EventMesh Core Engines Configuration Guide

EventMesh provides powerful core engines (Filter, Transformer, Router) to dynamically process messages. These engines are configured via MetaStorage (Governance Center, e.g., Nacos, Etcd), supporting on-demand loading and hot-reloading.

0. Core Concepts

Before configuration, it is important to understand the specific role of each engine in the message flow:

• Filter (The Gatekeeper): Decides “Whether to pass”.

  • It inspects the message (CloudEvent) attributes. If the message matches the rules, it passes; otherwise, it is dropped.
  • Use Case: Block debug logs from production traffic; Only subscribe to specific event types.

• Transformer (The Translator): Decides “What it looks like”.

  • It modifies the message content (Payload or Metadata) according to templates or scripts.
  • Use Case: Convert XML to JSON; Mask sensitive data (PII); Adapt legacy protocols to new standards.

• Router (The Dispatcher): Decides “Where to go”.

  • It dynamically changes the destination (Topic) of the message.
  • Use Case: Route traffic to a Canary/Gray release topic; Route high-priority orders to a dedicated queue.

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1. Overview

The configuration is not in local property files but distributed via the MetaStorage. EventMesh listens to specific Keys based on client Groups.

• Data Source: Configured via eventMesh.metaStorage.plugin.type.
• Loading Mechanism: Lazy loading & Hot-reloading.
• Key Format: {EnginePrefix}-{GroupName}.
• Value Format: JSON Array.

Engine	Prefix	Scope	Description
Router	router-	Pub Only	Routes messages to different topics.
Filter	filter-	Pub & Sub	Filters messages based on CloudEvent attributes.
Transformer	transformer-	Pub & Sub	Transforms message content (Payload/Header).

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2. Router (Routing)

Scope: Publish Only (Upstream)
Key: router-{producerGroup}

Decides the target storage topic for a message sent by a producer.

Configuration Example (JSON)

[
  {
    "topic": "original-topic",
    "routerConfig": {
      "targetTopic": "redirect-topic",
      "expression": "data.type == 'urgent'"
    }
  }
]

• topic: The original topic the producer sends to.
• targetTopic: The actual topic to write to Storage.
• expression: Condition to trigger routing (e.g., SpEL).

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3. Filter (Filtering)

Scope: Both Publish (Upstream) & Subscribe (Downstream)

A. Publish Side (Upstream)

Key: filter-{producerGroup}
Effect: Intercepts messages before they are sent to Storage.

B. Subscribe Side (Downstream)

Key: filter-{consumerGroup}
Effect: Intercepts messages before they are pushed to the Consumer.

Configuration Example (JSON)

[
  {
    "topic": "test-topic",
    "filterPattern": {
      "source": ["app-a", "app-b"],
      "type": [{"prefix": "com.example"}]
    }
  }
]

• filterPattern: Rules matching CloudEvent attributes. If a message doesn't match, it is dropped.

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4. Transformer (Transformation)

Scope: Both Publish (Upstream) & Subscribe (Downstream)

A. Publish Side (Upstream)

Key: transformer-{producerGroup}
Effect: Modifies message content before sending to Storage.

B. Subscribe Side (Downstream)

Key: transformer-{consumerGroup}
Effect: Modifies message content before pushing to the Consumer.

Configuration Example (JSON)

[
  {
    "topic": "raw-topic",
    "transformerConfig": {
      "transformerType": "template",
      "template": "{\"id\": \"${id}\", \"new_content\": \"${data.content}\"}"
    }
  }
]

• transformerType: e.g., original, template.
• template: The transformation template definition.

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5. Verification

1. Publish Config: Add the JSON config to your Governance Center (e.g., Nacos) with the Data ID router-MyGroup.
2. Send Message: Use EventMesh SDK to send a message from MyGroup.
3. Observe:
   • For Router: Check if the message appears in the targetTopic in your MQ.
   • For Filter: Check if blocked messages are skipped.
   • For Transformer: Check if the message body in MQ (for Pub) or Consumer (for Sub) is modified.