This section offers guidelines about choosing where to do work that could be initiated either from a task or a step procedure. (See Chapter 7 and Compaq ACMS for OpenVMS Writing Server Procedures for more information about step procedures.)

Use step procedures for:

• Database I/0
  Make database transactions as short as possible and include only the work appropriate to the procedure server. For example, if the procedure is in a server used for updates, do only database I/O required for updates. Move all other I/O to another procedure server, such as one used for reads.
• Calculations
  Be careful not to do intensive calculations in a procedure server that has limited server processes that also have I/O procedures. You do not want tasks waiting to do updates while another task does calculations.
• File opens or database binds
  By moving this work to a server initialization procedure, you can save the time that it takes to do this work when a task first calls a step procedure.

Use tasks for:

• Flow control
  It is usually preferable to have conditional logic in the task definition rather than in the procedure. (See Section 6.2.2.) However, sometimes conditional logic belongs in the procedure:
  • Complex business logic usually belongs in the procedure, not the task.
  • Conditional logic requiring array addressing belongs in the procedure, because the task does not allow for array addressing.
  • If logic internal to a procedure can preclude the need to return to the task definition, avoid the resulting reinvocation of the procedure by keeping the logic in the procedure.
• Terminal I/O
  When a step procedure does terminal I/O, the task ties up the procedure for the lengthiest part of the task's work. Also, the task cannot be distributed. The only reason to do terminal I/O in a procedure is if you must have access to some capability outside ACMS that requires terminal I/O.

Some types of work can be done either in the step procedure or the task depending on circumstances:

• Data moves
  Handle most data moves in the step procedure. However, do not have a task call a step procedure just to do a simple data move. Instead, use th MOVE statement in the task definition. However, a step procedure must handle array element manipulation.
• Data validation
  You can put data validation in either the form or the step procedure. The method you use can affect the performance and maintainability of the application.

A workspace is a buffer used to pass data between the form and the task, and between the task and the processing steps. A workspace can also hold application parameters and status information. Workspaces are passed to step procedures and tasks as parameters. You must define the workspace structure and store in CDO or DMU format.

6.4.1 Workspace Size

In the execution of an application, workspaces pass from the Command Process for exchange steps to the server process for processing steps, and back throughout the execution of a task. Information for use by these processes is stored within workspaces.

Part of the cost of communication across the network lies in the size of these workspaces. Consider this network cost when you design workspaces. Workspaces have to be stored in global sections or made up into packets to be passed over the network in a distributed transaction. Consider the total size of all the workspaces you are passing to the form or processing step.

Note that passing one workspace of 500 bytes is less expensive than passing 5 workspaces of 100 bytes each, providing all 500 bytes are required. Avoid passing unnecessary data, however large or small the workspace you use. Passing a 2250-byte workspace that requires three network packets is not unduly costly, if you intend to use most or all of the data in the workspace. Passing 250 bytes that require one network packet is wasteful, if you do not need the data.

6.4.2 Workspace Structure

You can design your workspaces:

• To match exactly your database or file record descriptions
  A workspace containing exactly the same fields as a relation in an Rdb database is easy to maintain. However, it may use more memory than necessary and may pass more data then necessary.
• To contain only the data necessary to do the work at hand
  Such a workspace minimizes the use of memory and passes only the data needed. It may be more difficult to maintain.

Note that no single workspace can be larger than 65,536 bytes. The total size of all workspaces (including system workspaces) cannot exceed 65,536 bytes for a task instance. This effectively limits the size of a single workspace to 65,536 bytes, minus the size of all ACMS system workspaces, because all system workspaces are always present (for example, the system workspace size is 519 bytes in ACMS Version 4.2).

6.4.3 Using Task, Group, or User Workspaces

There are three types of workspaces:

• Task workspace
  A task workspace stores database records and control information required by the task. ACMS system workspaces are a special type of task workspace, defined by ACMS.
• Group workspace
  Group workspaces store relatively static application-wide information (such as the interest rate for a financial application).
• User workspace
  User workspaces store static user information (such as a user security profile) or information specific to the user that changes during the session (such as an accounting of the kind of work done by the user).

Use task workspaces whenever possible. Task workspaces have the lowest CPU and memory costs. If you need group and user workspaces, use them for relatively static information. Table 6-2 summarizes the features of each type of workspace.

Table 6-2 Workspace Types

Characteristic	Task	Group	User
Availability	For the duration of the task instance	For as long as the application is started (active)	For the user's ACMS session within the application
Purpose	Passes information between: Steps in a task Exchange steps and forms Processing steps and procedure servers Parent tasks and called tasks	Stores relatively static information required by many tasks in a group	Stores user-specific information
CPU cost	Copied once from the .TDB at task start, not copied back at taskcompletion.	Copied from the permanent workspace pool to the temporary pool at task start. Copied back to the permanent workspace pool only if accessed for update by the task.	Copied from the permanent workspace pool to the temporary pool at task start. Copied back to the permanent workspace pool only if accessed for update by the task.
Memory cost	Exists only in the temporary workspace pool	One copy in the permanent pool and one copy for each task referring to the workspace in the temporary pool	One copy for each user in the permanent workspace pool, one copy for each task instance referring to the workspace for each user in the temporary workspace pool

6.4.4 Declaring and Referring to Workspaces

You can declare a workspace in the task definition using the WORKSPACE IS clause. Or you can declare the workspace in the task group definition with WORKSPACE IS and then refer to the workspace in the task definition with the USE WORKSPACE clause. It is usually more efficient to declare the workspaces in the task group definition rather than in the task definition. The difference s in these two approaches for each type of workspace are as follows:

• Task workspaces
  If you declare a workspace in the task definition, at the task group build, ADU stores in the .TDB file one copy of the workspace each time it is declared in a task. When you declare a workspace in the task group definition and refer to it in the task definition, the task refers to a single definition of the workspace that exists in the .TDB file.
  Therefore, declaring the workspace in the task group definition rather than the task definition decreases .TDB size (resulting in a decrease in the EXC and server process working set requirements) and improves run-time performance. For maintainability, define workspaces in the group and refer to them in the task definitions, even if the workspace is used in only one task.
• Group workspaces
  If you declare a group workspace in the task definition, there is one copy of the workspace in the permanent workspace pool for each task. In effect, the workspace becomes task-specific rather than being shared by all tasks. However, all users running a specific task see the same copy.
  If you declare a group workspace in the task group, each task uses a single copy of the workspace in the permanent workspace pool. As a result, all tasks referring to the workspace see the same copy.
  Figure 6_3 and Figure 6-4 illustrate the differences between declaring and referring to group workspaces.

  Figure 6-3 Declaring Group Workspaces in the Task Definition

  ---

  

  Figure 6-4 Declaring Group Workspaces in the Task Group Definition

  ---

  
• User workspaces
  If you declare a user workspace in multiple task definitions, the same user will have multiple copies of the workspace in the permanent workspace pool. In effect, the workspace becomes both user-specific and task-specific rather than just user-specific.
  If you declare a user workspace in the task group definition, each user has just one copy of that workspace in the permanent workspace pool. As a result, ACMS maintains a single copy of the workspace for each user. This copy is used by all tasks selected by a particular user. For example, use this method to store user profile information in a single workspace.
  Figure 6_5 and Figure 6-6 illustrate the differences between declaring user workspaces in the task, or declaring them in the task group and referring to them in the task.

  Figure 6-5 Declaring User Workspaces in the Task Definition

  ---

  

  Figure 6-6 Declaring User Workspaces in the Task Group Definition

  ---

  

The AVERTZ application declares all workspaces in the task group definition, and refers to them as needed in the task definition.

6.4.5 Specifying Access for Workspaces

ACMS allows a called task to accept arguments in the form of task workspaces. User-written agents or tasks may pass workspaces to other tasks. You can specify READ, WRITE, or MODIFY access for each workspace you include in a TASK ARGUMENT clause. Use MODIFY for passing and returning data, READ for passing data, and WRITE for returning data.

Specifying READ access on task workspace arguments can provide performance benefits, since ACMS does not have to return the workspace to the parent task or agent when the called task completes.

In creating workspaces for task calls, bear in mind the trade-off between workspace size and the various access types. For large workspaces, it is more efficient to pass data using a READ workspace and return data using a WRITE workspace than it is to pass and return data in a single MODIFY workspace. Conversely, it is more efficient to pass a single MODIFY workspace containing a small amount of data than it is to pass several separate READ and WRITE workspaces. The default is MODIFY.

The called tasks in the AVERTZ application are the Get Reservation task and the Complete Checkout task. Both tasks receive data from a parent task in the VR_SENDCTRL_WKSP workspace. However, neither task needs to return any data to the parent task in this workspace. Therefore, the VR_SENDCTRL_WKSP workspace is defined in both tasks as a task argument workspace with READ access. Both tasks also receive data from a parent task in the VR_CONTROL_WKSP workspace. Also, both tasks return information to the parent task in this workspace. Therefore, the VR_CONTROL_WKSP workspace is defined in both tasks as a task argument workspace with MODIFY access.

The Get Reservation task returns information to the parent task in the VR_RESERVATIONS_WKSP workspace after reading reservation information from the database. The VR_RESERVATIONS_WKSP workspace is defined as a task argument workspace with WRITE access in the Get Reservation task, because the task does not receive any data from the parent task in this workspace. However, the Complete Checkout task also receives data from the parent task in the VR_RESERVATIONS_WKSP workspace, in addition to returning information to the parent task in this workspace. Therefore, the VR_RESERVATIONS_WKSP workspace is defined as a task argument with MODIFY access in the Complete Checkout task.

See Compaq ACMS for OpenVMS Writing Applications for a complete description of workspaces.

6.5 Using Task Queuing

ACMS typically does interactive processing of the tasks in your application, but you may find that certain tasks have requirements that you can meet through the use of task queues. These requirements include:

• Data capture and deferred processing of data
  Queue the data with a task name in an ACMS task queue and process it at a later time when the demand for resources is less, if the application has the following requirements:
  • Needs to collect a large amount of data in a short time
  • Does not need to process the data immediately
  • Needs to allow the users to continue without delay
  
  For example, an application has hundreds of time cards that must be processed in a very short amount of time during a shift change. In this type of application, processing each data item immediately has adverse effects on the performance of the system, so it is useful to capture the data and store it for future processing. This type of processing is known as desynchronized processing, because the data capture and the data processing are not synchronized.
• High application availability
  Queuing is an important feature in distributed environments that allow users to continue working even if an application is not available. For example, an application uses a front-end node for data entry and queues these tasks for processing by a back-end node. When both nodes are up, the back-end node processes the queued tasks at regular intervals. If the back-end node goes down, the front-end node continues to queue the tasks, thus providing uninterrupted access to the application for terminal users entering data on the front end. These tasks remain queued until the back-end node becomes available and the front-end node can submit them.
• Transaction persistence
  If a system failure interrupts the processing of a queued task, the queuing facilities continue to dequeue and submit that task until the task succeeds. If the task fails for a different reason and cannot be resubmitted, ACMS writes the queue entry to an error queue, where it can be subsequently retrieved and processed by the application, based on the error status.

For these and other similar requirements, you can use the ACMS queuing facility to capture and initiate the tasks in your ACMS application.

Combine queuing with normal processing within a task when the data does not need to be synchronized. For example, in a task that ends with an update of multiple records, you can put the final processing step into a queued task; all the preceding steps can be interactive, even if they included write or modify operations. See the Compaq ACMS for OpenVMS Writing Applications for details about implementing a queued task, as well as a queued task example based on the AVERTZ application. The AVERTZ company offers a quick-checkin service for its customers. During peak periods, when many cars are being returned at the same time, customers can enter into the system their reservation information and the odometer reading from the car. The application stores the checkin information as a queued task that is processed at a later, less busy time. Section 6.6 describes the use of a queued task within a distributed transaction.

6.6 Designing Distributed Transactions

A business function usually involves several database operations. By including a set of operations within the bounds of a transaction, you are instructing ACMS to ensure that all updates are performed as an atomic transaction. ACMS uses the DECdtm services to ensure the integrity of a distributed transaction. Chapter 4 describes how to identify transactions for your business functions, including distributed transactions. This section offers details about the design of distributed transactions.

6.6.1 Locating Transaction Control

A distributed transaction must start and end in the same part of the application. You can start and stop a distributed transaction in a:

• Task
  Declaring the distributed transaction in the task definition ensures full ACMS functionality. For example, you can use the Oracle Trace software to track the distributed transaction if the transaction starts on a block step or a processing step. Also, if you always declare the distributed transaction in the task definition, you maintain consistent task definitions. Declaring the distributed transaction in the task definition contributes to ease of maintenance, because control of the transaction remains with the task and is insulated from changes to the procedure code. Within the task definition, you declare the distributed transaction either at the:
  • Block step
    If the task has more than one processing step participating in the distributed transaction, the transaction must start on the block step.
  • Processing step
    If only a single processing step participates in a distributed transaction, you can start the distributed transaction on the processing step. This method offers the advantage of allowing the use of one-phase commit if only a single local resource manager is being used. In such a case, two-phase commit is not required. For example, consider a banking transaction that consists of debiting of one account and crediting another. If the transaction involves the accounts of two patrons of the same branch (and the data is stored within the same resource manager), the local transaction requires only one-phase commit. If the transaction involves the accounts of patrons of different branches (and the data is in different resource managers), the two-phase commit functionality of a distributed transaction is required.
    If you start the transaction at the block step, the transaction starts in a different process (the Application Execution Controller) than the one in which the resource manager is executing (the server process). Therefore, you assume the full overhead of two phase commit. If you start the transaction at the processing step in the task, the transaction starts in the server process. By using conditional logic in your step procedure, you can make use of one-phase commit where appropriate.
• Step procedure
  Declaring the distributed transaction in a step procedure makes transaction retries easier to handle. However, the transaction is not under the control of ACMS. Therefore, you must code all the DECdtm service calls and assume responsibility for the integrity of the transaction. Also, you lose some ACMS functionality, such as the use of Oracle Trace.
• Agent
  Using a distributed transaction started in an agent, you can coordinate the database modifications made by a task with modifications made to a local database or file by the agent. For example, the ACMS Queued Task Initiator (QTI) uses a distributed transaction to coordinate the removal of a queued task element from an ACMS task queue with the modifications made to a database by the queued task instance. You can also use distributed transactions started in an agent in the design of a highly-available system using a fault-tolerant machine as a front-end system. For example, an agent on a front-end system collects information that is processed in a database by a system on a back-end VMScluster. After collecting the appropriate information, the agent starts a distributed transaction and then calls a task on a back-end system as part of the distributed transaction. When the task completes, the agents ends the distributed transaction. If the transaction commits successfully, the agent can process the next task. If the transaction aborts, perhaps because the back-end system fails, then the agent can retry the operation by calling the same task on a different system in the back-end VMScluster. By calling the task in a distributed transaction, the agents knows with absolute certainty whether or not the database operations performed by the task on the back-end system were successful, or if they failed and need to be retried.

You have access to remote data included in a distributed transaction by using:

• A task called from an agent in a server procedure
  If you use the ACMS$CALL or ACMS$START_CALL services in a step procedure called from a task, the procedure acts as an SI agent. As an agent, the procedure can call a task in another application, and has access to databases serving that task on the same or another node. Figure 6-7 shows pseudocode for the use of the ACMS$CALL service to call a task from a procedure. The example is a debit/credit banking operation between accounts on remote branches of the same bank. Figure 6-8 illustrates the use of the ACMS$CALL service to control processes and minimize network traffic.

  Figure 6-7 Including a Called Task Within a Procedure in a Distributed Transaction

  ---

  

  Figure 6-8 Remote Update Using Called Task from Procedure

  ---

  

  Note If you use task-call-task to call a task directly from another task, you do not have access to tasks in other task groups or applications. Both tasks must be in the same task group within the same application.

• Queued task
  A queued task can participate in a distributed transaction at enqueue or at dequeue. The distributed transaction encompasses the database transaction and the ensuing enqueue, or the dequeue and the ensuing database transaction:
  • Enqueue
    An application can store a queued task element in a task queue, including any data needed by the queued task.
  • Dequeue
    The QTI starts a distributed transaction, dequeues the queued task element and initiates the queued task in the specified application. The database processed by the dequeued task element can be remote from the source of the data used by the queued task element. In fact, the two databases, the applications, and the task queue can all be on the same or different nodes.
    If a queued task fails, the QTI places the failed queued task element on an error queue associated with the task queue so that remedial action can be taken. The QTI rolls back the transaction. If there is no error queue, the QTI removes the task from the queue, and commits the transaction.
• Remote Rdb, DBMS, or RMS
  In this case, you do not use ACMS$CALL from the step procedure for access to the called task. Instead, you use the remote access capabilities of the resource manager from within the local ACMS procedure.
  When you use the remote access capability of Rdb, for example, the initialization procedure might attach to a local database and a remote database when the server process starts. To handle the access to the remote database, Rdb creates a process on the remote node. This method has disadvantages:
  • Each local process accessing a remote database requires a process on the remote node.
  • The additional processes accessing the database on the remote node result in increased contention for resources in the remote database.
  • Each database access from the local node to the remote node requires a message exchange between the two nodes.
  • Operators on the remote node have less control over the database, because it is being accessed by processes on other nodes.
  
  In contrast, with ACMS$CALL, you do not need a process on the remote machine for every server process on the local machine; the database-access tasks called by the local machine can use the same server processes as the tasks on the remote machine. This reduces the number of processes required by the remote machine, resulting in a decrease on database contention. For example, in Figure 6-8 the remote task on Node 2 can use the same server processes as the local tasks to access the database.
  If you use the ACMS$CALL service to perform distributed transactions, you issue only two messages for each task call, regardless of the number of database accesses being performed by the server for the remote task. One message invokes the task, and one signals the completion.

  Note Additional messages are exchanged between the local and remote nodes as part of the two-phase commit protocol at the end of the transaction.

  
  Figure 6-9 shows pseudocode for a distributed transaction with Rdb remote access. Figure 6-10 illustrates the processes created and network traffic generated with the remote access method.

Figure 6-9 Using Rdb Remote Access for a Distributed Transaction

Figure 6_8 and Figure 6-10 illustrate the advantages of using the step procedure as an agent. In the example illustrated by Figure 6-8, using the ACMS SI:

• Minimizes server processes and lock contention on the remote node
  In Figure 6-8, the ACMS$CALL in the step procedure on Node 1 calls the task on Node 2. The remote task on Node 2 uses the same server process to access the database as the local tasks on Node 2. In Figure 6-10, each server process on Node 1 requires an Rdb server process on Node 2, in addition to the ACMS procedure server processes (not shown). These additional processes lead to increased contention in the database.
• Minimizes network traffic
  ACMS$CALL results in a single network transmission in each direction. Remote updates require network transmissions for each database verb. The result can be multiple network transmissions for each update.