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TWP: Add a small discussion of versioning.

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Mike Hearn 2019-07-08 13:30:32 +02:00
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docs/source/whitepaper/corda-technical-whitepaper.tex
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@ -347,14 +347,34 @@ so desired. Unlike in proof-of-work based block chain systems, there is no way t
change in the network parameters and remain on the `losing side'. Thus the notion of flag days is subtly different
to the notion of a hard fork.
\subsection{Protocol versioning}\label{subsec:protocol-versioning}
The network protocol is versioned using a simple incrementing integer version number. There are no minor or patch
versions in the protocol definition itself: the versioning scheme is based on a commitment for the protocol
to always be backwards compatible. All nodes publish their current highest supported version in their signed
\texttt{NodeInfo} structure, so peers can always known ahead of time which protocol features are available to them
before creating or receiving connections.
The protocol data structures are reflected in the platform API, which is therefore versioned in an identical
manner. Apps may specify the minimum platform version they require, such that nodes will refuse to load apps that
would need newer features than are supported. The decision to unify the protocol and API versions implies that some
protocol versions may be effectively identical despite having different version numbers: this was deemed preferable
to requiring developers to understand and keep track of two similar-but-different versioning streams.
\paragraph{Mandatory upgrades.}The network operator may choose to require nodes to meet a minimum version number.
In this way the network can continue to enable new protocol features that require consensus and control the rate of
In this way the network can continue to enable new transaction features that require, and control the rate of
growth-induced ossification. Nodes that aren't new enough to handle the network's minimum version shut down
automatically unless overridden.
automatically unless overridden. Because mandatory upgrades may be difficult to coordinate and enforce future
versions of the platform may support outsourcing of transaction verification to third party nodes or remote SGX
enclaves.
\subsection{Platform versions}\label{subsec:platform-versions}
The network protocol is versioned using a simple integer version number.
\paragraph{Target versioning.}The node supports `target versioning', in which the latest version the app was
tested against is advertised in its metadata. The node can use this to activate or deactivate workarounds for buggy
apps that may have accidentally baked in dependencies on undefined platform behaviours, or to enable the semantics
of an API to evolve without breaking backwards compatibility. For example, if an API returns a list of elements
that happened to be sorted in previous versions, but then ceases to be sorted due to performance optimisations,
this could potentially break apps that assumed a certain value would always be found at a certain index. Target
versioning can be used to keep apps working even as the platform underneath it evolves.
\section{Flow framework}\label{sec:flows}