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Planning Replication

Articles and Tips: article

01 Aug 1997


Explains how to plan replication, the rights required to set up replication, and which files to replicate. Gives detailed instructions on configuring a server for replication and defining the replication set.

Introduction

This article, which is adapted from the Novell Replication Services Administrator's Guide, explains what replication is and how to plan it, the rights required to set up replication, and which files to replicate. It also explains how to prevent a master version from being overwritten by updates to replica data, if required.

What Is Novell Replication Services?

Novell Replication Services is an IntranetWare application that provides an effective way of distributing information across a network of geographically separated IntranetWare servers. Novell Replication Services is specifically intended for, but not limited to, use over slow, high latency, or intermittent WAN links.

As a network administrator, you can use Novell Replication Services to manage information such as documents and other files that are normally updated by only one user or group at a time but are required simultaneously in several different locations. Typical examples are

  • Application documents, including word processing and desktop publishing documents, spreadsheets, and presentations

  • HTML files, including both text and images

  • Software updates

Novell Replication Services does not support replication of concurrently updated files, such as databases that would require selective replication of individual modified data fields within the entire database document.

Novell Replication Services consists of a single NetWare Loadable Module (NLM) program that runs on each server to or from which you want to distribute information. Administration is provided by a "snap-in" to the standard NetWare Administrator utility so that you can control Novell Replication Services from any network client. The snap-in adds pages to the Novell Directory Services (NDS) Server object dialog.

What Benefits Does Novell Replication Services Offer?

This section shows the benefits that Novell Replication Services software offers and how these are achieved.

Easy Dissemination of Information Across the WAN

Novell Replication Services is administered from a snap-in to the standard NetWare Administrator utility. This provides a consistent interface with other NetWare Administrator tools and shortens the learning curve for administrators.

Once Novell Replication Services is installed, replication is defined, and synchronization schedules have been set up as described in this guide, no further action is required by the user; the network administrator's subsequent involvement is generally confined to routine activities such as periodic inspection of logs. Any modifications made by the users to the data in the replication set are automatically synchronized to the defined servers.

Powerful synchronization algorithms provide real-time data consistency across multiple servers. There is no longer any need to spend time searching and tracking changes for replication or writing batch files or scripts. Novell Replication Services detects changes made in any of the replicated directories and automatically replicates only those files that have changed, creating a fully synchronized file and data image at each server. This creates a highly scalable replication architecture.

Novell Replication Services supports two-way synchronization between any pair of servers and supports simultaneous one-to-many data flows for replicating files to all of the critical servers on your network.

Improved Data Availability

Because information can be replicated to remote locations, users at those sites can now have access to WAN information at LAN speeds, even for graphics, audio, and video files. Access to critical corporate resources will no longer be subject to outage and disruption due to WAN faults or performance constraints.

With important information replicated across an enterprise, users can more effectively share and collaborate on their work. Key personnel can more easily access and act upon sales figures, engineering documents, software, and code changes.

Internet/Intranet Replication

Web sites can be replicated for content consistency while providing improved remote access.

Replicating Web pages/sites across multiple servers helps in balancing network and server load.

Help with Managing Bandwidth

Novell Replication Services has a powerful scheduling facility that can help smooth out WAN traffic. You can schedule replica synchronization to occur as often as every minute or during off-peak hours.

Replication puts information closer to the users, which reduces the need to constantly access the WAN and frees your WAN bandwidth for other critical applications.

Reduction in WAN Costs

Because Novell Replication Services automates the replication process, your organization can make more productive use of the existing staff who are involved with distribution of information.

The WAN-optimized design of Novell Replication Services minimizes WAN traffic. Better utilization of existing equipment, lines, and connections means that purchases of expensive additional capacity can be postponed.

What Is Replication?

Replication is the process by which a file or group of files is copied from its original source (on a master server) to other servers, known as replica servers within an organization's Novell Directory Services (NDS) tree. This process allows users local to a replica server to obtain the documents more quickly and optimizes the use of long-distance connections between servers.

A file or document that has been distributed in this way using Novell Replication Services software is called a replica. The original file residing on the master server is called a master.

What Happens During Replication?

During replication, the Novell Replication Services NLM module intercepts file system hierarchy updates on the servers and, using advanced replication technology, maintains a database on each master and replica server that records

  • The state of the server's file system hierarchy

  • Transaction log entries of changes made to the file system

The log entries include

  • Changes to file contents

  • Movement of files and directories from one directory/location to another

  • Deletion of files and directories

  • Addition of files and directories

Therefore, these database entries identify the system hierarchy and files modified since the last synchronization, and thus define those required for the next synchronization process.

Reconciliation

It is important to remember that the database on a master or replica server is only being updated when the Novell Replication Services NLM module is running. If the NLM module is not running, the entries are not being maintained.

This means that when the NLM module does start, one of its initial tasks is to reconcile the contents of the database and the server's file system hierarchy. The task is performed by a tree-walk on the server and may carry an operational overhead, depending on the volume of data so processed.

Configuring a Server for Replication

Before you start to set up servers using Novell Replication Services, you must

  • Log in to the network and start NetWare Administrator.

  • Ensure that you have Supervisor and Browse object rights to the servers you want to work with. You also require Read and Write property rights to the Supported Services property of these servers. Refer to your NDS documentation for an explanation of these rights.

  • Ensure that the servers you want to define are currently running Novell Replication Services.

  • Set Maximum Services Processes through SERVMAN under Miscellaneous for each master and link server. The value specified should be at least three times the number of replica servers. A default IntranetWare installation will therefore support 16 simultaneously synchronizing replicas.

How you define a new server to Novell Replication Services depends on the function of the server you want to add. This can be a master server, replica server, or link server.

SUGGESTION: When using volume-to-directory replication give the replica directory the same name as the master server. For example, if the master server is LIBRARY, then name the replica directory LIBRARY also.

Defining a Master Server

To define a server as a master server, complete these steps using the NetWare Administrator utility.

  1. From the Novell Directory Services tree, select the server object that you want to configure as the master.

    The Server object dialog will be displayed.

  2. Select Replication from the list of available operations on the right of the dialog.

    The Replication snap-in panel will be displayed.

  3. Ensure that the Master Server radio button is selected at the top of the dialog.

    This indicates that you are viewing this server as a master server.

  4. To define a server as a master server, you need to identify at least one replica server to which it can replicate data.

    See "Defining a Replica Server," below, for details. The remaining steps in this procedure can be performed once you have added at least one replica server for this master server.

  5. If you want to ensure that changes made to documents on replica servers do not overwrite the master copies of the documents on this server, select the Protect Master from Replica Server Updates check box.

    By default, replication works in both directions and these files would be updated.

  6. When you have finished defining details for this server object, select Apply Changes to save the details.

  7. To exit replication and return to the NDS tree, select OK.

Defining a Replica Server

To define a server as a replica server, complete these steps using the NetWare Administrator utility.

  1. From the Novell Directory Services (NDS) tree, select the Server object to replicate data from (the master server).

    The Server object dialog will be displayed.

  2. Select Replication from the list of available operations on the right of the dialog.

    The Replication snap-in panel will be displayed.

  3. Ensure that the Master Server radio button is selected at the top of the dialog.

  4. Select Add to display the New Replica Server dialog. Specify the details of the server that you want to act as replica, and select OK.

    The name of the server is added to the Replica Servers list in the Replication snap-in panel.

  5. Repeat Step 4 for each replica server that you want to define for this master server.

    You can now set further replication and synchronization options for this master server, including defining the set of files it will replicate.

  6. When you have finished defining details for this server object, select Apply Changes to save the details.

  7. To exit replication and return to the NDS tree, select OK.

Defining a Link Server

To define a server as a link server, acting as a replica of one master server and the master of one or more other servers, complete these steps using the NetWare Administrator utility.

  1. Configure the server as a replica of its master server, as shown in "Defining a Replica Server."

  2. Set the replication and synchronization options for this replica server, including defining the set of files it will receive as replicated data. See "Defining the Replication Set" and "Scheduling Synchronization."

  3. Return to the NDS tree and select the link server.

  4. Configure the link server as a master server, as shown in "Defining a Master Server." As part of this task, define the replica servers to which this link server will replicate data.

  5. When you have finished defining details for this server object, select Apply Changes to save the details.

  6. To exit replication and return to the NDS tree, select OK.

Removing a Replica Server

To remove a server acting as a replica, complete these steps using the NetWare Administrator utility.


Note: If you are removing a link server and wish to delete replicated files from it,you must first separately remove the servers to which the link replicates.

  1. From the Novell Directory Services (NDS) tree, select the server that is the master of the replica to be removed.

  2. Select Replication from the list of available operations on the right of the dialog box to display the Replication snap-in panel.

  3. Highlight the replica server to be removed.

  4. Select Remove.

  5. Select Remove All Replicated Files or Leave Replicated Files Intact as required.

  6. Select OK.

  7. Select OK to close the Replication panel.

Removing a Link Server

To remove a server acting as a link, complete these steps using the NetWare Administrator utility.


Note: If you intend to delete replicated files from a link server (Step 5 below), youmust first remove the servers to which the link replicates. When doing this, carry outserver removal from the replication tree from leaf objects upwards (that is, from those objects containing no other objects).

  1. From the Novell Directory Services (NDS) tree, select the server that is the master of the link server to be removed.

  2. Select Replication from the list of available operations on the right of the dialog box to display the Replication snap-in panel.

  3. Highlight the link server to be removed.

  4. Select Remove.

  5. Select Remove All Replicated Files or Leave Replicated Files Intact as required.

    However, you cannot, remove replicated files from a link server until you have removed the server(s) to which the link is replicating.

  6. Select OK.

  7. Select OK to close the Replication panel.

Defining the Replication Set

A replication set is the definition of data to be replicated. You specify this from the Replication snap-in panel, usually from the master server.

Once you have defined your master and replica servers, you need to define what data to replicate. You can perform this task from the master server or its replica server. Before you start, read "Choosing Directories to Replicate" to ensure that only the data you require is replicated.

To define the replication set from the master server, complete these steps using the NetWare Administrator utility.

  1. From the Novell Directory Services (NDS) tree, select the server object containing the data you want to replicate, and display the Server object dialog.

  2. Select Replication from the list of available operations on the right of the dialog to display the Replication snap-in panel.

  3. Ensure that the Master Server radio button is selected at the top of the dialog.

  4. Select the replica server(s) to which you want the data to be replicated.

    Selecting more than one server allows the same data to be conveniently replicated to multiple servers.

  5. Select Define to display the Define Replication dialog.

  6. Either type the name of the volume and directory you want to replicate, or select the tree button and browse for the directory. To identify the size of the data in this directory, select Space Needed.

    Space Needed displays the minimum amount of disk space that must be free on the servers to which you want to replicate data. Note that this figure is the physical storage requirement for the data, not the current logical size of the data. This method is used to allow for compressed and sparse files.

  7. If you want to replicate only some of the items in this directory:

    1. Select More and complete the remainder of the dialog with the subdirectories that you want to mask or exclude from replication (see "Other Replication Options," below).

    2. Type the subdirectory name (or browse and select a name) in the field, select the exclusion type (excluded or masked), and select Add to include it in the list of excluded/masked data. "Masked" is the default exclusion type.

    3. To reinstate a subdirectory that has been previously added to the list of excluded and/or masked items, select it from the list and select Remove. This data will then be replicated with the other included files.

  8. When you have finished defining the replication set, select OK to return to the Replication dialog.

    Any changes you have made to the replication paths are shown in the "Changes to Replicated Paths" list. Verify that the listed replication changes are those required. If not, select Define to edit the list.

  9. To replicate the data immediately, ensure that the Synchronize Servers when Applying Changes check box is checked (this is the default), and select Apply Changes.

    All data in the replication set is copied to the selected replica server(s). When you next synchronize the data, only changes to this data are replicated.

  10. To close the Replication snap-in panel and return to the NDS tree, select OK.

    You can now set further replication options for this server, such as scheduling the frequency and times for the servers to synchronize. For details, see Chapter 5, of the NRS Administrator's Guide, "Synchronization."


Warning: Once you have finalized your replication set, you must not subsequently rename any volume (on any master or replica) withinthe replication set.

Replication Item Errors

You may encounter replication item errors during the replication process. These will occur in circumstances where you might, for example, attempt to replicate to an unknown volume or to a server on which the Novell Replication Services NLM module is not running.

You can inspect any Replication Item Errors by selecting Details from the Replication Results panel when it appears. To interpret these errors, if necessary, see "Looking Up Messages in Help."

Other Replication Options

This section describes how to enable optional attributes for your replication network.

Protecting a Master Server from Updates on Replicas

To configure a master server as a protected master server (thereby disabling two-way synchronization), select Prevent Updates from Replica Servers check box on the Replication panel when configuring the server for replication.

Users of the replica servers without Supervisor rights are then not able to update replicated files. However, users with Supervisor rights will be allowed to update the replicated files, although these updates will not be copied to the master server to overwrite its existing content and will be treated as clashes on the replica server when synchronization next occurs.

This setting can be changed at any time, if required, to allow the updates to overwrite the master again.


Note: The default option for master server protection is OFF.

Replicating Preloaded Data


Note: If you have already set up your replication network, you can now begin topreload data.

When replicating to a server with preloaded data, if any files on the replica server are found that do not exist on the master server or that have been modified on the replica server, these files are moved to a subdirectory of the NRSLOCAL directory.

This subdirectory is named according to the date of replication, in the format mm-dd-yy.XXX, where mm represents the month, dd the day, yy the year (last two digits), and XXX is a string uniquely identifying directories created on the same day.

The master server version then overwrites the preloaded replica server version, so that the master and replica data match.

* Originally published in Novell AppNotes

Disclaimer

The origin of this information may be internal or external to Novell. While Novell makes all reasonable efforts to verify this information, Novell does not make explicit or implied claims to its validity.

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