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core/cosmosdb/cache-invalidator/README.md

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OpenWhisk Cache Invalidator Service

This service performs cache invalidation in an OpenWhisk cluster to enable cache event propagation in multi region setups.

Design

An OpenWhisk cluster uses a Kafka topic cacheInvalidation to communicate changes to any cached entity. Messages on this topic are of the form

json
{"instanceId":"controller0","key":{"mainId":"guest/hello"}}

When deploying OpenWhisk across multiple nodes which do not share a common Kafka instance, we need a way to propagate the cache-change events across the cluster. For CosmosDB based setups this can be done by using CosmosDB ChangeFeed support. It enables reading changes that are made to any specific collection.

This service makes use of change feed processor java library and listen to changes happening in whisks and subject collections and then convert them into Kafka message events which can be sent to cacheInvalidation topic local to the cluster

Usage

The service needs following env variables to be set

  • KAFKA_HOSTS - For local env it can be set to 172.17.0.1:9093. When using OpenWhisk Devtools based setup use kafka
  • COSMOSDB_ENDPOINT - Endpoint URL like https://<account>.documents.azure.com:443/
  • COSMOSDB_KEY - DB Key
  • COSMOSDB_NAME - DB name

Upon startup it would create a collection to manage the lease data with name cache-invalidator-lease. For events sent by this service instanceId are sent to cache-invalidator

Local Run

Setup the OpenWhisk cluster using devtools but have it connect to CosmosDB. This would also start the Kafka Topic UI at port 8001. Then when changes are made to the database, you should see events sent to the Kafka topic. For example, if a a package is created with wsk package create test-package using the guest account, the following event is generated:

json
 {"instanceId":"cache-invalidator","key":{"mainId":"guest/test-package"}}