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DMC

Data movement controller (DMC) is an event based dependency management system. It let's user define dependency / relationship among events and notifies when the dependency is met. It consists of

• a set of REST APIs to let user define event dependencies as a rule and configure callbacks/notifications.
• a http interface to listen to events
• a rule builder to express dependencies between events
• an async notifier.

Though, this can be viewed as a CEP system (listening to events and correlating them), DMC has purpose build intelligence to automically scope events in context of a rule and expire old events, hence very relevent in managing data-set dependencies in a data warehouse or similar.

In context of DMC, events are often Job triggers like START_DATASET_1 processing and DMC notifices the actual job once their dependencies (or) pre-conditions are met. Notifications are sent via web-hook easily extendable to something like Kafka. Events has headers (key, value pairs) using which one can be correlated with other event.

How to use it ?

All of the below are avaialble as an API and as an Java Library driven so you get the benefits of syntax check.

Define a Rule

Rule consists of

• spec (captures dependencies between events using their headers)
• callbacks that needs to be notified.

Spec will look like below

        // Create a spec capturing dependencies between events using their headers
        // More examples at com.thoongatechies.dmc.spec.service.SpecServiceImplTest
        Spec spec = SpecBuilder.newBuilder()
                .event("DATASET_1_COMPLETED")
                    .set("srcVersion").fromHeader("assemblyId") // rename & choose required headers from event
                    .set("customerId").fromHeader("customerId")
                .and()      // express and criteria between two events
                .event("DATASET_2_COMPLETED")
                    .set("srcVersion").fromHeader("assemblyId")
                    .set("customerId").fromHeader("customerId")
                .groupBy("srcVersion","customerId") // short cut for specifying e1.metadata1
                // in the notification message, you configure the required data
                    .addToResponse("version").fromHeader("srcVersion")  // ability to pick selective meta-data
                    .addToResponse("customerId").fromHeader("customerId")
                .build();

A callback (web-hook) will look like below

        // Create a callback URL to notify DATASET_3
        CallbackDefinition callback = CallbackDefinition.newBuilder()
                .withName("DataSet3_Processor")
                .withUrl("http://somedomain.com/post/jobs")
                .build();

Rule is a binding of both, which will look like below.

        // create a rule binding the spec and callback information together
        RuleDefinition rule = RuleDefinition.newBuilder()
                .withName("Trigger DataSet_3 on completion of 1 and 2")
                .withExpression(spec.toString())
                .withCallbacks(Lists.fixedSize.of(callback))
                .build();

RuleDefinition is a REST resource. A POST Operation to /v1/dependency/definition/rule will create this.

Publish events to DMC

A Event has following key information.

• name e.g., DATASET_1_COMPLETED
• qualifier ( a.k.a headers) , a set of key values
• data, a set of key-values but never understood by the system. Will be simply passed back in notifications. Typically used to pass some context information

Note that, a event could match n no. of rules based on the name. Evaluation will happen using headers, group by and other criteria's specified.

It looks like below

        Event.newBuilder()
                .withName("DATASET_1_COMPLETED")
                .withOwner("System1")
                .withOccuredAt(new Date())
                .withQualifier(UnifiedMap.newWithKeysValues("assemblyId","uuid","customerId",25))
                .build();

A POST operation to /v1/dependency/instance/event will create this.

Receive notifications

When the dependency condition specified in a rule mets, the corresponding callbacks will be notified with following payload structure

   {
       "rule" : { ... }, //rule_definition
       "events" : [ ], //all events matched against this rule
       "qualifier" : { } // key-values of response headers.  specified as the response metadata in the rule.
   }