Nats
The nats service is the event broker of the system. It distributes events among all other services and enables other services to communicate asynchronous.
Services can Publish events to the nats service and nats will store these events on disk and distribute these events to other services eventually. Services can Consume events from the nats service by registering to a ConsumerGroup. Each ConsumerGroup is guaranteed to get each event exactly once. In most cases, each service will register its own ConsumerGroup. When there are multiple instances of a service, those instances will usually use that ConsumerGroup as common resource.
Underlying Technology
As the service name suggests, this service is based on NATS specifically on NATS Jetstream to enable persistence.
Default Registry
By default, nats-js-kv is configured as embedded default registry via the MICRO_REGISTRY environment variable. If you do not want using the build-in nats registry, set MICRO_REGISTRY_ADDRESS to the address of the nats-js cluster, which is the same value as OCIS_EVENTS_ENDPOINT. Optionally use MICRO_REGISTRY_AUTH_USERNAME and MICRO_REGISTRY_AUTH_PASSWORD to authenticate with the external nats cluster.
Persistance
To be able to deliver events even after a system or service restart, nats will store events in a folder on the local filesystem. This folder can be specified by setting the NATS_NATS_STORE_DIR enviroment variable. If not set, the service will fall back to $OCIS_BASE_DATA_PATH/nats.
TLS Encryption
Connections to the nats service (Publisher/Consumer see above) can be TLS encrypted by setting the corresponding env vars NATS_TLS_CERT, NATS_TLS_KEY to the cert and key files and ENABLE_TLS to true. Checking the certificate of incoming request can be disabled with the NATS_EVENTS_ENABLE_TLS environment variable.
Certificate files can also be set via global variables starting with OCIS_, for details see the environment variable list.
Note that using TLS is highly recommended for productive environments, especially when using container orchestration with Kubernetes.