Tuesday, August 26, 2014

Existing Configuration Solutions

Existing Application Configuration Solutions

Within this blog I try to give some details on existing common configuration mechanisms in place. If I miss something, let me know, so I can add it here. Also not that the ordering of the section in this blog is completely random, it does not reflect any valuation from my side.

Also I explicitly do not focus on deployment configuration such as EJB deployment descriptors, CDI beans configuration, resource configuration, configuration for JPA, Bean Validation, web applications etc. This may be covered in another blog.

1. Play Framework

See http://www.playframework.com/documentation/2.0/Configuration

The default configuration file of a Play 2.0 application must be defined inconf/application.conf. It uses the HOCON format ( “Human-Optimized Config Object Notation”).

These system properties specify a replacement for application.conf. In the replacement config file, you can use include “application” to include the original default config file; after the include statement you could go on to override certain settings.

HOCON Syntax

Its basically a simplified JSON-Format with some changes, but not compatible with pure JSON.
  • Comments, with # or //
  • Allow omitting the {} around a root object
  • Allow = as a synonym for :
  • Allow omitting the = or : before a { so foo { a : 42 }
  • Allow omitting commas as long as there's a newline
  • Allow trailing commas after last element in objects and arrays
  • Allow unquoted strings for keys and values
  • Unquoted keys can use dot-notation for nested objects, foo.bar=42 means foo { bar : 42 }
  • Duplicate keys are allowed; later values override earlier, except for object-valued keys where the two objects are merged recursively
  • include feature merges root object in another file into current object, so foo { include "bar.json" }merges keys in bar.json into the object foo
  • include with no file extension includes any of .conf.json.properties
  • you can include files, URLs, or classpath resources; use include url("http://example.com") or file()or classpath() syntax to force the type, or use just include "whatever" to have the library do what you probably mean (Note: url()/file()/classpath() syntax is not supported in Play/Akka 2.0, only in later releases.)
  • substitutions foo : ${a.b} sets key foo to the same value as the b field in the a object
  • substitutions concatenate into unquoted strings, foo : the quick ${colors.fox} jumped
  • substitutions fall back to environment variables if they don't resolve in the config itself, so ${HOME} would work as you expect. Also, most configs have system properties merged in so you could use ${user.home}.
  • substitutions normally cause an error if unresolved, but there is a syntax ${?a.b} to permit them to be missing.
  • += syntax to append elements to arrays, path += "/bin"
  • multi-line strings with triple quotes as in Python or Scala
foo {
    bar = 10
    baz = 12

2. Typesafe Config Library

  • Formats supported: Java properties, JSON, and a human-friendly JSON superset.
  • merges multiple files across all formats
  • can load from files, URLs, or classpath
  • users can override the config with Java system properties
  • converts types, so if you ask for a boolean and the value is the string "yes", or you ask for a float and the value is an int, it will figure it out
  • Supports substitutions
  • API based on immutable Config instances, for thread safety and easy reasoning about config transformations
  • API Example
Config conf = ConfigFactory.load();
int bar1 = conf.getInt("foo.bar");
Config foo = conf.getConfig("foo");
int bar2 = foo.getInt("bar");
The convenience method ConfigFactory.load() loads the following (first-listed are higher priority):
  • system properties
  • application.conf (all resources on classpath with this name)
  • application.json (all resources on classpath with this name)
  • application.properties (all resources on classpath with this name)
  • reference.conf (all resources on classpath with this name)
The idea is that libraries and frameworks should ship with a reference.conf in their jar. Applications should provide an application.conf, or if they want to create multiple configurations in a single JVM, they could useConfigFactory.load("myapp") to load their own myapp.conf. (Applications can provide a reference.conf also if they want, but you may not find it necessary to separate it from application.conf.)
Finally it supports configuration merging based on so called fallbacks:
Config devConfig = originalConfig

3. Spring

Spring Boot allows you to externalize your configuration so you can work with the same application code in different environments. You can use properties files, YAML files, environment variables and command-line arguments to externalize configuration. Property values can be injected directly into your beans using the @Value annotation, accessed via Spring’s Environment abstraction or bound to structured objects.
Spring Boot uses a very particular PropertySource order that is designed to allow sensible overriding of values, properties are considered in the the following order:
  1. Command line arguments.
  2. Java System properties (System.getProperties()).
  3. OS environment variables.
  4. RandomValuePropertySource that only has properties in random.*.
  5. @PropertySource annotations on your @Configuration classes.
  6. Application properties outside of your packaged jar (application.properties including YAML and profile variants).
  7. Application properties packaged inside your jar (application.properties including YAML and profile variants).
  8. Default properties (specified using SpringApplication.setDefaultProperties).
SpringApplication will load properties from application.properties files in the following locations and add them to the Spring Environment:
  1. /config subdir of the current directory.
  2. The current directory
  3. A classpath /config package
  4. The classpath root
In addition to application.properties files, profile specific properties can also be defined using the naming convention application-{profile}.properties.

The values in application.properties are filtered through the existing Environment when they are used so you can refer back to previously defined values (e.g. from System properties).
app.description=${app.name} is a Spring Boot application
    url: http://dev.bar.com
    name: Developer Setup
    url: http://foo.bar.com
    name: My Cool App

Placeholders and dynamic resolution

Using the @Value("${property}") annotation to inject configuration properties can sometimes be cumbersome, especially if you are working with multiple properties or your data is hierarchical in nature. Spring Boot provides an alternative method of working with properties that allows strongly typed beans to govern and validate the configuration of your application. For example:
public class ConnectionSettings {

    private String username;

    private InetAddress remoteAddress;

    // ... getters and setters


Uses a Java based variant: 

// @Profile("production")
public class MyConfiguration {

4. Apache Deltaspike

The Apache DeltaSpike configuration system enables providing a default configuration inside the binary and allowing to amend this configuration (e.g. database credentials, some URLs from remote REST or SOAP endpoints, etc) from outside like environment settings, JNDI or the current ProjectStage.

Drop-In Configuration

The mechanism also allows for dynamic configuration in case of a JAR drop-in. By adding some JAR to the classpath, all it's contained configuration will get picked up and considered in the property value evaluation. You could also use this mechanism to switch implementations of some SPI (Service Provider Interface) in your own code.

CDI-Extension Configuration

In some cases low-level configs are needed e.g. during the bootstrapping process of the CDI container.
The good news: our DeltaSpike configuration mechanism does not rely on any other EE mechanism to be booted. Which means it can perfectly get used to even configure those parts itself. Since the mechanism doesn't rely on CDI it can for example be used to configure CDI-Extensions.
Currently this is e.g. used to configure the value of the current ProjectStage, configured values which can be used in the expressions for @Exclude, 'Deactivatable', etc. DeltaSpike needs such a low-level approach for several features internally, but users can utilize it for their own needs as well. This is done by using the ConfigResolver which resolves and caches ConfigSources per application.

Userland Configuration

DeltaSpike also provides a mechanism to inject those configured values using the @ConfigProperty CDI Qualifier.

ConfigSources provided by default

Per default there are implementations for the following config sources (listed in the lookup order):
  • System properties (deltaspike_ordinal = 400)
  • Environment properties (deltaspike_ordinal = 300)
  • JNDI values (deltaspike_ordinal = 200, the base name is "java:comp/env/deltaspike/")
  • Properties file values (apache-deltaspike.properties) (deltaspike_ordinal = 100, default filename is "META-INF/apache-deltaspike.properties")
It's possible to change this order and to add custom config sources.
To add a custom config-source, you have to implement the interface ConfigSource and register your implementation in a file /META-INF/services/org.apache.deltaspike.core.spi.config.ConfigSource by writing the fully qualified class name of the custom implementation/s into it.

Type-safe configuration

Finally DeltaSpike provides also a way to directly inject configured values into your code via the qualifier @ConfigProperty.
public class SomeRandomService
    @ConfigProperty(name = "endpoint.poll.interval")
    private Integer pollInterval;

    @ConfigProperty(name = "endpoint.poll.servername")
    private String pollUrl;


5. Apache Commons Configuration

Configuration parameters may be loaded from the following sources:
  • Properties files
  • XML documents
  • Windows INI files
  • Property list files (plist)
  • JNDI
  • JDBC Datasource
  • System properties
  • Applet parameters
  • Servlet parameters
Different configuration sources can be mixed using a ConfigurationFactory and a CompositeConfiguration. Additional sources of configuration parameters can be created by using custom configuration objects. This customization can be achieved by extendingAbstractConfiguration or AbstractFileConfiguration.
The full Javadoc API documentation is available here.
For manipulating properties or their values the following methods can be used:
Adds a new property to the configuration. If this property already exists, another value is added to it (so it becomes a multi-valued property).
Removes the specified property from the configuration.
Overwrites the value of the specified property. This is the same as removing the property and then calling addProperty() with the new property value.
Wipes out the whole configuration

6. Mechanism provided with the JDK

System Properties

Of course the well known Java system properties are always an easy way to configure things. By adding additional properties on the command line starting the Java process additional properties can be defined, e.g.

java -Dfoor.bar=notYet  <mainClass>

will start the given <mainClass>. Hereby the value from foor.bar can be extracted by calling System.getProperty("foo.bar"); This mechanism is widely used, but also has its limitations, mainly because it is a global configuration mechanism (the value is the same for the whole VM). Also adding very large amounts of system properties may lead to comlpex installation and deployment scenarios.


Of course, you could use simple property files. E.g. you can add a property file foobar.properties to your classpath (or a file, if you like)and then read it using the corresponding reader methods:

Properties props = new Properties();

Additionally the JDK also support storing properties using a XML format:

Properties props = new Properties();

Unfortunately there is no support for overriding of properties. So this mechanism is very limited.


Another API present in the JDK is java.util.preferences. It models a tree of nodes, where each node can have arbitrary attributes:

Prefereces systemPrefs = Preferences.systemRoot();
Prefereces userPrefs = Preferences.userRoot();

String configuredValue = systemPrefs.get("a.b.c.myKey", "myDefaultValue");
int configuredIntValue = userPrefs.get("my.foo.intValue", 190);

The key used above hereby resolve to paths in the tree, so "a.b.c.myKey", references the following node:

  \_ a
      \_ b
         \_ c

myKey is finally the key looked up in the node c.

Basically a node supports the following data types:
  • String
  • int
  • long
  • float
  • double
  • byte[]
So the API is simple but in practice has shown to be very limited and not appropriate as a base for modelling configuration, because
  • The API's absrtactions are modelled by abstract classes, which prevents the flexibility required.
  • Registering additional PreferencesFactory instances (SPI) may interfere with texisting code.
  • Access to the tree is widely synchronized, which is not acceptable for accessing configuration in a EE environment.
  • The Preferences SPI also supports writing configuration back, but is in combination with remote backends basically very cumbersome to implement and use.

7. JFig

See http://jfig.sourceforge.net/

JFig allows developers to:

  • Store application configuration in one common repository of XML files
  • Access configuration data using one common, convenient interface
  • Easily define multiple configurations, dynamically modifying those variables that need to change in different situations.
  • Eliminate the error prone practice of defining the same configuration variables in multiple locations
  • Ease the management, deployment, and control of configuration files
JFig structures configuration as a hierarchy of configuration files. It supports .xml and .ini files:

XML Format
<include name=”base.config.xml”/>
      <section name=”locos”>
<entry key=”instance” value=”development” />
<section name=”Paths”>
<entry key=”locosHome” value=”d:/[locos]{instance}/project/” />
<entry key=”locosExternal” value=”d:/external/[locos]{instance}/” />

<section name=”attachments”>
<entry key=”attachmentDirectory” value=”[paths]{locosExternal}attachments/”/>

INI File Format
Section names are in brackets, followed by key/value pairs.
Following is a sample file:



Hereby JFig allows to read multiple files. The “include” directive instructs the JFig parser to find the included configuration file. Files are parsed in reverse order. Thus, if a prod config file includes a base config file, the base config is parsed first.

Variable Substitution

The system provides substitution and cross referencing of variables. Substitution variables are in the format: [section]{key}. The second value, section Paths, key locosHome, uses the value of project, instance to build its value. So, locosHome resolves to d:/project/development . The next entry, attachmentDirectory, uses the previous entry to build its value. Thus, attachmentDirectory resolves to d:/project/development/attachment.
Also JFig allows to reference system properties:
<entry key=”docomentDir” value=”$user.home$/documents” />

Accessing Configuration

Configuration then can be accessed as follows:

JFig.getInstance().getValue(sectionName, key) Or 
JFig. getInstance().getValue(sectionName, key, defaultValue)

If you use a default value and the section/key is not found, the default value will return. If you don’t use a default value and the section/key is not found, ConfigException is thrown.

To set JFig values, use 
JFig.setConfigurationValue(sectionName, key, newValue);

For the most part, however, config values are set during the initial parsing of one or more ini files.

To show a complete listing of JFig values, use 

8. Carbon

See http://carbon.sourceforge.net/modules/core/docs/config/Usage.html
The Carbon configuration model takes two steps to improve the use of externalized data files for configuration. The first is to provide a simple configuration hierarchy based on the model of folders and documents. The second is to provide more advanced integration between the data and code using something called data binding. This provides much more advanced error checking as well as a consistent method for managing the data and most importantly strongly-typed access to the data.
The primary interface to configuration data is through extensions of the Configuration interface. The configuration service, through data binding, is able to implement any interface that extends the Configuration interface and adheres to the JavaBean™ Specification for properties.
package examples; 
public interface WebServiceConfiguration extends Configuration {
 String getServiceName(); void setServiceName(String value); String getServiceAddress(); void setServiceAddress(String serviceAddress);
 boolean isSecureConnection(); void setSecureConnection(boolean secureConnection);
 Class TransportClass = org.apache.soap.transport.SOAPHTTPConnection.class; Class getTransportClass(); void setTransportClass(Class value);
A Carbon configuration file then would look as follows:
<Configuration ConfigurationInterface="examples.WebServiceConfiguration">

The configuration can then be accessed as follows:

WebServiceConfiguration myConfig = (WebServiceConfiguration)
if (myConfig.isSecure()) {
    // ... use ssl socket factory
else {
    // ... use standard socket factory 
Transport transport = myConfig.getTransportClass().newInstance(); transport.send(new URL(myConfig.getServiceAddress()), "hello", // .....

Additionally there is also a weakly typed variant:

<Configuration ConfigurationInterface="org.sape.carbon.core.config.PropertyConfiguration">
 <MyFloat>6.6</MyFloat> <MyString>Hello, World!</MyString>
 <Name> <First>John</First> <Last>Doe</Last> </Name>
Which can be accessed as follows:

int num = myConfig.getIntProperty("MyInt");String firstname = myConfig.getProperty("Name.First");

9. Owner

See https://github.com/lviggiano/owner

The approach used by OWNER is to define a Java interface associated to a properties file.
Suppose your properties file is defined as ServerConfig.properties:
To access this property you need to define a convenient Java interface in ServerConfig.java:
public interface ServerConfig extends Config {
    int port();
    String hostname();
    int maxThreads();
OWNER calls this interface the Properties Mapping Interface or just Mapping Interface since its goal is to map Properties into a an easy to use piece of code. Finally, you can let OWNER read and assign your config as follows:
ServerConfig cfg = ConfigFactory.create(ServerConfig.class);
This looks rater simple and straight forward. But owner has a bunch of additional interesting features and is one of the most sophisticated solutions I have found so far

  • default values support
  • collection support, including custom tokenizers
  • type conversion support
  • overriding mechanisms
  • placeholders (called variable expansion) and dynamic resolution
  • mutability and configuration hot reload (both synchronous and asynchronous)
  • listening to reload events
  • explicit modelling of accessibility and mutability of configuration
  • Meta-Configuration

10. Other Configuration Mechanisms

I encountered several configuration frameworks and mechanism during my work life so far. Nevertheless I also want to describe one of the more powerful ones, just to show some of the features/requirements identified in bigger companies.
Typically such custom solution either use one or more of the mechanisms described above, or the companies decided to write a feasible configuration solution themselves.

Hereby it is noteable that
  • most of the solutions are based on simple key, value pairs, with String values only.
  • some companies added also support for different Collection types of String, but in general this concepts has shown to add more complexity and issues than it provides advantages.
  • some of them use flat keys, but most of them define a tree structure, similar to what the JDK's preferences API is doing.
  • the system differentiate between the execution context, such as
    • executed during system startup (system class loader)
    • executed during ear startup/shutdown (ear class loader)
    • executed during application startup (application class loader)
    • executed during a request, e.g. a http request, or an EJB called.
    • also a stage is defined, which may look very different for different companies
  • the system define different override rules, e.g.
    • system properties override everythging
    • application properties override ear config
    • ear config overrides system config
  • some of them also map environment,system properties and the command line arguments into the global configuration tree.
  • some also use additional environment settings, such as tier, network zone, security policy, server or JVM/app server instance names to define further possibilities for customizing the final configuration active.