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Management Model Referential Integrity and Suggestions

A significant improvement in WildFly 11 is much better support for referential integrity when one resource in your configuration refers to another resource. Going beyond just checking that your references are correct, the server provides reference information that our CLI and the HAL web console are able to use to suggest valid values to you as you set up your configuration.

Configuration references

When you are configuring a WildFly server, a common thing you need to do is configure attributes whose value refers to the name of some other resource. A common example of this is a resource that includes a socket-binding attribute:

[standalone@localhost:9999 /] /subsystem=undertow/server=default-server/ajp-listener=ajp:add(socket-binding=ajp)

When you do this, it’s because the services managed by the resource you are configuring need some capabilities provided by another resource. What you’re doing is configuring which one to use. But what kind of resource that socket-binding attribute refers to may not be obvious, and what the valid values are is also not obvious. And before the rollout of improved reference support, if you got it wrong the failure you’d see could be difficult to understand.

Here, using WildFly 9 with a config where the previously unused 'ajp' socket binding config had been removed, we try and add an AJP listener to the web container:

[standalone@localhost:9990 /] /subsystem=undertow/server=default-server/ajp-listener=ajp:add(socket-binding=ajp)
{
    "outcome" => "failed",
    "failure-description" => {"WFLYCTL0180: Services with missing/unavailable dependencies" => ["jboss.undertow.listener.ajp is missing [jboss.binding.ajp]"]},
    "rolled-back" => true
}

That error message says nothing about where to go to correct the mistake.

Worse, if you made that mistake when working with a server started in admin-only mode, that bad reference would not be detected when you entered it.

[standalone@localhost:9990 /] reload --admin-only=true
[standalone@localhost:9990 /] /subsystem=undertow/server=default-server/ajp-listener=ajp:add(socket-binding=ajp)
{"outcome" => "success"}

The configuration would be updated and the problem would only be detected when the server was reloaded or restarted not in admin-only mode. The server would boot but would not function correctly.

Referential Integrity Checks and Reference Suggestions

Starting in WildFly 10 and greatly expanded in WildFly 11, we’ve added reference description metadata to our resources and attributes, and we use that to proactively ensure that management operations that violate referential integrity fail immediately.

The same incorrect operation shown above will now fail immediately, with a message that gives a hint as to where you can configure the missing resource:

[standalone@embedded /] /subsystem=undertow/server=default-server/ajp-listener=ajp:add(socket-binding=ajp)
{
    "outcome" => "failed",
    "failure-description" => "WFLYCTL0369: Required capabilities are not available:
    org.wildfly.network.socket-binding.ajp; Possible registration points for this capability:
		/socket-binding-group=*/socket-binding=*",
    "rolled-back" => true
}

The same failure will happen if the server is running in admin-only mode (with some exceptions; see "Referential Integrity Checks in an admin-only Process" below.)

If you think the resource you need already exists, but you’re not sure of its name, you can use CLI tab completion to get a list of suggestions:

[standalone@embedded /] /subsystem=undertow/server=default-server/ajp-listener=ajp:add(socket-binding=
http  https  management-http  management-https  txn-recovery-environment  txn-status-manager
[standalone@embedded /] /subsystem=undertow/server=default-server/ajp-listener=ajp:add(socket-binding=

Once the needed socket binding resource is added, it is available in the tab completion results.

[standalone@embedded /] /socket-binding-group=standard-sockets/socket-binding=ajp:add(port=8009)
{"outcome" => "success"}
[standalone@embedded /] /subsystem=undertow/server=default-server/ajp-listener=ajp:add(socket-binding=
ajp                       https                     management-https          txn-status-manager
http                      management-http           txn-recovery-environment
[standalone@embedded /] /subsystem=undertow/server=default-server/ajp-listener=ajp:add(socket-binding=ajp)
{"outcome" => "success"}

The HAL console will also suggest valid values by means of a pull-down:

choose socket binding

If you try and remove a resource whose capabilities are depended upon by other resources, that will also result in a failed operation:

[standalone@embedded /] /socket-binding-group=standard-sockets/socket-binding=ajp:remove
{
    "outcome" => "failed",
    "failure-description" => "WFLYCTL0367: Cannot remove capability 'org.wildfly.network.socket-binding.ajp' as it is required by other capabilities:
capability 'org.wildfly.undertow.listener.ajp' requires it for attribute 'socket-binding' at address '/subsystem=undertow/server=default-server/ajp-listener=ajp'",
    "rolled-back" => true
}

Referential Integrity Checks in an admin-only Process

If your xml configuration file contains invalid references and you start the server normally, the server will fail to boot and the log will have an error message describing the problem. However, if you start the server with the --admin-only flag, the server boot will not fail. This is because starting in admin-only and manipulating the configuration via the CLI is the recommended way of correcting your configuration. If we didn’t allow the server to boot, the user would have no alternative to manually editing the xml.

When the server is started in this state, no operation will be rejected due to an invalid reference until all referential integrity problems have been corrected. Once the configuration reaches a state where there are no integrity issues, thereafter any changes that break integrity will be rejected. If a server is started in admin-only and has no integrity problems at boot, any changes that break integrity will be rejected. So, leniency in integrity checks is only enabled when the server’s configuration at boot has problems.

Further Work

The referential integrity functionality discussed here first began to appear in WildFly 10, but it’s use was greatly expanded in WildFly 11, and the use of it to drive CLI tab completion and HAL pulldowns is new in 11. But still, we don’t yet have complete coverage of all capabilities subsystems provide, although the bulk of cases are covered, particularly those involve configuration attributes. Rollout of the use of capabilities will continue in future WildFly releases.

More Information

If you are interested in learning more about how the capabilities and requirements system works from the point of view of someone working on developing WildFly, please see the Working with WildFly Capabilities document in the WildFly documentation.

Brian Stansberry

Brian Stansberry is a principal software engineer at Red Hat, working as a core engineer on WildFly. He is currently leading the development of the administration and management capabilities in WildFly and JBoss EAP 6. Previously, he was the technical lead for the application server’s high availability clustering features. Brian’s background is in international business and East Asian studies, with a bachelor’s degree from Michigan State and a master’s degree from Stanford. Before getting bitten by the software bug, Brian had a successful career in corporate finance in the semiconductor industry. He started working on JBoss technologies in 2003 and joined the company in 2005.

Location:  Ballwin, MO

Occupation:  Domain Management Lead

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