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From Apache Wiki <wikidi...@apache.org>
Subject [Incubator Wiki] Update of "Synapse/Architecture" by PaulFremantle
Date Wed, 12 Apr 2006 08:24:42 GMT
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The following page has been changed by PaulFremantle:
http://wiki.apache.org/incubator/Synapse/Architecture

New page:
== Architecture and Design of the Synapse ESB mediation framework ==

This paper is designed to capture the overall architecture and design of the Synapse ESB mediation
framework. The aim is to provide clear architectural and design guidance and direction as
we move towards Synapse 1.0.

=== Introduction ===

Synapse has a few simple design goals:

 1. Synapse is designed to help intermediate messages as they pass through it from a service
requester or client to a service provider or server, and back again. 
 1. Synapse is designed to support any message exchange pattern (MEP). 
 1. Synapse supports ''Connecting'' systems, ''Managing'' interactions and ''Transforming''
messages. The main function of Synapse can be categorized into these three areas. 
 1. Synapse is inherently extensible, but is designed to support a large set of useful function
out-of-the-box. 
 1. Synapse is designed to work ''with'' BPM/BPEL solutions, and explicitly leaves stateful
process-based integration out of scope.

This paper describes the architecture, design and framework, and also aims to capture the
intended 1.0 function. Not everything in this is available in the current release.

=== Use cases and capabilities: Connect, Manage, Transform ===
==== Connect ====

Synapse is designed to support connecting systems across different transports and with different
protocols and Quality of Service (QoS) applied. For example, Synapse will support the inter-connection
of any of the following models:
 * SOAP/HTTP, SOAP/HTTPS, SOAP+WSSec, SOAP+WSRM, SOAP/JMS, SOAP/SMTP, XML/HTTP, XML/JMS

For example, it allows termination or initiation or RM or WSSec sessions. Specific use cases
include:
 * Terminating RM and WSSec at a gateway and passing simple SOAP/HTTP or XML/JMS messages
into an internal system
 * Adding a REST-like interface to an existing SOAP service and vice-versa
 * Exposing existing XML/JMS queues as SOAP/WSRM endpoints.

Synapse supports ''virtualization'' by routing from virtual/logical URIs to real endpoints
(EPRs or URLs). 

Synapse supports routing of messages based on: headers such as source/destination (Regex matching),
content (XPath), user, and time (e.g. managed switchover).

==== Manage ====

Synapse supports failover and load-balancing to multiple endpoints. 

Synapse supports the collection and usage of stats (hit rate, failure rate, latency, etc)
to monitor and manage the use of services. 

It allows the logging and tracing of services. 

It supports fault handling. 

It allows the securing of services including XML schema validation, authentication, authorization
– including both service, operation and content based access control. 

==== Transform ====

Synapse supports transforming messages using:
 * XSLT, 
 * simple scripting languages (such as Javascript/E4X), and 
 * hand-coded Java. 

Transformations include namespace changes, handling different versions, thru to full XML rewriting.

=== What Synapse doesn’t support: ===

Synapse is designed to be per-message stateless. Mediators can aggregate information (e.g.
statistics) across messages, but there is NO support in the framework for message level state.
Synapse is '''not''' designed to support business process management, or complex stateful
message flows. Synapse mediators written to perform correlation or use per-message state must
do their own management of state and are not guaranteed to be clusterable. Synapse is designed
to work with other engines such as BPEL managers that support complex stateful message flow.
The reason for this is that Synapse is designed to be embedded in routers, clustered and to
perform very fast, and also is designed to address the simple “router” style case.

=== Mediation architecture ===

Synapse supports a very simple model for mediation. Each instance of a Synapse engine has
a single “master” mediator through which every message flows. That master mediator may
be comprised of one or more sub-mediators. Each mediator simply takes a message, reads or
modifies it, and passes it on. 

There are three main types of mediators: 
 * Grouping mediators (e.g. a list mediator which passes the message through a list of sub-mediators
in turn), 
 * Condition mediators (e.g. a mediator which only passes the message through the sub-mediators
if the condition is true), and 
 * Leaf mediators – i.e. mediators that actually do something like logging the message.

Each mediator (see org.apache.synapse.api.Mediator) must be thread-safe as many messages may
be simultaneously sent through the same instance of a mediator. 

 1. Mediators are effectively JavaBeans which implement the mediate() interface. 
 2. Any properties on a mediator are set() on them using the appropriate method. For example
the list of sub-mediators is set on a ListMediator by calling setList(List mediators). 
 3. Mediators are independent of any XML configuration format. There is an XML configuration
format but it is just one potential way of configuring those mediators. 

The simplest way of configuring mediators is simply the Java ''new'' instantiation. A key
design point of Synapse is that there is a programmatic way of instantiating a Synapse instance
(This is analogous to the Axis2 model where you can create an empty configuration and populate
it). 

The XML configuration model (see package org.apache.synapse.xml) is independent of the mediator
model. The XML configuration model is based on an xml.MediatorFactory, which simply takes
an XML tree and creates a mediator object. This is done recursively until the tree is built.
The Java2 JAR file service provider model (see [http://java.sun.com/j2se/1.4.2/docs/guide/jar/jar.html#Service%20Provider])
is used to automatically register new XML syntaxes. 

=== Relationship to Axis2 ===

The default distribution of Synapse is based on Axis2. The inbound and outbound service listeners
and invokers are Axis2 based. However, Synapse has a clean API model that is independent of
Axis2. Therefore alternative implementations are possible. However the main aim of this is
to make it unnecessary to understand the details of Axis2 to configure or code to Synapse
– i.e. this is about providing a clear ''separation of concerns''. 

 
=== Configuration scenarios ===
Synapse is designed to support multiple configuration patterns. 

{{synconfmodels.png}}

The simplest patterns are (1) it is embedded inside another system and is configured programmatically
without any XML or config file, and (2) is that it reads its config from an XML file. However,
Synapse is also designed so that it could be embedded in another solution, and might be configured
using JMX, WSDM or any other model.

=== Deployment patterns ===

Synapse supports several models of deployment:
  * Explicit proxy or gateway: In this model, service requests are targeted at the Synapse
instances endpoint. Synapse must be configured with explicit rules that identify the actual
destination of a message and route it there.
  * HTTP Proxy: In this model, clients are configured to use the Synapse instances endpoint
as an HTTP proxy. All messages are sent via Synapse and the actual endpoint of the request
is available to the Synapse engine, which it uses to route the message on. This is a simple
way of adding Synapse into an existing infrastructure because most service clients allow the
configuration of an HTTP proxy without code changes.
  * Transparent proxy: this is an extension of the previous model, where a network level router
transparently routes messages to Synapse. At this level, Synapse can mediate messages without
any change to an existing infrastructure.
  * “SOAP Intermediary”. In this model, the service client explicitly uses WS-A “To”
headers, but delivers the message to Synapse to route on. This model is the replacement for
the now dead WS-Routing. In Axis2 this can be enabled by explicitly setting the transport
headers to pass the message to the Synapse endpoint. 
  * Embedded: In this model Synapse is embedded in a client or server and takes the place
of a “smart” or rules-based handler. 

=== SynapseEnvironment ===

A given instance of Synapse is represented by a SynapseEnvironment (see org.apache.synapse.SynapseEnvironment).
This provides the interface from the mediators into the underlying environment and thus the
separation from – for example – Axis2.

The SynapseEnvironment offers a few simple abstractions: it offers the ability to inject messages
into Synapse or send them onwards (or back in the case of responses), it keeps track of the
classloader (for mediators to load new classes), it abstracts away access to other resources
(such as a registry), and it keeps track of any named mediators (for reuse). 

The SynapseEnvironment “in-use” for any given message is kept through a pointer in the
message. Thus, if the environment is reloaded or rebuilt (for example if the configuration
is changed), then any messages currently flowing through the system retain their flow through
the existing environment, while other messages could concurrently be flowing through the rebuilt
environment.

=== Registry interface ===
The SynapseEnvironment offers mediators access to configuration and the overall SOA “fabric”
through the concept of one or more abstract “registries”. The registry is abstractly something
that offers access to Strings, Properties, XMLs (including WSDLs, Policies, Schemas), URIs
and EndpointReferences. Each “registry” that Synapse has access to has a name, and in
addition there is a default registry.

The registry is accessed through a “Registry” interface:

<code java>
public interface SynapseRegistry {
	OMElement getXML(String uri);
	String getURI(String uri);
	String getString(String uri);
	Properties getProperties(String uri);
	List getURIList(String URI);
	EndpointReference getEPR(String URI);
}
</code>
A “registry” is accessible from the SynapseEnvironment with:
getRegistry(String name);

There is also a default registry associated with a SynapseEnvironment which can be accessed
using the getRegistry() method.

=== Metrics ===

In order to manage services, mediators need access to metrics about those services. For example,
Synapse can collect metrics about the underlying performance of endpoints that it is calling,
and then use that information to load-balance requests across those endpoints.

In general there are three types of metrics:
  * The actual metrics associated with external endpoints (which may or may not be visible
to Synapse)
  * The metrics for outbound services that Synapse invokes
  * The metrics for the actual service invocations that Synapse itself makes.


Because of the inherent separation of concerns in the Synapse design, Synapse may be implemented
on other systems than Axis2, and so the metrics available from that underlying system are
abstracted through the SynapseEnvironment interface:

''SynapseMetrics getMetrics(String URI);'' 

This method returns an object providing the usual metrics (average response time, hit rate,
failure rate, min response time, max response time, etc. Those metrics include metrics for
both the time spent inside the local Synapse environment as well as time spent on onbound
invocation) for any requests made to the given URI.  

The SynapseEnvironment allows multiple “metric” providers to be used. For example, a simple
internal system may be used, or a more extensive JMX or WSDM based system may be plugged in
via this model. The metric provider is plugged in through the SynapseMetricProvider interface.

 
=== Events ===

Another important aspect of an ESB/Mediation fabric is the concept of an event. For example,
a typical model is that a mediation engine may generate events – for example – if a failure
occurs, or a large transaction goes through.

In order to support a simple event model in Synapse we support a model compatible with WS-Eventing
([[http://www.w3.org/Submission/WS-Eventing/]]) spec in the engine.

The primary interface of interest to a mediator is the publish interface:
<code java>
EndpointReference fullEPR = SynapseEnvironment.createEventSource(EndpointReference epr);
EndpointReference fullEPR = SynapseEnvironment.createEventSource(String partialURI);
SynapseEnvironment.publish(EndpointReference epr, SynapseMessage sm);
SynapseEnvironment.deleteEventSource(EndpointReference epr);
</code>
As time goes on we will add a suitable internal subscription model. 

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