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Versioning
As the Corda platform evolves and new features are added it becomes important to have a versioning system which allows its users to easily compare versions and know what feature are available to them. Each Corda release uses the standard semantic versioning scheme of major.minor.patch. This is useful when making releases in the public domain but is not friendly for a developer working on the platform. It first has to be parsed and then they have three separate segments on which to determine API differences. The release version is still useful and every MQ message the node sends attaches it to the release-version header property for debugging purposes.
Platform Version
It is much easier to use a single incrementing integer value to represent the API version of the Corda platform, which is called the Platform Version. It is similar to Android's API Level. It starts at 1 and will increment by exactly 1 for each release which changes any of the publicly exposed APIs in the entire platform. This includes public APIs on the node itself, the RPC system, messaging, serialisation, etc. API backwards compatibility will always be maintained, with the use of deprecation to migrate away from old APIs. In rare situations APIs may have to be removed, for example due to security issues. There is no relationship between the Platform Version and the release version - a change in the major, minor or patch values may or may not increase the Platform Version.
The Platform Version is part of the node's NodeInfo object, which is available from the ServiceHub. This enables a CorDapp to find out which version it's running on and determine whether a desired feature is available. When a node registers with the Network Map Service it will use the node's Platform Version to enforce a minimum version requirement for the network.
Note
A future release may introduce the concept of a target platform version, which would be similar to Android's targetSdkVersion, and would provide a means of maintaining behavioural compatibility for the cases where the platform's behaviour has changed.
Flow versioning
In addition to the evolution of the platform, flows that run on top of the platform can also evolve. It may be that the flow protocol between an initiating flow and its initiated flow changes from one CorDapp release to the next in such a way to be backward incompatible with existing flows. For example, if a sequence of sends and receives needs to change or if the semantics of a particular receive changes.
The InitiatingFlow annotation (see flow-state-machine for more information on the flow annotations) has a version property, which if not specified defaults to 1. This flow version is included in the flow session handshake and exposed to both parties in the communication via FlowLogic.getFlowContext. This takes in a Party and will return a FlowContext object which describes the flow running on the other side. In particular it has the flowVersion property which can be used to programmatically evolve flows across versions.
@Suspendable
override fun call() {
val flowVersionOfOtherParty = getFlowContext(otherParty).flowVersion
val receivedString = if (flowVersionOfOtherParty == 1) {
receive<Int>(otherParty).unwrap { it.toString() }
} else {
receive<String>(otherParty).unwrap { it }
}
}The above shows an example evolution of a flow which in the first version was expecting to receive an Int, but then in subsequent versions was relaxed to receive a String. This flow is still able to communicate with parties which are running the older flow (or rather older CorDapps containing the older flow).
Warning
It's important that InitiatingFlow.version be incremented each time the flow protocol changes in an incompatible way.
FlowContext also has appName which is the name of the CorDapp hosting the flow. This can be used to determine implementation details of the CorDapp. See cordapp-build-systems for more information on the CorDapp filename.
Note
Currently changing any of the properties of a CordaSerializable type is also backwards incompatible and requires incrementing of InitiatingFlow.version. This will be relaxed somewhat once the AMQP wire serialisation format is implemented as it will automatically handle a lot of the data type migration cases.