Including mpHealth feature breaks standalone application in Open Liberty - open-liberty

Question

I'm currently playing with Open Liberty 18.0.0.4 and its Microprofile support. When I try to build a standalone runnable Jar which includes the mpHealth-1.0 feature, startup fails with
[ERROR ] CWWKF0033E: The singleton features com.ibm.websphere.appserver.javax.servlet-3.1 and com.ibm.websphere.appserver.javax.servlet-4.0 cannot be loaded at the same time. The configured features mpHealth-1.0 and jaxrs-2.1 include one or more features that cause the conflict. Your configuration is not supported; update server.xml to remove incompatible features.
[ERROR ] CWWKF0033E: The singleton features com.ibm.websphere.appserver.javax.annotation-1.2 and com.ibm.websphere.appserver.javax.annotation-1.3 cannot be loaded at the same time. The configured features mpHealth-1.0 and jaxrs-2.1 include one or more features that cause the conflict. Your configuration is not supported; update server.xml to remove incompatible features.
[ERROR ] CWWKF0033E: The singleton features com.ibm.websphere.appserver.javaeeCompatible-8.0 and com.ibm.websphere.appserver.javaeeCompatible-7.0 cannot be loaded at the same time. The configured features jsonb-1.0 and mpHealth-1.0 include one or more features that cause the conflict. Your configuration is not supported; update server.xml to remove incompatible features.
[ERROR ] CWWKF0033E: The singleton features com.ibm.websphere.appserver.javax.cdi-2.0 and com.ibm.websphere.appserver.javax.cdi-1.2 cannot be loaded at the same time. The configured features jsonb-1.0 and mpHealth-1.0 include one or more features that cause the conflict. Your configuration is not supported; update server.xml to remove incompatible features.
I'm using jaxrs-2.1 and jsonb-1.0 and everything works fine. As soon as I add the mpHealth-1.0 feature I get the above errors. Here's the relevant snippet of the server.xml:
<featureManager>
<feature>jaxrs-2.1</feature>
<feature>jsonb-1.0</feature>
<feature>mpHealth-1.0</feature>
</featureManager>
And this is what the started Jar says at then end (but no service will be available):
[AUDIT ] CWWKF0012I: The server installed the following features: [jsonb-1.0, servlet-4.0, jndi-1.0, mpHealth-1.0, json-1.0, cdi-1.2, jsonp-1.1, jaxrsClient-2.1, jaxrs-2.1].

Answer 1

This error occurs because not all OpenLiberty features are compatible with each other, and if you enable conflicting features you get an error indicating which features conflict. It should be resolvable by enabling additional features in your server.xml to help the runtime disambiguate which features should be enabled.
The two main reasons features conflict are:
Two versions of the same feature try to be enabled (e.g. foo-1.0 and foo-2.0)
A mix of Java EE 7 and Java EE 8 features are enabled (e.g. cdi-1.2 from EE 7 and jaxrs-2.1 from EE 8)
To break down the feature dependencies of what you have enabled, it looks something like this:
- jsonb-1.0 -> jsonp-1.1
- jaxrs-2.1 -> cdi-2.0
-> servlet-4.0
- mpHealth-1.0 -> cdi-1.2 (tolerates cdi-2.0)
The reason you are seeing these errors is because the OpenLiberty feature manager does not have sufficient evidence to know that it can failover to the mpHealth-1.0 -> cdi-2.0 dependency.
To fix this, you have two options:
Enable the cdi-2.0 feature in server.xml. This should help to disambiguate for the feature manager so it can failover to the mpHealth-1.0 -> cdi-2.0 dependency.
Instead of enabling individual features, enable the microProfile-2.0 convenience feature. Here you won't have to worry about feature conflicts, but it will load more features into the runtime (e.g. MP Metrics, MP Config, MP Fault Tolerance, etc), which will incur some additional startup time and memory footprint cost.