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Why validation belongs before a database write
Validation checks whether incoming data meets the requirements of the application before it is used. Done at a trusted boundary, it lets the receiving component reject malformed or semantically invalid input before that input enters business processing or storage. OWASP recommends not issuing a database command when validation fails: OWASP Secure Database Access guidance.
This applies to data arriving from browser requests, internal APIs, partner integrations, queues, or files. Data does not become trustworthy just because it came through an internal service or transport. Microsoft similarly recommends validating data before it enters a trusted tier and at trust boundaries in multitier systems: Microsoft Learn: SQL injection.
Rejecting a request early gives the application a clear place to return a useful error instead of letting a failed database write—or a later consumer—be the first place the problem appears. It also helps prevent invalid operations and excessive resource consumption. The effect on defects, attacks, or costs depends on the application; the cited guidance does not establish a universal measured reduction.
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Set rules for each field and operation. OWASP recommends checking both syntax—the shape of a value—and semantics—whether it makes sense for the intended operation. Use explicit rules rather than a vague test for whether input “looks safe.”
Field-level rules
- Type and format: Parse values as the types the application expects, and check formats where required.
- Presence and nullability: Decide whether a field may be omitted or set to null; do not treat those states as interchangeable by accident.
- Length and structure: Set minimum and maximum lengths and check the structure of strings, arrays, and nested objects.
- Allowed values and ranges: Use an allowlist for practical choices, and enforce numeric or date bounds that fit the operation.
- Nested content: Validate each item in an object or array, not only the outer container.
Relationships and operation context
Values can be individually valid yet invalid together. For example, a booking end date must follow its start date. Rules may also depend on the operation or current workflow state, so validate the combination in the context where it will be used.
Prefer defining what is accepted over trying to enumerate every suspicious string. Rejecting apostrophes, for instance, can block legitimate names and does not make SQL safe. Apply request-size and parser limits before buffering or parsing large input, parse safely, and validate the representation the application will actually use. When any required check fails, stop the write; return a clear error without exposing sensitive implementation details.
Use client, server, and database checks for different jobs
| Layer | Authority and timing | Best fit |
|---|---|---|
| Browser or client | Convenient feedback before submission; callers can bypass it. | Help people correct form errors quickly, while treating the result as untrusted. |
| Server or receiving service | Trusted enforcement when data enters that component, before business processing and writes. | Check request shape, operation-specific requirements, and context-aware rules; reject invalid input with a useful response. |
| Database | Enforcement at persistence time for every write path that reaches the database. | Protect durable structural invariants such as required values, uniqueness, keys, and relationships. |
Client-side checks improve usability, but a caller can send a request without using your form or alter the request before it arrives. Apply authoritative validation in the trusted server or receiving component. Microsoft’s guidance states: “In multitiered environments, all data should be validated before admission to the trusted zone.”
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Keep database constraints for rules that must remain true regardless of which application path performs a write. PostgreSQL 18 documents CHECK, NOT NULL, UNIQUE, primary key, and foreign key constraints; a write that violates a constraint raises an error: PostgreSQL 18: Constraints. The application can validate earlier and explain errors in context, while the database protects structural integrity at persistence time. Keep the corresponding rules aligned so those layers do not contradict each other.
What validation does not replace
Parameterized SQL
A value that passed validation is not safe to concatenate into a SQL statement. OWASP recommends parameterized queries as the primary defense against SQL injection. Bind values as parameters; validation can be an additional check, especially for query elements such as identifiers that cannot be bound as values: OWASP SQL Injection Prevention Cheat Sheet.
Authorization
A well-formed account ID does not prove that the caller is allowed to access that account. Check permissions for the authenticated caller and the requested resource separately from checking the input’s type or format.
Output encoding
Validating data on receipt does not determine how it must be safely rendered later. Encode output appropriately for its destination when displaying stored or submitted content.
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Business logic
Format checks alone cannot establish that an action is legitimate in the workflow. A client-submitted price may be correctly formatted but still untrusted, and a valid request must not be allowed to skip a required transaction step. Enforce workflow and business rules in the relevant trusted logic.
Quick Recap
Pre-write validation checklist
- Validate every input source at the receiving component’s trust boundary—not only browser forms.
- Define field-level and relationship rules for each operation, including null behavior and allowed ranges or values.
- Apply size and parser limits before processing input, and validate the representation the application will use.
- Stop processing and do not issue the database command when required validation fails.
- Use parameterized queries, separate authorization checks, and encode output for its context.
- Keep database constraints for invariants that must hold across write paths, and provide callers a clear error without leaking sensitive details.
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