Allow configuring Cassandra configuration file

[mp911de]

Spring Boot allows configuration of the most common configuration options for Cassandra driver 4 and allows customization of the session builder and DriverConfigLoader.

Configuring additional driver config options (e.g. driver profiles, additional settings) requires quite some code to do so. It would be convenient to use the Driver's config file format for an easier configuration mechanism since the majority of configuration examples use config files as using config files is the preferred mechanism.

Ideally, a config file can be specified through a property (e.g. spring.data.cassandra.config-file) that gets included if the property is set after defaultOverrides and before Spring Boot's config. Doing so allows picking up the config file. It also allows overriding Cassandra settings using system properties and the specified config file may override Spring Boot's properties which aligns nicely with the typical ordering of PropertySources.

Reference

• https://docs.datastax.com/en/developer/java-driver/4.9/manual/core/configuration/#concepts
• https://docs.datastax.com/en/developer/java-driver/4.9/manual/core/pooling/
• https://docs.datastax.com/en/developer/java-driver/4.9/manual/core/load_balancing/

[snicoll]

and the specified config file may override Spring Boot's properties which aligns nicely with the typical ordering of PropertySources.

I think that's inconsistent with "it gets included if the property is set after defaultOverrides and before Spring Boot's config.". Specifying the config file as a system property or in application.properties will give it the same precedence. For that reason, I think the file should come first and take precedence over anything else that has been defined.

[snicoll]

I've started hacking on this and I am not sure what I am doing wrong, perhaps @adutra could help me?

It looks like all values in the file are considered a String so "basic.request.timeout = 5 seconds" for instance leads to

basic.request.timeout has type STRING rather than NUMBER

Also, I am having second thought about introducing a property to a file. Users may want to use a different format for the file or different precedences rules. Maybe we need to reconsider our options here?

[adutra]

@snicoll I see here that you are trying to read the value of basic.request.timeout as a long with Config.getLong().

However the value for that key in the file is the string 100 milliseconds; this particular string can only be parsed as a string (obviously) or a Duration, not as a long. The following lines would both work:

driverConfig.getProfile("first").getString(DefaultDriverOption.REQUEST_TIMEOUT)
driverConfig.getProfile("first").getDuration(DefaultDriverOption.REQUEST_TIMEOUT)

I recommend going with getDuration since this setting expects its value to be convertible to a valid Duration instance.

[snicoll]

Thanks @adutra, that fixed it.

I am having a hard time figuring out what the precedence should be. From my experiment, there isn't any satisfactory outcome.

Spring's Environment takes precedence over the file

This looks the more natural option to me after having discussed this one with @wilkinsona. Whatever you put in the environment wins over a property in the file. Given our configurable Environment this is the most logical way of doing things (i.e. a user may set a spring.data.cassandra.contact-points as an env property to override the Cassandra cluster location).

I've tried to try this using:

private Config createConfig(CassandraProperties properties) {
	Config config = mapConfig(properties);
	Resource configFile = properties.getConfig();
	if (configFile != null) {
		config.withFallback(loadConfig(configFile));
	}
	return  config;
}

Two problems with this approach:

• Profiles are not loaded. I don't understand if that's a limitation of the API (you can't have a profile in a fallback Config) or something in the code above is wrong
• Any default in CassandraProperties takes precedence over a value in the file. That's really counter-intuitive as if there is nothing in the environment for the property, you'd expect the property in the file to win. Fixing this would be cumbersome as the whole point of CassandraProperties is that we don't have to care if the user has set the property or if the default should be used, if any).

File takes precedence over Spring's Environment

This was my first approach to ignore the precedence problem I've described previously. If you specify a file, it will take precedence on anything. That also means that you can't really use the file to specify some defaults that you may want to override using the specific property.

Only file is used when set

If a file is used, we use that and only that, ignoring any other property we currently handle. This sounds like a hard sell for a Spring Boot user who's used to the auto-completion experience of application.properties in the IDE.

What do you think? I am missing something?

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