NIFI-9967: Added FlowSerializationStrategy to determine which flow formats (XML,JSON) to save

[mattyb149]

Description of PR

Adds a FlowSerializationStrategy enum to determine whether to save the flow in XML and/or JSON formats. NiFi saves both, MiNiFi saves only the XML version (as this is what is overwritten by config.yml)

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[simonbence]

Some minimal remarks, otherwise it looks good to me.

[simonbence]

Minor suggestion: according to the WRITE_XML_AND_JSON value and the usage, we do not plan to use this like having a set of possible strategies, but always using only a single value. As of this, I would suggest to name the values like WRITE_XML_ONLY, and WRITE_JSON_ONLY.

[simonbence]

I think it would be useful to encapsulate the decision logic within the enum. The FlowSerializationStrategy could have a writesJSON and a writesXML methods with boolean return values. By this, the condition would change simply to serializationStrategy.writesJSON() and serializationStrategy.writesXML() accordingly.

[simonbence]

Due to simmerty and having the same responsibilities for standalone and clustered methods, I would add an argument for this as well. As I found, it is callled only in the StandardFlowServiceFactoryBean, and the used enum might be specified there (like in case of standalone)

[mattyb149]

Only NiFi itself runs in clustered mode, the reason we have an extra argument in the standalone instance is for MiNiFi, which needs WRITE_XML_ONLY and calls getStandaloneInstance() directly (not via Spring)

[bejancsaba]

+1 from me I just had a minor enum structuring related proposal. Thanks for the change, the issue came up at our current minifi c2 work as well.

[bejancsaba]

It is just syntactical but maybe better to read (but this could be really subjective), what do you think about:

public enum FlowSerializationStrategy {
    WRITE_XML_ONLY(false, true),
    WRITE_JSON_ONLY(true, false),
    WRITE_XML_AND_JSON(true, true);

    private final boolean writesJson;
    private final boolean writesXml;

    FlowSerializationStrategy(boolean writesJson, boolean writesXml) {
        this.writesJson = writesJson;
        this.writesXml = writesXml;
    }

    public boolean writesJson() {
        return writesJson;
    }

    public boolean writesXml() {
        return writesXml;
    }
}

[kevdoran]

I don't have any objection to this, and it is a small change that fixes key functionality, so overall I'm a +1.

That said, I hope that eventually we can move minifi to using our VersionedFlowSnapshot JSON format everywhere (in C2, in place of config.yml, in place of flow/xml.gz) which I feel will simplify a lot of the codebase in places where we have to deal with different formats.