MPC Client fails application startup when server is unavailable - need graceful degradation option

[astilt322]

When integrating Spring AI's MPC client in a Spring Boot application, if the MPC server is unavailable during client application startup, the entire application fails to start.

This creates an undesired hard dependency between client and server components, which is problematic in distributed systems. The client application should be able to start successfully even when the MPC server is temporarily unavailable, and should attempt to connect when the client is actually used and the server becomes available.

Error Details

The following exception occurs during application startup when the MPC server is unavailable:

ava.util.concurrent.CompletionException: java.net.ConnectException
at java.base/java.util.concurrent.CompletableFuture.encodeRelay(CompletableFuture.java:368) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture.completeRelay(CompletableFuture.java:377) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:1152) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:510) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run$$$capture(CompletableFuture.java:1773) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java) ~[na:na]
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) ~[na:na]
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) ~[na:na]
at java.base/java.lang.Thread.run(Thread.java:842) ~[na:na]
Caused by: java.net.ConnectException: null
at java.net.http/jdk.internal.net.http.common.Utils.toConnectException(Utils.java:1055) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.PlainHttpConnection.connectAsync(PlainHttpConnection.java:198) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.PlainHttpConnection.checkRetryConnect(PlainHttpConnection.java:230) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.PlainHttpConnection.lambda$connectAsync$1(PlainHttpConnection.java:206) ~[java.net.http:na]
at java.base/java.util.concurrent.CompletableFuture.uniHandle(CompletableFuture.java:934) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$UniHandle.tryFire(CompletableFuture.java:911) ~[na:na]
... 6 common frames omitted
Caused by: java.nio.channels.ClosedChannelException: null
at java.base/sun.nio.ch.SocketChannelImpl.ensureOpen(SocketChannelImpl.java:195) ~[na:na]
at java.base/sun.nio.ch.SocketChannelImpl.beginConnect(SocketChannelImpl.java:760) ~[na:na]
at java.base/sun.nio.ch.SocketChannelImpl.connect(SocketChannelImpl.java:848) ~[na:na]
at java.net.http/jdk.internal.net.http.PlainHttpConnection.lambda$connectAsync$0(PlainHttpConnection.java:183) ~[java.net.http:na]
at java.base/java.security.AccessController.doPrivileged(AccessController.java:569) ~[na:na]
at java.net.http/jdk.internal.net.http.PlainHttpConnection.connectAsync(PlainHttpConnection.java:185) ~[java.net.http:na]
... 10 common frames omitted

2025-05-18T16:51:05.442+08:00 ERROR 30076 --- [insight-ai] [onPool-worker-2] reactor.core.publisher.Operators : Operator called default onErrorDropped

reactor.core.Exceptions$ErrorCallbackNotImplemented: java.net.ConnectException
Caused by: java.net.ConnectException: null
at java.net.http/jdk.internal.net.http.common.Utils.toConnectException(Utils.java:1055) ~[java.net.http:na]
Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException:
Assembly trace from producer [reactor.core.publisher.MonoCompletionStage] :
reactor.core.publisher.Mono.fromFuture(Mono.java:629)
io.modelcontextprotocol.client.transport.HttpClientSseClientTransport.connect(HttpClientSseClientTransport.java:177)
Error has been observed at the following site(s):
*__Mono.fromFuture ⇢ at io.modelcontextprotocol.client.transport.HttpClientSseClientTransport.connect(HttpClientSseClientTransport.java:177)
Original Stack Trace:
at java.net.http/jdk.internal.net.http.common.Utils.toConnectException(Utils.java:1055) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.PlainHttpConnection.connectAsync(PlainHttpConnection.java:198) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.PlainHttpConnection.checkRetryConnect(PlainHttpConnection.java:230) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.PlainHttpConnection.lambda$connectAsync$1(PlainHttpConnection.java:206) ~[java.net.http:na]
at java.base/java.util.concurrent.CompletableFuture.uniHandle(CompletableFuture.java:934) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$UniHandle.tryFire(CompletableFuture.java:911) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:510) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run$$$capture(CompletableFuture.java:1773) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java) ~[na:na]
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) ~[na:na]
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) ~[na:na]
at java.base/java.lang.Thread.run(Thread.java:842) ~[na:na]
Caused by: java.nio.channels.ClosedChannelException: null
at java.base/sun.nio.ch.SocketChannelImpl.ensureOpen(SocketChannelImpl.java:195) ~[na:na]
at java.base/sun.nio.ch.SocketChannelImpl.beginConnect(SocketChannelImpl.java:760) ~[na:na]
at java.base/sun.nio.ch.SocketChannelImpl.connect(SocketChannelImpl.java:848) ~[na:na]
at java.net.http/jdk.internal.net.http.PlainHttpConnection.lambda$connectAsync$0(PlainHttpConnection.java:183) ~[java.net.http:na]
at java.base/java.security.AccessController.doPrivileged(AccessController.java:569) ~[na:na]
at java.net.http/jdk.internal.net.http.PlainHttpConnection.connectAsync(PlainHttpConnection.java:185) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.PlainHttpConnection.checkRetryConnect(PlainHttpConnection.java:230) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.PlainHttpConnection.lambda$connectAsync$1(PlainHttpConnection.java:206) ~[java.net.http:na]
at java.base/java.util.concurrent.CompletableFuture.uniHandle(CompletableFuture.java:934) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$UniHandle.tryFire(CompletableFuture.java:911) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:510) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run$$$capture(CompletableFuture.java:1773) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java) ~[na:na]
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) ~[na:na]
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) ~[na:na]
at java.base/java.lang.Thread.run(Thread.java:842) ~[na:na]

2025-05-18T16:51:05.444+08:00 ERROR 30076 --- [insight-ai] [onPool-worker-1] i.m.c.t.WebFluxSseClientTransport : Fatal SSE error, not retrying: null
2025-05-18T16:51:05.445+08:00 ERROR 30076 --- [insight-ai] [onPool-worker-1] reactor.core.publisher.Operators : Operator called default onErrorDropped

reactor.core.Exceptions$ErrorCallbackNotImplemented: org.springframework.web.reactive.function.client.WebClientRequestException
Caused by: org.springframework.web.reactive.function.client.WebClientRequestException: null
at org.springframework.web.reactive.function.client.ExchangeFunctions$DefaultExchangeFunction.lambda$wrapException$9(ExchangeFunctions.java:137) ~[spring-webflux-6.2.5.jar:6.2.5]
Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException:
Assembly trace from producer [reactor.core.publisher.MonoErrorSupplied] :
reactor.core.publisher.Mono.error(Mono.java:315)
org.springframework.web.reactive.function.client.ExchangeFunctions$DefaultExchangeFunction.wrapException(ExchangeFunctions.java:137)
Error has been observed at the following site(s):
*____________Mono.error ⇢ at org.springframework.web.reactive.function.client.ExchangeFunctions$DefaultExchangeFunction.wrapException(ExchangeFunctions.java:137)
*__Mono.onErrorResume ⇢ at org.springframework.web.reactive.function.client.ExchangeFunctions$DefaultExchangeFunction.exchange(ExchangeFunctions.java:106)
| Mono.map ⇢ at org.springframework.web.reactive.function.client.ExchangeFunctions$DefaultExchangeFunction.exchange(ExchangeFunctions.java:107)
| Mono.doOnNext ⇢ at org.springframework.web.reactive.function.client.DefaultWebClient$ObservationFilterFunction.filter(DefaultWebClient.java:745)
*_____Mono.defer ⇢ at org.springframework.web.reactive.function.client.DefaultWebClient$DefaultRequestBodyUriSpec.lambda$exchange$12(DefaultWebClient.java:467)
| checkpoint ⇢ Request to GET http://localhost:8081/sse [DefaultWebClient]
| Mono.switchIfEmpty ⇢ at org.springframework.web.reactive.function.client.DefaultWebClient$DefaultRequestBodyUriSpec.lambda$exchange$12(DefaultWebClient.java:472)
| Mono.doOnNext ⇢ at org.springframework.web.reactive.function.client.DefaultWebClient$DefaultRequestBodyUriSpec.lambda$exchange$12(DefaultWebClient.java:478)
| Mono.doOnError ⇢ at org.springframework.web.reactive.function.client.DefaultWebClient$DefaultRequestBodyUriSpec.lambda$exchange$12(DefaultWebClient.java:479)
| Mono.doFinally ⇢ at org.springframework.web.reactive.function.client.DefaultWebClient$DefaultRequestBodyUriSpec.lambda$exchange$12(DefaultWebClient.java:480)
| Mono.contextWrite ⇢ at org.springframework.web.reactive.function.client.DefaultWebClient$DefaultRequestBodyUriSpec.lambda$exchange$12(DefaultWebClient.java:486)
*Mono.deferContextual ⇢ at org.springframework.web.reactive.function.client.DefaultWebClient$DefaultRequestBodyUriSpec.exchange(DefaultWebClient.java:453)
| Mono.flatMapMany ⇢ at org.springframework.web.reactive.function.client.DefaultWebClient$DefaultResponseSpec.bodyToFlux(DefaultWebClient.java:601)
| Flux.retryWhen ⇢ at io.modelcontextprotocol.client.transport.WebFluxSseClientTransport.eventStream(WebFluxSseClientTransport.java:261)
| Flux.concatMap ⇢ at io.modelcontextprotocol.client.transport.WebFluxSseClientTransport.connect(WebFluxSseClientTransport.java:172)
*___________Flux.handle ⇢ at io.modelcontextprotocol.client.transport.WebFluxSseClientTransport.lambda$eventStream$5(WebFluxSseClientTransport.java:261)
Original Stack Trace:
at org.springframework.web.reactive.function.client.ExchangeFunctions$DefaultExchangeFunction.lambda$wrapException$9(ExchangeFunctions.java:137) ~[spring-webflux-6.2.5.jar:6.2.5]
at reactor.core.publisher.MonoErrorSupplied.subscribe(MonoErrorSupplied.java:55) ~[reactor-core-3.7.4.jar:3.7.4]
at reactor.core.publisher.Mono.subscribe(Mono.java:4576) ~[reactor-core-3.7.4.jar:3.7.4]
at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onError(FluxOnErrorResume.java:103) ~[reactor-core-3.7.4.jar:3.7.4]
at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.onError(FluxPeekFuseable.java:234) ~[reactor-core-3.7.4.jar:3.7.4]
at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.onError(FluxPeekFuseable.java:234) ~[reactor-core-3.7.4.jar:3.7.4]
at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onError(MonoIgnoreThen.java:280) ~[reactor-core-3.7.4.jar:3.7.4]
at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onError(FluxMapFuseable.java:142) ~[reactor-core-3.7.4.jar:3.7.4]
at reactor.core.publisher.MonoCompletionStage$MonoCompletionStageSubscription.apply(MonoCompletionStage.java:115) ~[reactor-core-3.7.4.jar:3.7.4]
at reactor.core.publisher.MonoCompletionStage$MonoCompletionStageSubscription.apply(MonoCompletionStage.java:67) ~[reactor-core-3.7.4.jar:3.7.4]
at java.base/java.util.concurrent.CompletableFuture.uniHandle(CompletableFuture.java:934) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$UniHandle.tryFire(CompletableFuture.java:911) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:510) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture.postFire(CompletableFuture.java:614) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$UniWhenComplete.tryFire(CompletableFuture.java:844) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$Completion.exec(CompletableFuture.java:483) ~[na:na]
at java.base/java.util.concurrent.ForkJoinTask.doExec$$$capture(ForkJoinTask.java:373) ~[na:na]
at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java) ~[na:na]
at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182) ~[na:na]
at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655) ~[na:na]
at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622) ~[na:na]
at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165) ~[na:na]
Caused by: java.net.ConnectException: null
at java.net.http/jdk.internal.net.http.common.Utils.toConnectException(Utils.java:1055) ~[java.net.http:na]
Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException:
Assembly trace from producer [reactor.core.publisher.MonoCompletionStage] :
reactor.core.publisher.Mono.fromCompletionStage(Mono.java:543)
org.springframework.http.client.reactive.JdkClientHttpConnector.lambda$connect$1(JdkClientHttpConnector.java:123)
Error has been observed at the following site(s):
*_Mono.fromCompletionStage ⇢ at org.springframework.http.client.reactive.JdkClientHttpConnector.lambda$connect$1(JdkClientHttpConnector.java:123)
| Mono.map ⇢ at org.springframework.http.client.reactive.JdkClientHttpConnector.lambda$connect$1(JdkClientHttpConnector.java:124)
*________________Mono.defer ⇢ at org.springframework.http.client.reactive.JdkClientHttpConnector.connect(JdkClientHttpConnector.java:117)
*_______________Mono.then ⇢ at org.springframework.http.client.reactive.JdkClientHttpConnector.connect(JdkClientHttpConnector.java:117)
| Mono.doOnRequest ⇢ at org.springframework.web.reactive.function.client.ExchangeFunctions$DefaultExchangeFunction.exchange(ExchangeFunctions.java:104)
| Mono.doOnCancel ⇢ at org.springframework.web.reactive.function.client.ExchangeFunctions$DefaultExchangeFunction.exchange(ExchangeFunctions.java:105)
Original Stack Trace:
at java.net.http/jdk.internal.net.http.common.Utils.toConnectException(Utils.java:1055) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.PlainHttpConnection.connectAsync(PlainHttpConnection.java:198) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.PlainHttpConnection.checkRetryConnect(PlainHttpConnection.java:230) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.PlainHttpConnection.lambda$connectAsync$1(PlainHttpConnection.java:206) ~[java.net.http:na]
at java.base/java.util.concurrent.CompletableFuture.uniHandle(CompletableFuture.java:934) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$UniHandle.tryFire(CompletableFuture.java:911) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:510) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run$$$capture(CompletableFuture.java:1773) ~[na:na]
at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java) ~[na:na]
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) ~[na:na]
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) ~[na:na]
at java.base/java.lang.Thread.run(Thread.java:842) ~[na:na]
Caused by: java.nio.channels.ClosedChannelException: null
at java.base/sun.nio.ch.SocketChannelImpl.ensureOpen(SocketChannelImpl.java:195) ~[na:na]
at java.base/sun.nio.ch.SocketChannelImpl.beginConnect(SocketChannelImpl.java:760) ~[na:na]
at java.base/sun.nio.ch.SocketChannelImpl.connect(SocketChannelImpl.java:848) ~[na:na]
at java.net.http/jdk.internal.net.http.PlainHttpConnection.lambda$connectAsync$0(PlainHttpConnection.java:183) ~[java.net.http:na]
at java.base/java.security.AccessController.doPrivileged(AccessController.java:569) ~[na:na]
at java.net.http/jdk.internal.net.http.PlainHttpConnection.connectAsync(PlainHttpConnection.java:185) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.PlainHttpConnection.checkRetryConnect(PlainHttpConnection.java:230) ~[java.net.http:na]
at java.net.http/jdk.internal.net.http.PlainHttpConnection.lambda$connectAsync$1(PlainHttpConnection.java:206) ~[java.net.http:na]

The error occurs in HttpClientSseClientTransport.connect() when attempting to establish an SSE connection to the MPC server at application startup.

Expected Behavior

The client application should:

1. Start successfully regardless of MPC server availability
2. Automatically connect to the MPC server when it becomes available
3. Provide appropriate error handling or fallback responses when the server is unavailable during actual usage

Current Workarounds

Currently, I have to implement complex workarounds such as:

• Custom ApplicationContextInitializer to check server availability before startup
• BeanFactoryPostProcessor to modify bean definitions
• Bean post-processors to replace or disable problematic beans
• Manual exception handling around every client usage

These workarounds add complexity and diverge from Spring's "convention over configuration" philosophy.

Feature Request

Please consider adding configuration options to enable graceful degradation when the MPC server is unavailable:

# Proposed configuration options
spring.ai.mcp.fail-fast=false                 # Don't fail application startup when server is unavailable
spring.ai.mcp.connect-on-startup=false        # Don't try to connect during initialization
spring.ai.mcp.lazy-connection=true            # Only connect when the client is actually used
spring.ai.mcp.reconnect-interval=5000         # Try reconnecting every 5 seconds
spring.ai.mcp.connection-failure-mode=silent  # Options: fail, warn, silent, retry

[lianneli]

LGTM! I'm also in great need of this function.

[ibrahimsahin]

@tzolov @markpollack
Is there anyway to manage sse connection manually? Reconnect or renew session etc. And update session info on client side. In production mcp is not sufficient.In local everything is ok but in cloud(gcp) there are timeouts and if one time connection lost program has crash.