ZooKeeper Configuration Source (Experimental)

The mConfigSourceZooKeeper module allows mConfig to retrieve configuration data from an Apache ZooKeeper ensemble. This is particularly useful for cluster-wide or organization-wide settings.

1. Bootstrap Configuration

Following the "self-configuration" principle, the ZooKeeper source doesn't require hardcoded connection strings in your application code. Instead, it looks for its own settings in a configuration named zookeeper.

You can provide these bootstrap settings in any standard mConfig location, such as a JAR resource (for defaults) or a local file (for overrides).

Example: .config/zookeeper.json (Bootstrap)

Place this file in src/main/resources/.config/zookeeper.json or /etc/metabit/myapp/zookeeper.json.

{
  "zookeeper": {
    "connectString": "zk-node-1:2181,zk-node-2:2181,zk-node-3:2181",
    "rootPath": "/mconfig/prod",
    "sessionTimeoutMs": 30000
  }
}

2. Data Structure in ZooKeeper

By default, the ZooKeeper source registers itself for the CLUSTER and ORGANIZATION scopes. It maps configuration names to ZNode paths relative to the rootPath.

If your rootPath is /mconfig/prod, and you request a configuration named database-settings, the source will look at: - /mconfig/prod/cluster/database-settings - /mconfig/prod/organization/database-settings

The content of the ZNode should be in a supported format (like JSON or Java Properties).

Example: ZNode content at /mconfig/prod/cluster/app-config

{
  "database": {
    "host": "prod-db-server",
    "port": 5432
  },
  "features": {
    "enableNewDashboard": true
  }
}

3. Usage in Code

To use the ZooKeeper source, simply include the mConfigSourceZooKeeper module in your classpath. mConfig will automatically discover and initialize it.

// Standard initialization
try (ConfigFactory factory = ConfigFactoryBuilder.create("metabit", "myapp").build())
    {
    // Requesting a configuration that might be in ZooKeeper
    Configuration config = factory.getConfig("app-config");

    // Accessing values (mConfig handles the layering)
    String dbHost = config.getString("database/host");
    boolean useDashboard = config.getBoolean("features/enableNewDashboard");
    }

4. How it Works (Self-Configuration)

The ZooKeeper source follows a specific startup sequence:

1. Local Initialization: JARConfigSource and FileConfigStorage are initialized first.
2. Bootstrap Lookup: ZooKeeperConfigStorage calls factory.getConfig("zookeeper"). This request is resolved using the already initialized local sources.
3. Activation: If a connectString is found, the storage connects to ZooKeeper and adds its networked locations to the global search list.
4. Transparent Layering: Subsequent calls to factory.getConfig("app-config") will now automatically include layers found in ZooKeeper, prioritized by scope (e.g., CLUSTER settings will override DEFAULT settings from the JAR).

5. Test Mode Isolation

In TEST_MODE, the ZooKeeper source will naturally point to your test resources (e.g., src/test/resources/.config/zookeeper.json). This ensures that unit tests do not accidentally connect to production ZooKeeper ensembles.

Related docs: - Test Mode