Events and subscriptions represent the cornerstone of modern Business Central development architecture. This publisher-subscriber pattern enables developers to extend system functionality without modifying base application code, ensuring upgrade compatibility and maintainable solutions.

The AL event system operates on a decoupled architecture where event publishers announce significant occurrences, and event subscribers respond to these announcements. This approach supports multiple extensions working together without direct dependencies, creating a flexible and scalable development environment.

Event Types and Their Applications

Integration Events

Integration events provide the most flexible extension points for custom functionality:

// Publishing an Integration Event
codeunit 50100 "Customer Management"
{
    [IntegrationEvent(false, false)]
    procedure OnBeforeCreateCustomer(var Customer: Record Customer; var IsHandled: Boolean)
    begin
    end;
    
    [IntegrationEvent(false, false)]
    procedure OnAfterCreateCustomer(Customer: Record Customer)
    begin
    end;
    
    procedure CreateCustomer(CustomerData: Record Customer)
    var
        IsHandled: Boolean;
    begin
        OnBeforeCreateCustomer(CustomerData, IsHandled);
        
        if not IsHandled then begin
            CustomerData.Insert(true);
            OnAfterCreateCustomer(CustomerData);
        end;
    end;
}

Integration event characteristics:

• Provide extension points without affecting base functionality
• Support conditional execution through "IsHandled" patterns
• Allow multiple subscribers without interference
• Maintain upgrade compatibility across Business Central versions

Business Events

Business events represent specific business operations and their outcomes:

// Business Event for sales processing
codeunit 50101 "Sales Order Processing"
{
    [BusinessEvent(false)]
    procedure OnSalesOrderShipped(SalesOrderNo: Code[20]; ShippingDate: Date; TrackingNo: Text[50])
    begin
    end;
    
    procedure ProcessShipment(SalesOrderNo: Code[20])
    var
        SalesHeader: Record "Sales Header";
        TrackingNumber: Text[50];
    begin
        // Shipment processing logic
        SalesHeader.Get(SalesHeader."Document Type"::Order, SalesOrderNo);
        TrackingNumber := GenerateTrackingNumber();
        
        // Raise business event
        OnSalesOrderShipped(SalesOrderNo, Today, TrackingNumber);
    end;
}

Business event applications:

• Workflow trigger points for business process automation
• Integration checkpoints for external system notifications
• Audit trail generation for compliance requirements
• Analytics data collection for business intelligence

Database Events

Database events respond to table operations automatically:

// Database events on Customer table
tableextension 50100 "Customer Events" extends Customer
{
    trigger OnAfterInsert()
    begin
        NotifyCustomerCreated();
    end;
    
    trigger OnAfterModify()
    begin
        if Rec."Credit Limit (LCY)" <> xRec."Credit Limit (LCY)" then
            NotifyCreditLimitChanged();
    end;
    
    local procedure NotifyCustomerCreated()
    var
        CustomerNotification: Codeunit "Customer Notification";
    begin
        CustomerNotification.SendWelcomeEmail(Rec);
    end;
    
    local procedure NotifyCreditLimitChanged()
    var
        ApprovalManagement: Codeunit "Custom Approval Management";
    begin
        if Rec."Credit Limit (LCY)" > 10000 then
            ApprovalManagement.RequestCreditLimitApproval(Rec);
    end;
}

Advanced Event Subscription Patterns

Conditional Event Handling

Implementing smart event handling with conditional logic:

// Sophisticated event subscriber with business logic
codeunit 50102 "Advanced Customer Handler"
{
    [EventSubscriber(ObjectType::Codeunit, Codeunit::"Customer Management", 'OnBeforeCreateCustomer', '', false, false)]
    local procedure ValidateCustomerData(var Customer: Record Customer; var IsHandled: Boolean)
    var
        DuplicateCustomer: Record Customer;
        ValidationError: Text;
    begin
        // Duplicate check
        DuplicateCustomer.SetRange("VAT Registration No.", Customer."VAT Registration No.");
        if not DuplicateCustomer.IsEmpty then begin
            ValidationError := 'Customer with VAT number %1 already exists';
            Error(ValidationError, Customer."VAT Registration No.");
        end;
        
        // Credit check for high-value customers
        if Customer."Credit Limit (LCY)" > 50000 then begin
            if not PerformCreditCheck(Customer) then begin
                IsHandled := true;
                CreatePendingApprovalRecord(Customer);
            end;
        end;
    end;
    
    local procedure PerformCreditCheck(Customer: Record Customer): Boolean
    var
        CreditCheckService: Codeunit "Credit Check Service";
    begin
        exit(CreditCheckService.ValidateCustomerCredit(Customer));
    end;
    
    local procedure CreatePendingApprovalRecord(Customer: Record Customer)
    var
        PendingApproval: Record "Pending Customer Approval";
    begin
        PendingApproval.Init();
        PendingApproval.TransferFields(Customer);
        PendingApproval."Approval Status" := PendingApproval."Approval Status"::Pending;
        PendingApproval.Insert();
    end;
}

Event Chaining and Orchestration

Creating complex workflows through event orchestration:

// Multi-step business process orchestration
codeunit 50103 "Order Processing Orchestrator"
{
    [EventSubscriber(ObjectType::Codeunit, Codeunit::"Sales-Post", 'OnAfterPostSalesDoc', '', false, false)]
    local procedure OnSalesOrderPosted(var SalesHeader: Record "Sales Header")
    begin
        case SalesHeader."Document Type" of
            SalesHeader."Document Type"::Order:
                ProcessPostedSalesOrder(SalesHeader);
        end;
    end;
    
    local procedure ProcessPostedSalesOrder(SalesHeader: Record "Sales Header")
    var
        InventoryManagement: Codeunit "Inventory Management";
        ShippingProcessor: Codeunit "Shipping Processor";
        InvoiceProcessor: Codeunit "Invoice Processor";
    begin
        // Step 1: Update inventory allocations
        InventoryManagement.UpdateAllocations(SalesHeader."No.");
        
        // Step 2: Generate shipping documentation
        ShippingProcessor.CreateShippingDocuments(SalesHeader."No.");
        
        // Step 3: Process invoicing if immediate billing
        if SalesHeader."Invoice Discount Calculation" = SalesHeader."Invoice Discount Calculation"::"% (Normal VAT)" then
            InvoiceProcessor.GenerateInvoice(SalesHeader."No.");
    end;
}

Error Handling and Performance Optimization

Robust Error Management in Event Subscribers

// Error handling best practices in event subscribers
codeunit 50104 "Resilient Event Handler"
{
    [EventSubscriber(ObjectType::Table, Database::Customer, 'OnAfterModifyEvent', '', false, false)]
    local procedure HandleCustomerModification(var Rec: Record Customer)
    var
        ErrorMessage: Text;
        TelemetryDictionary: Dictionary of [Text, Text];
    begin
        if not TryUpdateExternalSystems(Rec) then begin
            ErrorMessage := GetLastErrorText();
            LogError('Customer modification sync failed', ErrorMessage, Rec."No.");
            
            // Don't fail the main transaction for external system issues
            ClearLastError();
            
            // Queue for retry processing
            QueueForRetry(Rec);
        end;
    end;
    
    [TryFunction]
    local procedure TryUpdateExternalSystems(Customer: Record Customer)
    var
        ExternalCRM: Codeunit "External CRM Integration";
        WebService: Codeunit "Customer Web Service";
    begin
        ExternalCRM.UpdateCustomer(Customer);
        WebService.NotifyCustomerChange(Customer."No.");
    end;
    
    local procedure LogError(Context: Text; ErrorText: Text; CustomerNo: Code[20])
    var
        ActivityLog: Record "Activity Log";
    begin
        ActivityLog.LogActivity(
            Database::Customer,
            ActivityLog.Status::Failed,
            Context,
            ErrorText,
            CustomerNo);
    end;
    
    local procedure QueueForRetry(Customer: Record Customer)
    var
        RetryQueue: Record "Integration Retry Queue";
    begin
        RetryQueue.Init();
        RetryQueue."Record ID" := Customer.RecordId;
        RetryQueue."Retry Count" := 0;
        RetryQueue."Next Retry Time" := CurrentDateTime + 300000; // 5 minutes
        RetryQueue.Insert();
    end;
}

Performance Considerations

Optimizing event subscriber performance:

// Performance-optimized event handling
codeunit 50105 "Optimized Event Handler"
{
    var
        IsInitialized: Boolean;
        ConfigurationCache: Dictionary of [Text, Text];
    
    [EventSubscriber(ObjectType::Table, Database::"Sales Line", 'OnAfterModifyEvent', '', false, false)]
    local procedure OptimizedSalesLineHandler(var Rec: Record "Sales Line")
    begin
        // Early exit conditions to avoid unnecessary processing
        if not ShouldProcessSalesLine(Rec) then
            exit;
            
        // Initialize cache only when needed
        if not IsInitialized then
            InitializeCache();
            
        // Batch operations for multiple line changes
        if not IsProcessingBatch() then
            ProcessSalesLineChange(Rec)
        else
            QueueBatchOperation(Rec);
    end;
    
    local procedure ShouldProcessSalesLine(SalesLine: Record "Sales Line"): Boolean
    begin
        // Only process if meaningful fields changed
        exit(
            (SalesLine.Quantity <> xRec.Quantity) or
            (SalesLine."Unit Price" <> xRec."Unit Price") or
            (SalesLine."Line Discount %" <> xRec."Line Discount %")
        );
    end;
    
    local procedure InitializeCache()
    var
        Setup: Record "Sales & Receivables Setup";
    begin
        Setup.Get();
        ConfigurationCache.Add('AutoPostInvoice', Format(Setup."Auto Post Invoice"));
        ConfigurationCache.Add('RequireApproval', Format(Setup."Require Approval"));
        IsInitialized := true;
    end;
}

Event Documentation and Maintenance

Self-Documenting Event Patterns

// Well-documented event publisher with clear contracts
codeunit 50106 "Purchase Order Management"
{
    /// <summary>
    /// Raised before a purchase order is converted to an invoice.
    /// Allows subscribers to validate data or prevent conversion.
    /// </summary>
    /// <param name="PurchaseHeader">The purchase order being converted</param>
    /// <param name="IsHandled">Set to true to prevent default conversion</param>
    /// <param name="SkipValidation">Set to true to skip standard validations</param>
    [IntegrationEvent(false, false)]
    procedure OnBeforeConvertPurchaseOrderToInvoice(var PurchaseHeader: Record "Purchase Header"; var IsHandled: Boolean; var SkipValidation: Boolean)
    begin
    end;
    
    /// <summary>
    /// Raised after successful purchase order conversion to invoice.
    /// Use for notifications, logging, or triggering dependent processes.
    /// </summary>
    /// <param name="OriginalOrderNo">The original purchase order number</param>
    /// <param name="NewInvoiceNo">The generated invoice number</param>
    /// <param name="ConversionDetails">Additional conversion information</param>
    [IntegrationEvent(false, false)]
    procedure OnAfterConvertPurchaseOrderToInvoice(OriginalOrderNo: Code[20]; NewInvoiceNo: Code[20]; ConversionDetails: Dictionary of [Text, Text])
    begin
    end;
}

Testing Event-Driven Functionality

Comprehensive Event Testing Strategies

// Test codeunit for event functionality
codeunit 50150 "Event Handler Tests"
{
    Subtype = Test;
    
    var
        EventWasRaised: Boolean;
        EventParameters: List of [Text];
    
    [Test]
    procedure TestCustomerCreationEvent()
    var
        Customer: Record Customer;
        CustomerMgt: Codeunit "Customer Management";
    begin
        // Setup
        EventWasRaised := false;
        EventParameters.Clear();
        
        // Execute
        Customer.Init();
        Customer."No." := 'TEST001';
        Customer.Name := 'Test Customer';
        CustomerMgt.CreateCustomer(Customer);
        
        // Verify
        Assert.IsTrue(EventWasRaised, 'Customer creation event should have been raised');
        Assert.AreEqual('TEST001', EventParameters.Get(1), 'Event should contain customer number');
    end;
    
    [EventSubscriber(ObjectType::Codeunit, Codeunit::"Customer Management", 'OnAfterCreateCustomer', '', false, false)]
    local procedure CaptureCustomerCreationEvent(Customer: Record Customer)
    begin
        EventWasRaised := true;
        EventParameters.Add(Customer."No.");
        EventParameters.Add(Customer.Name);
    end;
}

Event-Driven Architecture Patterns

Microservice-Style Event Handling

// Domain-specific event handlers following microservice patterns
codeunit 50107 "Inventory Domain Handler"
{
    // Handle all inventory-related events in one domain handler
    
    [EventSubscriber(ObjectType::Table, Database::Item, 'OnAfterModifyEvent', '', false, false)]
    local procedure HandleItemChange(var Rec: Record Item)
    begin
        ProcessItemMasterDataChange(Rec);
    end;
    
    [EventSubscriber(ObjectType::Codeunit, Codeunit::"Item Jnl.-Post Line", 'OnAfterPostItemJnlLine', '', false, false)]
    local procedure HandleInventoryTransaction(var ItemJournalLine: Record "Item Journal Line")
    begin
        ProcessInventoryMovement(ItemJournalLine);
    end;
    
    local procedure ProcessItemMasterDataChange(Item: Record Item)
    var
        ChangeDetector: Codeunit "Item Change Detector";
        NotificationService: Codeunit "Inventory Notification Service";
    begin
        if ChangeDetector.IsPriceChange(Item, xRec) then
            NotificationService.NotifyPriceChange(Item);
            
        if ChangeDetector.IsAvailabilityChange(Item, xRec) then
            NotificationService.NotifyAvailabilityChange(Item);
    end;
}

Event Lifecycle Management

Dynamic Event Subscription Control

// Configurable event subscription management
codeunit 50108 "Event Subscription Manager"
{
    procedure EnableIntegrationFeature(FeatureName: Text)
    var
        IntegrationSetup: Record "Integration Feature Setup";
    begin
        IntegrationSetup.SetRange("Feature Name", FeatureName);
        if IntegrationSetup.FindFirst() then begin
            IntegrationSetup.Enabled := true;
            IntegrationSetup.Modify();
        end;
    end;
    
    [EventSubscriber(ObjectType::Codeunit, Codeunit::"Customer Management", 'OnAfterCreateCustomer', '', false, false)]
    local procedure ConditionalCustomerHandler(Customer: Record Customer)
    var
        IntegrationSetup: Record "Integration Feature Setup";
    begin
        IntegrationSetup.SetRange("Feature Name", 'EXTERNAL_CRM_SYNC');
        IntegrationSetup.SetRange(Enabled, true);
        
        if IntegrationSetup.FindFirst() then
            ProcessExternalCRMSync(Customer);
    end;
    
    local procedure ProcessExternalCRMSync(Customer: Record Customer)
    var
        ExternalSync: Codeunit "External CRM Sync";
    begin
        ExternalSync.SyncCustomer(Customer);
    end;
}

Understanding and effectively implementing events and subscriptions in AL enables developers to create sophisticated, maintainable Business Central extensions that integrate seamlessly with the base application while remaining upgrade-compatible. This event-driven architecture supports complex business requirements while maintaining clean code separation and testability.

The AL event system provides the foundation for building enterprise-grade solutions that can evolve with changing business needs while preserving existing functionality and ensuring long-term maintainability.