Configure a Gremlin Command Reaction

The Drasi Gremlin Command Reaction allows you to use the Continuous Query results as parameters to commands that run against a Gremlin database. You can specify a Gremlin Command for each type of query result (Added, Updated or Deleted). The Gremlin Reaction is compatible with Azure Cosmos DB for Gremlin, Apache TinkerPop Gremlin Servers, and JanusGraph.

Requirements

On the computer from where you will create the Drasi Gremlin Command Reaction, you need to install the following software:

• Drasi CLI

Supported Gremlin Servers

• Azure Cosmos DB for Gremlin
• Apache TinkerPop Gremlin Servers
• JanusGraph

Creating the Reaction

To create a Reaction, execute the drasi apply command as follows:

drasi apply -f my-reaction.yaml

The drasi apply command is how you create all new Drasi resources (in this case a Reaction). The -f flag specifies that the definition of the new Reaction is contained in the referenced YAML file my-reaction.yaml and the -n flag specifies the Drasi namespace in which to create the Reaction (Drasi must already be installed in that namespace).

Reaction Definitions

The YAML file passed to drasi apply can contain one or more Reaction definitions. Here is an example of a Drasi Gremlin Command Reaction definition:

kind: Reaction
apiVersion: v1
name: gremlin-reaction
spec:
  kind: Gremlin
  queries:
    query1:
  properties: 
    addedResultCommand: g.addV('Hello-world-from').property('MessageId', @MessageId).property('name',@MessageFrom)
    updatedResultCommand: g.addV('Vehicles').property('Id', @before.VehicleId).property('orders',@Orders)
    deletedResultCommand: g.V().has('MessageId', @MessageId).drop()
    gremlinHost: <hostname>
    gremlinPassword: <password>
    gremlinUsername: <username>
    gremlinPort: <port>

In this definition:

• the apiVersion must be v1.
• the kind property tells Drasi to create a Reaction resource.
• the spec.kind property tells Drasi the kind of Reaction to create, in this case a Gremlin Reaction.
• the name property tells Drasi the identity of the Reaction and must be unique within the scope of Reactions within the target Drasi environment. In the above example, the name of the Reaction is gremlin-reaction.

This table describes the other settings in the spec section of the Reaction definition:

Note: When defining the Gremlin commands, add @ before any parameter name to use a query’s return value as the parameter.

For the updatedResultCommand field, if the parameter has the prefix before., the value from the result set before the change will be used. If the parameter has the after. prefix, the value from the result set after the change will be used. By default, the result set after the change will be used. In this example, the parameter VehicleId will use the value from the before result set, whereas the parameter Orders will use the after result set

Secret Configuration

It is best practice to store private credentials for your database in a Kubernetes secret, which can be created using kubectl. The example below creates a Secret with the name gremlin-creds, containing one key called password in the drasi-system namespace.

kubectl create secret generic gremlin-creds --from-literal=password=<db-password> -n drasi-system

You can then reference the secret when you create a Gremlin Reaction as follows:

kind: Reaction
apiVersion: v1
name: gremlin-reaction
spec:
  kind: Gremlin
  queries:
    query1:
  properties: 
    addedResultCommand: g.addV('Hello-world-from').property('MessageId', @MessageId).property('name',@MessageFrom) 
    updatedResultCommand: g.V()  
    deletedResultCommand: g.V().has('MessageId', @MessageId).drop() 
    gremlinHost: <hostname>
    gremlinPassword: 
        kind: Secret
        name: gremlin-creds
        key: password
    gremlinUsername: <username>
    gremlinPort: <port>

Inspecting the Reaction

As soon as the Reaction is created it will start running, subscribing to the specified list of Continuous Queries and processing changes to the Continuous Query results.

You can check the status of the Reaction using the drasi list command:

drasi list reaction

Or including a target namespace:

drasi list reaction -n drasi-namespace

This will return a simple list of all Reactions in the default (or specified) namespace and their overall status. For example:

        ID               |    AVAILABLE
-------------------------+-----------------
    gremlin-reaction     |      true

If an error has occurred during the creation or operation of a Reaction, the AVAILABLE column will contain the error text instead of true or false.

For more details about the Reaction you can use the drasi describe command:

drasi describe reaction gremlin-reaction

This will return the full definition used to create the Reaction along with more detailed status information.

Modifying the Reaction

If you want to modify an existing reaction, you can use the drasi apply command to apply the updated YAML file. Ensure that the name of the reaction remains consistent.

Deleting the Reaction

To delete a Reaction you use the drasi delete command. There are two ways to do this.

Firstly, you can specify the type of resource (Reaction) and its name, for example:

drasi delete reaction gremlin-reaction

Secondly, you can refer to the YAML file(s) that contain the definitions used to create the Reaction(s):

drasi delete -f my-reaction.yaml <file2.yaml> <file3.yaml> <...>

This is a convenience, especially if a single YAML file contains multiple Reaction definitions.

If the Reaction is not in the default Drasi namespace, you should specific the target namespace using the -n flag as usual:

drasi delete -f my-reaction.yaml -n drasi-namespace