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At runtime the Ignite user installs all required bundles (including ignite-core, any optional Ignite bundles as well as the application bundles) using either the standard mechanisms defined by the OSGi spec, or relying on the container's implementation-specific capabilities. For example, Apache Karaf (an OSGi implementation) offers a packaging/deloyment concept called "Feature" which roughly speaking is a list of bundles to automatically deploy when the OSGi Framework starts.
The main problem we need to solve in order to allow Ignite OSGi enablement is the marshalling. More specifically the issue is with deserialization of the classes that are provided by the bundles other than the JDK and the Ignite bundle itself.
[raul.kripalani]: I've already modified the POMs to generate and package the MANIFEST.MF with the appropriate OSGi headers. The code is pushed to the ignite-1527 branch.
We should provide a Feature Repository to make it easier for Apache Karaf users to install Ignite and, optionally, Ignite modules. There should be one feature per module that also installs all necessary library dependencies.
The main problem we need to solve in order to allow Ignite OSGi enablement is the marshalling. More specifically the issue is with deserialization of the classes that are provided by the bundles other than the JDK and the Ignite bundle itself.
When the Ignite transport layer receives a message it needs to figure out how to deserialize the bytes and for that it needs to know the bundle that provides the class to be deserialized. To make things more complex, the class may contain other classes that come from other bundles, When the Ignite transport layer receives a message it needs to figure out how to deserialize the bytes and for that it needs to know the bundle that provides the class to be deserailized. To make things more complex, the class may contain other classes that come from other bundles, and so on recursively. In general, what is needed then is a way to map an FQN of a class to its bundle (and hence to the class loader).
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It's responsibility of the implementation to ensure that the encoded representation is sufficient to unambiguously identify the correct bundle during deserialization.
The ClassLoaderCodec
should be called for every Object during serialization and deserialization and should be part of the IgniteConfiguraiton
:
Code Block |
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public interface ClassLoaderCodec {
@Nullable public Object encodeClassLoader(Class<?> cls, ClassLoader clsLdr) throws IgniteException;
public ClassLoader decodeClassLoader(String fqn, @Nullable Object encodedClsLdr) throws IgniteException;
} |
Ignite will come with 2 OSGI class loader codecs out of the box, pessimistic
and optimistic
, leaving users with opportunity to provide their own custom class loader codecs as well (potentially for non-OSGI environments).
In general in OSGi, the same package may be exported by multiple bundles and therefore an FQN may not be sufficient to look up the correct class loader. In such cases, the codec implementation must employ a pessimistic
approach and encode enough information (for example, the bundle symbolic name, plus the bundle version) for the deserializer to be able to resolve the FQN to the correct class loader. Such implementation will work for all use cases, but it introduces some overhead and increases the size of the serialized messages.
[raul.kripalani]: The naming is confusing. We are actually not transmitting classloaders. In fact, we cannot do so. What we'd like to do is transmit deserialisation "hints", that are used in whatever form the marshaller deems appropriate. So if anything, I would call this class a DeserialisationHintsCodec with methods: generateHints and computeClassLoaderFromHints.
[dmitriy setrakyan]: I am not sure I see the reason for removing the word classLoader on serialization part and keeping it on deserialization. I also think that the method names should be symmetric. With that in mind, "encodeClassLoader" and "decodeClassLoader" may not be the best names, but they are consistent with each other and symmetric. My vote would be to keep the naming.
The ClassLoaderCodec
should be called for every Object during serialization and deserialization and should be part of the IgniteConfiguraiton
:
Code Block |
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public interface ClassLoaderCodec {
@Nullable public Object encodeClassLoader(Class<?> cls, ClassLoader clsLdr) throws IgniteException;
public ClassLoader decodeClassLoader(String fqn, @Nullable Object encodedClsLdr) throws IgniteException;
} |
[raul.kripalani]: See my comment above.
Ignite will come with 2 OSGI class loader codecs out of the box, pessimistic
and optimistic
, leaving users with opportunity to provide their own custom class loader codecs as well (potentially for non-OSGI environments).
In general in OSGi, the same package may be exported by multiple bundles and therefore an FQN may not be sufficient to look up the correct class loader. In such cases, the codec implementation must employ a pessimistic
approach and encode enough information (for example, the bundle symbolic name, plus the bundle version) for the deserializer to be able to resolve the FQN to the correct class loader. Such implementation will work for all use cases, but it introduces some overhead and increases the size of the serialized messages.
However, for the applications that can enforce one-to-one mapping of packages to bundles, a simplified (optimistic) approach can be used instead. With this approach, no encoding of the class loader is required (encodeClassLoader()
returns null
), and only the FQN is used for decoding of the class loader.
[raul.kripalani]: I don't like transmitting bundle symbolic names over the wire, as it couples the serialising party with the deserialising party, forcing both to contain the class inside the same bundle. As I said in the mailing list, making this assumption would be a short-sighted strategy, as users may be sharing caches across applications across multiple containers, where classes live in different bundles in different containers.
I also don't think it's necessary. We just need the package name + package version. An OSGi container cannot expose the same package under the same version number twice, so the tuple (package name, package version) is enough to unambiguously locate the Bundle that exports our class.
Now, what we need to do is determine HOW we locate the Bundle. I have two ideas in mind:
With either of these approaches, I think we don't need pessimistic and/or optimistic strategies. Just a single strategy would be enoughHowever, for the applications that can enforce one-to-one mapping of packages to bundles, a simplified (optimistic) approach can be used instead. With this approach, no encoding of the class loader is required (encodeClassLoader()
returns null
), and only the FQN is used for decoding of the class loader.
Here's how the pessimistic codec implementation might look like (in pseudo-code):
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TBD >= 5.0
On the write side this approach require you to capture the bundle symbolic name and its version. This is something easy to do as in OSGi all classloader except the system classloader implements the BundleReference. The pessimis codec can look like that:
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In this strategy we start with a more strict assumption: We will never serialized a
packages of all serialized classes come from one and only one bundle.
You may think this option is more simple but it is not true.
TODO