# Transaction Support

## Introduction

Programmers used to working with relational databases coming to the LDAP world often express surprise to the fact that there is no notion of transactions. It is not specified in the protocol, and thus no servers support it. Recognizing that this may be a major problem, Spring LDAP provides support for client-side, compensating transactions on LDAP resources.

LDAP transaction support is provided by `ContextSourceTransactionManager`, a `PlatformTransactionManager` implementation that manages Spring transaction support for LDAP operations. Along with its collaborators it keeps track of the LDAP operations performed in a transaction, making record of the state before each operation and taking steps to restore the initial state should the transaction need to be rolled back.

In addition to the actual transaction management, Spring LDAP transaction support also makes sure that the same `DirContext` instance will be used throughout the same transaction, i.e. the `DirContext` will not actually be closed until the transaction is finished, allowing for more efficient resources usage.

It is important to note that while the approach used by Spring LDAP to provide transaction support is sufficient for many cases it is by no means "real" transactions in the traditional sense. The server is completely unaware of the transactions, so e.g. if the connection is broken there will be no hope to rollback the transaction. While this should be carefully considered it should also be noted that the alternative will be to operate without any transaction support whatsoever; this is pretty much as good as it gets.
The client side transaction support will add some overhead in addition to the work required by the original operations. While this overhead should not be something to worry about in most cases, if your application will not perform several LDAP operations within the same transaction (e.g. a `modifyAttributes` followed by a `rebind`), or if transaction synchronization with a JDBC data source is not required (see below) there will be nothing to gain by using the LDAP transaction support.

## Configuration

Configuring Spring LDAP transactions should look very familiar if you're used to configuring Spring transactions. You will create a `TransactionManager` instance and wrap your target object using a `TransactionProxyFactoryBean`. In addition to this, you will also need to wrap your `ContextSource` in a `TransactionAwareContextSourceProxy`.

    <beans>
       ...
       <bean id="contextSourceTarget" class="org.springframework.ldap.core.support.LdapContextSource">
          <property name="url" value="ldap://localhost:389" />
          <property name="base" value="dc=example,dc=com" />
          <property name="userDn" value="cn=Manager" />
          <property name="password" value="secret" />
       </bean>

       <bean id="contextSource"
        class="org.springframework.ldap.transaction.compensating.manager.TransactionAwareContextSourceProxy">
          <constructor-arg ref="contextSourceTarget" />
       </bean>

       <bean id="ldapTemplate" class="org.springframework.ldap.core.LdapTemplate">
          <constructor-arg ref="contextSource" />
       </bean>

       <bean id="transactionManager"
        class="org.springframework.ldap.transaction.compensating.manager.ContextSourceTransactionManager">
          <property name="contextSource" ref="contextSource" />
       </bean>

       <bean id="myDataAccessObjectTarget" class="com.example.MyDataAccessObject">
          <property name="ldapTemplate" ref="ldapTemplate" />
       </bean>

       <bean id="myDataAccessObject"
                class="org.springframework.transaction.interceptor.TransactionProxyFactoryBean">
          <property name="transactionManager" ref="transactionManager" />
          <property name="target" ref="myDataAccessObjectTarget" />
          <property name="transactionAttributes">
             <props>
                <prop key="*">PROPAGATION_REQUIRES_NEW</prop>
             </props>
          </property>
       </bean>
       ...

In a real world example you would probably apply the transactions on the service object level rather than the DAO level; the above serves as an example to demonstrate the general idea.
You'll notice that the actual `ContextSource` and DAO instances get ids with a "Target" suffix. The beans you will actually refer to are the Proxies that are created around the targets; `contextSource` and `myDataAccessObject`

## JDBC Transaction Integration

A common use case when working against LDAP is that some of the data is stored in the LDAP tree, but other data is stored in a relational database. In this case, transaction support becomes even more important, since the update of the different resources should be synchronized.

While actual XA transactions is not supported, support is provided to conceptually wrap JDBC and LDAP access within the same transaction using the `ContextSourceAndDataSourceTransactionManager`. A `DataSource` and a `ContextSource` is supplied to the `ContextSourceAndDataSourceTransactionManager`, which will then manage the two transactions, virtually as if they were one. When performing a commit, the LDAP part of the operation will always be performed first, allowing both transactions to be rolled back should the LDAP commit fail. The JDBC part of the transaction is managed exactly as in `DataSourceTransactionManager`, except that nested transactions is not supported.

Once again it should be noted that the provided support is all client side. The wrapped transaction is not an XA transaction. No two-phase as such commit is performed, as the LDAP server will be unable to vote on its outcome. Once again, however, for the majority of cases the supplied support will be sufficient.

## LDAP Compensating Transactions Explained

Spring LDAP manages compensating transactions by making record of the state in the LDAP tree before each modifying operation (`bind`, `unbind`, `rebind`, `modifyAttributes`, and `rename`).

This enables the system to perform compensating operations should the transaction need to be rolled back. In many cases the compensating operation is pretty straightforward. E.g. the compensating rollback operation for a `bind` operation will quite obviously be to unbind the entry. Other operations however require a different, more complicated approach because of some particular characteristics of LDAP databases. Specifically, it is not always possible to get the values of all `Attributes` of an entry, making the above strategy insufficient for e.g. an `unbind` operation.

This is why each modifying operation performed within a Spring LDAP managed transaction is internally split up in four distinct operations - a recording operation, a preparation operation, a commit operation, and a rollback operation. The specifics for each LDAP operation is described in the table below:

LDAP Operation | Recording | Preparation | Commit | Rollback
--- | --- | --- | --- | ---
`bind` | Make record of the DN of the entry to bind. | Bind the entry. | No operation. | Unbind the entry using the recorded DN.
`rename` | Make record of the original and target DN. | Rename the entry. | No operation. | Rename the entry back to its original DN.
`unbind` | Make record of the original DN and calculate a temporary DN. | Rename the entry to the temporary location. | Unbind the temporary entry. | Rename the entry from the temporary location back to its original DN.
`rebind` | Make record of the original DN and the new `Attributes`, and calculate a temporary DN. | Rename the entry to a temporary location. | Bind the new `Attributes` at the original DN, and unbind the original entry from its temporary location. | Rename the entry from the temporary location back to its original DN.
`modifyAttributes` | Make record of the DN of the entry to modify and calculate compensating `ModificationItem`s for the modifications to be done. | Perform the `modifyAttributes` operation. | No operation. | Perform a `modifyAttributes` operation using the calculated compensating `ModificationItem`s.

A more detailed description of the internal workings of the Spring LDAP transaction support is available in the javadocs.

### Renaming Strategies

As described in the table above, the transaction management of some operations require the original entry affected by the operation to be temporarily renamed before the actual modification can be made in the commit. The manner in which the temporary DN of the entry is calculated is managed by a `TempEntryRenamingStrategy` supplied to the `ContextSourceTransactionManager`. Two implementations are supplied with Spring LDAP, but if specific behaviour is required a custom implementation can easily be implemented by the user. The provided `TempEntryRenamingStrategy` implementations are:

- `DefaultTempEntryRenamingStrategy` (the default). Adds a suffix to the least significant part of the entry DN. E.g. for the DN `cn=john doe, ou=users`, this strategy would return the temporary DN `cn=john doe_temp, ou=users`. The suffix is configurable using the `tempSuffix` property

- `DifferentSubtreeTempEntryRenamingStrategy`. Takes the least significant part of the DN and appends a subtree DN to this. This makes all temporary entries be placed at a specific location in the LDAP tree. The temporary subtree DN is configured using the `subtreeNode` property. E.g., if `subtreeNode` is `ou=tempEntries` and the original DN of the entry is `cn=john doe, ou=users`, the temporary DN will be `cn=john doe, ou=tempEntries`. Note that the configured subtree node needs to be present in the LDAP tree.

There are some situations where the `DefaultTempEntryRenamingStrategy` will not work. E.g. if your are planning to do recursive deletes you'll need to use `DifferentSubtreeTempEntryRenamingStrategy`. This is because the recursive delete operation actually consists of a depth-first delete of each node in the sub tree individually. Since it is not allowed to rename an entry that has any children, and `DefaultTempEntryRenamingStrategy` would leave each node in the same subtree (with a different name) in stead of actually removing it, this operation would fail. When in doubt, use `DifferentSubtreeTempEntryRenamingStrategy`.