Frequently Asked Questions

This section provides answers to commonly asked questions pertaining to the Eventing Service and Functions.

Generic

  • What kind of nodes do I run my Eventing Service on?

    Eventing leverages the latest trends in multi-core CPUs. The nodes for the Eventing Service should optimally have a higher number of cores than those for indexing.

  • Can the Eventing Service be co-located with other services?

    Yes, the Eventing Service node can be co-located with other MDS-enabled services.

  • Is it supported for both text and binary formats of the documents?

    The value or the document must always be text in the form of a JSON document.

Change Capture

  • Will all the updates to a document be seen in DCP?

    When a document is updated multiple times in a small time-window, not all updates are seen in DCP. This means that the event handlers see only those deduplicated events that are published in DCP.

  • Can a Function listen to all changes across Couchbase Server?

    A defined Function listens only to changes that are exposed using the DCP for the buckets (Couchbase and Ephemeral) in the data-nodes. Memcached buckets are not supported. The Function cannot listen to changes happening in other Couchbase components, such as FTS, Views, or GSI; nor can it listen to system events.

  • Can we determine what has changed on the document?

    No. But you can implement versioning as part of the application logic. If versioning is important, include it as part of the data architecture.

  • Can I get old and new values of the document inside the Function?

    No. We do not support versioning of documents, therefore this feature is not available out of the box. However, you can have a parent bucket and its replica version. The replica bucket would retain the version of documents before the current set of changes.

  • Is the ordering of document mutations enforced?

    All changes from a document are always processed in order.

Functions

  • Do I need a separate middleware to consume the Functions? How do I consume changes in my middleware/application?

    Database changes are consumed using the defined Function. There is no other programmatic way of accessing the changes (such as by using an SDK, or some other form of middleware). REST endpoints are exposed, to perform administrative operations

  • Can I import my own JS libraries or Node Modules?

    No. We do not allow import of node modules or external JS libraries.

  • Are Functions supported for both Data Node and Document storage?

    The Eventing Service listens to changes that appear in the DCP. DCP is valid for the Data Service, and Functions operate on documents that are either in key-value or in document (JSON) format.

  • How to perform Functions lifecycle operations from CI/CD?

    To perform Functions lifecycle operations from CI/CD, refer to CLI Eventing section.

  • How to invoke a REST endpoint from inside the Function?

    To invoke a REST Endpoint from inside the Function, refer to Functions REST API section.

  • How does the Functions offering compare with the Couchbase’s Kafka Connector?

    The Functions offering is about server-side processing or compute; it does not require any middleware to be deployed or managed. Couchbase’s Kafka connector is an SDK component that needs an application container or middleware to run.

Function Handler Code

  • Why can’t I create global variables?

    We restrict the language model in such a way that chances of going wrong are minimized. As Functions are stateless compute entities, global variables do not have a good use-case, and therefore, they are not supported. Though you can define Javascript functions inside a Function (outside the scope of OnUpdate and OnDelete) that can be invoked any number of times from either of the event handlers.

  • What is in the "meta" Function parameter (OnUpdate, OnDelete)? Is this the metadata we currently write in order to figure out what has changed in the document?

    These are the meta fields associated with the document. For more information, refer to the Link section.

  • Can there be more than one Function listening to changes to a bucket?

    Yes. More than one Function can be defined for the same bucket. This lets you process the change according to the business logic that you enforce. But there is no ordering enforced; for example, if bucket 'wine' has three different Functions, which are FunctionA, FunctionB, and FunctionC, you cannot enforce the order in which these Functions are executed. Also, database triggers suffered from scalability and diagnosability issues. Functions offer multiple diagnosability solutions and is highly scalable and performant.

  • Is it possible to get additional state during a Function execution? For example, can you read from the data service in a Function to fetch related data? For example, can we enrich the updated document with data from another document (using a document id)?

    Yes. You can read from any other bucket, and enrich the document.

Cluster Behavior

  • What happens to the Eventing Service during a failover condition?

    When the Data service experiences a failover condition, mutations may be lost and these lost mutations are not processed by the Eventing service. When the Eventing node experiences a failover condition, few mutations may be processed more than once.

  • Does a rebalance have any effect on the firing of events?

    No. Functions do not lose any mutations during a rebalance operation.