To diagnose a PostgreSQL deadlock in a Spring Boot app, start with PostgreSQL’s complete deadlock report, identify the backend processes and statements involved, then match those details to the application’s request and transaction path. A request that appears stuck can also be waiting for a connection from an exhausted pool; that is a different failure and requires different evidence.
Start with PostgreSQL’s deadlock report
For a deadlock that PostgreSQL has already detected, the server log is the durable record. An application exception may tell you that a transaction failed, but it often does not preserve the full details needed to determine which sessions and statements formed the cycle. Ask for the complete server-log entry around the failure, including all deadlock detail and context lines.
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Preserve the timestamp, PostgreSQL process IDs, database and user, and the SQL statements in the report. Those fields let you connect the database event to application logs and transaction traces. A useful log_line_prefix can include:
%mfor the timestamp;%pfor the process ID;%afor the application name;%ufor the user;%dfor the database; and%efor the SQLSTATE.
Whether you can change this setting depends on your permissions and deployment. Check your PostgreSQL version and managed-service logging controls. PostgreSQL documents these logging options in its Error Reporting and Logging reference.
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Enable evidence for recurring lock waits
If the incident recurs, PostgreSQL can log lock waits that last longer than deadlock_timeout when log_lock_waits is enabled. That setting is off by default. deadlock_timeout also determines when PostgreSQL checks a lock wait for a deadlock; its documented default is one second. It is a detection and logging threshold, not a fix for conflicting lock order or excessive contention.
For a targeted investigation, a shorter threshold may make lock-wait messages appear sooner, but account for server permissions, the deployed environment, and the resulting logging volume before changing it. The PostgreSQL 18 references explain lock-wait logging and deadlock timeout behavior.
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Inspect sessions while the incident is active
A query against pg_locks joined to pg_stat_activity can show outstanding locks alongside the activity of the backend that owns or awaits them. Join the lock view’s pid to pg_stat_activity.pid, then examine whether locks are granted and what SQL and application identity appear for the associated sessions.
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SELECT a.pid,
a.application_name,
a.usename,
a.datname,
a.state,
a.query,
l.locktype,
l.mode,
l.granted,
l.relation
FROM pg_locks AS l
JOIN pg_stat_activity AS a ON a.pid = l.pid
ORDER BY a.pid, l.granted, l.locktype;This is a live snapshot, not a historical record. PostgreSQL detects a deadlock and aborts a transaction to break the cycle, so the conflicting state may no longer be visible after the event; use the server log in that case. If you resolve relation OIDs through pg_class, do so in the relevant database context. Check view columns against your PostgreSQL major version: the linked pg_locks reference documents PostgreSQL 16.
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Trace each backend to a Spring call path
Use the database timestamp, backend process ID, statement, database, and application name to align the deadlock report with request or job logs. Then inspect the code that issued each statement and establish the real transaction boundary: identify the method, the transaction manager, and the order in which the competing transactions acquire database resources.
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Distinguish a database deadlock from pool exhaustion
Both failures can leave a request waiting or cause a transaction to fail, but they are not interchangeable diagnoses. A PostgreSQL deadlock requires evidence of a database lock cycle. A pool stall can occur when application threads hold connections and wait for more, even if PostgreSQL has not reported a deadlock.
| Evidence | PostgreSQL lock deadlock | Spring connection-pool exhaustion |
|---|---|---|
| Primary signal | PostgreSQL deadlock report with participating processes and lock details. | Pool acquisition delays or timeouts, along with connections held by transactions. |
| Database view | Lock-wait or cycle evidence in server logs, or live lock state while the incident is occurring. | Sessions may hold connections without a matching PostgreSQL deadlock report. |
| Potential Spring clue | Concurrent transaction statements acquire conflicting resources in different sequences. | PROPAGATION_REQUIRES_NEW or another nested call path requests an additional connection while an outer transaction retains one. |
| First investigation | Preserve the deadlock report, identify its SQL and backend processes, and trace the call paths. | Inspect pool metrics, transaction lifetimes, and connection demand per thread. |
Spring documents that REQUIRES_NEW creates an independent physical transaction while resources from the outer transaction remain bound. If multiple threads hold outer connections and wait for inner connections, a pool that cannot supply them can be exhausted. Treat this as a pool-resource problem unless PostgreSQL’s evidence establishes a lock cycle. The details are in Spring’s Transaction Propagation reference.
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Check exception translation and rollback rules
The exception surfaced by Spring is not necessarily a one-to-one representation of the database failure. Spring’s JdbcTransactionManager translates database locking failures during commit or rollback into DataAccessException subclasses; DataSourceTransactionManager behaves differently. Check which transaction manager the application actually uses and inspect the underlying cause, rather than diagnosing from the top-level exception type alone. Spring describes the distinction in Controlling Database Connections.
Also check the method’s @Transactional rollback rules. By default, Spring rolls back on unchecked exceptions and Error, but not on checked exceptions; configured rollback rules can change that behavior. A transaction’s rollback outcome can affect what the application observes after a database error, but it does not identify the lock cycle by itself. See Spring’s declarative rollback rules.
A practical incident checklist
- Capture the full PostgreSQL report. Retain the deadlock details, SQL, process IDs, timestamps, and nearby log lines.
- Align database and application records. Use the timestamp and backend identity, then correlate the statements with request or job identifiers.
- Inspect live locks if the problem is still happening. Join
pg_lockstopg_stat_activityand examine granted status, session identity, and SQL. - Reconstruct transaction behavior. Find the methods that issued the statements, confirm the proxy and transaction boundaries, and compare the order in which transactions acquire resources.
- If there is no deadlock report, check pool evidence. Review acquisition delays, timeouts, active and idle connections, transaction duration, and whether nested work needs another connection.
- Verify how failures are handled. Identify the transaction manager, inspect the underlying exception, and confirm the applicable rollback rules.
PostgreSQL’s references above are for version 18 except for the explicitly linked PostgreSQL 16 pg_locks page. Spring references include current documentation and a Spring Framework 7.1 propagation page. Confirm settings, view columns, transaction behavior, and provider-specific controls against the versions and deployment actually running your application.
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