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How to send and receive messages as CloudEvents

The RockLib.Messaging.CloudEvents package allows messages to be sent, received, and validated according to the CloudEvents spec.

Sending CloudEvents with an ISender

To make it easy to send CloudEvents with any ISender or ITransactionalSender, the CloudEvent class and all of its inheritors are implicitly convertible to SenderMessage. Simply instantiate a CloudEvent and pass it anywhere that needs a sender message.

ISender sender = // TODO: Initialize

// Source and Type must be provided.
CloudEvent cloudEvent = new CloudEvent
{
    Source = "example.org/sample/123456",
    Type = "example"
};

await sender.SendAsync(cloudEvent);

Alternatively, explicitly convert a CloudEvent by calling its ToSenderMessage() method:

await sender.SendAsync(cloudEvent.ToSenderMessage());

Validating messages and events

To ensure that a CloudEvent is valid, call its Validate() method - it will throw a CloudEventValidationException if the event has missing or invalid attribute values.

To ensure that a SenderMessage is valid for a given IProtocolBinding, call the static Validate method on the specified CloudEvent type - it will throw a CloudEventValidationException if the sender message has missing or invalid headers given the specified protocol binding.

To ensure that all messages sent by an ISender are in the correct format for a given IProtocolBinding, wrap it in a ValidatingSender and call a static Validate(SenderMessage, IProtocolBinding) method in the callback. There is also a AddValidation<TCloudEvent>() extension method for ISenderBuilder that calls the static Validate method of type TCloudEvent:

public void ConfigureServices(IServiceCollection services)
{
    services.AddNamedPipeSender("exampleSender")
        .AddValidation<SequentialEvent>(ProtocolBindings.Default);
}

Receiving CloudEvents from an IReceiver

To receive CloudEvents from any IReceiver, start it using the Start<TCloudEvent> extension method, where TCloudEvent is CloudEvent or its inheritor.

public class MyService : IHostedService
{
    private readonly IReceiver _receiver;

    public MyService(IReceiver receiver) =>
        _receiver = receiver ?? throw new ArgumentNullException(nameof(receiver));

    public Task StartAsync(CancellationToken cancellationToken)
    {
        _receiver.Start<CorrelatedEvent>(OnCorrelatedEventReceived, ProtocolBindings.Default);
        return Task.CompletedTask;
    }

    public Task StopAsync(CancellationToken cancellationToken)
    {
        _receiver.Dispose();
        return Task.CompletedTask;
    }

    private async Task OnCorrelatedEventReceived(CorrelatedEvent correlatedEvent,
        IReceiverMessage receiverMessage)
    {
        Console.WriteLine(correlatedEvent.StringData);
        Console.WriteLine($"  id:            {correlatedEvent.Id}");
        Console.WriteLine($"  source:        {correlatedEvent.Source}");
        Console.WriteLine($"  correlationid: {correlatedEvent.CorrelationId}");

        await receiverMessage.AcknowledgeAsync();
    }
}

CloudEvent class

The CloudEvent class is the base class for all CloudEvents. Three additional implementations, SequentialEvent, CorrelatedEvent, and PartitionedEvent are included in this package.

CloudEvent Constructors

The CloudEvent class defines four constructors:

  • public CloudEvent()
    • The default constructor.
    • This constructor does not set any properties.
  • public CloudEvent(CloudEvent source)
    • The copy constructor.
    • Creates a new instance of CloudEvent based on an existing instance of CloudEvent.
    • Copies all CloudEvent attributes except Id and Time from the source parameter to the new instance.
    • Copies all non-CloudEvent headers to the new instance.
    • Does not copy the event data (StringData or BinaryData properties).
    • Alternative: Instead of invoking this constructor directly, call the .Copy() extension method on an instance of CloudEvent. For example, cloudEvent.Copy().
  • public CloudEvent(IReceiverMessage receiverMessage, IProtocolBinding protocolBinding = null)
    • The message constructor.
    • Creates an instance of CloudEvent that is equivalent to the specified IReceiverMessage.
    • Uses the specified IProtocolBinding (or DefaultProtocolBinding if null) to map message headers to event attributes.
    • Alternative: Instead of invoking this constructor directly, call the .To<CloudEvent>() extension method on an instance of IReceiverMessage. For example, receiverMessage.To<CloudEvent>().
  • public CloudEvent(string jsonFormattedCloudEvent)
    • The json constructor.
    • Creates a CloudEvent by parsing the JSON-formatted CloudEvent string.
    • Sets CloudEvent attributes and data.
    • Does not set non-CloudEvent headers.

Inheritors of the CloudEvent class (and inheritors of those classes) are expected to have a default constructor, a copy constructor, a message constructor, and a json constructor that each call their base constructor.

CloudEvent Attributes

The following CloudEvent attributes are defined by the CloudEvent class:

Property (CloudEvent attribute) Type Required? Default Value Notes
Id (id) string Yes Guid.NewGuid().ToString()
Source (source) string Yes N/A Must be valid relative or absolute URI.
SpecVersion (specversion) string Yes "1.0"
Type (type) string Yes N/A
DataContentType (datacontenttype) string No N/A Must be valid Content-Type according to RFC 2616.
DataSchema (dataschema) string No N/A Must be valid relative or absolute URI.
Subject (subject) string No N/A
Time (time) DateTime No DateTime.UtcNow

CloudEvent Data

The raw data (or payload) of an event is available from the StringData and BinaryData properties, as well as from the GetData<T> and TryGetData<T> extension methods. The data of a CloudEvent, can be set by calling one of the SetData extension method overloads.

The following example demonstrates usage of these properties and extension methods:

// Note that all of the SetData extension method overloads return the same
// CloudEvent, allowing for an event to be initialized on a single line.

// Setting the event's data as a string:
CorrelatedEvent cloudEvent = new CorrelatedEvent()
    .SetData("Hello, world!");

Console.WriteLine(cloudEvent.StringData ?? "<null>"); // Prints "Hello, world!"
Console.WriteLine(ToHexString(cloudEvent.BinaryData) ?? "<null>"); // Prints "<null>"

// Setting the event's data as a byte object:
cloudEvent.SetData(new byte[] { 0x00, 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80, 0xFF });

Console.WriteLine(cloudEvent.StringData ?? "<null>"); // Prints "<null>"
Console.WriteLine(ToHexString(cloudEvent.BinaryData) ?? "<null>"); // Prints "01-02-04-08-10-20-40-80"

// Setting the event's data as type T:
Client client = new Client { FirstName = "Brian", LastName = "Friesen" };
cloudEvent.SetData(client, DataSerialization.Json);

// Prints "{'FirstName':'Brian','LastName':'Friesen'}":
Console.WriteLine(cloudEvent.StringData ?? "<null>");

// Prints "<null>":
Console.WriteLine(ToHexString(cloudEvent.BinaryData) ?? "<null>");

// The same instance of T can be retrieved with GetData:
Client retrievedClient = cloudEvent.GetData<Client>(DataSerialization.Json);

// Prints "same"
Console.WriteLine(ReferenceEquals(client, retrievedClient) ? "same" : "different");

// TryGetData also retrieves the same instance of T:
if (cloudEvent.TryGetData(out Client anotherClient, DataSerialization.Json))
    // Prints "retrieved, same"
    Console.Write("retrieved, "
        + (ReferenceEquals(client, anotherClient) ? "same" : "different"));
else
    // Not executed
    Console.WriteLine("not found");

// To clear the event data, pass null to any of the SetData extension methods:
string nullData = null;
cloudEvent.SetData(nullData);

// Prints "<null>":
Console.WriteLine(cloudEvent.StringData ?? "<null>");

// Prints "<null>":
Console.WriteLine(ToHexString(cloudEvent.BinaryData) ?? "<null>");

if (cloudEvent.TryGetData(out Client notFoundClient, DataSerialization.Json))
    // Not executed
    Console.Write("retrieved "
        + (ReferenceEquals(client, notFoundClient) ? "same" : "different"));
else
    // Prints "not found"
    Console.WriteLine("not found");

// If the StringData is set to a serialized object...
cloudEvent.SetData("{'FirstName':'Brian','LastName':'Friesen'}");

// ...then an object of that type can be retrieved with GetData or TryGetData:
Client deserializedClient = cloudEvent.GetData<Client>(DataSerialization.Json);

// Prints "different"
Console.WriteLine(ReferenceEquals(client, deserializedClient) ? "same" : "different");

static string ToHexString(byte[] binaryData) =>
    binaryData is null ? null : BitConverter.ToString(binaryData);

class Client
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

CloudEvent Headers

This property exists to store key/value pairs that are not specific to the CloudEvent specification. It also allows for a lossless conversion from CloudEvent to SenderMessage and from IReceiverMessage to CloudEvent.

ProtocolBinding property

This property is used to determine the header name of a sender or receiver message in order to map it to/from a CloudEvent.

DefaultProtocolBinding static property

The DefaultProtocolBinding property determines which IProtocolBinding to use when not otherwise specified.

Its value is used...

  • ...by the default constructor as the initial value of the ProtocolBinding property.
  • ...by the message constructor - but only if its protocolBinding parameter is null - as the initial value of the ProtocolBinding property, and also to map the receiver message headers to CloudEvent attributes.
  • ...by the static Validate method (and methods that hide it) - but only if its protocolBinding parameter is null - to map the CloudEvent attributes to sender message headers.

ToJson method

This method serializes the CloudEvent in the JSON Event Format for CloudEvents.

ToSenderMessage method

The CloudEvent class defines a virtual ToSenderMessage that creates a new SenderMessage based on the event's data and attributes, using the event's ProtocolBinding to map those attributes to sender message headers. Note that before this method creates the SenderMessage, it first calls the Validate instance method.

ToHttpRequestMessage method

To convert any CloudEvent to an HttpRequestMessage (to be used by HttpClient), call one of the ToHttpRequestMessage overloads, optionally passing an HttpMethod or request URI.

Validate instance method

This virtual method ensures that the CloudEvent instance is valid - throws a CloudEventValidationException if invalid. This method is called at the beginning of the ToSenderMessage() method.

Inheritors of the CloudEvent class (and inheritors of those classes) are expected to override this method, call base.Validate(), and add additional validation according to the CloudEvent subclass type.

Validate static method

This static method ensures that the SenderMessage instance is valid according to the IProtocolBinding - throws a CloudEventValidationException if invalid.

Inheritors of the CloudEvent class (and inheritors of those classes) are expected to hide this method, call CloudEvent.Validate(senderMessage, protocolBinding), and add additional validation according to the CloudEvent subclass type.

Protected methods

In order to implement custom validation logic, subclasses of CloudEvent will need to query the headers of outgoing sender messages. However, the SenderMessage class wasn't really designed to have its headers queried, so accessing its headers can be cumbersome. To make it easier for subclasses, the CloudEvent class contains two protected helper methods:

  • protected static bool ContainsHeader<T>(SenderMessage senderMessage, string headerName)
    • Returns whether the senderMessage has a header with a name matching the headerName and a value of either type T or a type convertible to type T.
  • protected static bool TryGetHeaderValue<T>(SenderMessage senderMessage, string headerName, out T value)
    • Gets the value of the header with the specified name as type T.

SequentialEvent class

The SequentialEvent class defines two additional CloudEvent attributes, Sequence, and SequenceType.

Property (CloudEvent attribute) Type Required? Default Value
Sequence (sequence) string Yes N/A
SequenceType (sequencetype) string No N/A

If the SequenceType attribute is set to "Integer", the Sequence attribute has the following semantics:

  • The values of sequence are string-encoded signed 32-bit Integers.
  • The sequence MUST start with a value of 1 and increase by 1 for each subsequent value (i.e. be contiguous and monotonically increasing).
  • The sequence wraps around from 2,147,483,647 (2^31 - 1) to -2,147,483,648 (-2^31).

The SequenceTypes static class defines a string constant, Integer, with the value "Integer".

SequentialEvent Constructors

The SequentialEvent class defines four constructors:

  • public SequentialEvent()
    • The default constructor.
    • This constructor does not set any properties.
  • public SequentialEvent(SequentialEvent source)
    • The copy constructor.
    • Creates a new instance of SequentialEvent based on an existing instance of SequentialEvent.
    • Copies all CloudEvent attributes except Id and Time from the source parameter to the new instance.
    • If the value of the source parameter's SequenceType is "Integer" and its Sequence is a numeric string, then the new instance will have a Sequence equal to the source's Sequence, plus one.
    • Does not copy the event data (StringData or BinaryData properties).
    • Alternative: Instead of invoking this constructor directly, call the .Copy() extension method on an instance of SequentialEvent. For example, sequentialEvent.Copy().
  • public SequentialEvent(IReceiverMessage receiverMessage, IProtocolBinding protocolBinding = null)
    • The message constructor.
    • Creates an instance of SequentialEvent that is equivalent to the specified IReceiverMessage.
    • Uses the specified IProtocolBinding (or DefaultProtocolBinding if null) to map message headers to event attributes.
    • Alternative: Instead of invoking this constructor directly, call the .To<SequentialEvent>() extension method on an instance of IReceiverMessage. For example, receiverMessage.To<SequentialEvent>().
  • public SequentialEvent(string jsonFormattedCloudEvent)
    • The json constructor.
    • Creates a CloudEvent by parsing the JSON-formatted CloudEvent string.
    • Sets CloudEvent attributes and data.
    • Does not set non-CloudEvent headers.

Inheritors of the SequentialEvent class are expected to have a default constructor, a copy constructor, a message constructor, and a json constructor that each call their base constructor.

CorrelatedEvent class

The CorrelatedEvent class defines one additional CloudEvent attribute, CorrelationId.

Property (CloudEvent attribute) Type Required? Default Value
CorrelationId (correlationid) string Yes Guid.NewGuid().ToString()

CorrelatedEvent Constructors

The CorrelatedEvent class defines four constructors:

  • public CorrelatedEvent()
    • The default constructor.
    • This constructor does not set any properties.
  • public CorrelatedEvent(CorrelatedEvent source)
    • The copy constructor.
    • Creates a new instance of CorrelatedEvent based on an existing instance of CorrelatedEvent.
    • Copies all CloudEvent attributes except Id and Time from the source parameter to the new instance.
    • Does not copy the event data (StringData or BinaryData properties).
    • Alternative: Instead of invoking this constructor directly, call the .Copy() extension method on an instance of CorrelatedEvent. For example, correlatedEvent.Copy().
  • public CorrelatedEvent(IReceiverMessage receiverMessage, IProtocolBinding protocolBinding = null)
    • The message constructor.
    • Creates an instance of CorrelatedEvent that is equivalent to the specified IReceiverMessage.
    • Uses the specified IProtocolBinding (or DefaultProtocolBinding if null) to map message headers to event attributes.
    • Alternative: Instead of invoking this constructor directly, call the .To<CorrelatedEvent>() extension method on an instance of IReceiverMessage. For example, receiverMessage.To<CorrelatedEvent>().
  • public CorrelatedEvent(string jsonFormattedCloudEvent)
    • The json constructor.
    • Creates a CloudEvent by parsing the JSON-formatted CloudEvent string.
    • Sets CloudEvent attributes and data.
    • Does not set non-CloudEvent headers.

Inheritors of the CorrelatedEvent class (and inheritors of those classes) are expected to have a default constructor, a copy constructor, a message constructor, and a json constructor that each call their base constructor.

PartitionedEvent class

The PartitionedEvent class defines one additional CloudEvent attribute, PartitionKey.

Property (CloudEvent attribute) Type Required? Default Value
PartitionKey (partitionkey) string Yes N/A

PartitionedEvent Constructors

The PartitionedEvent class defines four constructors:

  • public PartitionedEvent()
    • The default constructor.
    • This constructor does not set any properties.
  • public PartitionedEvent(PartitionedEvent source)
    • The copy constructor.
    • Creates a new instance of PartitionedEvent based on an existing instance of PartitionedEvent.
    • Copies all CloudEvent attributes except Id and Time from the source parameter to the new instance.
    • Does not copy the event data (StringData or BinaryData properties).
    • Alternative: Instead of invoking this constructor directly, call the .Copy() extension method on an instance of PartitionedEvent. For example, correlatedEvent.Copy().
  • public PartitionedEvent(IReceiverMessage receiverMessage, IProtocolBinding protocolBinding = null)
    • The message constructor.
    • Creates an instance of PartitionedEvent that is equivalent to the specified IReceiverMessage.
    • Uses the specified IProtocolBinding (or DefaultProtocolBinding if null) to map message headers to event attributes.
    • Alternative: Instead of invoking this constructor directly, call the .To<PartitionedEvent>() extension method on an instance of IReceiverMessage. For example, receiverMessage.To<PartitionedEvent>().
  • public PartitionedEvent(string jsonFormattedCloudEvent)
    • The json constructor.
    • Creates a CloudEvent by parsing the JSON-formatted CloudEvent string.
    • Sets CloudEvent attributes and data.
    • Does not set non-CloudEvent headers.

Inheritors of the PartitionedEvent class (and inheritors of those classes) are expected to have a default constructor, a copy constructor, a message constructor, and a json constructor that each call their base constructor.

Protocol Bindings

The ProtocolBindings static class defines the following bindings (corresponding to the CloudEvents spec):

  • Default
    • Basically does nothing.
  • Kafka
    • Attributes are prefixed with "ce_".
    • Remaps "partitionkey" to/from "Kafka.Key".