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  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 runsonly once and if it fails the container is unable to start. No provision to restart / retry. (So if it runs before the application is not ready then it will eventually fail)

  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 container itself and then execute some code (which is creating a dataspace in our scenario)

    Example

    Code Block
    languagejava
    titleApplicationReadyEvent
    @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.
    We can execute some script , may be wait for some time and then try to create a dataspace.


Summary


Sr No.ApproachAnalysisComments
1init-containerWould require dependency on SQL (as app container doesn't come up until this point)
2k8s-hooksPostStart hook : executed once, if failed we cant retry.
3Events in SpringApplication specific solution. We can listen the event and trigger the action.
4Sidecar ContainerBoth container runs in parallel (if we cant establish relationship i.e sidecar container wait for main container to start then difficult to execute command on a non ready container)

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