Version: v2.0.0

Apache EventMesh Client Guide

Audience: Java application developers using eventmesh-sdk-java (CloudEventsClient / A2AClient). Backend-agnostic — the complete API surface. Configuration: Configuration; architectural context: Architecture.

The new architecture ships with one client SDK that exposes two surface APIs:

CloudEventsClient — HTTP + CloudEvents 1.0 pub/sub, request-reply, SSE / WebSocket push, RocketMQ 5.x Lite Topic, LLM streaming. This is the primary user path and the recommended way to integrate with EventMesh.
A2AClient — Agent-to-Agent task dispatch on top of the same Runtime (docs/feature/a2a.md). Used by multi-agent systems that need a durable task lifecycle.

Both clients talk only to the EventMesh Runtime (HTTP). The underlying storage backend — RocketMQ 4.x, RocketMQ 5.x, Kafka, or any other MeshStoragePlugin implementation — is completely transparent to the client. Switching backends is a server-side configuration change, not a client change.

Legacy EventMeshHttpClient / EventMeshTCPClient are kept for protocol compatibility but are no longer extended. New integrations must use CloudEventsClient (or A2AClient for A2A workloads). See the migration notes at the end of this guide.

Quick orientation
CloudEventsClient API reference
Builder and configuration
Publish / subscribe patterns
Request-reply (synchronous RPC)
Subscriber transports: long-poll / SSE / WebSocket
RocketMQ 5.x Lite Topic
LLM streaming call (Mode 1 / Mode 2)
Security: tokens, signatures, the unified gate
Reliability: ACK, retries, dead-letter, idempotency
A2AClient for agent workloads
Backend selection: RocketMQ 4.x / 5.x / Kafka
End-to-end example
Operational checklist
Legacy compatibility
Code locations

1. Quick orientation

A typical client looks like this:

CloudEventsClient client = CloudEventsClient.builder()
    .runtimeUrl("http://localhost:10105")          // EventMesh Runtime HTTP endpoint
    .clientId("order-svc")                         // unique per JVM
    .pollIntervalMs(500L)                          // long-poll cadence
    .build();

client.subscribe("orders", "BROADCAST", event -> {
    System.out.println("got " + event.getId() + " type=" + event.getType());
});

CloudEvent e = CloudEventsClient.event(
    "evt-1", "order-svc", "order.created", "{\"amt\":99}".getBytes(StandardCharsets.UTF_8));
client.publish("orders", e);

That's the entire model. The rest of this document covers edge cases and the optional surfaces (A2A, streaming, security, Lite Topic).

2. `CloudEventsClient` API reference

org.apache.eventmesh.client.cloudevents.CloudEventsClient

Method	Returns	Notes
`builder()`	`CloudEventsClientBuilder`	Entry point — see §3
`publish(topic, CloudEvent)`	`boolean`	Single publish; 202 on success
`publish(topic, List<CloudEvent>)`	`boolean`	Batched publish (HTTP body batching)
`request(topic, CloudEvent, timeoutMs)`	`CloudEvent`	Blocking request-reply; `null` on timeout; late replies are dropped
`reply(correlationId, CloudEvent)`	`boolean`	Reply side of request-reply; uses the `emcorrelationid` extension
`subscribe(topic, mode, Consumer<CloudEvent>)`	—	Long-poll subscribe; handler return ⇒ auto-ACK
`subscribeWithAck(topic, mode, Predicate<CloudEvent>)`	—	Long-poll subscribe; `Predicate` returns `true` = ACK, `false` = no-ACK (at-least-once re-delivery on dispatcher timeout)
`subscribeSse(topic, mode, Consumer<CloudEvent>)`	—	SSE push subscribe; runs over HTTP on `/events/stream`
`subscribeWs(topic, mode, Consumer<CloudEvent>)`	—	WebSocket push subscribe; needs `wsUrl` (separate port)
`unsubscribe(topic)`	—	Unsubscribe one topic; stops the long-poll loop if no topics remain
`unsubscribe()`	—	Unsubscribe all; stop all loops / pushes
`createLiteTopic(parent, lite)`	`boolean`	RocketMQ 5.x only. Idempotent create of a Lite Topic (RIP-83)
`publishLite(parent, lite, CloudEvent)`	`boolean`	5.x only. Publish to LMQ inside the Lite Topic
`subscribeLite(parent, lite, Consumer<CloudEvent>)`	—	5.x only. Background poll loop; offset managed inside the storage plugin (no ACK / no DLQ)
`unsubscribeLite(parent, lite)`	—	5.x only. Stop the background poll loop for one Lite Topic
`streaming()`	`StreamingOperations`	Entry point for LLM streaming — see §8
`shutdown()`	—	Stop everything: long-poll, SSE, WS, Lite loops, streaming sessions
`static event(id, source, type, byte[] data)`	`CloudEvent`	Convenience constructor

Subscriber mode (`DistributionMode`)

mode is one of the string constants from org.apache.eventmesh.runtime.subscription.DistributionMode:

Mode	Semantics
`BROADCAST`	Every subscriber gets every message
`LOAD_BALANCE`	Each message goes to exactly one subscriber in the group
`MULTICAST`	Multi-cast delivery
`LOAD_BALANCE_STICKY`	Hash by `partitionKey` extension for stable affinity (preserves per-key order)

3. Builder and configuration

CloudEventsClient.builder()
    .runtimeUrl("http://localhost:10105")     // required — Runtime HTTP endpoint
    .clientId("my-service")                  // required — unique per JVM
    .pollIntervalMs(500L)                    // long-poll cadence (default: builder default)
    .wsUrl("http://localhost:10107")          // optional — required for subscribeWs
    .build();

Builder key	Required	Default	Notes
`runtimeUrl`	yes	—	HTTP(S) base URL of the Runtime
`clientId`	yes	—	Used in subscription registration, `SubscriptionStore`, quota key
`pollIntervalMs`	no	builder default	Long-poll cadence. Larger value = more idle time per round; smaller = more requests
`wsUrl`	no (yes for WS)	—	WebSocket endpoint. Runtime exposes WS on a separate port (configured at server start)

Environment variables are honored via System.getProperty for tests:

CloudEventsClient client = CloudEventsClient.builder()
    .runtimeUrl(System.getProperty("eventmesh.runtime.url", "http://localhost:10105"))
    .clientId("demo-" + System.currentTimeMillis())
    .build();

TLS / mTLS is configured at the Runtime (server side), not the client. The client just talks to https://... once TLS is enabled. See docs/quickstart/configuration.md.

client.subscribe("orders", "BROADCAST", event -> {
    System.out.println("got " + event.getId());
    // any thrown exception still counts as ACK — use subscribeWithAck for at-least-once
});

4.2 Manual ACK for at-least-once

client.subscribeWithAck("orders", "LOAD_BALANCE", event -> {
    try {
        process(event);          // your business logic
        return true;             // ACK → offset advances
    } catch (Exception ex) {
        return false;            // no-ACK → re-delivery on dispatcher timeout
    }
});

Business idempotency is your responsibility. EventMesh guarantees at-least-once, not exactly-once.

4.3 Batch publish

List<CloudEvent> batch = ...;
boolean ok = client.publish("orders", batch);

The Runtime splits the batch into per-partition writes inside the storage plugin. A single failure inside the batch surfaces as false and the Runtime returns 502 for that call.

5. Request-reply (synchronous RPC)

The request-reply pattern uses the emcorrelationid CloudEvents extension (all-lowercase, no hyphens — CloudEvents disallows hyphens in extension names).

// requester
CloudEvent req = CloudEventsClient.event("req-1", "caller", "query.price", payload);
CloudEvent reply = client.request("price-req", req, 10_000L);   // up to 10s
if (reply != null) { /* use reply */ }

// replier
responder.subscribe("price-req", "LOAD_BALANCE", event -> {
    Object corr = event.getExtension("emcorrelationid");
    if (corr != null) {
        CloudEvent r = CloudEventsClient.event("reply-1", "price-svc", "query.price.reply",
            priceJson(event).getBytes(StandardCharsets.UTF_8));
        responder.reply(corr.toString(), r);
    }
});

request(...) is blocking on the client thread. Late replies arriving after timeoutMs are dropped at the Runtime. Use subscribeSse or subscribeWs when you need to keep the channel open.

6. Subscriber transports: long-poll / SSE / WebSocket

All three transports produce the same CloudEvent payload to the handler; the only difference is the push direction.

Transport	Endpoint	Push direction	Port
Long-poll	`POST /events/subscribe`	client-driven	Runtime HTTP port (default 10105)
SSE	`GET /events/stream` (text/event-stream)	server push	Runtime HTTP port (default 10105)
WebSocket	runtime WS endpoint	server push, bi-directional	Runtime WS port (default 10107, configurable)

WebSocket needs a separate port because the WS upgrade is a different protocol negotiation than plain HTTP. The Runtime starts the WS server on its own port (server-side configuration), and the client must configure wsUrl explicitly. Pointing wsUrl at the HTTP port will fail the WS handshake.

// SSE — same port as HTTP
client.subscribeSse("orders", "BROADCAST", event -> { /* server-push */ });

// WebSocket — separate port
CloudEventsClient wsClient = CloudEventsClient.builder()
    .runtimeUrl("http://localhost:10105")    // HTTP (publish / long-poll / SSE)
    .wsUrl("http://localhost:10107")         // WS push
    .clientId("ws-sub").build();
wsClient.subscribeWs("orders", "BROADCAST", event -> { /* WS push */ });

All three transports auto-ACK on handler return (like subscribe). Use the manual-ACK variant only for the long-poll transport.

7. RocketMQ 5.x Lite Topic

Lite Topic (RIP-83) is RocketMQ 5.5+'s hierarchical message container. A Lite Topic lives inside a normal parent topic; the parent must be declared LITE type, then individual lite queues inside it share the parent's storage budget. Useful for session / sub-class fan-out at very high cardinality.

Backend-only feature. createLiteTopic / publishLite / subscribeLite return false (or no callbacks fire) on RocketMQ 4.x, Kafka, or any non-LiteTopicCapable storage backend. The Runtime returns 501 Not Implemented for the corresponding endpoints.

// 1. Declare — idempotent, call once at startup
client.createLiteTopic("orders", "user-42");

// 2. Subscribe — background poll loop, push-style callback (no ACK, no DLQ)
client.subscribeLite("orders", "user-42", event -> { /* process lite event */ });

// 3. Publish — routes to LMQ via __LITE_TOPIC property
client.publishLite("orders", "user-42",
    CloudEventsClient.event("lt-1", "order-svc", "order.lite", payload));

Differences from ordinary topic subscribe:

No ACK / no DLQ — offset is managed inside the storage plugin. At-least-once is best-effort.
Client-driven polling — subscribeLite runs a GET /events/lite/poll loop on the client side. Use unsubscribeLite(parent, lite) to stop one Lite subscription; unsubscribe() / shutdown() stop everything.

8. LLM streaming call (Mode 1 / Mode 2)

EventMesh provides two streaming patterns for LLM-style use cases (token chunks flowing back, multi-turn conversation context).

Mode	Use case	Direction	Entry
Mode 1 — streaming call	client → agent (LLM), agent streams tokens back	request/response, push	`client.streaming().openSession(...)`
Mode 2 — pub/sub on a session	producer writes chunks; consumer reads via SSE	publish/subscribe	`client.subscribeSession(sessionId)` / `client.openSessionPublisher(sessionId)`

8.1 Mode 1 — single call

CloudEventsClient client = CloudEventsClient.builder()
    .runtimeUrl("http://localhost:10105").clientId("my-app").build();

try (StreamingResponse r = client.streaming()
        .openSession(OpenSession.builder().clientId(client.clientId()).build())
        .call("Introduce EventMesh in three sentences")) {
    r.forEach(chunk -> System.out.print(chunk.getChunk())).join();
}

forEach fires once per token (or delta). .join() blocks until the stream ends. Closing the StreamingResponse ends one round; it does not close the session.

8.2 Mode 1 — multi-turn conversation

StreamingSession session = client.streaming()
    .openSession(OpenSession.builder().clientId("my-app").build());
try {
    try (StreamingResponse r1 = session.call("I'm Zhang San, a Java engineer")) {
        r1.forEach(c -> System.out.print(c.getChunk())).join();
    }
    // session remembers the previous round
    try (StreamingResponse r2 = session.call("What's my name and job?")) {
        r2.forEach(c -> System.out.print(c.getChunk())).join();
    }
} finally {
    session.close();
}

Multi-turn context is owned by the agent's ConversationStore, keyed by sessionId.

8.3 Mode 2 — pub/sub on a session

Useful when chunks need to be persistent (durable across process restarts) or fan-out to multiple consumers. Internally uses the storage plugin's Lite Topic.

// consumer side
StreamingResponse sub = client.subscribeSession("my-session-id");
sub.forEach(chunk -> System.out.println("[" + chunk.getSeq() + "] " + chunk.getChunk())).join();
sub.close();

// producer side
SessionPublisher pub = client.openSessionPublisher("my-session-id");
pub.publish("Hello", false);   // non-terminal frame
pub.publish(" world", false);
pub.publish("", true);         // terminal frame — consumer's forEach completes
pub.close();

8.4 Implementing an agent

An agent that participates in Mode 1 follows a four-step contract:

parse sessionId, prompt, replyTo from the inbound CloudEvent
on each LLM token → emit a non-terminal frame {chunk: token, done: false}
on normal completion → emit a terminal frame {chunk: "", done: true}
on error → emit a terminal error frame {chunk: "", done: true, error: "..."}

Reference implementation: eventmesh-agent/.../StreamingAgent.java (instantiate with an LLM client, an agentParent topic, the agent's agentId, and a ConversationStore).

8.5 Server-side configuration for streaming

The Runtime pre-creates the agent / client parent topics. For Mode 2 also pre-create sessionStreamParent. The 6-arg SessionRouter enables sessionTtlMs + sessionStreamParent; the 4-arg variant is Mode 1 only.

9. Security: tokens, signatures, the unified gate

By default, the Runtime is open. Production deployments must enable the unified security gate (issue #5304) on the server. From the client side, the only practical change is that you may need to attach credentials as HTTP headers / CloudEvents extensions:

Authorization: Bearer <token> — picked up by the built-in TokenAuthFilter and recorded into the RequestContext as principal / scopes
CloudEvents extension emtenantid — drives per-tenant quota in TenantQuotaManager
CloudEvents extension emcorrelationid — request-reply correlation

The gate runs FilterChain (TokenAuth → SignatureVerifier → Acl) → QuotaManager (per-Resource counter, default UnlimitedQuotaManager) → AuditSink (default LoggingAuditSink) on every ingress. The Operation enum is recorded in the context, so quota can distinguish a publish from a subscribe from an A2A call.

For configuration and the three wiring points (UniHttpServer.withSecurityGate, A2AGatewayHttpHandler.withSecurityGate, ConnectorScheduler.withSecurityGate) see docs/quickstart/configuration.md and docs/architecture/overview.md §4.

The client SDK does not need to "know" about the gate. A deployment that enables the gate is a server-side change. The client sends the same CloudEvent and the Runtime decides. If the Runtime requires auth, it returns 401 and your handler can re-authenticate and retry.

10. Reliability: ACK, retries, dead-letter, idempotency

Concept	Where it lives	Client responsibility
At-least-once	Runtime `DeliveryStateStore`	Use `subscribeWithAck` and return `true` only after success
Retries	Runtime retry policy	`false` from your predicate triggers a re-delivery after the dispatcher timeout
Dead-letter	Runtime `DeadLetterStore`	Inspect / replay via admin endpoints (see eventmesh-configuration.md)
Idempotency	—	You. Use `event.getId()` as the dedup key. The Runtime does not deduplicate.
Offset	Runtime `OffsetStore` (L1)	—
Subscription state	Runtime `SubscriptionStore` (L2)	Survives Runtime restart via the meta store
Task state (A2A)	Runtime `TaskStore` (L3)	—

For the storage-state taxonomy (L1 / L2 / L3) and the MeshStoragePlugin / MeshStoragePluginTCK contract, see docs/architecture/overview.md §3.

Configuration knobs: see docs/quickstart/configuration.md (eventmesh.runtime.delivery.*).

11. `A2AClient` for agent workloads

For multi-agent systems, the A2A protocol gives you a durable task lifecycle (submitted → working → completed | failed | canceled) on top of the same storage substrate. The client side is org.apache.eventmesh.protocol.a2a.A2AClient (in the eventmesh-protocol-a2a module).

11.1 Builder

A2AClient client = A2AClient.builder()
    .gatewayUrl("http://localhost:10105")    // Runtime A2A gateway (port 10105 by default)
    .namespace("default")
    .agentName("order-agent")
    .heartbeatInterval(30_000L)
    .build();

Builder key	Required	Notes
`gatewayUrl`	yes	Runtime HTTP base URL (A2A is served on the same HTTP port)
`namespace`	recommended	A2A namespace for topic isolation
`agentName`	recommended	Local agent identity; used in topic factory and `AgentCard`
`heartbeatInterval`	no	Heartbeat to the Runtime; default 30s
`socketTimeoutMs`	no	Underlying HTTP client socket timeout

11.2 Core operations

Method	Returns	Notes
`sendTask(task)`	`TaskResult`	Submit a task; returns immediately with `taskId` + initial state
`sendTaskSync(task, timeoutMs)`	`TaskResult`	Submit and block until terminal state (or timeout)
`sendTaskAsync(task, Consumer<TaskResult>)`	—	Submit and stream intermediate states via callback
`getTaskStatus(taskId)`	`TaskResult`	Re-query the current state of a task
`cancelTask(taskId)`	`boolean`	Request cancellation; the target agent stops work if it can
`streamTaskStatus(taskId, Consumer<TaskResult>)`	—	SSE push of state transitions until terminal
`listAgents()`	`List<AgentCard>`	Browse the agent registry
`registerAgentCard(AgentCard)`	`boolean`	Publish this agent's capability description

TaskResult exposes taskId, state, data, error, and targetAgent.

11.3 Idempotency: `taskEpoch`

Each task has a taskEpoch field that is set at creation and never reset. Stale writes with a taskEpoch different from the create value are rejected by the Runtime. Use the same taskEpoch across retries so the same logical task always lands in the same slot.

11.4 Server-side wiring

The A2A gateway is enabled at the Runtime by booting the A2AGatewayServer (Netty) on a configurable port (defaults to the main HTTP port). The endpoint surface is:

POST /a2a/tasks/send — submit a task
POST /a2a/tasks/sync — submit and block
GET /a2a/tasks/{id} — query state
POST /a2a/tasks/{id}/cancel — cancel
GET /a2a/tasks/{id}/stream — SSE stream of state transitions
GET /a2a/agents — list agents
POST /a2a/agents — register an agent card

See docs/feature/a2a.md for the wire contract and docs/architecture/overview.md §5 for the runtime architecture.

12. Backend selection: RocketMQ 4.x / RocketMQ 5.x / Kafka

The client code is identical across backends. Switching from one storage backend to another is a Runtime configuration change; the same CloudEventsClient (and A2AClient) bytes run unchanged.

12.1 Server-side configuration matrix

	RocketMQ 4.x	RocketMQ 5.x	Kafka
Plugin SPI key	`rocketmq`	`rocketmq5`	`kafka`
Storage module	`eventmesh-storage-plugin/eventmesh-storage-rocketmq`	`eventmesh-storage-plugin/eventmesh-storage-rocketmq5`	`eventmesh-storage-plugin/eventmesh-storage-kafka`
Connection	`NettyRemotingClient` direct (no `rocketmq-client` JAR)	Same — pure 5.5 remoting	`kafka-clients` (assign+seek+poll, no consumer group; EventMesh owns offsets)
Auth	ACL (optional)	ACL (optional)	SASL/SSL pass-through (`security.protocol` / `sasl.mechanism` / `sasl.jaas.config` are passed verbatim to `kafka-clients`)
Lite Topic	—	yes (`LiteTopicCapable`)	—

Per-backend keys are listed in docs/quickstart/configuration.md. Pick one eventmesh.storage.type at Runtime startup:

# 4.x
EVENTMESH_STORAGE_TYPE=rocketmq EVENTMESH_ROCKETMQ_NAMESRV=127.0.0.1:9876 bin/start.sh

# 5.x
EVENTMESH_STORAGE_TYPE=rocketmq5 EVENTMESH_ROCKETMQ5_NAMESRV=127.0.0.1:9876 bin/start.sh

# Kafka
EVENTMESH_STORAGE_TYPE=kafka bin/start.sh     # eventmesh.properties has bootstrap + SASL

12.2 Client-visible behavioral differences

The HTTP contract is the same — but the storage-plugin choices have client-visible consequences for subscription semantics under failure:

Dimension	RocketMQ 4.x	RocketMQ 5.x	Kafka
Consumption model	Classic PULL (EventMesh owns offset + partition ownership)	POP (broker allocates queues + lease gate)	assign + seek + poll (no consumer group; EventMesh owns offset)
Multi-instance de-dup	EventMesh `PartitionOwnership`	broker POP + lease	EventMesh `PartitionOwnership` (Kafka assign)
Offset ACK semantics	offset advances only on ACK	same	same (Kafka offset not committed; EventMesh-managed)
`publish` / `subscribe` / `request` / `reply`	consistent	consistent	consistent
Lite Topic	not supported	supported	not supported

The publish / subscribe / subscribeWithAck / request / reply API contract is identical across all three backends — that is the point of the abstraction.

12.3 Kafka + SASL example

For SASL-enabled Kafka clusters (e.g. wemq-kafka), set in eventmesh.properties:

eventMesh.server.kafka.namesrvAddr=127.0.0.1:9094
security.protocol=SASL_PLAINTEXT
sasl.mechanism=PLAIN
sasl.jaas.config=org.apache.kafka.common.security.plain.PlainLoginModule required username="<user>" password="<pass>";

The KafkaMeshStoragePlugin forwards security.* / sasl.* / ssl.* keys verbatim to the underlying KafkaProducer / KafkaConsumer / AdminClient. Plain-text Kafka clusters need none of these.

13. End-to-end example

public class Demo {
    public static void main(String[] args) throws Exception {
        CloudEventsClient client = CloudEventsClient.builder()
            .runtimeUrl(System.getProperty("eventmesh.runtime.url", "http://localhost:10105"))
            .clientId("demo-" + System.currentTimeMillis())
            .pollIntervalMs(500L)
            .build();

        client.subscribeWithAck("demo-topic", "LOAD_BALANCE", event -> {
            System.out.println("processing: " + event.getId() + " type=" + event.getType());
            return true;     // ACK
        });

        for (int i = 0; i < 10; i++) {
            CloudEvent e = CloudEventsClient.event(
                "e" + i, "demo", "demo.tick",
                ("tick-" + i).getBytes(StandardCharsets.UTF_8));
            client.publish("demo-topic", e);
        }

        Thread.sleep(60_000L);
        client.shutdown();
    }
}

Switching the backend is a server-side change only:

# 4.x
EVENTMESH_STORAGE_TYPE=rocketmq EVENTMESH_ROCKETMQ_NAMESRV=127.0.0.1:9876 bin/start.sh

# 5.x (Lite Topic capable)
EVENTMESH_STORAGE_TYPE=rocketmq5 EVENTMESH_ROCKETMQ5_NAMESRV=127.0.0.1:9876 bin/start.sh

# Kafka (SASL in eventmesh.properties)
EVENTMESH_STORAGE_TYPE=kafka bin/start.sh

The same Demo class runs unchanged on all three.

14. Operational checklist

Check	Where	What to look for
Runtime reachable	client log	First `publish` returns `true`; first `subscribe` callback fires within `pollIntervalMs`
`clientId` uniqueness	Runtime log	A `clientId` collision prints a warning; use a different `clientId` per JVM
Backend connection	Runtime startup log	`[storage] connected to <backend>` line; otherwise no subscriptions will fire
Security gate	Runtime response	`401` on first request → auth header missing; `429` → quota exhausted; `403` → ACL denied
Quota exhaustion	Runtime metrics	`eventmesh_security_gate_quota_*` per-tenant counters
Dead-letter inspection	admin HTTP (port 10106)	`GET /admin/dlq?topic=<topic>`
A2A agent registry	`A2AClient.listAgents()`	Should return at least one `AgentCard` for `agentName` you registered

See docs/feature/deployment.md for SLOs and runbooks.

15. Legacy compatibility

The legacy EventMeshHttpClient and EventMeshTCPClient continue to work against the current Runtime, but they are legacy-compatible in the capability status table and are not extended.

Old client	New client	Migration
`EventMeshHttpClient.publish(CloudEventMessage)`	`CloudEventsClient.publish(topic, CloudEvent)`	Switch the event from `CloudEventMessage` to `CloudEvent`; topic is a string
`EventMeshTCPClient.subscribe(topic, EventListener)`	`CloudEventsClient.subscribe(topic, mode, Consumer<CloudEvent>)`	Add a `mode`; switch the callback to `Consumer<CloudEvent>`
TCP subscribe with custom `Session`	WebSocket	WebSocket is the modern bi-directional transport
OpenMessaging SDK	CloudEventsClient	The OpenMessaging wire is not supported in the new Runtime; use the HTTP + CloudEvents path

For TCP / gRPC SDK migration details, see the legacy-compat section of this guide (§15) — this document is the authoritative home for the new client API; the old guide is preserved in git history for the migration notes.

16. Code locations

CloudEventsClient — eventmesh-sdks/eventmesh-sdk-java/.../cloudevents/CloudEventsClient.java
- CloudEventsClientBuilder
Streaming — eventmesh-sdks/eventmesh-sdk-java/.../cloudevents/stream/ (operations, response, session, request, publisher, exception)
A2AClient — eventmesh-protocol-plugin/eventmesh-protocol-a2a/.../A2AClient.java
A2ATopicFactory — eventmesh-protocol-plugin/eventmesh-protocol-a2a/.../A2ATopicFactory.java (agentInbox(agentId), gatewayResponseTopic(ns, gw, taskId), + wildcard)
Runtime HTTP entry — eventmesh-runtime/.../http/UniHttpServer.java (/events/* endpoints; withSecurityGate(...) wiring point)
A2A HTTP handler — eventmesh-runtime/.../a2a/A2AGatewayHttpHandler.java (/a2a/* endpoints; withSecurityGate(...) wiring point)
Streaming agent — eventmesh-agent/.../StreamingAgent.java
Storage plugins —
- eventmesh-storage-plugin/eventmesh-storage-rocketmq/ (SPI key rocketmq)
- eventmesh-storage-plugin/eventmesh-storage-rocketmq5/ (SPI key rocketmq5, LiteTopicCapable)
- eventmesh-storage-plugin/eventmesh-storage-kafka/ (SPI key kafka, assign+seek+poll, SASL pass-through)
Security gate — eventmesh-runtime/.../security/gate/ (SecurityGate, RequestContext, QuotaManager, AuditSink, GateDecision)
Architecture — eventmesh-architecture-guard/.../guard/ArchitectureRules.java (ArchUnit layered-architecture enforcement)

Apache EventMesh Client Guide

Table of contents​

1. Quick orientation​

2. CloudEventsClient API reference​

Subscriber mode (DistributionMode)​

3. Builder and configuration​

4. Publish / subscribe patterns​

4.1 Auto-ACK subscribe (the simple case)​

4.2 Manual ACK for at-least-once​

4.3 Batch publish​

5. Request-reply (synchronous RPC)​

6. Subscriber transports: long-poll / SSE / WebSocket​

7. RocketMQ 5.x Lite Topic​

8. LLM streaming call (Mode 1 / Mode 2)​

8.1 Mode 1 — single call​

8.2 Mode 1 — multi-turn conversation​

8.3 Mode 2 — pub/sub on a session​

8.4 Implementing an agent​

8.5 Server-side configuration for streaming​

9. Security: tokens, signatures, the unified gate​

10. Reliability: ACK, retries, dead-letter, idempotency​

11. A2AClient for agent workloads​

11.1 Builder​

11.2 Core operations​

11.3 Idempotency: taskEpoch​

11.4 Server-side wiring​

12. Backend selection: RocketMQ 4.x / RocketMQ 5.x / Kafka​

12.1 Server-side configuration matrix​

12.2 Client-visible behavioral differences​

12.3 Kafka + SASL example​

13. End-to-end example​

14. Operational checklist​

15. Legacy compatibility​

16. Code locations​

Table of contents