Sling Distribution

AEM Cloud Service Sling Distribution Event Handling

Monitor and react to content distribution lifecycle events using the Sling Distribution API.

When to Use This Skill

Use Sling Distribution event handling for:

• Monitoring distribution lifecycle: Track package creation, queueing, and delivery
• Custom post-distribution actions: Cache warming, notifications, analytics
• Distribution auditing: Log and track all distribution events
• Failure handling: React to dropped packages and distribution failures
• Integration workflows: Trigger external systems after successful distribution

For programmatic publishing, use the Replication API instead.

Official API Documentation

Javadoc: https://developer.adobe.com/experience-manager/reference-materials/cloud-service/javadoc/org/apache/sling/distribution/package-summary.html

Key Packages:

• org.apache.sling.distribution - Core distribution interfaces
• org.apache.sling.distribution.event - Event topics and properties
• org.apache.sling.distribution.queue - Queue monitoring

Understanding Sling Distribution in Cloud Service

What is Sling Distribution?

Sling Distribution is the underlying transport mechanism for content replication in AEM Cloud Service. When you use the Replication API (Replicator.replicate()), Sling Distribution handles:

1. Package Creation: Content is assembled into distribution packages
2. Queueing: Packages are queued for delivery
3. Transport: Packages are sent to Adobe Developer pipeline service
4. Import: Target tier imports and applies the content

Architecture Flow

Replication API Call
       ↓
[AGENT_PACKAGE_CREATED] - Package assembled
       ↓
[AGENT_PACKAGE_QUEUED] - Package added to queue
       ↓
[AGENT_PACKAGE_DISTRIBUTED] - Package sent to pipeline
       ↓
[IMPORTER_PACKAGE_IMPORTED] - Package imported on target tier

OR

[AGENT_PACKAGE_DROPPED] - Package failed and was removed

Distribution Event Topics

Available Event Topics

Sling Distribution fires events at each stage of the distribution lifecycle:

Event Topic Constants

import org.apache.sling.distribution.event.DistributionEventTopics;

// Event topic strings
DistributionEventTopics.AGENT_PACKAGE_CREATED      // Package created
DistributionEventTopics.AGENT_PACKAGE_QUEUED       // Package queued
DistributionEventTopics.AGENT_PACKAGE_DISTRIBUTED  // Package distributed
DistributionEventTopics.AGENT_PACKAGE_DROPPED      // Package dropped
DistributionEventTopics.IMPORTER_PACKAGE_IMPORTED  // Package imported

Listening to Distribution Events

Example: Basic Distribution Event Logger

import org.apache.sling.distribution.event.DistributionEventTopics;
import org.apache.sling.distribution.event.DistributionEventProperties;
import org.osgi.service.component.annotations.Component;
import org.osgi.service.event.Event;
import org.osgi.service.event.EventHandler;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

@Component(
    service = EventHandler.class,
    property = {
        org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
            DistributionEventTopics.AGENT_PACKAGE_CREATED,
        org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
            DistributionEventTopics.AGENT_PACKAGE_QUEUED,
        org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
            DistributionEventTopics.AGENT_PACKAGE_DISTRIBUTED,
        org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
            DistributionEventTopics.AGENT_PACKAGE_DROPPED,
        org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
            DistributionEventTopics.IMPORTER_PACKAGE_IMPORTED
    }
)
public class DistributionEventLogger implements EventHandler {
    
    private static final Logger LOG = LoggerFactory.getLogger(DistributionEventLogger.class);
    
    @Override
    public void handleEvent(Event event) {
        String topic = event.getTopic();
        
        // Extract event properties
        String componentName = (String) event.getProperty(
            DistributionEventProperties.DISTRIBUTION_COMPONENT_NAME
        );
        String componentKind = (String) event.getProperty(
            DistributionEventProperties.DISTRIBUTION_COMPONENT_KIND
        );
        String distributionType = (String) event.getProperty(
            DistributionEventProperties.DISTRIBUTION_TYPE
        );
        String packageId = (String) event.getProperty(
            DistributionEventProperties.DISTRIBUTION_PACKAGE_ID
        );
        String[] paths = (String[]) event.getProperty(
            DistributionEventProperties.DISTRIBUTION_PATHS
        );
        Long timestamp = (Long) event.getProperty(
            DistributionEventProperties.DISTRIBUTION_ENQUEUE_TIMESTAMP
        );
        
        LOG.info("Distribution Event: topic={}, component={}, type={}, packageId={}, paths={}, timestamp={}",
            topic, componentName, distributionType, packageId, 
            paths != null ? String.join(",", paths) : "null",
            timestamp
        );
    }
}

Event Properties

Available Event Properties

Every distribution event contains these properties:

Accessing Event Properties

import org.apache.sling.distribution.event.DistributionEventProperties;

@Override
public void handleEvent(Event event) {
    // Component information
    String componentName = (String) event.getProperty(
        DistributionEventProperties.DISTRIBUTION_COMPONENT_NAME
    );
    String componentKind = (String) event.getProperty(
        DistributionEventProperties.DISTRIBUTION_COMPONENT_KIND
    );
    
    // Distribution details
    String type = (String) event.getProperty(
        DistributionEventProperties.DISTRIBUTION_TYPE
    );
    String packageId = (String) event.getProperty(
        DistributionEventProperties.DISTRIBUTION_PACKAGE_ID
    );
    
    // Content paths
    String[] paths = (String[]) event.getProperty(
        DistributionEventProperties.DISTRIBUTION_PATHS
    );
    String[] deepPaths = (String[]) event.getProperty(
        DistributionEventProperties.DISTRIBUTION_DEEP_PATHS
    );
    
    // Timing
    Long enqueueTime = (Long) event.getProperty(
        DistributionEventProperties.DISTRIBUTION_ENQUEUE_TIMESTAMP
    );
}

Common Use Cases

Use Case 1: Alert on Distribution Failures

import org.apache.sling.distribution.event.DistributionEventTopics;

@Component(
    service = EventHandler.class,
    property = {
        org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
            DistributionEventTopics.AGENT_PACKAGE_DROPPED
    }
)
public class DistributionFailureAlertHandler implements EventHandler {
    
    private static final Logger LOG = LoggerFactory.getLogger(
        DistributionFailureAlertHandler.class
    );
    
    @Reference
    private AlertService alertService;  // Custom alert service
    
    @Override
    public void handleEvent(Event event) {
        String packageId = (String) event.getProperty(
            DistributionEventProperties.DISTRIBUTION_PACKAGE_ID
        );
        String[] paths = (String[]) event.getProperty(
            DistributionEventProperties.DISTRIBUTION_PATHS
        );
        
        LOG.error("Distribution package dropped: packageId={}, paths={}", 
            packageId, String.join(",", paths));
        
        // Send alert to operations team
        alertService.sendAlert(
            "CRITICAL: Distribution Failed",
            String.format("Package %s was dropped. Paths: %s", 
                packageId, String.join(",", paths))
        );
    }
}

Use Case 2: CDN Cache Warming After Distribution

@Component(
    service = EventHandler.class,
    property = {
        org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
            DistributionEventTopics.IMPORTER_PACKAGE_IMPORTED
    }
)
public class CdnCacheWarmingHandler implements EventHandler {
    
    private static final Logger LOG = LoggerFactory.getLogger(
        CdnCacheWarmingHandler.class
    );
    
    @Reference
    private HttpClient httpClient;
    
    @Override
    public void handleEvent(Event event) {
        String[] paths = (String[]) event.getProperty(
            DistributionEventProperties.DISTRIBUTION_PATHS
        );
        String distributionType = (String) event.getProperty(
            DistributionEventProperties.DISTRIBUTION_TYPE
        );
        
        // Only warm cache on ADD (activation)
        if ("ADD".equals(distributionType) && paths != null) {
            for (String path : paths) {
                warmCache(path);
            }
        }
    }
    
    private void warmCache(String path) {
        try {
            // Convert JCR path to public URL
            String publicUrl = "https://www.example.com" + 
                path.replace("/content/mysite", "") + ".html";
            
            LOG.info("Warming CDN cache for: {}", publicUrl);
            
            HttpGet request = new HttpGet(publicUrl);
            httpClient.execute(request);
            
        } catch (Exception e) {
            LOG.error("Cache warming failed for path: " + path, e);
        }
    }
}

Use Case 3: Distribution Analytics Tracking

@Component(
    service = EventHandler.class,
    property = {
        org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
            DistributionEventTopics.AGENT_PACKAGE_CREATED,
        org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
            DistributionEventTopics.AGENT_PACKAGE_DISTRIBUTED,
        org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
            DistributionEventTopics.AGENT_PACKAGE_DROPPED
    }
)
public class DistributionAnalyticsHandler implements EventHandler {
    
    @Reference
    private AnalyticsService analyticsService;
    
    @Override
    public void handleEvent(Event event) {
        String topic = event.getTopic();
        String packageId = (String) event.getProperty(
            DistributionEventProperties.DISTRIBUTION_PACKAGE_ID
        );
        String[] paths = (String[]) event.getProperty(
            DistributionEventProperties.DISTRIBUTION_PATHS
        );
        Long timestamp = (Long) event.getProperty(
            DistributionEventProperties.DISTRIBUTION_ENQUEUE_TIMESTAMP
        );
        
        // Track metrics
        if (DistributionEventTopics.AGENT_PACKAGE_CREATED.equals(topic)) {
            analyticsService.trackEvent("distribution.package.created", 
                Map.of("packageId", packageId, "pathCount", paths.length));
        } 
        else if (DistributionEventTopics.AGENT_PACKAGE_DISTRIBUTED.equals(topic)) {
            long duration = System.currentTimeMillis() - timestamp;
            analyticsService.trackEvent("distribution.package.distributed", 
                Map.of("packageId", packageId, "duration", duration));
        }
        else if (DistributionEventTopics.AGENT_PACKAGE_DROPPED.equals(topic)) {
            analyticsService.trackEvent("distribution.package.failed", 
                Map.of("packageId", packageId, "paths", String.join(",", paths)));
        }
    }
}

Use Case 4: Slack Notifications for Production Publishes

@Component(
    service = EventHandler.class,
    property = {
        org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
            DistributionEventTopics.IMPORTER_PACKAGE_IMPORTED
    }
)
public class ProductionPublishNotificationHandler implements EventHandler {
    
    @Reference
    private SlackService slackService;
    
    @Reference
    private SlingSettingsService slingSettings;
    
    @Override
    public void handleEvent(Event event) {
        // Only notify for production environment
        if (!slingSettings.getRunModes().contains("prod")) {
            return;
        }
        
        String[] paths = (String[]) event.getProperty(
            DistributionEventProperties.DISTRIBUTION_PATHS
        );
        String componentName = (String) event.getProperty(
            DistributionEventProperties.DISTRIBUTION_COMPONENT_NAME
        );
        
        // Only notify for publish agent (not preview)
        if (!"publish".equals(componentName)) {
            return;
        }
        
        String message = String.format(
            ":rocket: Content published to production: %s",
            String.join(", ", paths)
        );
        
        slackService.postToChannel("#content-releases", message);
    }
}

Use Case 5: Audit Log for All Distribution Events

import javax.jcr.Node;
import org.apache.sling.api.resource.ResourceResolver;
import org.apache.sling.api.resource.ResourceResolverFactory;

@Component(
    service = EventHandler.class,
    property = {
        org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
            DistributionEventTopics.AGENT_PACKAGE_CREATED,
        org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
            DistributionEventTopics.AGENT_PACKAGE_QUEUED,
        org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
            DistributionEventTopics.AGENT_PACKAGE_DISTRIBUTED,
        org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
            DistributionEventTopics.AGENT_PACKAGE_DROPPED,
        org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
            DistributionEventTopics.IMPORTER_PACKAGE_IMPORTED
    }
)
public class DistributionAuditHandler implements EventHandler {
    
    @Reference
    private ResourceResolverFactory resolverFactory;
    
    @Override
    public void handleEvent(Event event) {
        try (ResourceResolver resolver = getServiceResolver()) {
            
            String topic = event.getTopic();
            String packageId = (String) event.getProperty(
                DistributionEventProperties.DISTRIBUTION_PACKAGE_ID
            );
            String[] paths = (String[]) event.getProperty(
                DistributionEventProperties.DISTRIBUTION_PATHS
            );
            Long timestamp = (Long) event.getProperty(
                DistributionEventProperties.DISTRIBUTION_ENQUEUE_TIMESTAMP
            );
            
            // Create audit log entry under /var/audit/distribution
            String auditPath = "/var/audit/distribution/" + 
                System.currentTimeMillis();
            
            Node auditNode = resolver.getResource("/var/audit/distribution")
                .adaptTo(Node.class)
                .addNode(String.valueOf(System.currentTimeMillis()), "nt:unstructured");
            
            auditNode.setProperty("topic", topic);
            auditNode.setProperty("packageId", packageId);
            auditNode.setProperty("paths", paths);
            auditNode.setProperty("timestamp", timestamp);
            auditNode.setProperty("auditTime", System.currentTimeMillis());
            
            resolver.commit();
            
        } catch (Exception e) {
            LOG.error("Failed to create audit log", e);
        }
    }
    
    private ResourceResolver getServiceResolver() throws Exception {
        Map<String, Object> param = Map.of(
            ResourceResolverFactory.SUBSERVICE, "distributionAuditor"
        );
        return resolverFactory.getServiceResourceResolver(param);
    }
}

Distribution Request Types

When monitoring events, the DISTRIBUTION_TYPE property indicates the type of operation:

Example: Handle Different Distribution Types

@Override
public void handleEvent(Event event) {
    String distributionType = (String) event.getProperty(
        DistributionEventProperties.DISTRIBUTION_TYPE
    );
    String[] paths = (String[]) event.getProperty(
        DistributionEventProperties.DISTRIBUTION_PATHS
    );
    
    switch (distributionType) {
        case "ADD":
            LOG.info("Content activated: {}", String.join(",", paths));
            break;
        case "DELETE":
            LOG.info("Content deleted: {}", String.join(",", paths));
            break;
        case "INVALIDATE":
            LOG.info("Cache invalidated: {}", String.join(",", paths));
            break;
        case "TEST":
            LOG.debug("Distribution test executed");
            break;
        default:
            LOG.warn("Unknown distribution type: {}", distributionType);
    }
}

Best Practices

1. Filter Events Appropriately

Only listen to events you need:

// Good: Listen to specific events
property = {
    org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
        DistributionEventTopics.AGENT_PACKAGE_DROPPED
}

// Avoid: Listening to all events if not needed

2. Handle Events Asynchronously

Event handlers should be fast. Offload heavy processing:

@Reference
private JobManager jobManager;

@Override
public void handleEvent(Event event) {
    // Queue a job for async processing
    Map<String, Object> jobProperties = new HashMap<>();
    jobProperties.put("packageId", event.getProperty(
        DistributionEventProperties.DISTRIBUTION_PACKAGE_ID));
    
    jobManager.addJob("com/myapp/distribution/process", jobProperties);
}

3. Use Service Users

Event handlers should use service users, not admin sessions:

<!-- Service user mapping -->
{
  "user.mapping": [
    "com.myapp.core:distributionEventHandler=myapp-distribution-service"
  ]
}

4. Handle Null Properties

Not all properties are available in all events:

String[] paths = (String[]) event.getProperty(
    DistributionEventProperties.DISTRIBUTION_PATHS
);

if (paths != null && paths.length > 0) {
    // Process paths
}

5. Log Appropriately

Use appropriate log levels:

// INFO for normal flow
LOG.info("Package distributed: {}", packageId);

// WARN for unexpected situations
LOG.warn("Package queued longer than expected: {}", packageId);

// ERROR for failures
LOG.error("Package dropped: {}", packageId);

Monitoring Distribution Queue

While you can't directly query distribution queues via the API in Cloud Service, you can monitor via:

1. Events: Track AGENT_PACKAGE_QUEUED and AGENT_PACKAGE_DISTRIBUTED events
2. Felix Console: View Sling jobs at /system/console/slingjobs
3. Logs: Check distribution logs in Cloud Manager

Example: Queue Depth Monitoring

import java.util.concurrent.ConcurrentHashMap;
import java.util.Map;

@Component(
    service = {EventHandler.class, QueueMonitor.class},
    property = {
        org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
            DistributionEventTopics.AGENT_PACKAGE_QUEUED,
        org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
            DistributionEventTopics.AGENT_PACKAGE_DISTRIBUTED,
        org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
            DistributionEventTopics.AGENT_PACKAGE_DROPPED
    }
)
public class QueueMonitor implements EventHandler {
    
    private final Map<String, Long> queuedPackages = new ConcurrentHashMap<>();
    
    @Override
    public void handleEvent(Event event) {
        String topic = event.getTopic();
        String packageId = (String) event.getProperty(
            DistributionEventProperties.DISTRIBUTION_PACKAGE_ID
        );
        
        if (DistributionEventTopics.AGENT_PACKAGE_QUEUED.equals(topic)) {
            queuedPackages.put(packageId, System.currentTimeMillis());
            LOG.info("Queue depth: {}", queuedPackages.size());
            
            // Alert if queue is too deep
            if (queuedPackages.size() > 50) {
                LOG.warn("Distribution queue depth exceeds threshold: {}", 
                    queuedPackages.size());
            }
        } 
        else if (DistributionEventTopics.AGENT_PACKAGE_DISTRIBUTED.equals(topic) ||
                 DistributionEventTopics.AGENT_PACKAGE_DROPPED.equals(topic)) {
            Long queuedTime = queuedPackages.remove(packageId);
            
            if (queuedTime != null) {
                long duration = System.currentTimeMillis() - queuedTime;
                LOG.info("Package processed in {}ms. Queue depth: {}", 
                    duration, queuedPackages.size());
            }
        }
    }
    
    public int getQueueDepth() {
        return queuedPackages.size();
    }
}

Troubleshooting

Issue: Event Handler Not Firing

Causes:

1. Wrong event topic constant
2. Component not activated
3. Event topic not registered

Solution:

// Verify event topic matches exactly
property = {
    org.osgi.service.event.EventConstants.EVENT_TOPIC + "=" + 
        DistributionEventTopics.AGENT_PACKAGE_CREATED  // Exact constant
}

// Check component is active in Felix console
// /system/console/components

Issue: Missing Event Properties

Cause: Not all properties are available in all events

Solution: Always null-check:

String[] paths = (String[]) event.getProperty(
    DistributionEventProperties.DISTRIBUTION_PATHS
);

if (paths == null) {
    LOG.warn("No paths in distribution event");
    return;
}

Issue: Event Handler Slowing Down Distribution

Cause: Synchronous processing in event handler

Solution: Use async job processing:

@Reference
private JobManager jobManager;

@Override
public void handleEvent(Event event) {
    // Don't do heavy work here
    jobManager.addJob("com/myapp/process", eventData);
}

Relationship with Replication API

Key Understanding:

• Replication API (com.day.cq.replication.Replicator) - What you call to publish content
• Sling Distribution Events (org.apache.sling.distribution.event) - What fires during the distribution lifecycle

Workflow:

Your Code: replicator.replicate(...)
    ↓
Sling Distribution: Creates package → [AGENT_PACKAGE_CREATED]
    ↓
Sling Distribution: Queues package → [AGENT_PACKAGE_QUEUED]
    ↓
Sling Distribution: Sends package → [AGENT_PACKAGE_DISTRIBUTED]
    ↓
Target Tier: Imports package → [IMPORTER_PACKAGE_IMPORTED]

References

• Sling Distribution Javadoc: https://developer.adobe.com/experience-manager/reference-materials/cloud-service/javadoc/org/apache/sling/distribution/package-summary.html
• Sling Distribution Docs: https://sling.apache.org/documentation/bundles/content-distribution.html
• Replication API Skill: replication/SKILL.md
• Adobe AEM Replication: https://experienceleague.adobe.com/docs/experience-manager-cloud-service/content/operations/replication.html