References:

CPS-1005 - Getting issue details... STATUS

Overview:

This POC is for finding ways to create a resource (NCMP for this instance).

Few possibilities were discussed during the Grooming session which are listed as below.

1. init-container
2. k8s hooks
3. Events in Spring.
4. Sidecar container

Issues and decisions

Analysis

1. init-container
   
   Init containers are exactly like regular containers, except:
       -  Init containers always run to completion.
       -  Each init container must complete successfully before the next one starts.
   
   Since our application container would not be running the before the init-container so we cant invoke any endpoint to create a resource and we dont want to go with DB related script(SQL) to create a resource (insert a row in the database) so I am not exploring this further for this use case.
   
   
2. k8s hooks
   
   - postStart hook : It runs only once and if it fails the container is unable to start. No provision to restart / retry.
   
   

3. Events in Spring
   
   We can use the ApplicationReadyEvent which is published when the application is ready to service requests.
   We can listen to this event , which is thrown by the SpringApplication itself and then execute some code (which is creating a dataspace in our scenario)
   
   Example
   
   

   ApplicationReadyEvent

   @EventListener(ApplicationReadyEvent.class)
       public void initializeNcmp() {
           log.info("Initializing NCMP when the ApplicationReadyEvent is received");
           try {
               cpsAdminService.createDataspace("NCMP-CmHandle-Operational-Data");
           } catch (final AlreadyDefinedException ex) {
               log.info("Already defined : {} ", ex.getMessage());
           } catch (final Exception ex) {
               log.warn("Failed to create dataspace, cause : {}", ex.getMessage());
           }
       }

4. Sidecar container
   
    - Wait for the readiness check of the main container to pass and then execute the script to create a dataspace.
    - Looks like we are unable to wait for the other container readiness check and then execute the command. (Connection Refused ) - need more analysis on this.
    - If we are unable to wait for the main container to start then dont go ahead with this approach (as all the containers run in parallel)
   
   Also general use case of sidecar container is for doing continuous help of main container (eg. log shipping , monitoring etc) , but our use case is kind off a one-off activity.

Summary

References

https://docs.spring.io/spring-boot/docs/current/api/org/springframework/boot/context/event/ApplicationReadyEvent.html

https://kubernetes.io/docs/concepts/workloads/pods/init-containers/

https://medium.com/bb-tutorials-and-thoughts/kubernetes-learn-sidecar-container-pattern-6d8c21f873d

https://kubernetes.io/docs/tasks/configure-pod-container/attach-handler-lifecycle-event/