idempotency_key text NOT NULL,
amount_minor_units bigint NOT NULL CHECK (amount_minor_units > 0),
currency text NOT NULL,
status text NOT NULL,
provider_payment_id text,
created_at timestamptz NOT NULL DEFAULT now(),
UNIQUE (account_id, idempotency_key)
);This example assumes the key is unique within an account. Use UNIQUE (idempotency_key) instead if it must be globally unique. The integer amount is expressed in minor units here, but the correct monetary representation depends on the application’s currency and accounting rules. This is an example operation table, not a complete accounting schema.
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Use the unique constraint in the insert
A preliminary SELECT followed by an INSERT is not a safe duplicate guard by itself. Two concurrent requests can both see no row before either inserts. Let the unique constraint arbitrate the conflict as part of the insert:
INSERT INTO donations (
account_id, idempotency_key, amount_minor_units, currency, status
)
VALUES ($1, $2, $3, $4, 'pending')
ON CONFLICT (account_id, idempotency_key) DO NOTHING
RETURNING id, status;If this returns a row, this request created the local donation record. If it returns no row, retrieve the existing record by (account_id, idempotency_key) and return its current state, subject to authorization checks. PostgreSQL’s RETURNING clause reports rows actually inserted or updated; with DO NOTHING, a conflict skips the insert.
Store enough normalized request data, or a fingerprint of it, to detect reuse of the same key with different meaningful parameters. If the key already exists but the amount, currency, recipient, or other material terms differ, return a clear conflict instead of silently treating the changed request as the original gift. Do not expose or return a donation merely because a caller knows its key; apply the normal authorization rules.
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Choose the conflict action deliberately
ON CONFLICT DO NOTHINGplus retrieval is appropriate when a repeated request should leave the original donation unchanged.ON CONFLICT DO UPDATEis appropriate only when the operation’s defined behavior is to update the existing row. PostgreSQL documents an atomic insert-or-update outcome under concurrency when no independent error occurs.
For a donation retry, overwriting a confirmed amount, recipient, or other settled detail is usually unsafe. Prefer a no-op conflict and comparison unless changing the existing operation is explicitly part of the business rules.
Keep the four idempotency boundaries separate
1. The donor’s application request
Create an unpredictable key once for each intended donation attempt and retain it across retries, including a retry after a timeout. A timeout does not prove that the server failed to commit. Do not derive keys from sensitive personal information. Stripe recommends random keys such as UUID v4 and advises against placing sensitive data in them; the same principle is prudent for an application-generated key.
2. The local PostgreSQL donation row
Persist the application key and enforce its scope with the unique constraint. This is the durable local guard that prevents concurrent or repeated requests from creating multiple rows for one operation. It remains necessary even if the payment provider also supports idempotency.
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3. The payment-provider request
Use the provider’s own idempotency mechanism when creating or updating payment objects. It protects provider-side operations, while the PostgreSQL constraint protects local rows; neither substitutes for the other.
For Stripe, the API documentation says the first status and body are saved after endpoint execution begins, and later calls with the same key return that result, including a saved 500 response. Stripe also compares parameters on key reuse and may prune keys once they are at least 24 hours old. Keep the local operation identity for as long as the application needs to recognize the donation; do not rely on a provider key as a permanent ledger.
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Webhook delivery is a separate retry path: a provider can send an event again even if the donor’s original request was handled correctly. Stripe advises webhook consumers that the same event can arrive more than once and suggests recording processed event IDs. It also notes that separate Event objects can describe duplicate underlying activity; where relevant, use the underlying object ID together with the event type to identify that semantic duplicate.
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Make recording a webhook receipt and applying its local state change one atomic operation, or use a durable processing state with a recovery strategy. The exact transaction or outbox design depends on the application. A receipt table could begin with a uniqueness guard such as:
CREATE TABLE processed_webhook_events (
provider text NOT NULL,
event_id text NOT NULL,
received_at timestamptz NOT NULL DEFAULT now(),
PRIMARY KEY (provider, event_id)
);When processing, insert the event identity with ON CONFLICT DO NOTHING; only the worker that successfully records a new receipt should apply the event’s effect. If a handler can fail after recording the receipt but before completing the state change, add a durable status and retry/recovery path rather than leaving the event permanently marked complete.
What to do when a request fails or repeats
- Timeout after a possible commit: retry with the same application key and the same provider request key. Look up and return the already-persisted operation rather than generating a fresh donation.
- Two simultaneous submissions: allow the unique constraint to select the winner. Handle the conflict by retrieving the existing operation and checking that the retried request matches it.
- Same key, changed terms: reject the request or return a conflict response. Do not alter the original donation as an accidental side effect of key reuse.
- Duplicate webhook delivery: skip an event whose provider event identity has already been recorded; also account for distinct event IDs that can represent the same underlying activity.
Nulls, composite keys, and conditional uniqueness
By default, PostgreSQL treats null values as distinct for unique constraints. A nullable idempotency key can therefore appear in multiple rows without violating uniqueness. If every donation must have an operation key, declare it NOT NULL, as in the example. PostgreSQL also supports UNIQUE NULLS NOT DISTINCT when null values should compare as equal, but a mandatory key is often easier to reason about.
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Use a composite unique constraint when the business scope is a combination, such as account plus key. A partial unique index can enforce uniqueness only for rows meeting a predicate—for example, a genuinely state-defined category of active records—but use one only after accounting for how records enter and leave that category and how historical rows should behave. Neither composite nor partial uniqueness should be based on donor, campaign, or time fields unless that rule preserves legitimate repeat and recurring gifts.
Isolation levels do not replace the key constraint
PostgreSQL 18 documents Read Committed as the default isolation level. Under Read Committed, INSERT ... ON CONFLICT DO UPDATE has an insert-or-update outcome for each proposed row even when a conflicting concurrent transaction was not visible to the statement’s initial snapshot; DO NOTHING can likewise skip an insert because of a concurrent transaction.
Serializable isolation can help with broader invariants involving multiple rows, but it is not a substitute for defining a unique operation key. Serializable transactions can fail and need to be retried; PostgreSQL also notes that an earlier absence check can still be followed by a unique violation when Serializable transactions overlap. If using Serializable, retry serialization failures such as SQLSTATE 40001 and retain the unique constraint for the key invariant.
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