System Services

/**
 * Abstract class for system services. System services can only be used in 
 * applications that are deployed in the system namespace.
 */
@Beta
public abstract class AbstractSystemService 
    extends AbstractService<SystemServiceConfigurer, SystemServiceContext> {
  
  /**
   * Creates a system table that conforms to the given table specification 
   * when the application is deployed.
   * @param tableSpecification the specification for the system table
   */
  protected void createTable(StructuredTableSpecification tableSpecification);
}

import io.cdap.cdap.api.service.AbstractSystemService;
import io.cdap.cdap.api.service.SystemServiceConfigurer;
import io.cdap.cdap.spi.data.StructuredTableSpecification;

public class MySystemService extends AbstractSystemService {
  @Override
  public void configure(SystemServiceConfigurer configurer) {
    setName("my-system-service");
    setDescription("Example system service");
    
    // Create a system table
    StructuredTableSpecification tableSpec = StructuredTableSpecification.builder()
      .withId("system-metadata")
      .withFields(/* field definitions */)
      .build();
    createTable(tableSpec);
  }

  @Override
  protected void startUp() throws Exception {
    // Service startup logic
  }

  @Override
  protected void shutDown() throws Exception {
    // Service shutdown logic
  }
}

/**
 * Configurer for system application services. Allows additional capabilities 
 * beyond those available to user services.
 */
@Beta
public interface SystemServiceConfigurer extends ServiceConfigurer, SystemTableConfigurer {
  // Inherits all methods from ServiceConfigurer and SystemTableConfigurer
}

/**
 * A System ServiceContext that exposes capabilities beyond those available 
 * to service contexts for user services.
 */
public interface SystemServiceContext extends ServiceContext, TransactionRunner, SystemNamespaceAdmin {
  // Inherits all methods from ServiceContext, TransactionRunner, and SystemNamespaceAdmin
}

/**
 * Interface for listing all namespaces for system service.
 */
public interface SystemNamespaceAdmin {
  /**
   * Lists all the namespaces.
   * @return list of namespace summaries
   * @throws Exception if listing namespaces fails
   */
  List<NamespaceSummary> listNamespaces() throws Exception;
}

import io.cdap.cdap.api.service.SystemServiceContext;
import io.cdap.cdap.proto.NamespaceSummary;

public class NamespaceManagerService extends AbstractSystemService {
  @Override
  protected void startUp() throws Exception {
    SystemServiceContext context = getContext();
    
    // List all namespaces
    List<NamespaceSummary> namespaces = context.listNamespaces();
    for (NamespaceSummary namespace : namespaces) {
      // Process each namespace
      LOG.info("Found namespace: {}", namespace.getName());
    }
  }
}

/**
 * Allows registering system tables for creation.
 */
@Beta
public interface SystemTableConfigurer {
  /**
   * Creates a system table that conforms to the given specification when 
   * application is deployed.
   * @param tableSpecification the table specification
   */
  void createTable(StructuredTableSpecification tableSpecification);
}

import io.cdap.cdap.api.SystemTableConfigurer;
import io.cdap.cdap.spi.data.StructuredTableSpecification;
import io.cdap.cdap.spi.data.table.field.FieldType;
import io.cdap.cdap.spi.data.table.field.Fields;

public void configureSystemTable(SystemTableConfigurer configurer) {
  StructuredTableSpecification tableSpec = StructuredTableSpecification.builder()
    .withId("system-audit-log")
    .withFields(
      Fields.stringField("timestamp"),
      Fields.stringField("user_id"),
      Fields.stringField("action"),
      Fields.stringField("details")
    )
    .withPrimaryKeys("timestamp", "user_id")
    .withIndexes("action")
    .build();
    
  configurer.createTable(tableSpec);
}