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Choose a mechanism for the invariant
Start by describing what must remain true when requests or workers run at the same time. The right protection depends on whether the race concerns one value, one entity, a set of known rows, or a broader condition.
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| Race or invariant | Starting point | What to account for |
|---|---|---|
| A value must be unique, even when another process writes it | PostgreSQL unique constraint or unique index | The database rejects the losing write; translate that conflict into an appropriate application response. Validation can improve the usual user experience but is not enforcement. Symfony’s UniqueEntity documentation |
| A user may submit an entity changed since they first loaded it | Doctrine optimistic locking with a version field | Check the version the client originally saw; handle the conflict rather than silently applying stale data. Doctrine’s Transactions and Concurrency documentation |
| A short operation must exclude competing changes to known rows | Doctrine pessimistic locking within an explicit transaction | Other operations may block; lock only the necessary rows and keep the transaction short. Doctrine’s Transactions and Concurrency documentation |
| A rule depends on a predicate, range, or broader read set | Review the database constraint and isolation design; consider PostgreSQL Serializable transactions | Serializable transactions can be aborted and require retrying the complete transaction. PostgreSQL’s Transaction Isolation documentation |
| Workers need exclusive access to an application-level named resource | Symfony Lock’s PostgreSQL advisory-lock store may fit | Its lock lifecycle depends on the database session and connection; it is not a replacement for data-integrity constraints. Symfony’s Lock documentation |
Doctrine ORM supports both pessimistic and optimistic locking, but neither is a universal substitute for identifying what the business rule actually protects. Locking one row, for example, does not automatically protect a condition about rows that do not yet exist.
Prevent duplicate records with a database constraint
Why validation alone loses a race
UniqueEntity checks whether a matching value exists when validation runs. Two requests can both pass that check before either inserts its row. A similar race can involve a command, worker, or another application that does not run Symfony validation. Symfony explicitly warns that the constraint “doesn’t provide any protection against race conditions.” Symfony: UniqueEntity
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Enforce uniqueness in PostgreSQL
Add a PostgreSQL unique constraint (or a unique index) over the column or columns that define uniqueness in the domain. For example, if an email must be unique within a tenant, the invariant is normally about the pair of tenant and email, not email alone:
ALTER TABLE account
ADD CONSTRAINT account_tenant_email_unique
UNIQUE (tenant_id, email);Choose the columns and any normalization rules to match the actual business rule. If existing data violates the constraint, PostgreSQL will not be able to add it until those conflicts are resolved. Keep UniqueEntity if it provides useful form feedback, but treat the database constraint as the final authority.
Translate the losing write into a useful result
When concurrent inserts target the same unique key, PostgreSQL accepts one and rejects the conflicting write. Catch the expected database conflict at the application boundary and map it to a domain-appropriate outcome, such as explaining that the account already exists. PostgreSQL reports unique-violation SQLSTATE 23505; where the driver and exception chain expose it, check the violated constraint name too, so unrelated integrity errors are not mistaken for this case. Exception-wrapping details differ across Doctrine DBAL versions, so verify the handling against the installed stack.
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Detect stale edits with optimistic locking
Optimistic locking is suited to edits that span multiple requests: a user loads an entity, spends time editing it, and submits after someone else has changed it. Doctrine stores a version for the entity and compares it with the database version when the change is persisted. A mismatch raises OptimisticLockException. Doctrine supports integer and datetime version fields; an integer version is preferable where timestamp resolution could allow collisions. Doctrine: Transactions and Concurrency
The important part is checking the version the user actually saw. Include that version with the submitted form or command and use it as the expected version when applying the update. Fetching a fresh entity at submission time and blindly applying old form data can erase the evidence that the user edited a stale copy.
- On a version conflict, explain that the record changed and let the user reload or reconcile their edits.
- If the operation can safely be repeated, reload and redo the decision-making work in a new transaction; do not simply retry the same stale write.
Use pessimistic locks for short, contested operations
When an operation must temporarily exclude competing changes to known rows, Doctrine pessimistic locking uses database-level row locks. Doctrine requires an active transaction for pessimistic locking. PESSIMISTIC_WRITE protects the underlying rows against concurrent read/write operations; PESSIMISTIC_READ blocks concurrent requests attempting update or write-mode locking. Confirm that the query locks every row the invariant depends on, and account for blocking and deadlocks. Doctrine: Transactions and Concurrency
Keep the critical section narrow. Do not hold a database lock while waiting for user input, making network calls, or doing unrelated work: those delays keep other transactions waiting and increase the chance of contention.
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Doctrine’s UnitOfWork queues ORM changes until flush(); its ORM write set is handled transactionally. That implicit behavior may be enough for a straightforward group of ORM writes. Explicitly demarcate a transaction when the business operation also includes custom DBAL work or requires an API such as pessimistic locking that needs an active transaction. Doctrine ORM: Architecture Doctrine ORM: Transactions and Concurrency
Doctrine documents transaction helpers including Connection#transactional() and EntityManager#wrapInTransaction(), which manage commit and rollback around work. Keep the reads that inform a decision and the writes that enact it inside the same relevant transaction. The exact API surface can vary with ORM and DBAL versions; use the documentation matching the installed versions. Doctrine DBAL: Transactions
// Illustrative only: verify method signatures for your Doctrine version.
$connection->transactional(function ($connection) use ($entityManager) {
// Read the state needed for the decision.
// Apply the write(s) that preserve the invariant.
$entityManager->flush();
});A transaction makes its operations atomic, but it does not by itself protect every invariant from concurrent transactions. The isolation level and the rows or predicates involved still matter.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Protect rules that depend on multiple rows or predicates
A row lock cannot lock a matching row that does not exist. This matters for rules such as “there must be fewer than five active reservations” or “no overlapping booking exists.” Two transactions may both observe that a condition is currently true and then each make a change that violates it.
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Where possible, encode the invariant as a database constraint. If the rule cannot be expressed that way, assess the complete read set and transaction isolation needed. PostgreSQL’s default isolation level is Read Committed: each statement sees a snapshot from the start of that statement, so two successive selects in one transaction can see different committed data. PostgreSQL 18: Transaction Isolation
Serializable isolation offers PostgreSQL’s strictest isolation behavior, but it does not mean every transaction will commit. PostgreSQL can abort one with a serialization failure when concurrent work cannot be represented as a serial execution. Retry the whole transaction in that case: repeat the reads, decision-making, and writes in a new transaction. Do not rely on reads from an aborted transaction as valid results. PostgreSQL 18: Transaction Isolation
Retry conflicts without repeating unsafe side effects
A retry is safe only if it reruns the work needed to make a correct decision. For an optimistic-lock conflict, that generally means reloading current state and asking the user to reconcile or repeating a repeatable operation against the new state. For a PostgreSQL serialization failure, it means rerunning the entire transaction, not just the final write.
Keep non-database side effects—such as sending a message or charging an external service—out of a retryable transaction unless they are made idempotent or otherwise coordinated. A transaction rollback cannot undo an action performed by another system. Identify the specific retryable failure rather than treating every database exception as transient. DBAL provides transaction and isolation controls, but choosing an isolation level does not remove the need to design failure handling for the actual invariant. Doctrine DBAL: Transactions
Use advisory locks only for the coordination they provide
Symfony Lock includes PostgreSQL advisory-lock stores, including one backed by Doctrine DBAL. They can coordinate workers around a named application resource when the database session and connection lifecycle are acceptable. Symfony’s documentation says these locks are released when the session ends and may be lost if PostgreSQL restarts or the TCP connection drops. They therefore should not stand in for unique constraints, transactions, or other database protections for persisted data. Symfony: Dealing with Concurrency with Locks
Account for Doctrine read replicas
If Doctrine DBAL is configured with read replicas, its wrapper routes according to the DBAL method called: reads go to replicas, while writes and transactions go to the primary. The documented connection-level routing behavior also matters when choosing how to perform a read. Do not make an invariant-critical concurrency decision from a potentially stale replica read when the decision needs current primary state. Symfony: How to Use Doctrine DBAL
Check the deployed stack before relying on an API detail
The Symfony and Doctrine references linked here are their current documentation, while the PostgreSQL isolation link is the PostgreSQL 18 documentation. Installed Symfony, ORM, DBAL, and PostgreSQL major versions may differ; confirm lock-mode names, transaction methods, exception handling, and isolation configuration against the deployed versions. The key design choice remains invariant-specific: database constraints for integrity rules that can be expressed as constraints, and locking or isolation with explicit conflict handling for the races they are meant to address.
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