All,
A criticism that I have heard about the OSGi Service Layer,
particularly in discussions where it is compared with the Eclipse
Extension Registry, is that services "do not scale".
I would like to pin down exactly what is meant by this, and try to
quantify:
a) How bad the problem is, if it exists
b) Whether the problem can be fixed by an implementation, or is
intrinsic to the design
c) Whether and how the Eclipse Extension Registry achieves better
scalability.
The problem I have is that "scalability" can mean so many things. For
example it might mean that service registry lookup time becomes
unfeasible when the total number of services becomes very large. It's
difficult to see how a single lookup could be worse than linear, so
presumably the problem here is when lookups must be done repeatedly
-- and if this is the case, would some form of caching or indexing
help? Alternatively the problem might the dynamic "churn" that the
Spring-OSGi guys are so concerned about, i.e. when dependent services
are added and removed so quickly that a significant amount of time is
spent merely reacting to ServiceListener events rather than doing
useful work. But surely the same problem would affect extensions?
It seems to me that one potential difference is scoping. In Eclipse,
an extension point defines a search scope, and the definer of the
extension point tends scans the registry for extensions contributed
to that particular point. This means an O(n) operation, with n being
the number of extensions contributed to the extension point.
With OSGi services, one can simulate a scope simply by adding a
registration property to the service. If the service registry knew
that this property was being used for scoping, then it could create
an index for it. However, the registry does not have such knowledge,
so in theory it needs to perform an O(n) search where n is the size
of the entire registry. However this is only a guess; I haven't
examined the source code or performed any benchmarks.
Would anybody like to comment on whether and why they think that OSGi
services either do or do not scale?
Thanks,
Neil
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