NetKernel News Volume 7 Issue 2

February 26th 2016

• Repository Updates
• JVM Constraint
• New: JSON Schema Validator
• Talk: Resource Oriented Microservices - London Microservices Meetup

Repository Updates

We're in that twilight zone where library developers are increasingly tempted by the bells and whistles offered by Java 8 but operationally its good to stick with what you know and stay on Java 7.

Our general strategy for the NetKernel core and modules is to target the mature Java generation - currently all our libraries target Java 7. However we're constantly adding to the modules available for NetKernel in the repositories and want to provide useful new tools as they become available - even if they are using libraries designed for the latest Java generation.

To square this circle, we've updated the Standard Module to enable Java runtime version constraint checking for modules.

You can now provide a <jvm> tag in the <system> declaration for a module...

When the module is commissioned a check is made to ensure that the current executing JVM is at least this version (or higher). If it is not, then the module will not be commissioned and a warning is issued...

18:25:43 StandardModu~ !file:/home/dexter/latest/urn.org.netkernel.json.schema-1.1.1.jar :
Java version must be 1.8 or higher !

The system health check will also show a warning that you are trying to commission a module that requires a newer version of JVM runtime.

The first repository module to use this constraint checking is today's new json-schema module (see below) which requires Java 8.

As part of ongoing enhancements to the JSON tool set, today we've published a new json-schema module in the repos.

This provides the active:jsonSchema validator to validate JSON against JSON Schema.

After install the docs are here.

Examples

Here's an example of what it does...

Given a JSON resource r1.json, representing a rectangle data structure...

{
    "rectangle" : {
        "a" : -5,
        "b" : 10
    }
}

Then we can write a JSON schema rectangle-schema.json for the rectangle structure with a constraint that size must not be negative (as shown by, "minimum" : 0)...

{
    "type" : "object",
    "properties" : {
        "rectangle" : {"$ref" : "#/definitions/Rectangle" }
    },
    "definitions" : {
        "size" : {
            "type" : "number",
            "minimum" : 0
        },
        "Rectangle" : {
            "type" : "object",
            "properties" : {
                "a" : {"$ref" : "#/definitions/size"},
                "b" : {"$ref" : "#/definitions/size"}
            }
        }
    },
    "required": ["rectangle"]
}

We can validate r1.json against the schema with this resource request...

active:jsonSchema
      +operand@r1.json
      +operator@rectangle-schema.json

Since r1.json has "a" as a negative number the JSON fails to validate and an execption is thrown..

org.everit.json.schema.ValidationException
#/rectangle/a: -5.0 is not higher or equal to 0

Validation as Resource

By default the validator will throw an exception if the JSON does not validate. However often it is a better model to consider the validation as a resource (whether or not the JSON is valid).

To support rich validation resources, an optional hds argument may be provided. For example...

active:jsonSchema
      +operand@r1.json
      +operator@rectangle-schema.json
      +hds@true

The validation will now return a full validation representation as an HDS structure like this...

The advantage of this mode is that if the JSON fails to validate for multiple reasons, then all of the errors are returned, with a JSON Path reference, as nodes in the HDS and appropriate steps can be taken to deal with each one.

If you only care about boolean validity then you can simply use IHDSNode getFirstValue("/validation/valid") to test validity.

ROC URI Resolver

JSON schema supports URI references. The transreptor that does the schema compilation provides the contextual ROC domain as the resolution scope for schema references.

Therefore you may seamlessly reference schemas as resources resolvable in the ROC domain.

So for example, you could have active: URI references to dynamically generated schema or, with a suitable import of the http:client, remote schemas over HTTP etc.

Concurrency

The underlying Schema transreptor produces a cacheable compiled Schema representation that is threadsafe. Therefore active:jsonSchema is very high performance and can concurrently validate very large numbers of JSON documents against the same schema.

JSON Schema XUnit Assert

urn:org:netkernel:json:schema also provides an XUnit assertion library, which can be imported to an XUnit testlist with...

This provides the following assertion...

which validates the test result against the specified JSON schema.

Java 8 Requirement

Due to the underlying schema engine using Java 8 features, this library is constrained to Java 8 or higher (see above).

I'm giving a public talk on Wednesday evening to the London Microservices Meetup group. Attendance is open and you're very welcome to come along.

Details here: Resource Oriented Microservices, Wednesday 2nd March 2016, 6:30pm

There are 200 people signed up already so it should be a great evening.

---

Have a great weekend, hopefully I'll see some of you in London..

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