Cascade Delete

This example illustrates how to leverage the Eventing Service to perform a cascade delete operation.

Need for Cascade Delete Operation

Couchbase is a NoSQL data platform and therefore does not provide referential integrity. When a user gets deleted, the data platform should provide a reliable method to delete all dependent transactions and documents of the user.

Developing your Handler Code

Create a JavaScript Function that contains an OnDelete handler. The handler listens to data-changes within a specified source bucket, the users bucket. When a user within the source bucket gets deleted, the handler executes a routine to remove the deleted user.

In this example, the handler executes the routine when a delete operation is performed. The handler then uses a N1QL query to fetch all associated documents/transactions of this user. All the fetched documents are removed from the cluster. The handler then records the delete operation in a Function-specific application log file.

A sample handler code for cascade delete is provided.

function OnUpdate(doc, meta) {
    log('OnUpdate document:', meta.id);
}

function OnDelete(meta) {
    log('Document Deleted:', meta.id);
    if(meta.id < 10)
    {
       try
       {
            var this_user_id = meta.id;
            delete from `transactions` where user_id = TONUMBER($this_user_id);
            log('Deleted Orphaned Transactions for User:', this_user_id);
       }
       catch(e)
       {
           log('Exception:', e)
       }
    }
}

Prerequisites

Before you begin, you’ll need to ensure that two buckets, metadata and users buckets, are created. See Creating Buckets.

Procedure

Proceed as follows:

From the Couchbase Web Console > Eventing page, click ADD FUNCTION, to add a new Function.
In the ADD FUNCTION dialog, for individual Function elements, provide the below information:
1. For the Source Bucket drop-down, select the Users bucket option that was created for this purpose.
2. For the Metadata Bucket drop-down, select the metadata bucket option that was created for this purpose.
3. Enter delete_orphaned_txns as the name of the Function you are creating in the Function Name text-box.
4. Enter Delete Orphaned Transactions from the `transactions’ bucket when user_id is less than 10 in the Description text-box.
5. For the Settings option, use the default values.
6. For the Bindings option, specify users as the name of the bucket and specify src_user as its associated value.
In the ADD FUNCTION dialog, click Next: Add Code. The delete_orphaned_txns dialog appears. The delete_orphaned_txns dialog initially contains a placeholder code block. You will substitute your actual delete_orphaned_txns code in this block.
Copy the sample Function handler code, and paste it in the placeholder code block of the delete_orphaned_txns screen:
After pasting, the screen appears as displayed below:
To return to the Eventing screen, click Eventing. The Function delete_orphaned_txns is listed as a defined Function.
Click Deploy.
In the Confirm Deploy Function dialog, from the Feed boundary drop-down, select Everything and click Deploy Function.
From this point, the defined Function is executed on all existing documents and on subsequent mutations.
Navigate to the Couchbase Web Console > Query page. Before deleting a user, a snapshot of Query Resultfrom the users bucket is displayed for reference:
The Query Results display users with user_ids from 1 to 10.
Navigate to the Couchbase Web Console > Buckets page. Delete two users from the Users bucket:
1. Select User4 from the list and click the delete icon.
2. Select User10 from the list and click the delete icon.
From the Query Editor, execute an N1QL query to check that all related records for the deleted users are removed from the cluster.
```
SELECT user_id, COUNT(1) FROM `Users` GROUP BY user_id ORDER BY user_id ASC;
```
In the Query Results pane notice that user_ids, user_id4 and user_id 10 are removed as part of the cascade user delete operation.

To summarize, since Eventing Service is a homegrown offering, we uses N1QL queries in the Function handler code. The transpiler in Couchbase Server converted the N1QL query to a code that the JavaScript engine understands, and thereby made the cascade delete operation look easy.