Allow for consul services to be used as configuration

[kesor]

When services are registered as consul services, the service has several properties that can be queried via consul APIs. For example a MongoDB registered service has an Address, Port, Tags as part of the consul service.

The current configuration of endpoints for Spring Cloud applications is hardcoded in Consul K/V store. But some of these (see examples in http://docs.spring.io/spring-boot/docs/current/reference/html/common-application-properties.html) can be Consul services, not hardcoded non-moving parts.

Allow for using Consul services as application configuration.

[dsyer]

Can you be a bit more explicit about the use case? I'm not sure what you mean by "current configuration of endpoints". Is it just that the ServiceInstance doesn't contain "tags" metadata from Consul? Or do you want to bridge from ServiceInstance to the Spring Environment?

[kesor]

@dsyer At the moment, spring-cloud-consul-config loads configuration from the Consul Key-Value store. These configuration values are used by multiple Spring frameworks/libraries, for example MongoDB looks for spring.data.mongodb.host and spring.data.mongodb.port, another example RabbitMQ looks for spring.rabbitmq.host and spring.rabbitmq.port.

On the other hand, you have Consul services, which are not Key-Value entries. The request is to use the service entries as configuration for Spring libraries.

[dsyer]

I see (I think). There are potential lifecycle issues (e.g. MongoDB might be needed before the DiscoveryClient is available), but maybe that's nothing new. You would probably want your MongoDB connection to be @RefreshScope, to protect you against changes in the service catalog (so not the one that is created by Spring Boot). I recommend you play around with that a bit and find out if it at least works for your use case, and then we will have some code to talk about, and we can decide if it is a new feature or just a customization that users can apply if they choose.

P.S. if we can implement something using DiscoveryClient it wouldn't be Consul-specific (that's why I mentioned it). But if Consul has some unique features that make this work better without that abstraction I suppose that's also worth looking into (can't think what they would be though).

[spencergibb]

I don't know that there is anything consul specific about the request, except that it's easier to register services like a db in consul since it's already a sidecar and you just need a bit of configuration.

[dhait]

Currently in Spring Boot, we can use interpolated values in properties:

app.description=${app.name}

Perhaps we could do it this way: Register the database as an external service, so that 'mongodb.service.consul' resolves to the MongoDB service. Then, in the config on consul:

spring:
  data:
    mongodb:
       host: ${service/mongodb/host}

Or something along those lines?

[spencergibb]

@dhait what happens when there is more than one instance registered?