Provide an example of how to configure multiple datasources and configuration consumption

[odrotbohm]

It seems that there are a couple of tricks needed to get a manual setup of multiple DataSources working nicely including the externalization of properties. Ideally the sample would show:

1. How to structure properties so that each DataSource has its own distinct configuration.
2. Show how to declare @Bean methods to that they pick up the configuration from the declared property sets.
3. Optionally (if possible and recommended), show how to share some of the properties declared (e.g. user and password) to avoid mistakes when configuration changes.

[philwebb]

Perhaps we can extend http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#howto-two-datasources

[odrotbohm]

That's a nice find actually. When skimmed the docs, I quickly found http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#boot-features-configure-datasource scrolled through it, reached the Spring Data JPA specific bits and thought that was it. I didn't even think there could be anything further below. Linking to the HowTo from the reference section would be very helpful.

[snicoll]

I am looking at building the sample and some things are bugging me. I can see others in the community are confused by it.

Spring Boot advertizes that we can automatically detect the driver class name based on the URL and it will automatically provide an embedded URL if no url is provided. These are pretty interesting feature but we can't cleanly apply them when one wants to configure two datasources.

The main issue is that DataSourceProperties does it and it does more things: you can specify the jndi-name of the datasource (only used by the auto-configuration) and several attributes used to initialize the datasource.

The main goal of such arrangement is to keep the auto-completion in the IDE for the custom namespaces. With the current design, a bunch of keys would be exposed that have no effect. I've start to hack something in a branch and the end-result is quite concise and only exposes the key that actually do something in practice.

Still, we're not there yet. DataSourceAutoConfiguration exposes a DataSourceInitializer almost unconditionally and the latter makes a weird link between DatasourceProperties and DataSource. We should fix that (typically not do anything at all if a custom DataSource is provided).

All in all, I don't think that sample can go in 1.4. ping @philwebb

[cptwunderlich]

As an affected user, I'd like to comment on this discussion. I was just trying to implement a feature using two datasources.

I found the documentation mentioned above, but it has two issues:

1. They use prefixes like 'datasource.primary', where the normal property is called spring.datasource.* (e.g., spring.datasource.initialize). So it looks like the resulting key would be 'spring.datasource.primary.', but looking at other examples, it would be 'datasource.primary.datasource.'?

2. There is one section called 'Configure Two DataSources' and later 'Use Two EntityManagers'. The latter references code from the first one. But if you try out the code like that, you'll find out that something else is missing.

In fact, the best source I've found, was a stackoverflow posting referencing the docs and this example: https://github.com/snicoll-demos/demo-multi-entity-managers

The docs should also mention, that when you configure an additional DataSource/EntityManager, the auto-configuration won't work anymore. I added a secondary DataSource+EntityManager like in the example, but now I get a warning "No bean named 'entityManagerFactory' available" and a follow-up Exception

[snicoll]

Thanks for the feedback.

I didn't get 1. If the prefix is datasource.primary, it is datasource.primary and none of the Spring Boot features apply since you're taking control over the configuration. It's a common principle in Spring Boot: if you configure things yourself, you are in control.

As of 2, I am actually keen on moving the sample from my repo to the main Spring Boot repo and reference it in the doc.