feat: include notification into polling

backend/src/main/java/rocks/inspectit/gepard/agentmanager/application/config/EventConfig.java

@@ -0,0 +1,20 @@
+/* (C) 2024 */
+package rocks.inspectit.gepard.agentmanager.application.config;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.context.event.ApplicationEventMulticaster;
+import org.springframework.context.event.SimpleApplicationEventMulticaster;
+import org.springframework.core.task.SimpleAsyncTaskExecutor;
+
+@Configuration
+public class EventConfig {
+
+  @Bean
+  public ApplicationEventMulticaster simpleApplicationEventMulticaster() {
+    SimpleApplicationEventMulticaster eventMulticaster = new SimpleApplicationEventMulticaster();
+
+    eventMulticaster.setTaskExecutor(new SimpleAsyncTaskExecutor());
+    return eventMulticaster;
+  }
+}

backend/src/main/java/rocks/inspectit/gepard/agentmanager/configuration/controller/ConfigurationController.java

@@ -3,6 +3,7 @@
 
 import io.swagger.v3.oas.annotations.Operation;
 import jakarta.validation.Valid;
+import java.util.Map;
 import java.util.Objects;
 import lombok.RequiredArgsConstructor;
 import org.springframework.http.ResponseEntity;
@@ -17,10 +18,15 @@ public class ConfigurationController {
 
   private final ConfigurationService configurationService;
 
-  @GetMapping
-  @Operation(summary = "Get the agent configuration.")
-  public ResponseEntity<InspectitConfiguration> getAgentConfiguration() {
-    InspectitConfiguration configuration = configurationService.getConfiguration();
+  @GetMapping("/{agentId}")
+  @Operation(
+      summary =
+          "Get the agent configuration and register the agent with the given id and agent info in the configuration server.")
+  public ResponseEntity<InspectitConfiguration> getAgentConfiguration(
+      @PathVariable String agentId, @RequestHeader Map<String, String> headers) {
+
+    InspectitConfiguration configuration =
+        configurationService.handleConfigurationRequest(agentId, headers);
 
     // No config available
     if (Objects.isNull(configuration)) {

backend/src/main/java/rocks/inspectit/gepard/agentmanager/configuration/events/ConfigurationRequestEvent.java

@@ -0,0 +1,19 @@
+/* (C) 2024 */
+package rocks.inspectit.gepard.agentmanager.configuration.events;
+
+import java.util.Map;
+import lombok.Getter;
+import org.springframework.context.ApplicationEvent;
+
+@Getter
+public class ConfigurationRequestEvent extends ApplicationEvent {
+
+  private final String agentId;
+  private final Map<String, String> headers;
+
+  public ConfigurationRequestEvent(Object source, String agentId, Map<String, String> headers) {
+    super(source);
+    this.agentId = agentId;
+    this.headers = headers;
+  }
+}

backend/src/main/java/rocks/inspectit/gepard/agentmanager/configuration/service/ConfigurationService.java

@@ -3,8 +3,11 @@
 
 import com.fasterxml.jackson.core.JsonProcessingException;
 import com.fasterxml.jackson.databind.ObjectMapper;
+import java.util.Map;
 import lombok.AllArgsConstructor;
+import org.springframework.context.ApplicationEventPublisher;
 import org.springframework.stereotype.Service;
+import rocks.inspectit.gepard.agentmanager.configuration.events.ConfigurationRequestEvent;
 import rocks.inspectit.gepard.agentmanager.exception.JsonParseException;
 import rocks.inspectit.gepard.config.model.InspectitConfiguration;
 
@@ -16,6 +19,8 @@ public class ConfigurationService {
 
   private final ObjectMapper objectMapper;
 
+  private final ApplicationEventPublisher applicationEventPublisher;
+
   /**
    * Retrieves the current configuration from the local Git repository.
    *
@@ -44,4 +49,23 @@ public void updateConfiguration(InspectitConfiguration configuration) {
       throw new JsonParseException("Failed to serialize InspectitConfiguration to JSON", e);
     }
   }
+
+  /**
+   * Handles a configuration request from an agent. If the agent is not connected, it will be
+   * connected. The last fetch time of the agent will be updated. The configuration will be
+   * returned.
+   *
+   * @param agentId the id of the agent requesting the configuration
+   * @param headers the request headers, which should contain the agent information
+   * @return the inspectit configuration for this agent
+   */
+  public InspectitConfiguration handleConfigurationRequest(
+      String agentId, Map<String, String> headers) {
+
+    // Event Emitter instead
+    ConfigurationRequestEvent event = new ConfigurationRequestEvent(this, agentId, headers);
+    applicationEventPublisher.publishEvent(event);
+
+    return getConfiguration();
+  }
 }

backend/src/main/java/rocks/inspectit/gepard/agentmanager/configuration/validation/ConfigurationRequestHeaderValidator.java

@@ -0,0 +1,23 @@
+/* (C) 2024 */
+package rocks.inspectit.gepard.agentmanager.configuration.validation;
+
+import java.util.Map;
+import rocks.inspectit.gepard.agentmanager.exception.MissingHeaderException;
+
+public class ConfigurationRequestHeaderValidator {
+  public static void validateConfigurationRequestHeaders(Map<String, String> headers) {
+    validateHeader(headers, "x-gepard-service-name");
+    validateHeader(headers, "x-gepard-vm-id");
+    validateHeader(headers, "x-gepard-gepard-version");
+    validateHeader(headers, "x-gepard-otel-version");
+    validateHeader(headers, "x-gepard-java-version");
+    validateHeader(headers, "x-gepard-start-time");
+  }
+
+  private static void validateHeader(Map<String, String> headers, String headerName) {
+    String value = headers.get(headerName);
+    if (value == null) {
+      throw new MissingHeaderException(headerName + " header is required");
+    }
+  }
+}

backend/src/main/java/rocks/inspectit/gepard/agentmanager/connection/controller/ConnectionController.java

@@ -12,10 +12,7 @@
 import lombok.RequiredArgsConstructor;
 import org.springframework.http.ResponseEntity;
 import org.springframework.web.bind.annotation.*;
-import org.springframework.web.servlet.support.ServletUriComponentsBuilder;
-import rocks.inspectit.gepard.agentmanager.connection.model.Connection;
 import rocks.inspectit.gepard.agentmanager.connection.model.dto.ConnectionDto;
-import rocks.inspectit.gepard.agentmanager.connection.model.dto.CreateConnectionRequest;
 import rocks.inspectit.gepard.agentmanager.connection.model.dto.QueryConnectionRequest;
 import rocks.inspectit.gepard.agentmanager.connection.model.dto.UpdateConnectionRequest;
 import rocks.inspectit.gepard.agentmanager.connection.service.ConnectionService;
@@ -32,15 +29,6 @@ public class ConnectionController {
 
   private final ConnectionService connectionService;
 
-  @PostMapping("/{id}")
-  @Operation(summary = "Connect an agent to the agent manager.")
-  public ResponseEntity<Void> connect(
-      @PathVariable String id, @Valid @RequestBody CreateConnectionRequest connectRequest) {
-    Connection connection = connectionService.handleConnectRequest(id, connectRequest);
-    return ResponseEntity.created(ServletUriComponentsBuilder.fromCurrentRequest().build().toUri())
-        .build();
-  }
-
   @PatchMapping("/{id}")
   @Operation(summary = "Update the agent connection.")
   public ResponseEntity<ConnectionDto> update(

backend/src/main/java/rocks/inspectit/gepard/agentmanager/connection/model/Connection.java

@@ -19,6 +19,9 @@ public class Connection {
   /** The registration time. * */
   private Instant registrationTime;
 
+  /** The time of the last communication. */
+  private Instant lastFetch;
+
   /** The status of the connection. */
   private ConnectionStatus connectionStatus;
 

backend/src/main/java/rocks/inspectit/gepard/agentmanager/connection/model/dto/ConnectionDto.java

@@ -2,6 +2,7 @@
 package rocks.inspectit.gepard.agentmanager.connection.model.dto;
 
 import jakarta.validation.constraints.NotNull;
+import java.time.Duration;
 import java.time.Instant;
 import java.util.Map;
 import rocks.inspectit.gepard.agentmanager.connection.model.Connection;
@@ -11,6 +12,7 @@
 public record ConnectionDto(
     @NotNull(message = "Registration Time missing.") Instant registrationTime,
     @NotNull(message = "Connection status is missing") ConnectionStatus connectionStatus,
+    @NotNull(message = "Time since last fetch is missing.") Duration timeSinceLastFetch,
     @NotNull(message = "Service Name missing.") String serviceName,
     @NotNull(message = "Gepard Version missing.") String gepardVersion,
     @NotNull(message = "OpenTelemetry Version missing.") String otelVersion,
@@ -23,6 +25,7 @@ public static ConnectionDto fromConnection(Connection connection) {
     return new ConnectionDto(
         connection.getRegistrationTime(),
         connection.getConnectionStatus(),
+        Duration.between(connection.getLastFetch(), Instant.now()),
         connection.getAgent().getServiceName(),
         connection.getAgent().getGepardVersion(),
         connection.getAgent().getOtelVersion(),

backend/src/main/java/rocks/inspectit/gepard/agentmanager/connection/service/ConnectionService.java

@@ -1,15 +1,20 @@
 /* (C) 2024 */
 package rocks.inspectit.gepard.agentmanager.connection.service;
 
+import static rocks.inspectit.gepard.agentmanager.configuration.validation.ConfigurationRequestHeaderValidator.validateConfigurationRequestHeaders;
+
+import java.time.Instant;
 import java.util.*;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.stream.Collectors;
 import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
+import org.springframework.context.event.EventListener;
 import org.springframework.stereotype.Service;
+import rocks.inspectit.gepard.agentmanager.configuration.events.ConfigurationRequestEvent;
 import rocks.inspectit.gepard.agentmanager.connection.model.Connection;
+import rocks.inspectit.gepard.agentmanager.connection.model.ConnectionStatus;
 import rocks.inspectit.gepard.agentmanager.connection.model.dto.ConnectionDto;
-import rocks.inspectit.gepard.agentmanager.connection.model.dto.CreateConnectionRequest;
 import rocks.inspectit.gepard.agentmanager.connection.model.dto.QueryConnectionRequest;
 import rocks.inspectit.gepard.agentmanager.connection.model.dto.UpdateConnectionRequest;
 import rocks.inspectit.gepard.agentmanager.connection.validation.RegexQueryService;
@@ -26,18 +31,18 @@ public class ConnectionService {
   private final RegexQueryService regexQueryService;
 
   /**
-   * Handles a connection request from an agent.
+   * Handles a ConfigurationRequestEvent. If the agent is not connected, it will be connected. If it
+   * is already connected, the last fetch time of the agent will be updated.
    *
-   * @param connectionId The id for the created connection.
-   * @param connectRequest The request for the new connection to be created.
-   * @return Connection The response containing all saved information.
+   * @param event The configuration request event.
    */
-  public Connection handleConnectRequest(
-      String connectionId, CreateConnectionRequest connectRequest) {
-    Connection connection = CreateConnectionRequest.toConnection(connectRequest);
-    connectionCache.put(connectionId, connection);
-
-    return connection;
+  @EventListener
+  public void handleConfigurationRequestEvent(ConfigurationRequestEvent event) {
+    if (!isAgentConnected(event.getAgentId())) {
+      connectAgent(event.getAgentId(), event.getHeaders());
+    } else {
+      updateConnectionLastFetch(event.getAgentId());
+    }
   }
 
   /**
@@ -95,6 +100,29 @@ public ConnectionDto getConnection(String id) {
     return ConnectionDto.fromConnection(connection);
   }
 
+  /**
+   * Determines if an agent is connected.
+   *
+   * @param agentId The id of the agent to be searched for.
+   * @return true if the agent was found in the cache, false otherwise.
+   */
+  private boolean isAgentConnected(String agentId) {
+    return connectionCache.get(agentId) != null;
+  }
+
+  /**
+   * Updates the last fetch time of an agent connection.
+   *
+   * @param agentId The id of the agent to be updated.
+   */
+  private void updateConnectionLastFetch(String agentId) {
+    Connection connection = connectionCache.get(agentId);
+    if (connection != null) connection.setLastFetch(Instant.now());
+    else
+      throw new NoSuchElementException(
+          "No connection for agent id " + agentId + " found in cache.");
+  }
+
   /**
    * Checks if a connection matches the given query.
    *
@@ -165,4 +193,48 @@ private boolean matchesAttributes(
                   && regexQueryService.matches(actualValue, queryEntry.getValue());
             });
   }
+
+  /**
+   * Handles a connection request from an agent.
+   *
+   * @param connectionId The id for the created connection.
+   * @param headers The request headers, which should contain the agent information.
+   */
+  private void connectAgent(String connectionId, Map<String, String> headers) {
+
+    validateConfigurationRequestHeaders(headers);
+
+    String serviceName = headers.get("x-gepard-service-name");
+    String vmId = headers.get("x-gepard-vm-id");
+    String gepardVersion = headers.get("x-gepard-gepard-version");
+    String otelVersion = headers.get("x-gepard-otel-version");
+    String javaVersion = headers.get("x-gepard-java-version");
+    String startTime = headers.get("x-gepard-start-time");
+
+    Map<String, String> attributes =
+        headers.entrySet().stream()
+            .filter(entry -> entry.getKey().startsWith("x-gepard-attribute-"))
+            .collect(
+                Collectors.toMap(
+                    entry ->
+                        entry
+                            .getKey()
+                            .substring("x-gepard-attribute-".length()), // remove the prefix
+                    Map.Entry::getValue));
+
+    Agent agent =
+        new Agent(
+            serviceName,
+            vmId,
+            gepardVersion,
+            otelVersion,
+            Instant.parse(startTime),
+            javaVersion,
+            attributes);
+
+    Connection connection =
+        new Connection(Instant.now(), Instant.now(), ConnectionStatus.CONNECTED, agent);
+
+    connectionCache.put(connectionId, connection);
+  }
 }

backend/src/main/java/rocks/inspectit/gepard/agentmanager/exception/GlobalExceptionHandler.java

@@ -157,4 +157,17 @@ public ResponseEntity<ApiError> handleInvalidPatternSyntax(
 
     return new ResponseEntity<>(apiError, HttpStatus.BAD_REQUEST);
   }
+
+  @ExceptionHandler(MissingHeaderException.class)
+  public ResponseEntity<ApiError> handleMissingHeaderException(
+      MissingHeaderException ex, HttpServletRequest request) {
+    ApiError apiError =
+        new ApiError(
+            request.getRequestURI(),
+            List.of(ex.getMessage()),
+            HttpStatus.BAD_REQUEST.value(),
+            LocalDateTime.now());
+
+    return new ResponseEntity<>(apiError, HttpStatus.BAD_REQUEST);
+  }
 }

backend/src/main/java/rocks/inspectit/gepard/agentmanager/exception/MissingHeaderException.java

@@ -0,0 +1,8 @@
+/* (C) 2024 */
+package rocks.inspectit.gepard.agentmanager.exception;
+
+public class MissingHeaderException extends RuntimeException {
+  public MissingHeaderException(String message) {
+    super(message);
+  }
+}