[jboss-jira] [JBoss JIRA] (JGRP-2218) New payload interface

[Bela Ban (JIRA)]

    [ https://issues.jboss.org/browse/JGRP-2218?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13464517#comment-13464517 ] 

Bela Ban commented on JGRP-2218:
--------------------------------

[~dan.berindei] Actually, I take back what I said on {{size()}}: some protocols such as {{FRAG2}} need the size to be more or less exact, to determine whether or not to fragement the message!

If {{size()}} returns a number that's less than what's actually serialized later, the transport may fail to send the message, e.g. when using UDP the datagram packet might be bigger than the max datagram size (65K). If greater, there may be unnecessary fragmentation, but this latter case would at least not cause messages to get dropped.

So the way things look, you'd have to implement {{size()}} to use {{ObjectPayload}} or do eager marshalling and use {{ByteArrayPayload}}. But aren't you already able to get the size, ie. IIRC you do implemnt {{Marshaller.estimatedSize(Object)}}, don't you?

> New payload interface
> ---------------------
>
>                 Key: JGRP-2218
>                 URL: https://issues.jboss.org/browse/JGRP-2218
>             Project: JGroups
>          Issue Type: Feature Request
>            Reporter: Bela Ban
>            Assignee: Bela Ban
>             Fix For: 5.0
>
>
> h3. Goal
> Change payload in {{Message}} from byte[] arrays to a {{Payload}} interface which can have multiple implementations.
> h3. Reason
> Currently, having to pass a byte[] array to a message leads to unnecessary copying:
> * When an application has a ref to an NIO (direct) {{ByteBuffer}}, the bytes in the byte buffer have to be copied into a byte[] array and then set in the message
> * When the application sends around byte[] arrays, but also wants to add some additional metadata, e.g. type (1000-byte requests/responses), it needs to create a new byte[] array of (say) 1001 bytes and copy the data (1000 bytes) plus the request type (1 byte) into the new copy. Example: {{MPerf}} and {{UPerf}}
> * When an object has to be sent (e.g. in Infinispan), the object has to be marshalled into a byte[] array (first allocation) and then added to the message. With the suggested {{ObjectPayload}} (below), marshalling of the object would occur late, and it would be marshalled directly into the output stream of the bundler, eliminating the byte[] array allocation made by the application.
> h3. Design
> Instead of copying, the application creates an instance of {{Payload}} and sets the payload in {{Message}}. The {{Payload}} is then passed all the way down into the transport where it is marshalled and sent. There can be a number of payload implementations, e.g.
> * {{ByteArrayPayload}}: wraps a byte[] array with an offset and length
> * {{NioDirectPayload}}: wraps an NIO direct {{ByteBuffer}}
> * {{NioHeapPayload}}: wraps an NIO heap-based {{ByteBuffer}}
> * {{CompositePayload}}: wraps multiple Buffers. E.g. type (1 byte) and data (1000 bytes) as described above
> * {{IntPayload}}: a single integer
> * {{ObjectPayload}}: has an Object and a ClassLoader (for reading), plus a Marshaller which know how to marshal the object, this allows for objects to be passed in payloads and they're only marshalled at the end (transport).
> * {{PartialPayload}}: a ref to a {{Payload}}, with an offset and length
> The {{Payload}} interface has methods:
> * {{size()}}
> * {{writeTo(DataOutput)}}
> * {{readFrom(DataInput)}}
> * {{getInput()}}: this provides a {{DataInput}} stream for reading from the underlying payload
> and  possibly also 
> * {{acquire()}} and 
> * {{release()}} (for ref-counting)
> * {{copy()}}
> Each payload impl has an ID and it should be possible to register new impls. A {{PayloadFactory}} maintains a mapping between IDs and impl classes.
> When marshalling a {{Payload}}, the ID is written first, followed by the payload's {{writeTo()}} method. When reading payloads, the {{PayloadFactory}} is used to create instances from IDs.
> h4. Fragmentation
> When fragmenting a buffer, the fragments are instances of {{PartialPayload}} which maintains an offset and length over an underlying payload. When marshalling a {{PartialPayload}}, only the part between offset and offset+length is written to the output stream.
> h4. Reference counting
> If we implement ref-counting, then payloads can be reused as soon as the ref-count is 0. For example, when sending a message, the payload's ref-count could be incremented by the app calling {{acquire()}}. (Assuming the message is a unicast message), {{UNICAST3}} would increment the count to 2. This is needed because {{UNICAST3}} might have to retransmit the message if it was lost on the network, and meanwhile the payload cannot be reused (changed). The app calls {{release()}} when the {{JChannel.send()}} call returns, but the payload cannot be reused until {{UNICAST3}} calls {{release()}} as well. This will happen when an {{ACK}} for the given message has been received.
> h4. Payload factory
> When a request is received, the buffer is created from the bytes received on the network, based on the ID. This should be done by asking a {{PayloadFactory}} component for a new buffer. A naive implementation might create a new buffer every time, and more sophisticated one might use a pool of payloads.
> The {{PayloadFactory}} instance could be replaced by one's own implementation; this allows for an application to control the lifecycle of payloads: thus the creation of buffers by the application and of payloads received over the network can be controlled by the same payload management impl.
> h4. Symmetry
> When sending a {{CompositePayload}} of a 500 byte {{ByteArrayPayload}} and a 1000 byte {{NioDirectPayload}}, would we want to also get the same {{CompositePayload}} consisting of 2 payloads on the receiver side, or would we want to combine the 2 payloads into one and make the 2 payloads refer to the same combined byte[] array (or NIO buffer)? Should this be made configurable?
> h4. Misc
> * Since this issue includes API changes, the version will be 5.0



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