REPLICATION GROUPS

What is a replication group

Replication Groups are units of organization that we use to manage replication of certain repositories between certain sites. In order to replicate a SVN repository between two or more nodes, you would need to associate by adding them to a replication group:

Example Replication Groups

An organization with developers working in Chengdu and San Fransisco need to collaborate on projects stored in two SVN repositories, Repo1 and Repo3. An administrator in the Chengdu office creates a replication group called "DragonGroup". The SVN MultiSite Plus nodes corresponding with each of the two sites are added to the group.

edit repository 2

Replication Group Example 1

The Chengdu office is the location of largest development team, where most repository changes occur. For this reason the node is assigned the role of Tie-breaker. In the event that there is disagreement between the nodes in the group over transaction ordering, NodeChengdu will carry the deciding vote.

The node in San Fransisco hosts a standard active node. Changes to the local repository are replicated to NodeChengdu, changes made on the Chengdu node are replicated back to San Fransisco.


In addition to the two active nodes, a third node, NodeParis is added, located at a management site that plays no active part in development. However, the node has been added to the group as a "pure voter". This means that NodeParis takes part in the vote for transaction ordering, even though the payload of those transactions are not written to repository replicas stored at the Paris office. The purpose of NodeParis is simply to add resilience to the replication system, in the event of a short-term disruption to traffic from one of the other two nodes, agreement can still be reached and replication could continue.

The organization might choose to make the Paris node "Passive" instead. With ParisNode running a passive node, replicas of Repo1 and Repo3 would also be stored in Paris, although these copies could only be changed by updates from the other (active) repository copies, they wouldn't be accessible to SVN clients.

Types of node

There's another element controlled by replication groups, the role that each repository replica plays the replication system, where each replica may be casting a vote in order to determine the correct order in which proposals are played out. See more about Types of Sites.

CREATE REPLICATION GROUP

You can create a replication group providing that you have at least two sites connected.

  1. Click CREATE REPLICATION GROUP.
    edit repository 2

    2. Replication Groups


  2. Enter a Group Name, it can be anything but it's a good idea to keep it simple.
    group name

    3. Group names


  3. Select from the available sites. As you select sites they'll be listed in the add node's field. group name

    Add some sites - you need a minimum of two

    4. ** Alert! **The local node is automatically made the first member
    It's not possible to create a replication group remotely, the initiating node is alsways added automatically.

  4. Added sites are by default given the role Active Voter. Click on a node's label to bring up a list of available roles. See Types of Sites.
    site type

    5. Replication groups

  5. When you have added all the nodes that you need, click on the CREATE REPLICATION GROUP. Replication groups are listed in boxes on the Replication Group tab.

VIEW

You can view and partly edit a replication group by clicking on the view button.

  1. Click on the Quick View button.
    site type
  2. The replication group's screen will open showing the member nodes of the group.
    site type
  3. Each node is desplayed as a color-coded circle. Click on the circle to see what other node types are available. Read more about node types.
    site type
  4. The Configuration screen provides access to the each node's type, along with a list of repositories and a link to the Configure Schedule screen.

Add Nodes

It's possible to add additonal nodes to a replication group. Click on the Add Nodes button to start the procedure, you can read about it in the Admin Guide - 13. Adding a node to a replication group

Save Node Roles

Use this button to save any changes that you make to the member nodes.

Configure Schedule

The Schedule screen lets you set the roles of sites to change over time, specifically changing according to a schedule.

Why change a node's role?

At the heart of WANdisco's DConE2 replication technology is an agreement engine that ensures that SVN operations are performed in exactly the same order on each replica, on each node. Any node that has the role of voter becomes part of the agreement engine and together with other voters determine the correct ordering. If there's high latency between any voters this may adversely affect replication performance. Fortunately it isn't a requirement that every site takes part in forming agreement. An Active site can still create proposals (i.e. instigate repository changes) but the agreement engine doesn't need to wait for its vote. Read more about how replication works in the Reference Section.

Follow the Sun

So in order to optimize replication performance it's common for administrators to remove voter status from node after their staff leave for the day - a practice commonly known as "Follow The Sun" where far-flung organisations transfer roles and privileges between locations so that they are always held by an active, manned site.

site type

Role Schedule
The Role Schedule window shows all the sites in the replication group, along with each node's current roll (denoted by a circular buttons). It's possible to click on a node and change its role as a Schedule Manager.
site type
If you change any sites you need to click the SAVE SITE ROLES button. to ensure that your changes are saved.
There's an indicator as to whether a Role Schedule has been enabled on the group. If there's a schedule in place the indicator will show ENABLED, otherwise it will show DISABLED.
site type
Clicking on the indicator will open up the Schedule screen, the same as if you clicked on the CONFIGURE SCHEDULE button.
Schedule
A drop-down selector for viewing a particular day of the week. This filter is only for viewing an established schedule, it isn't used to select the day that you intend to schedule.
Clear Schedule
Use this button to blank out settings you have changed, returning to the default schedule.
Save Schedule
After making any changes to the schedule, you need to click the SAVE SCHEDULE to apply them. Next to the button is a reminder of the server's timezone, this is derived from the server's system clock, not the location information provided during installation.

To see how to setup a schedule, see the SVN MultiSite Plus Admin Guide - How to configure a schedule