Eventeum

A bridge between your Ethereum smart contract events and backend microservices. Eventeum listens for specified event emissions from the Ethereum network, and broadcasts these events into your middleware layer. This provides a distinct seperation of concerns and means that your microservices do not have to subscribe to events directly to an Ethereum node.

Features

• Dynamically Configurable - Eventeum exposes a REST api so that smart contract events can be dynamically subscribed / unsubscribed.

• Highly Available - Eventeum instances communicate with each other to ensure that every instance is subscribed to the same collection of smart contract events.

• Resilient - Node failures are detected and event subscriptions will continue from the failure block once the node comes back online.

• Fork Tolerance - Eventeum can be configured to wait a certain amount of blocks before an event is considered 'Confirmed'. If a fork occurs during this time, a message is broadcast to the network, allowing your services to react to the forked/removed event.

Supported Broadcast Mechanisms

• Kafka
• HTTP Post

Getting Started

Follow the instructions below in order to run Eventeum on your local machine for development and testing purposes.

Prerequisites

• Java 8
• Maven
• Docker (optional)

Build

1. After checking out the code, navigate to the root directory

$ cd /path/to/eventeum/

2. Compile, test and package the project

$ mvn clean package

Run

a. If you have a running instance of MongoDB, Kafka, Zookeeper and an Ethereum node:

Executable JAR:

$ cd server
$ export SPRING_DATA_MONGODB_HOST=<mongodb-host:port>
$ export ETHEREUM_NODE_URL=http://<node-host:port>
$ export ZOOKEEPER_ADDRESS=<zookeeper-host:port>
$ export KAFKA_ADDRESSES=<kafka-host:port>

$ java -jar target/eventeum-server.jar

Docker:

$ cd server
$ docker build  . -t kauri/eventeum:latest

$ export SPRING_DATA_MONGODB_HOST=<mongodb-host:port>
$ export ETHEREUM_NODE_URL=http://<node-host:port>
$ export ZOOKEEPER_ADDRESS=<zookeeper-host:port>
$ export KAFKA_ADDRESSES=<kafka-host:port>

$ docker run -p 8060:8060 kauri/eventeum

b. If you prefer build an all-in-one test environment with a parity dev node, use docker-compose:

$ cd server
$ docker-compose -f docker-compose.yml build
$ docker-compose -f docker-compose.yml up

Registering Events

REST

Eventeum exposes a REST api that can be used to register events that should be subscribed to / broadcast.

• URL: /api/rest/v1/event-filter
• Method: POST
• Headers:

Key	Value
content-type	application/json

• URL Params: N/A
• Body:

{
	"id": "event-identifier",
	"contractAddress": "0x1fbBeeE6eC2B7B095fE3c5A572551b1e260Af4d2",
	"eventSpecification": {
		"eventName": "TestEvent",
		"indexedParameterDefinitions": [
		  {"position": 0, "type": "UINT256"},
		  {"position": 1, "type": "ADDRESS"}],
		"nonIndexedParameterDefinitions": [
		  {"position": 2, "type": "BYTES32"},
		  {"position": 3, "type": "STRING"}] },
	"correlationIdStrategy": {
		"type": "NON_INDEXED_PARAMETER",
		"parameterIndex": 0 }
}

Name	Type	Mandatory	Default	Description
id	String	no	Autogenerated	A unique identifier for the event.
contractAddress	String	yes		The address of the smart contract that the address will be emitted from.
eventSpecification	json	yes		The event specification
correlationIdStrategy	json	no	null	Define a correlation id for the event (only used with the Kafka broadcaster). See the advanced section for details.

eventSpecification:

Name	Type	Mandatory	Default	Description
eventName	String	yes		The event name within the smart contract
indexedParameterTypes	String array	no	null	The array of indexed parameter types for the event.
nonIndexedParameterTypes	String array	no	null	The array of non-indexed parameter types for the event.

parameterDefinition:

Name	Type	Mandatory	Default	Description
position	Number	yes		The zero indexed position of the parameter within the event specification
type	String	yes		The type of the event parameter.

Currently supported parameter types: UINT256, ADDRESS, BYTES32, STRING

correlationIdStrategy:

Name	Type	Mandatory	Default	Description
type	String	yes		The correlation id strategy type.
parameterIndex	Number	yes		The parameter index to use within the correlation strategy.

• Success Response:
  • Code: 200
    Content:

{
    "id": "event-identifier"
}

Hard Coded Configuration

Static events can be configured within the application.yml file of Eventeum.

eventFilters:
  - id: RequestCreated
    contractAddress: ${CONTRACT_ADDRESS:0x4aecf261541f168bb3ca65fa8ff5012498aac3b8}
    eventSpecification:
      eventName: RequestCreated
      indexedParameterDefinitions:
        - position: 0
          type: BYTES32
        - position: 1
          type: ADDRESS
      nonIndexedParameterDefinitions:
        - position: 2
          type: BYTES32
    correlationId:
      type: NON_INDEXED_PARAMETER
      index: 0

Un-Registering Events

REST

• URL: /api/rest/v1/event-filter/{event-id}

• Method: DELETE

• Headers: N/A

• URL Params: N/A

• Body: N/A

• Success Response:

  • Code: 200
    Content: N/A

Broadcast Messages Format

Contract Events

When a subscribed event is emitted, a JSON message is broadcast to the configured kafka topic (contract-events by default), with the following format:

{
	"id":"unique-event-id",
	"type":"CONTRACT_EVENT",
	"details":{
		"name":"DummyEvent",
		"filterId":"63da468c-cec6-49aa-bea4-eeba64fb1df4",
		"indexedParameters":[{"type":"bytes32","value":"BytesValue"},
			{"type":"address","value":"0x00a329c0648769a73afac7f9381e08fb43dbea72"}],
		"nonIndexedParameters":[{"type":"uint256","value":10},
			{"type":"string","value":"StringValue"}],
		"transactionHash":"0xe4fd0f095990ec471cdf40638336a73636d2e88fc1a240c20b45101b9cce9438",
		"logIndex":0,
		"blockNumber":258,
		"blockHash":"0x65d1956c2850677f75ec9adcd7b2cfab89e31ad1e7a5ba93b6fad11e6cd15e4a",
		"address":"0x9ec580fa364159a09ea15cd39505fc0a926d3a00",	
		"status":"UNCONFIRMED",
		"eventSpecificationSignature":"0x46aca551d5bafd01d98f8cadeb9b50f1b3ee44c33007f2a13d969dab7e7cf2a8",
		"id":"unique-event-id"},
		"retries":0
}

Block Events

When a new block is mined, a JSON message is broadcast to the configured kafka topic (block-events by default), with the following format:

 {
 	"id":"0x79799054d1782eb4f246b3055b967557148f38344fbd7020febf7b2d44faa4f8",
	"type":"BLOCK",
	"details":{
		"number":257,
		"hash":"0x79799054d1782eb4f246b3055b967557148f38344fbd7020febf7b2d44faa4f8"},
	"retries":0
}

Configuration

Many values within Eventeum are configurable either by changing the values in the application.yml file or by setting the associated environment variable.

Env Variable	Default	Description
SERVER_PORT	8060	The port for the eventeum instance.
ETHEREUM_BLOCKSTRATEGY	POLL	The strategy for obtaining block events from an ethereum node (POLL or PUBSUB)
ETHEREUM_NODE_URL	http://localhost:8545	The ethereum node url.
ETHEREUM_NODE _HEALTHCHECK_POLLINTERVAL	2000	The interval time in ms, in which a request is made to the ethereum node, to ensure that the node is running and functional.
EVENTSTORE_TYPE	DB	The type of eventstore used in Eventeum. (See the Advanced section for more details)
BROADCASTER_TYPE	KAFKA	The broadcast mechanism to use. (KAFKA or HTTP)
BROADCASTER_CACHE _EXPIRATIONMILLIS	6000000	The eventeum broadcaster has an internal cache of sent messages, which ensures that duplicate messages are not broadcast. This is the time that a message should live within this cache.
BROADCASTER_EVENT _CONFIRMATION _NUMBLOCKSTOWAIT	12	The number of blocks to wait (after the initial mined block) before broadcasting a CONFIRMED event
BROADCASTER_EVENT _CONFIRMATION _NUMBLOCKSTOWAITFORMISSINGTX	200	After a fork, a transaction may disappear, and this is the number of blocks to wait on the new fork, before assuming that an event emitted during this transaction has been INVALIDATED
BROADCASTER_MULTIINSTANCE	false	If multiple instances of eventeum are to be deployed in your system, this should be set to true so that the eventeum communicates added/removed filters to other instances, via kafka.
BROADCASTER_HTTP CONTRACTEVENTSURL		The http url for posting contract events (for HTTP broadcasting)
BROADCASTER_HTTP BLOCKEVENTSURL		The http url for posting block events (for HTTP broadcasting)
ZOOKEEPER_ADDRESS	localhost:2181	The zookeeper address
KAFKA_ADDRESSES	localhost:9092	Comma seperated list of kafka addresses
KAFKA_TOPIC_CONTRACT_EVENTS	contract-events	The topic name for broadcast contract event messages
KAFKA_TOPIC_BLOCK_EVENTS	block-events	The topic name for broadcast block event messages
SPRING_DATA_MONGODB_HOST	localhost	The mongoDB host (used when event store is set to DB)
SPRING_DATA_MONGODB_PORT	27017	The mongoDB post (used when event store is set to DB)

INFURA Support Configuration

Connecting to an INFURA node is only supported if connecting via websockets (wss://<...> node url). The blockstrategy must also be set to PUBSUB.

Advanced

Correlation Id Strategies (Kafka Broadcasting)

Each subscribed event can have a correlation id strategy association with it, during subscription. A correlation id strategy defines what the kafka message key for a broadcast event should be, and allows the system to be configured so that events with particular parameter values are always sent to the same partition.

Currently supported correlation id strategies are:

Indexed Parameter Strategy - An indexed parameter within the event is used as the message key when broadcasting. Non Indexed Parameter Strategy - An non-indexed parameter within the event is used as the message key when broadcasting.

Event Store

Eventeum utilises an event store in order to establish the block number to start event subscriptions from, in the event of a failover. For example, if the last event broadcast for event with id X had a block number of 123, then on a failover, eventeum will subscribe to events from block 124.

There are currently 2 supported event store implementations:

MongoDB

Broadcast events are saved and retrieved from a mongoDB database.

Required Configuration

Env Variable	Default	Description
EVENTSTORE_TYPE	DB	MongoDB event store enabled
SPRING_DATA_MONGODB_HOST	localhost	The mongoDB host
SPRING_DATA_MONGODB_PORT	27017	The mongoDB post

REST Service

Eventeum polls an external REST service in order to obtain a list of events broadcast for a specific event specification. It is assumed that this REST service listens for broadcast events on the kafka topic and updates its internal state...broadcast events are not directly sent to the REST service by eventeum.

The implemented REST service should have a pageable endpoint which accepts a request with the following specification:

• URL: Configurable, defaults to /api/rest/v1/event
• Method: GET
• Headers:

Key	Value
content-type	application/json

• URL Params:

Key	Value
page	The page number
size	The page size
sort	The results sort field
dir	The results sort direction
signature	Retrieve events with the specified event signature

• Body: N/A

• Success Response:

  • Code: 200
    Content:

{
	"content":[
		{"blockNumber":10,"id":<unique event id>}],
	"page":1,
	"size":1,
	"totalElements":1,
	"first":false,
	"last":true,
	"totalPages":1,
	"numberOfElements":1,
	"hasContent":true
}

Required Configuration

Env Variable	Default	Description
EVENTSTORE_TYPE	REST	REST event store enabled
EVENTSTORE_URL	http://localhost:8081/api/rest/v1	The REST endpoint url
EVENTSTORE_EVENTPATH	/event	The path to the event REST endpoint

Integrating Eventeum into Third Party Spring Application

Eventeum can be embedded into an existing Spring Application via an annotation.

Steps to Embed

1. Add the Consensys Kauri bintray repository into your pom.xml file:

<repositories>
  <repository>
    <id>bintray-consensys-kauri</id>
    <url>https://consensys.bintray.com/kauri</url>
  </repository>
</repositories>

2. Add the eventeum-core dependency to your pom.xml file:

<dependency>
  <groupId>net.consensys.eventeum</groupId>
  <artifactId>eventeum-core</artifactId>
  <version>*LATEST_EVENTEUM_VERSION*</version>
</dependency>

3. Within your Application class or a @Configuration annotated class, add the @EnableEventeum annotation.

Known Caveats / Issues

• In multi-instance mode, where there is more than one Eventeum instance in a system, your services are required to handle duplicate messages gracefully, as each instance will broadcast the same events.