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A PostgreSQL transaction can protect related database changes, but it cannot make an external image-generation request part of the same all-or-nothing commit. Treat prompt moderation, generation, and any second generation or edit as recoverable job-state transitions: record the job and the prompt or policy version, check that state before accepting each result, and define separate handling for database conflicts, API failures, and moderation decisions.
What can PostgreSQL protect—and what can’t it protect?
A transaction makes a group of database operations atomic: they commit together or roll back together. PostgreSQL’s transactions tutorial describes this as bundling multiple steps into a “single, all-or-nothing operation.” That guarantee applies to PostgreSQL work, not to a side effect at an external image-generation service.
If an application keeps a transaction open while it calls a generation API, the transaction still cannot roll back an image the provider has already created. A timeout can also leave the application uncertain whether the provider accepted the request. The practical consequence is to keep API calls outside database transactions and represent the workflow as durable states the application can resume or reconcile.
Model the work as a job
Persist a unique job or generation identifier before starting external work. Record the prompt or an immutable reference to it, the applicable policy/version, and each stage’s status. Store provider request identifiers and result references as they become available. Then each short database transaction can make a conditional state change, instead of pretending that the whole workflow is one transaction.
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Make the boundary recoverable
A provider may finish a request just as the application loses its connection or fails to save the result. If the outcome is ambiguous, reconcile using the provider’s request identifier or other documented provider mechanism before submitting again. Do not assume every provider supports idempotency keys or request lookup; verify those capabilities in the provider’s current documentation. This recovery design follows from the separation between the database commit and the external API side effect; it is not a PostgreSQL feature guarantee.
Which PostgreSQL isolation level fits the state checks?
Isolation determines what a transaction can see while other transactions are changing data. In PostgreSQL’s default READ COMMITTED mode, each statement sees data committed before that statement began. A later statement in the same transaction can therefore see newer committed state than an earlier statement. REPEATABLE READ preserves a stable transaction snapshot, while SERIALIZABLE aims to make committed transactions behave as if they ran one at a time and can abort a transaction when concurrent activity creates a serialization conflict.
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|---|---|---|---|
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| REPEATABLE READ | The transaction uses a stable snapshot rather than adopting later commits for its reads. | Reads remain tied to the transaction snapshot. | It does not guarantee serial execution. Handle transaction failures where they can occur and still validate that the application is acting on the intended job state. |
| SERIALIZABLE | PostgreSQL detects execution patterns that could produce a result inconsistent with serial execution. | Successful transactions have serializable behavior. | A transaction may fail with a serialization error. Retry the complete database transaction when appropriate; do not automatically repeat an external generation request as part of that retry. |
These behaviors are documented in the PostgreSQL 18 transaction isolation documentation. The right choice depends on the invariants and contention in the application; SERIALIZABLE is not automatically necessary, and it does not remove the need for a retry path.
Use a conditional transition for each stage
Regardless of isolation level, carry the job identifier and expected state into the write that accepts a stage result. A conditional update can move a job from an expected state to its next state only if the relevant version still matches. Check the affected-row count: if no row changed, another worker may have advanced, cancelled, or superseded the job. Do not mark a stale result as current merely because the provider returned successfully.
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For example, before accepting a generated image, verify that the job remains eligible for that result and that the prompt or policy version used for the request is still the one the application intends to honor. This compare-and-check pattern is an engineering recommendation based on PostgreSQL’s documented visibility rules, not a built-in guarantee that validates application policy.
How should the two generation stages be sequenced?
Keep the same job identity across the initial generation and any follow-up generation or edit. Treat a second stage as a new transition with its own input, status, request identifier, and result. This makes it possible to distinguish an unfinished first request from a failed edit or a result that no longer belongs to the current job version.
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- Create the job: In a short transaction, save the request, the prompt or immutable prompt reference, the policy/version, and an initial status. Assign a stable job identifier.
- Moderate the prompt: Evaluate the input before generation according to application policy. Persist the moderation outcome and move the job forward only if the decision permits it. On moderation-service failure, use an explicit failure or review state rather than treating missing moderation as approval.
- Call the image provider: After the database transition commits, submit the request outside the transaction. Save the provider request identifier and resulting stage status in a separate short transaction.
- Validate before accepting: When a result arrives, conditionally confirm that the job is still in the expected state and that its prompt/policy version is valid. If it has been cancelled or superseded, retain or discard the result according to the application’s data-retention policy, but do not publish it as the current output.
- Moderate and release the result: Apply any required output checks before making the image visible. If a second generation or edit is needed, record it as the next stage and repeat the state-checking pattern.
Make transition handlers idempotent so that processing the same completion event twice does not advance the job twice or publish duplicate results. Keep retry policies separate: a PostgreSQL serialization failure calls for retrying the database transaction; a transient provider error may call for provider-specific retry or reconciliation; a moderation outage needs its own policy; and a content-policy block is a decision outcome, not a transient error to retry unchanged.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Where should moderation happen?
The application must decide which stages need checks and what to do with each outcome. Provider filtering, a separate moderation endpoint, and application-level policy are not interchangeable, and their exact behavior varies by provider. An image-generation workflow may need input review before sending a prompt, output review before release, or both, depending on product policy and provider capabilities.
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| Approach | Stage coverage | Control and decision point | Key handling concern |
|---|---|---|---|
| Provider-side generation filtering | Depends on the provider and the specific generation API; do not assume it covers every stage or returns the same details across vendors. | Filtering is part of the provider workflow. The application still needs to handle blocks and decide whether a result may be released. | Consult that provider’s current documentation for blocked-request signals, output behavior, and retry guidance. |
| Separate moderation call | Can be applied to the prompt, image, or both when the selected service supports those inputs. | The application receives a moderation result and applies its own allow, reject, or review policy. | Handle service failure, latency, and the gap between moderation and subsequent generation or publication. |
| Combined application workflow | Can coordinate input moderation, provider filtering, and output review across the stages the product requires. | Policy decisions are made at explicit job transitions, with results persisted for audit and recovery. | More orchestration is required; ensure no result becomes user-visible before the required checks finish. |
OpenAI as a provider-specific example
OpenAI’s image-generation guide says that prompts and generated images are filtered under its content policy. Its documentation also describes moderation blocks that can identify whether the input or output stage was blocked, and warns against blindly retrying user-correctable image-generation errors without changing the prompt or input. These are OpenAI-specific details, not guarantees about an unnamed or different provider. Model identifiers, parameters, supported models, and error behavior can change, so confirm them against the deployed provider’s current documentation.
OpenAI also documents a separate text-and-image moderation endpoint. Its guidance says moderation scores are signals for an application’s policy, not automatic authorization decisions. The application must define whether a result is rejected, allowed, or routed for review, and what happens when moderation is unavailable. A score alone does not settle that decision.
What changes when the schema or application is changing?
Schema migrations and application deployments can overlap with jobs already in flight. A worker may read a job under one application version, call an external service, and return after another version or migration has changed the job’s state or policy. Use explicit job/version checks at transition time so a late result cannot silently overwrite newer intent.
Protect name resolution and privileges
PostgreSQL warns that schemas writable by untrusted users in search_path can affect name resolution. Use deliberate schema privileges and avoid unsafe writable schemas in the search path used by application roles. Review privileges as part of deployment and migration planning; a transaction isolation setting does not protect against an unsafe privilege configuration.
Do not assume one online-migration recipe fits
The safe procedure and lock impact depend on the exact DDL operation, PostgreSQL major version, deployment topology, and allowed lock time. The PostgreSQL documentation surfaced for these behaviors includes PostgreSQL 18 documentation, while its transactions tutorial is on the PostgreSQL 19 documentation branch. Check the documentation and operational impact for the version actually deployed and the specific migration; there is no universal lock budget or safe migration sequence established here.
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