Web Services Reliable Messaging (WS-RM)

Web Services Reliable Messaging (WS-RM)
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Motivation

Observations About TCP:
- Limited definition of "reliable"
- Does not know about the application protocol (e.g., whether a request must always result in a response)
Delivery Assurances Required by Some Applications:
- At most once/At least once/Exactly once
- In order

Objectives of WS-RM

Reliability:
- Ensure messages are delivered even in the event of failures of software components, the network, etc...
Independence from Transport Protocol:
- Should be implementable using different network transport technologies

Mechanisms

Sequences:
- Each message is part of a (monotonically increasing) sequence that has a unique URI
- Each message has a unique number in the sequence
Acknowledgements:
- Each message that is successfully transferred (though not necessarily processed) is acknowledged
- Negative acknowledgements are allowed
Persistance:
- Optional
- If provided, acknowledgements are only sent after the message was "saved"

The Process

Establish Preconditions:
- Policies, trust, ...
Create Sequence Identifier:
- Either source-side creation or destination-side creation
Communication:
- Source sends messages in sequence
- Destination sends acknowledgements (perhaps consolidating them before sending)
- Source re-sends if necessary...
Sequence Termination:
- Explicit for destination-side creation

Sequence Faults

The wsrm:Sequence Element

Content:
- wsrm:Identifier (required)
- wsrm:MessageNumber (required)
- wsrm:LastMessage (required in last message)
Requirements:
- Must be exactly one such element

The wsrm:Acknowledgement Element

Content:
- wsrm:Identifier (required)
- wsrm:AcknowledgementRange (optional)
- wsrm:Nack (optional)
Requirements:
- Ranges must not overlap
- AcknowledgementRange and Nack can not both appear

The wsrm:CreateSequence Element

Content:
- wsrm:AcksTo (required)
- wsrm:Expires (optional)
- wsrm:Offer (optional offer of a corresponding sequence)
Issues:
- May contain security tokens

The wsrm:AckRequested Element