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This chapter describes the programming interfaces of the Distributed Transaction Manager (DECdtm). You use these interfaces to implement distributed transactions or when you write resource managers that participate in distributed transactions. Examples of single and multiple branch applications are also presented. Additionally, this chapter describes the implementation of the X/Open Distributed Transaction Processing XA interface. This interface allows DECdtm to coordinate XA-compliant resource managers and XA-compliant transaction processing systems to coordinate resource managers compliant with DECdtm.
DECdtm system services are documented in the HP OpenVMS System Services Reference Manual.
This chapter contains the following sections:
Section 30.1 provides an overview of the DECdtm programming interfaces.
Section 30.2 describes single branch applications.
Section 30.3 describes multiple branch applications.
Section 30.4 describes default transactions.
Section 30.5 describes the Resource Manager interface.
Section 30.6 describes the Communication Resource Manager interface.
Section 30.7 describes the XA interface (Alpha only).
Section 30.8 provides program examples that use DECdtm.
DECdtm provides a basic infrastructure for a distributed transaction processing system. A transaction is a collection of operations that change the system from one valid state to another. A transaction performs operations on resources. Examples of resources are databases and files.
Specifically, a transaction has the ACID properties:
|Atomicity||Either all of the changes for a transaction are made, or none are. If the changes for a transaction cannot be completed, partial changes by the transaction must be undone.|
|Consistency||A transaction is expected to change the system from one consistent state to another.|
|Isolation||Intermediate changes by a transaction must not be visible to other transactions.|
|Durability||The changes made by a transaction should survive computer and media failures.|
A transaction often needs to use more than one resource on one or more system. This type of transaction is called a distributed transaction.
Individual OpenVMS systems within the distributed system are called nodes in this chapter.
The DECdtm model constructs a distributed transaction processing system from three types of component:
DECdtm implements a two-phase commit protocol. This is a simple consensus protocol that allows a collection of participants to reach a single conclusion. The two-phase commit protocol makes sure that all of the operations can take effect before the transaction is committed. If any operation cannot take effect, for example if a network link is lost, then the transaction is aborted, and none of the operations take effect. Given a list of participants and a designated coordinator, the protocol proceeds as follows:
|Phase 1:||The coordinator asks each participant if it can agree to commit. Each participant examines its internal state. If the answer is yes, it does whatever it requires to ensure that it can either commit or abort the transaction, regardless of failures. Typically, this requires logging information to disk. It then votes either yes or no.|
The coordinator records the outcome on disk: yes, if all the votes were
positive, or no, if any votes were negative or missing.
The coordinator then informs each participant of the final result.
Note that this protocol reaches a single decision while it allows the coordinator and participants to fail. Any failure during phase 1 causes the transaction to be aborted. If the coordinator fails during phase 2, participants wait for it to recover and read the decision from disk. If a participant fails, it can ask the coordinator for the decision on recovery.
While DECdtm is not complex in itself, construction of a full-function
resource manager needs knowledge of more techniques than can be given
in this manual. Transaction Processing: Concepts and
Techniques by Jim Gray and Andreas Reuter (Morgan Kaufman
Publishers, 1993) may be helpful.
30.2 Single Branch Application
A sequence of AP operations that occurs within a single transaction is called a branch of the transaction. In the simplest use of DECdtm, a single AP invokes two or more RMs.
The AP uses just three of the DECdtm services: $START_TRANS, $END_TRANS, and $ABORT_TRANS. These services are documented in the HP OpenVMS System Services Reference Manual. They have not changed, but additional information is given in this manual.
$START_TRANS initiates a new transaction and returns a transaction identifier (TID) that is passed to other DECdtm services. $END_TRANS ends a transaction by attempting to commit it and returns the outcome of the transaction with either a commit or abort. $ABORT_TRANS ends the transaction by aborting it.
During the transaction, the AP passes the TID to each RM that it uses. The TID may be passed explicitly, or through the default transaction mechanism described in Section 30.4. Internally, each RM calls the DECdtm RM services. It also uses the branch services if parts of the transaction can be executed by different processes or on different nodes.
DECdtm aborts a transaction if the process executing a branch
terminates. By default, it also aborts a transaction if the current
program image terminates.
30.2.1 Calling DECdtm System Services for a Single Branch Application
An application using the DECdtm system services follows these steps:
Edward Jessup, an employee of a computer company in Italy, is transferring to a subsidiary of the company in Japan. An application must remove his personal information from an Italian DBMS database and add it to a Japanese Rdb database. Both of these operations must happen, otherwise Edward's personal information may either end up cyber space (the application might remove him from the Italian database but then lose a network link while trying to add him to the Japanese database) or find that he is in both databases at the same time. Either way, the two databases would be out of step.
If the application used DECdtm to execute both operations as an atomic transaction, then this error could never happen; DECdtm would automatically detect the network link failure and abort the transaction. Neither of the databases would be updated, and the application could then try again.
Figure 30-1 shows the participants in the distributed transaction discussed in this sample transaction. The application is on node ITALY.
Figure 30-1 Participants in a Distributed Transaction
A transaction may have multiple branches. A separate branch is required for each process that takes part in a transaction, regardless of whether the processes run on the same node or on different nodes of the system.
The top branch of the transaction is created by $START_TRANS. A new branch can be requested in the following ways:
Note that in the last two cases, the RM or TP framework make the necessary branch service calls on behalf of the application. There is no difference in the three cases from the viewpoint of DECdtm.
The top branch of a transaction is created by calling $START_TRANS. A subordinate branch is authorized when an existing branch calls $ADD_BRANCH. This returns a globally unique branch identifier (BID). The application passes the BID and TID with an application-specific request to another process or node of the system. $START_BRANCH is then called on the target node to add a new branch to the transaction. A subordinate branch of a transaction may in turn create further branches.
DECdtm can connect the two parts of the transaction together because $ADD_BRANCH specifies the name of the target node while $START_BRANCH specifies the name of the parent node. Either the two nodes must be in the same OpenVMS Cluster or they must be able to communicate by DECnet. DECdtm operation is more efficient within an OpenVMS Cluster.
Unless DECdtm operation is confined to a single cluster, you must configure each node with the same DECnet node name as its cluster node name.
An application may complete its processing within a branch by calling $END_BRANCH.
On $START_BRANCH, DECdtm checks that the two nodes are able to communicate, but it does not validate that the branch is authorized until $END_BRANCH is called. At that point, an unauthorized branch is aborted without affecting the ability of the authorized branches to commit.
Be careful in situations in which an application attempts to access the same resource from different branches of a transaction. Some RMs can recognize that the branches form part of the same transaction and allow concurrent access to the resource. In that case, just like multiple threads in a process, the application may need to serialize its own operations on the shared resource. Other RMs may lock one branch against another. In that case, the application is likely to deadlock.
Multiple branches in a transaction can serialize their operations on a shared resource within an OpenVMS Cluster using the Lock Manager. Care is needed if two branches outside an OpenVMS Cluster implicitly share a resource, perhaps by each creating a subordinate branch on a third system.
A single process may have multiple branches. For example, a server
process may execute parallel operations on behalf of different
30.3.1 Resource Manager Use of the Branch Services
Strictly defined, an RM provides access to resources on the same process as an AP that has started a transaction or added a branch. However an RM may perform work for a transaction in a different process to the original request. In that case, it must use the branch services to join the transaction in the worker process.
Similarly, an RM such as Oracle Rdb may provide an application
interface that allows remote resources to be accessed. In that case,
the RM uses the branch services to add a branch on the local node and
start a branch on the remote node.
30.3.2 Branch Synchronization
Processing in all branches of a transaction must be complete before calling $END_TRANS.
Normally DECdtm is used to ensure branch completion. In this case:
In other words, when a transaction completes successfully, all synchronized branches complete together. When a transaction aborts, all synchronized branches on a single node complete together, but branches on different nodes complete at different times. Using synchronized branches does not add extra message overhead, because the synchronization events are implicit in the normal DECdtm commitment protocol.
DECdtm branch synchronization is redundant when branch processing is initiated by a synchronous call to a process or remote node, and that call does not return until processing is complete. For example, remote operations may be requested by Remote Procedure Call (RPC). In this case:
A default transaction TID is maintained for each process. Some DECdtm services act on the default transaction if no transaction is explicitly specified in the call. The default transaction of a process has two states:
The default transaction is cleared during the processing that occurs when the transaction commits or aborts.
Some operations ($START_TRANS, $START_BRANCH) that set the default transaction of a process will fail if the default transaction of the process was not previously clear. Such operations will update the default transaction without error if it is still set but commit or abort processing that is already in progress.
The default transaction TID is read by the $GET_DEFAULT_TRANS service.
Some RMs check if a default transaction has been started by the
application. If there is none, the requested operation is performed as
a single atomic operation. Do not use unsynchronized branches with such
RMs. The problem is that a transaction might be aborted asynchronously
(by another branch) before the branch calls the RM in question. The RM
would then perform the operation separately instead of joining the
transaction and then receiving an abort notification. This problem
cannot occur with a synchronized branch because the default transaction
TID is not cleared until $END_BRANCH is called.
30.4.1 Multithreaded Applications
Because the default transaction TID is per-process, not per-thread, it is preferable to use explicit TIDs in multithreaded processes.
However, you must use the default transaction with RMs that do not
provide an interface that allows the AP to specify the TID. In this
case, use the $SET_DEFAULT_TRANS service to set the appropriate TID in
each thread. Take care to serialize each sequence of operations that
sets and uses the default transaction.
30.5 Resource Manager Interface
A resource manager provides transaction operations on one or more resources. The RM must have the following characteristics:
DECdtm recognizes two components of an RM:
The RMI and its RM participants share a single event handler, but each participant may have a different name and context. The name is used to find relevant transactions on recovery. The context is a handle, opaque to DECdtm, which is passed to the event handler and may be used to address RM-specific data.
An RM uses the following DECdtm services during normal execution of transactions:
|$DECLARE_RM||Creates an RM instance in the current process.|
|$JOIN_RM||Adds an RM participant to a transaction.|
|$ACK_EVENT||Acknowledges an event reported to an RMI or RM participant.|
|$FORGET_RM||Deletes an RMI from the current process.|
An RM uses the following DECdtm services during recovery from an RM or system failure:
|$GETDTI||Gets distributed transaction information. Used to get information about the state of transactions.|
|$SETDTI||Sets distributed transaction information. Used to remove RM participants from a transaction.|