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feat: implement demand-based request handling in ResponseSubscribers #516

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Merged
merged 1 commit into from
Aug 26, 2025
+27 −9
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feat: implement demand-based request handling in ResponseSubscribers #516

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36 changes: 27 additions & 9 deletions mcp/src/main/java/io/modelcontextprotocol/client/transport/ResponseSubscribers.java
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Original file line number Diff line number Diff line change
Expand Up @@ -223,6 +223,8 @@ static class AggregateSubscriber extends BaseSubscriber<String> {
*/
private ResponseInfo responseInfo;

volatile boolean hasRequestedDemand = false;

/**
* Creates a new JsonLineSubscriber that will emit parsed JSON-RPC messages.
* @param sink the {@link FluxSink} to emit parsed {@link ResponseEvent} objects
Expand All @@ -236,7 +238,13 @@ public AggregateSubscriber(ResponseInfo responseInfo, FluxSink<ResponseEvent> si

@Override
protected void hookOnSubscribe(Subscription subscription) {
sink.onRequest(subscription::request);

sink.onRequest(n -> {
if (!hasRequestedDemand) {
subscription.request(Long.MAX_VALUE);
}
hasRequestedDemand = true;
});

// Register disposal callback to cancel subscription when Flux is disposed
sink.onDispose(subscription::cancel);
Expand All @@ -249,8 +257,11 @@ protected void hookOnNext(String line) {

@Override
protected void hookOnComplete() {
String data = this.eventBuilder.toString();
this.sink.next(new AggregateResponseEvent(responseInfo, data));

if (hasRequestedDemand) {
String data = this.eventBuilder.toString();
this.sink.next(new AggregateResponseEvent(responseInfo, data));
}

this.sink.complete();
}
Expand All @@ -271,6 +282,8 @@ static class BodilessResponseLineSubscriber extends BaseSubscriber<String> {

private final ResponseInfo responseInfo;

volatile boolean hasRequestedDemand = false;

public BodilessResponseLineSubscriber(ResponseInfo responseInfo, FluxSink<ResponseEvent> sink) {
this.sink = sink;
this.responseInfo = responseInfo;
Expand All @@ -280,7 +293,10 @@ public BodilessResponseLineSubscriber(ResponseInfo responseInfo, FluxSink<Respon
protected void hookOnSubscribe(Subscription subscription) {

sink.onRequest(n -> {
subscription.request(n);
if (!hasRequestedDemand) {
subscription.request(Long.MAX_VALUE);
}
hasRequestedDemand = true;
});

// Register disposal callback to cancel subscription when Flux is disposed
Expand All @@ -291,11 +307,13 @@ protected void hookOnSubscribe(Subscription subscription) {

@Override
protected void hookOnComplete() {
// emit dummy event to be able to inspect the response info
// this is a shortcut allowing for a more streamlined processing using
// operator composition instead of having to deal with the CompletableFuture
// along the Subscriber for inspecting the result
this.sink.next(new DummyEvent(responseInfo));
if (hasRequestedDemand) {
// emit dummy event to be able to inspect the response info
// this is a shortcut allowing for a more streamlined processing using
// operator composition instead of having to deal with the
// CompletableFuture along the Subscriber for inspecting the result
this.sink.next(new DummyEvent(responseInfo));
}
this.sink.complete();
}

Expand Down