2017-11-25

The Tamaya High Level Design

Though Tamaya is a very powerful and flexible solution there are basically only a few simple core concepts required that build the base of all the other mechanisms:

The API (package org.apache.tamaya) provides

  • A simple but complete SE API for accessing key/value based Configuration:

    • Configuration hereby models configuration, the main interface of Tamaya, providing key/value pairs as raw (String-based) key/value pairs, allowing also access to typed values.

    • ConfigurationProvider provides the static entry point for accessing configuration.

The SPI (package org.apache.tamaya.spi) provides: A simple minimalistic model for configuration data, called PropertySource. Several extension points for adding additional configuration property sources or adapting the internal workings of the overall system. ** A ServiceContext / ServiceContextManager that controls the loading of the components in Tamaya. This allows to adapt the behaviour depending on the runtime environment in use, e.g. a Java standalone application, an OSGI container or a Java EE application server.

Tamaya Modules finally allow to add additional functionality to customize your configuration solution with the functionality you want. E.g. modules are providing features such as

  • Configuration injection

  • Dynamic placeholders and resolution mechanism for configuration values

  • Abstractions for reusable configuration formats

  • Dynamic configuration updates and change events

  • Support for OSGI/Java EE Classloading

  • A configuration server/client

  • and more…​

How Tamaya organizes Configuration

Overview

All the mentioned artifacts are used to organize configuration in a higly flexible and extendable way. Hereby the PropertySource is the key artifact. In general Tamaya organizes Configuration as follows:

CoreDesign

Key abstraction hereby is the ConfigurationContext, which basically

  • an ordered chain of PropertySource instances. This chain is used to evaluate raw configuration values.

  • a set of PropertyFilter instances that filter the raw values evaluated from the property source chain.

  • a set of PropertyConverter that convert String values into typed values when needed.

In most standalone use cases only one ConfigurationContext will be active at a time. But in more complex scenarios, such as Java EE also multiple contexts could be active that are active depending on the current runtime context (e.g. attached to the corresponding classloader(s)). These aspects are basically handled by the ConfigurationProvider and its corresponding SPIs.

Loading the current ConfigurationContext

The ConfigurationContext is the core of Tamaya. It manages all configuration sources and additional components required to evaluate a concrete configuration value:

  • Tamaya loads all available PropertySource instances. Hereby PropertySource instances can be

    • Directly registered (using the mechanism defined by the current ServiceContext implementation, by default the Java ServiceLoader.

    • Provided by a registered instance of PropertySourceProvider.

  • All loaded property sources are ordered based on each ordinal, returned from PropertySource.getOrdinal() as an ordered chain of PropertySources, building up the ordered chain of PropertySource instances used for raw configuration value evaluation.

  • Tamaya loads all available PropertyFilter instances. Hereby PropertyFilter instances can be registered by default using the Java ServiceLoader API. The PropertyFilter instances loaded are ordered based on the @Priority annotations found on each filter. If no priority annotation is present, 0 is assumed.

  • Tamaya loads all available PropertyConverter instances. Hereby PropertyConverter instances can be registered by default using the Java ServiceLoader API. The PropertyConverter instances loaded are ordered based on the @Priority annotations found on each filter. If no priority annotation is present, 0 is assumed. It is possible to register multiple converters for the same target type.

Evaluating raw property values

When evaluating a concrete configuration value for a given key, Tamaya iterates through this chain of registered PropertySources. Hereby the final value, by default, is determined by the last non-null value returned from a PropertySource.

Since the ladder may not always be appropriate, e.g. when values should be combined instead of overridden, a instance of PropertyValueCombinationPolicy can be registered, which allows to add more detailed behaviour how values are combined.

Access to the complete configuration Map is performing the same resolution and combination algorithm, but for all key/value pairs available.

Filtering the raw properties:

Each raw configuration value evaluated is filtered by the ordered filter chain, as long as there are any changes applied by any of the filters called. This ensures that also transitive replacements by filters are possible. If, after a configurable number of evaluation loops still values are changes during each loop, the filtering process is aborted, since a non-resolvable circular filter issue is assumed.

The output is the final configuration value as type String.

Applying type conversion:

Finally, if the required target type, does not match Java.ui.lang.String, all registered PropertyConverter instances targeting the corresponding target type are asked to convert the given (String-based) configuration entry to the required (non String) target type.

Hereby the first non-null value returned by a PropertyConverter is used as the final typed configuration value and returned to the caller.

Advanced Features

Basically the bahaviour of Tamaya can be customized using the following mechanisms. Basically configuration can be provided using the following mechanism:

  • Registering additional (default) PropertySource instances. Depending on their ordinal value they will override or extend existing configuration.

  • Registering additional (default) PropertySourceProvider instances.that can provide multiple PropertySource instances.

Additionally Tamaya provides hooks for further adapting the internal workings:

  • Adapting the way how multiple entries with the same key are combined (PropertyValueCombinationPolicy). This may be useful, if overriding is not the way how entries of the same key should be combined. An example, where such an alternate scenario is useful are list entries, that combine all entries encountered to a collecting list entry.

  • Adding additional support for new target types configurable by registering additional PropertyConverter instances. This can be used for adding support for new types as well as for adding support for additional formats.

  • Complex extensions may adapt the complete ConfigurationContext, using the ConfigurationContextBuilder and reapply the changed instance using ConfigurationProvider.setConfigurationContext(ConfigurationContext). This is one example how to react on dynamic changes detected on configuration files read.

  • Registering additional PropertyFilter instances, that filter the configuration values extracted.

  • Registering an alternate ServiceContext to support alternate runtime containers, e.g. a CDI container.

  • A combination of all above.

Additionally instances of ConfigOperator, ConfigQuery can be provided that provide additional functionality that should not be globally visible. It is recommended to provide them from a singleton accessor, hereby hiding the effective implementation classes.

Component Loading

As mentioned the component loading of Tamaya can be adapted. By default the JDK ServiceLoader API is used to determine a ServiceContext implementation that should control Tamaya’s overall component loading. If not found, a default implementation is registered, which relies on the Java hava.util.ServiceLoader mechanism. This behaviour can be changed by implementing your own version of the ServiceContext interface, annotating it with a @Priority annotation and registering it using the java.util.ServiceLoader mechanism.

Compatibility

The Tamaya API is compatible with Java 8 and beyond.

Further Documentation

Being here we recommend to have a look at the more detailed documentation of Tamaya’s API and SPI, and of its current available modules.