oe-kafka

Table of Contents

• Need
• Implementation
• Setup
• [Event Format](#Event Format)
• Configuration
• [Custom Suffix](#Custom Suffix)

Need

This is a requirement primarily for oeCloud applications that work with Finacle. Finacle uses Kafka for passing events and messages between some of its modules. There is thus a need for Finacle-oeCloud apps to also be able to use Kafka as an event queue.

Specifically, the following requirements are to be met:

1. Kafka Publisher: It should be possible for an oeCloud application to publish messages to a Kafka topic whenever a create, update or delete event occurs on application models.
2. Kafka Subscriber: It should be possible for an oeCloud application to create, update or delete application model instances whenever appropriate Kafka messages are received.

Implementation

The oe-kafka module provides the infrastructure for catering to the above need. It is implemented as an app-list module for oe-Cloud based applications. This module further provides a oeCloud mixin (KafkaMixin), a boot script, and a Model (KafkaFailQueue).

The mixin is named KafkaMixin and it provides the Kafka Publisher feature.

The boot script in the oe-kafka module provides the Kafka Subscriber feature.

The two features can work independent of each other but share some Configuration options.

Kafka Publisher

It provides the ability to automatically publish Create, Update and Delete operation data to a configured Kafka topic. The topic is composed of a prefix and a suffix, concatenated by a period (.). Thus the topic is of the form <PREFIX>.<SUFFIX>.

The prefix is configurable in the application's config.json, while the suffix is automatically set to the Model Name by default. The suffix may be overridden at the model level through KafkaMixin properties.

The application Models for which this feature applies, is configurable. By default, this feature is turned off for all Models. To enable it, KafkaMixin needs to be explicitly attached to models that need the Kafka Publisher feature. If it is attached to a Base Model, then all derived Models will automatically have this feature enabled.

Failure to publish to Kafka Topic results in the event being logged to an error queue Model named KafkaFailQueue.

Kafka Subscriber

It provides the ability to automatically Create, Update and Delete Model instances (data) whenever appropriate messages are received on a configured Kafka topic.

There are 2 ways to configure the topic for this feature -

1. Have a common topic for receiving Kafka messages - this works for all Models, i.e., any model instance can be created/updated/deleted if the appropriate message is received on this common topic. In this case, the message needs to provide the modelName in msg.type field.
2. Have Model-specific topics for receiving Kafka messages. In this case, the message should not specify the modelName. It is taken from the topic-model mapping in the configuration.

Both the above configurations are done via config.json. These configurations can both exist simultaneously, in which case, appropriate messages (with/without msg.type (see above)) may be sent to either the default topic or the topics specified as part of the topic-model mapping.

Setup

To get the Kafka features, the following changes need to be done in the oe-Cloud based application:

1. This (oe-kafka) module needs to be added as application package.json dependency.
2. The above modules need to be added to the server/app-list.json file in the app.

The code snippets below show how steps 1 and 2 can be done:

package.json (only part of the file is shown here, with relevant section in bold):

server/app-list.json (Relevant section in bold):

Note the app-list parameter KafkaMixin: true. This is required to be set to true to enable the KafkaMixin from the oe-kafka module.

The aplication models that need the Kafka Publisher feature should declare the KafkaMixin explicitly as shown below, in bold:

common/models/contact.json (Relevant section in bold):

Event Format

The oe-kafka module uses the CloudEvent v1.0 specification as the format of data that is published to the Kafka topic by the oeCloud based app when using the Kafka Publisher feature. This is the same format that is expected by oe-kafka when using the Kafka Subscriber feature as well.

An example of an event published/subscribed to/from Kafka in CloudEvent format is as follows:

Example CloudEvent message

{
   "specversion":"1.0",                          // required
   "type":"Contact",                             // optional. see above
   "source":"",                                  // optional
   "subject":"",                                 // optional
   "id":"5dee166581edba6f8680ed9f",              // optional
   "time":"2019-12-09T09:39:49.161Z",            // required
   "operation":"CREATE",                         // required
   "datacontenttype":"application/json",         // required  
   "data":"{\"FirstName\":\"Ajith\",\"LastName\":\"Vasudevan\",\"_isDeleted\":false,\"_type\":\"Contact\",\"_createdBy\":\"system\",\"_createdOn\":\"2019-12-09T09:39:49.156Z\",\"_modifiedBy\":\"system\",\"_modifiedOn\":\"2019-12-09T09:39:49.156Z\",\"_version\":\"f636df1f-2231-4232-b022-5dd259cf2077\",\"id\":\"5dee166581edba6f8680ed9f\"}"        // required. Contains actual payload published
}

Configuration

The oe-kafka module features can be configured via the server/config.json file. The following example shows the structure of the kafka configuration in this file:

server/config.json

A minimal config is as follows:

Here, the value of clientOpts is an object having the same properties as the KafkaClient of the npm module kafka-node. Within the clientOpts object, only kafkaHost is mandatory. kafkaHost is of the form <host>:<port>. For explanations of the rest of the clientOpts parameters, see the documentation of the kafka-node npm module.

producerOpts is an object having the same properties as Producer of the npm module kafka-node.

producerOpts and all properties within it are optional.

topicPrefix is used by oe-kafka as the prefix of the Kafka topic to which events are published. It is a mandatory field.

consumerGroupOpts is an object having the same properties as ConsumerGroup of the npm module kafka-node.

consumerGroupOpts and all properties within it are optional. consumerGroupOpts.kafkaHost is ignored, if provided. kafkaHost will be taken from clientOpts.kafkaHost

consumerGroupOpts.groupId is set by default as topicPrefix + '-group'

subscriber is an object with the following keys -

• disabled - Optional boolean indicating whether the Kafka Subscriber feature is disabled or not. Default is false (feature is enabled)
• topicSuffix - Optional String used as a suffix to arrive at a default topic to receive Kafka messages. Any message received on this topic, in the [Event Format](#Event Format) will be examined for msg.value.type to see if it is a valid model name. If so, the operation specified in msg.value.operation would be performed on the model using the data in msg.value.data
• mappings - Optional Object whose keys are topics to subscribe to and values the name of the Model which needs to be created/updated or deleted when a suitable message is received on the corresponding topic. The expected message format and interpretation is the same as above.

Custom suffix

By default, for the Kafka Publisher feature, the topic suffix is automatically set to the model name. Thus, for a model named Contact, the topic is calculated as <options.topicPrefix>.<Model Name> resulting in oe-demo-app.Contact as the topic

However, the topic suffix can be changed on a per Model basis, by specifying the same in the Model Definition's KafkaMixin options, as follows:

common/models/contact.json (Relevant section in bold):

Here, the topic is calculated as <options.topicPrefix>.<Model.settings.definition.mixin.KafkaMixin.topicSuffixOverride> resulting in oe-demo-app.CTCT as the topic