The Container uses OSGi to provide a modular platform
for developing applications that can be composed of many reusable components.
The user can deploy, upgrade and remove these components at runtime.
OSGi
OSGi is a framework for modular development of Java applications, where a
set of resources called bundles can be installed. OSGi allows the developer to
control which resources (Java packages) in a bundle that should be available to
other bundles. Hence, you can explicitly declare a bundle's public API,
and also ensure that internal implementation details remain hidden.
Unless you're already familiar with OSGi, we recommend reading Richard S. Hall's presentation
Learning
to ignore OSGi, which explains the most important aspects that you must relate to as a bundle
developer. There are other good OSGi tutorials available:
JDisc uses OSGi's module and lifecycle layers, and does not provide any functionality
from the service layer.
OSGi bundles
An OSGi bundle is a regular JAR file with a MANIFEST.MF file that describes its content,
what the bundle requires (imports) from other bundles, and what it provides (exports) to other
bundles. Below is an example of a typical bundle manifest with the most important headers:
Bundle-SymbolicName: com.yahoo.helloworld
Bundle-Description: A Hello World bundle
Bundle-Version: 1.0.0
Export-Package: com.yahoo.helloworld;version="1.0.0"
Import-Package: org.osgi.framework;version="1.3.0"
The meaning of the headers in this bundle manifest is as follows:
Bundle-SymbolicName - The unique identifier of the bundle.
Bundle-Description - A human-readable description of the bundle's
functionality.
Bundle-Version - Designates a version number to the
bundle.
Export-Package - Expresses which Java packages contained
in a bundle will be made available to the outside world.
Import-Package - Indicates which Java packages will be
required from the outside world to fulfill the dependencies needed in a
bundle.
Note that OSGi has a strict definition of version numbers that need to
be followed for bundles to work correctly. See the
OSGi
javadoc for details. As a general advice, never use more than three numbers
in the version (major, minor, micro).
Building an OSGi bundle
As long as the project was created by following steps in the
Developer Guide,
the code is already being packaged into an OSGi bundle by the
Maven bundle plugin.
However, if migrating an existing Maven project, change the packaging statement to:
<packaging>container-plugin</packaging>
and add the plugin to the build instructions:
<plugin><groupId>com.yahoo.vespa</groupId><artifactId>bundle-plugin</artifactId><!-- Find latest version at <a href="https://search.maven.org/search?q=g:com.yahoo.vespa%20a:bundle-plugin">search.maven.org/search?q=g:com.yahoo.vespa%20a:bundle-plugin</a> --><version>8.500.20</version><extensions>true</extensions><configuration><failOnWarnings>true</failOnWarnings></configuration></plugin>
Because OSGi introduces a different runtime environment from what Maven provides when running unit tests,
one will not observe any loading and linking errors until trying to deploy the application onto a running Container.
Errors triggered at this stage will be the likes of
ClassNotFoundException and NoClassDefFoundError.
To debug these types of errors, inspect the stack traces in the
error log, and refer to
troubleshooting.
vespa-logfmt
with its --nldequote option is useful when reading logs.
The test suite needs to cover deployment of the application bundle to
ensure that its dynamic loading and linking issues are covered.
Depending on non-OSGi ready libraries
Unfortunately, many popular Java libraries have yet to be bundled with the
appropriate manifest that makes them OSGi-compatible.
The simplest solution to this is to set the scope of the problematic
dependency to compile in your pom.xml file. This will cause the bundle
plugin to package the whole library into your bundle's JAR file. Until the
offending library becomes available as an OSGi bundle, it means that your
bundle will be bigger (in number of bytes), and that classes of that
library can not be shared across application bundles.
The practical implication of this feature is that the bundle plugin
copies the compile-scoped dependency, and its transitive dependencies,
into the final JAR file, and adds
a Bundle-ClassPath instruction to its manifest that
references those dependencies.
Although this approach works for most non-OSGi libraries, it only works for
libraries where the jar file is self-contained. If, on the other hand, the
library depends on other installed files, it must be treated as if it was a
JNI library.
Depending on JNI Libraries
This section details alternatives for using native code in the container.
OSGi bundles containing native code
OSGi jars may contain .so files, which can be loaded in the standard way from Java
code in the bundle. Note that since only one instance of an .so can be loaded at any time,
it is not possible to hot swap a jar containing .so files - when such jars are changed
the new configuration will not
take effect until the container is restarted.
Therefore, it is often a good idea to package a .so file and its Java API
into a separate bundle from the rest of your code to avoid having to restart the container on all code changes.
Add JNI code to the global classpath
When the JNI dependency cannot be packaged in a bundle,
and you run on an environment where you can install files locally
on the container nodes,
you can add the dependency to the container's classpath and explicitly
export the packages to make them visible to OSGi bundles.
Add the following configuration in the top level services
element in services.xml:
Adding the config at the top level ensures that it's applied to all jdisc clusters.
The packages are now available and visible,
but they must still be imported by the application bundle that uses the library.
Here is how to configure the bundle plugin to enforce an import of the packages to the bundle:
When adding a library to the classpath it becomes globally visible, and
exempt from the package visibility management of OSGi.
If another bundle contains the same library, there will be class loading issues.
The bundle plugin automates generation of configuration classes by invoking the maven step
generate-resources - read more in
configuring-components.html
Including Your Own Maven Submodules
You can include your own Maven submodules as dependencies within your Vespa component bundle.
This allows you to share code and functionality between different components within your project.
To include a submodule as a dependency, add it to your bundle's pom.xml in scope compile:
Then, add the jar to the components folder of your application
package, along with your own bundles. In this case,
only packages exported by the author of the library will be
available for use by your bundle (see the section below).
Exporting, Importing and Including Packages from Bundles
OSGi features information hiding — by default all the classes
used inside a bundle are invisible from the outside. Also, the bundle
will by default only see (all) the packages in the Java and Container
+ Vespa APIs. If any other package is needed by the bundle, then it
must happen in one of three ways:
Some additional packages are exported by the container and may
be imported explicitly by a bundle
In addition, any deployed bundle may export packages on its own,
which may then be imported by another bundle
Finally, the bundle may include its own JAR libraries
One can export packages from a bundle by annotating the package.
E.g. to export com.mydomain.mypackage, create package-info.java
in the package directory with:
The Maven plugin will place such information in the manifest of the
plugin JAR built to be picked up by the Container.
Note that this may also be used with bundles that do not contain any
searchers but libraries used by other searchers - a bundle may
just exist to export some libraries and never have any searchers
instantiated.
Bundles may import packages (exported by some other bundle or
by the container). The maven plugin will automatically import any
package used from bundles it compiles against(i.e. maven dependencies
with scope provided).
As mentioned above, each exported package has a version associated
with it. Similarly, an import of a package has a version range
associated with it. The version range determines which exported
packages can be used. The range used by the maven plugin is the
current version(i.e. the version of the package available at compile
time) up to the next major version (not including).
To learn more about OSGi manifests and bundle packaging (e.g. how to
include Java libraries and native code), please refer to the OSGi spec
at the OSGi home page.
The bundle plugin will emit warnings for the following common issues that may cause problems at runtime:
[WARNING] This project uses packages that are not part of Vespa's public api
Only Vespa types that are in Java packages annotated with
@PublicApi
should be used in application code, as other types are not guaranteed to be stable across Vespa releases.
[WARNING] This project does not have 'container' as provided dependency
All application bundles must have the com.yahoo.vespa:container artifact as a provided scoped dependency,
to ensure that the generated 'Import-Package' OSGi header contains the Java packages provided by the Vespa runtime.
[WARNING] Artifacts provided from Vespa runtime are included in compile scope
This makes the bundle unnecessarily large and may cause problems at runtime, as these artifacts will be embedded in the bundle.
Run mvn dependency:tree to identify the source of transitive dependencies, and add the necessary exclusions in
pom.xml. If an artifact must be included, e.g. because a specific version is needed, an exception can be added
with the configuration parameter allowEmbeddedArtifacts.
[WARNING] This project defines packages that are also defined in provided scoped dependencies
Overlapping Java packages between bundles will usually cause problems at runtime, because the OSGi framework will only be able
to resolve classes from one of the bundles.
Configuring the Bundle-Plugin
The bundle plugin can be configured to tailor the resulting bundle to specific needs.
If true, the maven build will fail upon warnings for e.g. using Vespa types
that are not annotated with @PublicApi.
This should always be set to true to ensure that your project
will compile successfully on future Vespa releases.
Default is false
allowEmbeddedArtifacts
A comma-separated list of maven artifacts to allow embedding in the bundle, on the
format groupId:artifactId
attachBundleArtifact
Whether to attach the bundle jar artifact to the build.
Use this if you want to install and deploy the bundle jar along with the default jar.
Default is false
bundleClassifierName
If attachBundleArtifact is true,
this will be used as classifier for the bundle jar artifact.
Default is bundle
discApplicationClass
The fully qualified class
name of the Application to be started by JDisc
discPreInstallBundle
The name of the bundles
that jDISC must pre-install
bundleVersion
The version of this bundle.
Defaults to the Maven project version
bundleSymbolicName
The symbolic name of this bundle.
Defaults to the Maven artifact ID
bundleActivator
The fully qualified class name of the bundle activator
configGenVersion
The version of com.yahoo.vespa.configlib.config-class-plugin
that will be used to generate config classes
configModels
List of config models
Bundle Plugin Troubleshooting
A package p is imported if all of this hold:
Using a class in p directly (i.e. not with reflection) in the
bundle
There's no classes in the bundle that is in p
There's a bundle that exports p, and compiling against this bundle
To debug, run
$ mvn -X package
and look at Defined packages (=packages in the bundle), Exported packages of
dependencies, Referenced packages(= packages used).
A package is imported if it is in Exported packages and Referenced packages but not in Defined packages.
Troubleshooting
This section describes how to troubleshoot the most common errors when working with bundles:
Bundles that are uninstalled between re-configs are logged like this:
INFO : qrserver Container.com.yahoo.container.core.config.ApplicationBundleLoader
Bundles to schedule for uninstall: [com.yahoo.vespatest.ExtraHitSearcher [67]]
And in case there are none, it shows the empty set:
INFO : qrserver Container.com.yahoo.container.core.config.ApplicationBundleLoader
Bundles to schedule for uninstall: []
Could not create component
The Container fails to start if it cannot load bundles.
Example, using wrong bundle name in the
multiple-bundles sample app:
Looking at what is actually deployed in multiple-bundles:
$ ls -1 target/*.jar
target/multiple-bundles-1.0.0-deploy.jar
target/multiple-bundles-1.0.0-without-dependencies.jar
target/multiple-bundles-lib-1.0.1-deploy.jar
Error in log:
[2020-01-23 14:28:01.367] WARNING : qrserver Container.com.yahoo.container.di.Container
Failed to set up new component graph. Retaining previous component generation.
exception=
java.lang.IllegalArgumentException: Could not create a component with id 'com.mydomain.lib.FibonacciProducer'.Tried to load class directly, since no bundle was found for spec: multiple-bundles-typo.
If a bundle with the same name is installed, there is a either a version mismatch or the installed bundle's version contains a qualifier string.
at com.yahoo.osgi.OsgiImpl.resolveFromClassPath(OsgiImpl.java:74)
at com.yahoo.osgi.OsgiImpl.resolveClass(OsgiImpl.java:65)
at com.yahoo.container.di.Container.addNodes(Container.java:228)
at com.yahoo.container.di.Container.createComponentsGraph(Container.java:217)
at com.yahoo.container.di.Container.getConfigAndCreateGraph(Container.java:160)
at com.yahoo.container.di.Container.getNewComponentGraph(Container.java:84)
at com.yahoo.container.core.config.HandlersConfigurerDi.getNewComponentGraph(HandlersConfigurerDi.java:145)
at com.yahoo.container.jdisc.ConfiguredApplication.lambda$startReconfigurerThread$1(ConfiguredApplication.java:275)
at java.base/java.lang.Thread.run(Thread.java:834)
[2020-01-23 14:28:01.367] ERROR : qrserver Container.com.yahoo.container.jdisc.ConfiguredApplication
Reconfiguration failed, your application package must be fixed, unless this is a JNI reload issue: Could not create a component with id 'com.mydomain.lib.FibonacciProducer'. Tried to load class directly, since no bundle was found for spec: multiple-bundles-typo. If a bundle with the same name is installed, there is a either a version mismatch or the installed bundle's version contains a qualifier string.
exception=
java.lang.IllegalArgumentException: Could not create a component with id 'com.mydomain.lib.FibonacciProducer'. Tried to load class directly, since no bundle was found for spec: multiple-bundles-typo. If a bundle with the same name is installed, there is a either a version mismatch or the installed bundle's version contains a qualifier string.
at com.yahoo.osgi.OsgiImpl.resolveFromClassPath(OsgiImpl.java:74)
at com.yahoo.osgi.OsgiImpl.resolveClass(OsgiImpl.java:65)
at com.yahoo.container.di.Container.addNodes(Container.java:228)
at com.yahoo.container.di.Container.createComponentsGraph(Container.java:217)
at com.yahoo.container.di.Container.getConfigAndCreateGraph(Container.java:160)
at com.yahoo.container.di.Container.getNewComponentGraph(Container.java:84)
at com.yahoo.container.core.config.HandlersConfigurerDi.getNewComponentGraph(HandlersConfigurerDi.java:145)
at com.yahoo.container.jdisc.ConfiguredApplication.lambda$startReconfigurerThread$1(ConfiguredApplication.java:275)
at java.base/java.lang.Thread.run(Thread.java:834)
Make sure that the jar files (i.e. bundles) are actually deployed with correct names per services.xml.
Could not load class
If a component is added to services.xml, and its class cannot be found in the declared bundle,
the container will fail to start. For example:
java.lang.IllegalArgumentException: Could not load class 'com.example.MissingClass' from bundle my-bundle
If you see this error, please make sure that the class actually exists in the given bundle. Also, verify that
the id (or class) tag refers to the component class, and not e.g. a java package
or the bundle name.
Class not found
All classes that are referred to in a user bundle must either be embedded in the bundle,
or imported from another bundle by an Import-Package statement in the bundle manifest.
When this rule has been breached, we get one of the most commonly seen exceptions when working with OSGi bundles:
...
exception=
java.lang.NoClassDefFoundError: com/acme/utils/Helper
...
java.lang.ClassNotFoundException: com.acme.utils.Helper not found by my_bundle [29]
For the bundle-plugin to automatically add an
Import-Package statement to the bundle's manifest,
that package must be exported from another bundle
that is declared as a 'provided' scope dependency in pom.xml.
If the dependency that contains the missing class is under your own control,
make sure it's packaged as an OSGi bundle,
and export the package
from that bundle.
If not, the simplest way to resolve the issue is to embed the dependency in your own bundle,
by setting its scope to 'compile' instead of 'provided'.
If the strategy above does not resolve the case,
it's most likely because the class in question is loaded by reflection,
e.g. Class.forName("com.acme.utils.Helper").
This is quite common when working with libraries for pluggable frameworks,
for which there is a separate troubleshooting doc.
Slow Container start
In the vespa log, a container startup looks like:
[2021-01-07 10:13:35.325] INFO : container Container.com.yahoo.container.core.config.ApplicationBundleLoader
Installed bundles: {[0]org.apache.felix.framework:6.0.3, [1]container-disc:7.335.22 ...
...
[2021-01-07 10:26:57.291] INFO : container Container.com.yahoo.container.jdisc.ConfiguredApplication
Switching to the latest deployed set of configurations and components. Application config generation: 1
The container is ready at the last log line - note the long startup time.
To get more details on what the container is doing at startup, inspect the ComponentGraph debug log.
Find the container service name (here: "container"), set debug logging and restart the container:
$ vespa-sentinel-cmd list
vespa-sentinel-cmd 'sentinel.ls' OK.
container state=RUNNING mode=AUTO pid=246585 exitstatus=0 id="default/container.0"
$ vespa-logctl container:com.yahoo.container.di.componentgraph.core debug=on
$ vespa-stop-services && vespa-start-services
# Find DEBUG log messages for component creation, like:
[2021-01-07 10:13:37.006] DEBUG : container Container.com.yahoo.container.di.componentgraph.core.ComponentGraph
Trying the fallback injector to create component of class com.yahoo.container.jdisc.messagebus.SessionCache to inject
into component 'chain.mychain in MbusServer' of type 'com.yahoo.container.jdisc.messagebus.MbusServerProvider'.
[2021-01-07 10:14:14.082] DEBUG : container Container.com.yahoo.container.di.componentgraph.core.ComponentNode
Constructing 'com.yahoo.search.query.profile.compiled.CompiledQueryProfileRegistry'
[2021-01-07 10:26:54.669] DEBUG : container Container.com.yahoo.container.di.componentgraph.core.ComponentNode
Finished constructing 'com.yahoo.search.query.profile.compiled.CompiledQueryProfileRegistry'
In this particular example, query profile compilation takes a long time.
Unresolved constraint
If the bundle has an Import-Package for a package that is not available at runtime,
the OSGi framework will report an unresolved constraint error.
The symptom as seen in the log is:
org.osgi.framework.BundleException: Unresolved constraint in bundle my_bundle [29]:
Unable to resolve 29.0:
missing requirement [29.0] osgi.wiring.package; (osgi.wiring.package=com.acme.utils)
at org.apache.felix.framework.Felix.resolveBundleRevision(Felix.java:3974)
This means that the missing class resides in a 'provided' dependency referred to from
the bundle's pom.xml, either directly or transitively.
In order to make the dependency available at runtime, there are two options:
The easiest is to set the dependency as 'compile' scope (instead of 'provided') to embed it in your own bundle.
This works fine in most cases, unless two of the dependencies need two different versions of the same library.
Add the missing jar file to the components/ folder of the application package,
along with your own bundles.
The maven-dependency-plugin has a goal called 'copy-dependencies' to help with this.
If the missing jar is a transitive dependency, maven can help visualize the dependency graph of the project:
$ mvn dependency:tree
Multiple implementations of the same class
When two bundles interact via their public APIs, it is crucial that both bundles resolve
each and every participating class to the same Class object.
If not, we will get error messages like:
java.lang.LinkageError: loader constraint violation: when resolving field
"DATETIME" the class loader (instance of
org/apache/felix/framework/BundleWiringImpl$BundleClassLoaderJava5) of the referring
class, javax/xml/datatype/DatatypeConstants, and the class loader (instance of
<bootloader>) for the field's resolved type, pe/DatatypeConstants, have different Class
objects for that type
or:
java.lang.LinkageError: loader constraint violation: loader (instance of <bootloader>)
previously initiated loading for a different type with name "javax/xml/namespace/QName"
or (less frequently):
java.lang.ClassCastException: com.acme.utils.Helper cannot be cast to com.acme.utils.Helper
All these error messages indicate that multiple implementations of one or more classes are used at runtime -
possible root causes:
Two interacting user bundles embed the same Java package.
A user bundle embeds a Java package that is exported from one of the JDisc bundles.
Usually, the "duplicate" package is pulled in by the user bundle transitively from a library dependency.
Multiple implementations example
Let's take a look at an example resolving the duplicate javax.xml.namespace.QName class
from the error message above.
All 'javax.xml' packages in the JDK are exported by the JDisc core bundle.
This means that they should be imported by user bundles, instead of embedded inside them.
Hence, ensure that there are no classes from packages prefixed by 'javax.xml' in the bundle.
Find out which library that pulls in the package:
Extract the full component jar, including any embedded jars.
One tool that does the job is rjar.
Search the folder where the jar was extracted for 'javax.xml' classes:
Find out which libraries that pulled in the offending classes - here it was stax-api-1.0.1.
Usually, these libraries are not pulled in by the pom as direct dependencies,
but rather transitively via another library being used.
Use maven's dependency plugin from the application directory to find the direct dependency:
This is similar to the previous example, but logging libraries are maybe the most common problem teams encounter.
Here we will see the symptom, use dependency:tree and add an exclusion. The symptom:
java.lang.RuntimeException: An exception occurred while
constructing 'com.acme.utils.Helper in acme-utils'
Caused by: java.lang.LinkageError: loader constraint violation: when resolving method
"org.slf4j.impl.StaticLoggerBinder.getLoggerFactory()Lorg/slf4j/ILoggerFactory;"
the class loader (instance of org/apache/felix/framework/BundleWiringImpl$BundleClassLoaderJava5) of the
current class, org/slf4j/LoggerFactory, and the class loader (instance of
sun/misc/Launcher$AppClassLoader) for the method's defining class,
org/slf4j/impl/StaticLoggerBinder, have different Class objects for the type
org/slf4j/ILoggerFactory used in the signature
at
org.slf4j.LoggerFactory.getILoggerFactory(LoggerFactory.java:299)
at
org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:269)
Running mvn dependency:tree in the previous example gives:
Added the right excludes for application and used mvn dependency:tree and verified that all references were gone,
except the ones for container-dev. Still found:
$ jar -tf mailsearch-docprocs-deploy.jar | grep slf
dependencies/slf4j-api-1.7.5.jar
One can make it work by managing this dependency explicitly - add this at POM top-level: