Service definition
Internet now has twenty years of experience with providing services.
Initiating a service (the first specification of a service) is usually
quite simple, but once it gets users it gets incredible hard to
upgrade them: mechanisms to handle interface upgrades are simply
missing.
In the XML world, WSDL files are used to describe the service.
These combine an end-point (URL) with an operation (function name)
and in- and outgoing messages. You can specify alternative end-point
sets, usually for the test infrastructure.
WSDL files are already quite complex already, but lack any kind of
interface maintenance needs. How do you change the interface? One
approach is to require regular reload at URL "$endpoint?WSDL".
Without pre-notice, your application may pick-up changes. Nice for
developers, horrifying for operations.
An other approach to publish interfaces was implemented by
OASIS UDDI (which seems to have stalled)
and Apache jUDDI (which is
maintained).
TODO: describe differences between UDDI and MSI.
Interface maintenance features
- Deprecation cycle
- It is always easy to add functionality to an interface, but it is
much harder to get rid of outdated interfaces. MSI adds support
for deprecation and removal of end-points. Humans can be warned
by their client applications that changes are due.
- Parallel versions
- When the deprecation cycle works well, it gets easier for designers
to upgrade their features. Old versions of operations MUST stay
supported for some time while newer versions are already available.
This gets complicated by XML's traditional use of namespaces.
- Client-side sharding
- Service-side sharing is common practice: service applications use
a key to distribute their data-set over multiple databases, or
processing over multiple hosts. When services need to stick to normal
society regulations, you will need to distribute and access knowledge
based on jurisdiction. But also, you may have separate redundant
networks for parts of the data-set. This can be implemented with
server instigated redirections (late) or smarter clients (early).
- Supported features
- The service lists the transport features it supports, like packaging
and compression. This may differ per end-point. Read more on the
transport of data.
- Pricing
- MSI focuses on automated optimizations. One of the best mechanisms
to lure people into spending time implementing optimizations, is to
differentiate in price. For instance, loading huge data-sets at
night PST could be cheaper than at daytime.
- Tracing
- Not to be bothered by the development efforts of dozens of developers
of client applications, the service will offer debugging and
processing logs on request.
- Flow control
- Per end-point static and dynamic flow control is needed to be able
to keep agreed service levels.
TODO: describe each of the ideas on a separate page.
mark@overmeer.net
Web-pages generated on 2023-12-19
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