Skip to main content
Version: Next

Schema Validation at Publish Time

The Hermodr.Publisher.EventSchema package adds a middleware component that automatically validates every event's payload against its registered schema before the event reaches any publish channel.

Prerequisites​

dotnet add package Hermodr.Publisher.EventSchema

Quick start​

Register the schema validation middleware on the publisher builder:

using Hermodr;

builder.Services
    .AddEventPublisher(opts => opts.Source = new Uri("https://example.com"))
    .UseSchemaValidation()
    .AddChannel<RabbitMqPublishChannel>();

UseSchemaValidation() auto-registers all schema services (IEventSchemaRegistry, IEventSchemaValidator, deserializers) if they are not already registered.

Registering schemas​

Schemas must be registered before validation can run. Pass them via the configureSchemaServices callback:

builder.Services
    .AddEventPublisher()
    .UseSchemaValidation(
        configureOptions: null,
        configureSchemaServices: schema =>
        {
            schema.SchemaRegistrations.Add(registry =>
            {
                var schema = new EventSchema("order.placed", "1.0", "application/json");
                schema.Properties.Add(new EventProperty("orderId", "string", "1.0"));
                schema.Properties[0]!.Constraints.Add(new PropertyRequiredConstraint());
                schema.Properties.Add(new EventProperty("amount", "decimal", "1.0"));
                schema.Properties[1]!.Constraints.Add(new PropertyRequiredConstraint());
                registry.Register(schema);
            });
        });

You can also register schemas from CLR types using assembly scanning:

schema.AssembliesToScan.Add(typeof(OrderPlacedData).Assembly);

Configuration​

Missing schema behavior​

Controls what happens when no schema is registered for an event type:

BehaviorDescription
Allow (default)The event is published without validation
BlockA SchemaValidationException is thrown
FallbackA schema with no constraints is used (all properties pass)
.UseSchemaValidation(opts =>
{
    opts.MissingSchemaBehavior = MissingSchemaBehavior.Block;
})

Validation failure behavior​

Controls what happens when the payload does not match the schema:

BehaviorDescription
Throw (default)A SchemaValidationException is thrown with all validation errors
LogErrors are logged but the event is still published
.UseSchemaValidation(opts =>
{
    opts.ValidationFailureBehavior = ValidationFailureBehavior.Log;
})

Version-aware validation​

The middleware supports schema versioning through the dataversion CloudEvent extension attribute. When present, the exact version is looked up; when absent, the latest registered version is used.

var @event = new CloudEvent
{
    Type = "order.placed",
    Source = new Uri("https://example.com"),
    Data = data
};

// Set the version extension attribute
var attr = CloudEventAttribute.CreateExtension("dataversion", CloudEventAttributeType.String);
@event[attr] = "2.0";

How it works​

The middleware runs after enrichment and before channel dispatch:

  1. Looks up the schema by CloudEvent.Type (and optional dataversion extension)
  2. Deserializes the event payload using a format-agnostic deserializer (JSON, XML, etc.)
  3. Validates every property against the schema's constraints (required, range, enum)
  4. On failure: throws or logs according to ValidationFailureBehavior
  5. On success: calls next(context) to continue the pipeline