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INT-1552 doc polishing

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This commit is contained in:
Mark Fisher
2010-11-22 15:36:31 -05:00
parent a7b2263dac
commit 5418f0d7f5
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109
docs/src/reference/docbook/transactions.xml
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@@ -21,7 +21,7 @@
<para><emphasis>MessageChannel</emphasis> - Direct interactions with MessageChannel methods (e.g., channel.send(message))</para>
</listitem>
<listitem>
<para><emphasis>Message Publisher</emphasis> - the way to initiate message flow as a bi-product of method invocations on Spring beans</para>
<para><emphasis>Message Publisher</emphasis> - the way to initiate message flow as the by-product of method invocations on Spring beans</para>
</listitem>
<listitem>
<para><emphasis>Inbound Channel Adapters/Gateways</emphasis> - the way to initiate message flow based on connecting third-party
@@ -38,75 +38,74 @@
</itemizedlist>
</para>
<para>
These 6 cold be split in 2 general categories:
These 6 could be split in 2 general categories:
<itemizedlist>
<listitem>
<para><emphasis>Message flows initiated by a USER process</emphasis> - Example scenarios in this category
would be invoking a Gateway method or explicitly sending a Message to a MessageChannel. In other words these message flows depend on third
party process (e.g., some code that we wrote) to be initiated</para>
would be invoking a Gateway method or explicitly sending a Message to a MessageChannel. In other words, these message flows depend on a third
party process (e.g., some code that we wrote) to be initiated.</para>
</listitem>
<listitem>
<para><emphasis>Message flows initiated by the DAEMON process</emphasis> - Example scenarios in this category would be a Poller
polling for a Message queue to initiate a new Message flow with the polled Message or a Scheduler scheduling the
process, by creating a new Message and initiating a message flow at a predefined time</para>
<para><emphasis>Message flows initiated by a DAEMON process</emphasis> - Example scenarios in this category would be a Poller
polling a Message queue to initiate a new Message flow with the polled Message or a Scheduler scheduling the
process by creating a new Message and initiating a message flow at a predefined time.</para>
</listitem>
</itemizedlist>
</para>
<para>
Clearly the <emphasis>Gateway Proxy</emphasis>, <emphasis>MessageChannel.send(..)</emphasis> and <emphasis>MessagePublisher</emphasis> are
all belong to the 1st category and <emphasis>Inbound Adapters/Gateways</emphasis>, <emphasis>Scheduler</emphasis> and <emphasis>Poller</emphasis> belong to the 2nd.
Clearly the <emphasis>Gateway Proxy</emphasis>, <emphasis>MessageChannel.send(..)</emphasis> and <emphasis>MessagePublisher</emphasis>
all belong to the 1st category and <emphasis>Inbound Adapters/Gateways</emphasis>, <emphasis>Scheduler</emphasis> and <emphasis>Poller</emphasis>
belong to the 2nd.
</para>
<para>
So, how do we address transactional needs in various scenarios within each category and is there a need for Spring Integration
to provide something explicitly with regard to transaction for a particular scenario or Spring's Transaction Support could be leveraged instead?.
to provide something explicitly with regard to transactions for a particular scenario? Or, can Spring's Transaction Support be leveraged instead?.
</para>
<para>
First of all, the first and obvious goal is NOT to re-invent something that has already been invented unless you can provide a beter solution.
In our case Spring itself provides a first class support for transaction management. So our goal here is not to provide something new but rather
The first and most obvious goal is NOT to re-invent something that has already been invented unless you can provide a better solution.
In our case Spring itself provides first class support for transaction management. So our goal here is not to provide something new but rather
delegate/use Spring to benefit from the existing support for transactions. In other words as a framework we must expose hooks to the Transaction management functionality
provided by Spring. But since Spring Integration configuration is based on Spring Configuration it is not always neccessery to expose these hooks as they already
expposed via Spring natively. Remeber every Spring Integration component is a Spring Bean after all.
provided by Spring. But since Spring Integration configuration is based on Spring Configuration it is not always necessary to expose these hooks as they
are already exposed via Spring natively. Remember every Spring Integration component is a Spring Bean after all.
</para>
<para>
With this goal in mind let's look at the two scenarios. 
</para>
<para>
If you think about it, Message flows that are initiated by the <emphasis>USER process</emphasis> (Category 1) and obviously configured in Spring Application Context,
are subject to transactional configuration of such process and therefore don't need to be explicitly configured by Spring Integration to support transactions.
The transaction could and should be initiated by such process through standard Transaction support provided by Spring and Spring Integration message flow will honor
transactional semantics of the components naturally because it is Spring configured. For example; A Gateway or ServiceActivator methods could
be annotated with <classname>@Transactional</classname> or <classname>TransactionInterceptor</classname> could be configured in XML configuration
with point-cut expression pointing to specific methods that should be transactional.
The bottom line you have full control over transaction configuration and boundaries in these scenarios.
If you think about it, Message flows that are initiated by the <emphasis>USER process</emphasis> (Category 1) and obviously configured in a Spring Application Context,
are subject to transactional configuration of such processes and therefore don't need to be explicitly configured by Spring Integration to support transactions.
The transaction could and should be initiated through standard Transaction support provided by Spring. The Spring Integration message flow will honor
the transactional semantics of the components naturally because it is Spring configured. For example, a Gateway or ServiceActivator method could
be annotated with <classname>@Transactional</classname> or <classname>TransactionInterceptor</classname> could be defined in an XML configuration
with a point-cut expression pointing to specific methods that should be transactional.
The bottom line is that you have full control over transaction configuration and boundaries in these scenarios.
</para>
<para>
However, things are a bit different when it comes to Message flows initiated by the <emphasis>DAEMON process</emphasis> (Category 2).
Although configured by the developer these flows do not directly involve human or some other process to be initiated. These are trigger-based flows
Although configured by the developer these flows do not directly involve a human or some other process to be initiated. These are trigger-based flows
that are initiated by a trigger process (DAEMON process) based on the configuration of such process. For example, we could have a Scheduler
initiating a message flow every Friday night of every week. We can also configure a trigger that initiates a Message flow every second, etc.
So, we obviously need the same way to let these trigger-based processes know of our intention to make these Message flows transactional so
Transaction context could be created whenever a new Message flow is initiated. In other words we need to expose some Transaction configuration, but ONLY enough
So, we obviously need a way to let these trigger-based processes know of our intention to make the resulting Message flows transactional so that
a Transaction context could be created whenever a new Message flow is initiated. In other words we need to expose some Transaction configuration, but ONLY enough
to delegate to Transaction support already provided by Spring (as we do in other scenarios).
</para>
<para>
Spring Integration provides transactional support for Pollers. Pollers are a special case comoponents becouse
Spring Integration provides transactional support for Pollers. Pollers are a special type of component because
we can call receive() within that poller task against a resource that is itself transactional thus including <emphasis>receive()</emphasis>
call in the the boundaries of the Transaction allowing it to be rolled back in case of a task failure. If we were to add the same support
for channels, the added transactions would affect all downstream components starting with that <emphasis>send()</emphasis> call. That is
providing a rather wide scope for transaction demarcation without any strong reason especially when Spring already provides several way to
address transactional needs of any component downstream. However the <emphasis>receive()</emphasis> method being included in a transaction
boundary is the "strong reason" for pollers. 
providing a rather wide scope for transaction demarcation without any strong reason especially when Spring already provides several ways to
address the transactional needs of any component downstream. However the <emphasis>receive()</emphasis> method being included in a transaction
boundary is the "strong reason" for pollers.
</para>
<section id="transaction-poller">
<title>Poller Transaction Support</title>
<para>
Any time you configure a Poller you can provide transactional configuration via <emphasis>transactional</emphasis> element and its attributes:
Any time you configure a Poller you can provide transactional configuration via the <emphasis>transactional</emphasis> sub-element and its attributes:
<programlisting language="xml"><![CDATA[<poller max-messages-per-poll="1" fixed-rate="1000">
<transactional transaction-manager="txManager" 
isolation="DEFAULT"
@@ -114,27 +113,27 @@
read-only="true" 
timeout="1000"/>
</poller>]]></programlisting>
As you can see this configuration looks evry similar to native Spring transaction configuration. You must still provide reference to Transaction manager and specify
transaction attributes or rely on defauls (e.g., if 'transaction-manager'' attribute is not specified then it will default to the bean with the name 'transactionManager').
Internally the process would be wrapped in the Spring's native Transaction where <classname>TransactionInterceptor</classname> is responsible to handle transactions.
For more information on how to configure Transaction Manager, the types of Transaction Managers (e.g., JTA, Datasource etc.) and other details related to
As you can see this configuration looks very similar to native Spring transaction configuration. You must still provide a reference to a Transaction manager and specify
transaction attributes or rely on defaults (e.g., if the 'transaction-manager' attribute is not specified, it will default to the bean with the name 'transactionManager').
Internally the process would be wrapped in Spring's native Transaction where <classname>TransactionInterceptor</classname> is responsible for handling transactions.
For more information on how to configure a Transaction Manager, the types of Transaction Managers (e.g., JTA, Datasource etc.) and other details related to
transaction configuration please refer to Spring's Reference manual (Chapter 10 - Transaction Management).
</para>
<para>
With the above configuration all Message flows initiated by this poller will be transactional. For more information and details on
With the above configuration all Message flows initiated by this poller will be transactional. For more information and details on a
Poller's transactional configuration please refer to section - <emphasis>21.1.1. Polling and Transactions</emphasis>.
</para>
<para>
There times when besides transaction several more cross cutting concerns needs to be addressed when running Poller. To help with that,
Poller element defines <emphasis>&lt;advice-chain&gt; </emphasis> sub-element which allows you to define a custom chain of Advices
to be applied on the Poller. (see section 4.4 for more details)
In Spring Integration 2.0 Poller went through the major  refactoring effort and is now using proxy mechanism to address transactional
concerns as well as other cross cutting concerns, one of the significant changes evolving from this effort is that we
made <emphasis>&lt;transactional&gt;</emphasis> and <emphasis>&lt;advice-chain&gt;</emphasis> elements mutually exclusive.
The rational behind this is; If you need more then one advice, and one of them is Transaction advice, then you can simply
Along with transactions, several more cross cutting concerns might need to be addressed when running a Poller. To help with that,
the Poller element accepts an <emphasis>&lt;advice-chain&gt; </emphasis> sub-element which allows you to define a custom chain of Advice
instances to be applied on the Poller. (see section 4.4 for more details)
In Spring Integration 2.0, the Poller went through the a refactoring effort and is now using a proxy mechanism to address transactional
concerns as well as other cross cutting concerns. One of the significant changes evolving from this effort is that we
made <emphasis>&lt;transactional&gt;</emphasis> and <emphasis>&lt;advice-chain&gt;</emphasis> elements mutually exclusive.
The rationale behind this is that if you need more than one advice, and one of them is Transaction advice, then you can simply
include it in the <emphasis>&lt;advice-chain&gt;</emphasis> with the same convenience as before but with much more control
since you now have an option to position any advice in the desired order. 
since you now have an option to position any advice in the desired order.
<programlisting language="xml"><![CDATA[<poller max-messages-per-poll="1" fixed-rate="10000">
<advice-chain>
<ref bean="txAdvice"/>
@@ -153,23 +152,25 @@
As yo can see from the example above, we have provided a very basic XML-based configuration of Spring Transaction advice  - "txAdvice" and
included it within the <emphasis>&lt;advice-chain&gt;</emphasis> defined by the Poller.
And if you only need to address transactional concerns of the Poller, then you can still use <emphasis>&lt;transactional&gt;</emphasis> element
If you only need to address transactional concerns of the Poller, then you can still use the <emphasis>&lt;transactional&gt;</emphasis> element
as a convinience.
</para>
</section>
</section>
<section id="transaction-boundaries">
<title>Transaction Boundaries</title>
<para>
Another important factor that needs to be understood is the boundaries of the Transactions within the Message flow.
When transaction is started, transaction context is bound to the current thread. So regardless of how many endpoints and channels you have in your
Message flow you transaction context will be preserved as long as you are ensuring that the flow continues on the same thread.
As soon as you break it by introducing a <emphasis>Pollable Channel</emphasis> or <emphasis>Executor Channel</emphasis> or initiate a new thread manually in some
service, the Transactional boundary will be broken as well. Essentially the Transaction will END right there and if
successfull hand of happened between the threads, the flow would be considered a success and COMMIT signal would be sent
even though the flow might still result in the exception somewhere downstream. If such flow was synchronous the exception would be thrown back to the
initiator of the Message flow who is also the initiator of the transactional context and transaction would result in a ROLLBACK.
Another important factor is the boundaries of Transactions within a Message flow.
When a transaction is started, the transaction context is bound to the current thread. So regardless of how many endpoints and channels you have in your
Message flow your transaction context will be preserved as long as you are ensuring that the flow continues on the same thread.
As soon as you break it by introducing a <emphasis>Pollable Channel</emphasis> or <emphasis>Executor Channel</emphasis> or initiate a new thread manually
in some service, the Transactional boundary will be broken as well. Essentially the Transaction will END right there, and if
a successful handoff has transpired between the threads, the flow would be considered a success and a COMMIT signal would be sent
even though the flow will continue and might still result in an Exception somewhere downstream. If such a flow were synchronous, that Exception could
be thrown back to the initiator of the Message flow who is also the initiator of the transactional context and the transaction would result in a ROLLBACK.
The middle ground is to use transactional channels at any point where a thread boundary is being broken. For example, you can use a Queue-backed Channel
that delegates to a transactional MessageStore strategy, or you could use a JMS-backed channel.
</para>
</section>
</chapter>