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This protocol is out of scope of this proposal.

Proposed changes

Cluster version

Validate joining node version

Server nodes stores ClusterVersion - set of Ignite versions in cluster (server and clients nodes). The joining node version must be checked versus the version (see actual Ignite versions in a cluster (within OnDiscoveryNodeValidationProcessor):

• If ClusterVersion a cluster contains nodes of only one version (all nodes are run same version), then the joining node can be greater/less than this version by up to 1 minor version.
• If ClusterVersion a cluster contains nodes of two versions, then joining node version must be one of the ClusterVersionthem.

Message

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serialization framework

There should be one serialization framework for communication and discovery protocol. 

1. MessageWriter, MessageReader logic is depends on a remote IgniteProductVersion.
2. Message#writeTo, Message#readFrom is auto-generated and stored separately from Message DTO classes classes.
3. Add Message#writeObject - for serializing Objects (java functions, and user objects) :
   1. MessageWriter for communication protocol use BinaryMarshaller
   2. MessageWriter for discovery protocol use JdkMarshaller
    with  BinaryMarshaller.
4. Message fields may containMessage fields contain:
   1. primitive classes.
   2. known collections.
   3. POJO - that are other others Message.
   4. Users classes and java functions (e.g. ComputeJob).
   5. byte[] fields (objects serialized externally) must be avoided as much as possible, because their compatibility can't be guaranteed.
5. Order of fields in Message is fixed with @Order annotation.
6. Add  Adding and removing fields must be go along with @Since, @Until  annotations  annotations for Message classes and fields.

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1. Ignite CI notifies for IF-clauses with condition based on IgniteVersion older than (curVer - 1).
2. Ignite CI forbids code changes if Message DTO fields changed without corresponding @Since, @Until annotations.
3. Ignite CI forbids code changes if Message DTO contains byte[] fields.
4. Ignite CI forbids code changes if Message field changes type.
5. Ignite CI checks @Order annotation of fields - starts with 0, no lags.

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1. Data - set of declared Message classes, including ser/des algorithm.
2. Transport - algorithm of transport the Messages to remote node.

Data compatibility

Message - is base class for all messages transported between nodes. Proposed changes:

1. Deprecate Message#writeTo and Message#readFrom in favor generated MessageSerializer. 
2. MessageSerializer#writeTo Message#writeTo consumes MessageWriter that stores IgniteProductVersion of destination a receiver node and use it for serializing data for this version (mostly, for ignoring some fields).

3. Message#readFrom MessageSerializer#readFrom consumes MessageReader that stores IgniteProductVersion of source node and use it for deserializing data (mostly, for setting default values of new fields).

   Code Block
   language java title Message
   public interface MessageSerializer { public boolean writeTo(Message msg, ByteBuffer buf, MessageWriter writer); public boolean readFrom(Message msg, ByteBuffer buf, MessageReader reader); } public interface MessageWriter { public IgniteProductVersion destinationVersionreceiverVersion(); } public interface MessageReader { public IgniteProductVersion sourceVersionsenderVersion(); }

   
   

4. Introduce annotations @Since and @Until for Message classes and Message fields, to use it for generating code for Message#writeTo and Message#readFrom:
   
   

// Package where the Message is defined.
package org.apache.ignite.internal.my.message;

// package private class.
@Since(version = "2.19.0")
public class MyMessage implements Message {
    @Order(0)
    private int id;
    
 	/** Remove field. */
    @Until(version = "2.20.0")
	    @Order(1)
     private String rmFld;

    /** New field. */
    @Since(version = "2.20.0")
 	@Order(2)
    private String newFld;

	// Message Delegatesmust #writeTohave methodsetters/getters callfor toall generated@Ordered serializerfields.
 Methods  names  @Overrideare publicequal booleanto writeTo(ByteBuffer buf, MessageWriter writer) {
        return MyMessageSerializer.writeTo(this, buf, writer);
    }

 	// Delegates #readFrom method call to generated serializer.
    @Override public boolean readFrom(ByteBuffer buf, MessageReader reader) {
         return MyMessageSerializer.readFrom(this, buf, reader);
    a corresponding field name. 
	public void id(int id) {
		this.id = id;
	}

  	public int id() {
		return id;
	}

  	public void newFld(String newFld) {
		this.newFld = newFld;
	}
 
 	public String newFld() {
		return newFld;
	}
}

// Generated code from the message ^ for Ignite version 2.20.0.
// Use the same package as corresponding message.
package org.apache.ignite.internal.my.message;

class MyMessageMyMessageSerializer {
    public static boolean writeTo(MyMessageMessage msgm, MByteBufferByteBuffer buf, MessageWriter writer) {
		MyMessage        IgniteProductVersion destVer msg = writer.version(MyMessage)m;

		IgniteProductVersion rcvVer       if (destVer.lessThan(2, 19, 0))
            throw new IgniteException("Must not send the message to destination node")= writer.receiverVersion();

        writer.writeString(msg.id());

        if (destVerrcvVer.lessThan(2, 20, 0))
            writer.writeString(msg.rmFld());

        if (destVerrcvVer.greaterThanEqual(2, 20, 0))
            writer.writeString(msg.newFld());
    
        return true;
    }

    public static boolean readFrom(MyMessageMessage msgm, ByteBuffer buf, MessageReader reader) {
 		MyMessage msg = (MyMessage)m;
 
        IgniteProductVersion srcVer = reader.versionsenderVersion();

        msg.id(reader.readString());

		if (srcVer.lessThan(2, 20, 0))
			msg.rmFld(reader.readString());

        if (srcVer.greaterThanEqual(2, 20, 0))
            msg.newFld(reader.readString());

        return true;
    }
}

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1. Do not remove Message class or Message fields, but annotate it with @Until. 
2. Do not change types or @Order of fields.
3. New fields must be annotated with @Since. All such fields must be optional, default value is null. Handling the nulls is care of Message consumer on the reader side.
4. Setters and getters must follow name of the field.

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1. .

Transport compatibility 

Communication handshake

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