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A long-lived Server-Sent Events (SSE) response can magnify the cost of request-scoped persistence behavior. In a September 21, 2026 incident account, Jo4 Team says Spring Boot’s spring.jpa.open-in-view=true setting kept a Hibernate session—and, in their setup, a Hikari connection—tied to each open stream. When enough streams remained open, other database-backed requests could no longer get a connection. That is the authors’ account of their configuration, not a universal result for every Spring Boot application.
What happened in the reported incident
Jo4 Team describes an SSE notification endpoint that returned Flux<ServerSentEvent<...>>. The stream could stay open for 30 minutes, sent a heartbeat every 30 seconds, and delivered an initial unread count plus live in-memory updates. Their account attributes the failure to Open EntityManager in View (OSIV), enabled with spring.jpa.open-in-view=true. Read the incident account.
In the authors’ explanation, OSIV keeps the Hibernate persistence context open over the request, and the connection borrowed through it remains occupied while the response is being written. A request that normally finishes quickly can therefore retain a scarce database connection for as long as its SSE response stays open. The article’s authors summarize their scenario this way: “For a 30-minute SSE connection, it’s lethal: spring.jpa.open-in-view=true pins a Hikari connection per open tab for the entire lifetime of the stream.” That wording describes their incident, not a guarantee about all configurations or versions.
How connection exhaustion can spread beyond SSE
The incident article gives an example pool size of 10 connections and says ten open SSE tabs could leave no connections available for other work. A further request then waits for a connection and, in the example, reaches a reported 30-second acquisition timeout. The authors describe those figures as their scenario; the account does not establish the Spring Boot, HikariCP, database, or deployment versions, so neither number should be treated as a current universal default.
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If SSE streams consume the connections in a pool shared with ordinary application routes, any database-backed endpoint using that pool can stall while waiting for a connection. The symptom may appear to affect the whole application even though the long-lived SSE requests are what retain the resource.
How to check whether OSIV is the cause
Look for evidence of database connection acquisition delays alongside active, long-lived SSE requests. Pool metrics can help show whether connections are all in use and whether threads are waiting to borrow one. Then check whether the stream’s request retains persistence state after its initial database work. The incident account supports this mechanism as a possible cause; it does not establish how frequently it occurs across Spring applications.
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- Connection-pool exhaustion: connection usage reaches the configured limit, acquisition waits rise, and database-backed routes sharing that pool slow or fail.
- Servlet executor saturation: response-writing work is starved or queued. This is a distinct capacity issue from a retained database connection.
- Timeout or disconnect: an async request timeout, proxy idle timeout, or client disconnect can end a stream without indicating database-pool exhaustion. Spring MVC’s async timeout is container-dependent when not explicitly configured.
What to change—and what to verify first
The incident article proposes setting spring.jpa.open-in-view=false. That can avoid relying on a request-lifetime persistence context, but changing it safely requires an audit of where JPA data is accessed. In particular, later stream events must not depend on lazy-loaded state after the transaction that loaded it has ended.
- Confirm that each database read or write occurs inside an explicit transaction boundary.
- Fetch the data needed for an event within that boundary and map it to a DTO or other detached value.
- Build and send later SSE events from those values, not by traversing JPA entities or lazy relationships after the transaction closes.
- Only after verifying those conditions, set
spring.jpa.open-in-view=falseand test both initial event delivery and later updates.
This separates persistence work from the lifetime of the network response: the database operation can finish and release its connection even while the SSE stream remains open.
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Remember the separate Spring MVC streaming limit
Disabling OSIV does not solve every streaming-capacity problem. Spring Framework documents that Servlet-stack writes for reactive streaming remain blocking and are performed through a configured AsyncTaskExecutor. It also warns that the default executor used for streaming reactive types and Callable execution is not suitable for production under load. Treat executor capacity as a separate issue to assess, rather than as the cause established by the OSIV incident. Spring MVC asynchronous requests reference.
For MVC applications using the dedicated SSE API, Spring provides SseEmitter, a ResponseBodyEmitter for Server-Sent Events. It supports an open response that sends multiple events; it does not by itself address database connection retention or executor sizing.
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