Access Control A set of functionality implemented as a proxy, providing more granular authentication and authorization rules for SVN than can be implemented with Apache alone.

Acceptor The recipient node for an incoming Agreement message initiated by a Proposer.

Admin Console Gives administrators complete control of Access the Admin Console by pointing a browser at the machine hosting the WANdisco product: http://[HOST MACHINE's IP]:6444
Login using the username 'admin' and password.

Agreement Each step that the DCone replicated state machine executes is called an agreement.

Agreement Manager Individual agreement steps of the replicated state machine are executed under the purview of one or more Agreement Managers.
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Batched Replication Objects changed in a batch of user transactions are replicated in a single replication transaction. Irrespective of whether a replication is transactional or batch based only the part of the transaction that is of interest to that site is applied at the receiving site. Batched transactions can utilize the self-healing features offered by WANdisco. For example, the network connection can be disabled between the distinguished node site and the other sites on a periodic basis (e.g., twice a day). When the network is reconnected, the remote replicator will reach out to the distinguished node to pick up all of the commits that were done at the distinguished node site during the network outage.

Bypassable Transaction Is bypassable if all nodes in Replication Group fail on this exact transaction.
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CentOS is a community-supported, free and open source operating system based on Red Hat Enterprise Linux. It exists to provide a free enterprise class computing platform and strives to maintain 100% binary compatibility with its upstream distribution. CentOS stands for Community ENTerprise Operating System.

CVS Concurrent Versions System is a popular open source code versioning system. CVS, like most source code versioning systems, is designed to run as a central server to which multiple CVS clients connect using a CVS protocol over TCP.

Clustering WANdisco's Clustering products are implemented as a transparent gateway between the SVN/CVS clients and the servers, and acts as a network proxies for the local server. There is a one-to-one relationship between replicator and SVN Server. With clustering, a load balancer directs a user to one node in the cluster but clients write activity must be replicated from one node to the others in the cluster.

Current Node The High Availability node the Failover Agent is communicating with within the HA group.
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DConE WANdisco's Distributed Coordinated engine, the software engine underlying replication. Read the Whitepaper on our DConE technology.

Failover Agent is the intermediary that acts as an application proxy/gateway between the SVN client and the High Availability replicators. The Failover Agent keeps track of which High Availability replicas are available, and issues the SVN client's request to one of them.
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Group Evolution A feature that allows changes to a previously defined Replication Group.
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GUID Globally Unique Identifier. Each node is assigned a GUID when added to a Replication Group. The nodes identify each other by their GUIDs.
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Heartbeat Mechanism used by High Availability's Failover Agent to monitor node availability.

Heartbeat connection timeout The time the Failover Agent waits before assuming the nonresponding node is unavailable. If the current active primary is unavailable, this triggers failover, and the Failover Agent begins.

Helper Node A node that is used as part of a repository repair procedure. The helper node is suspended from the replication group so that a copy of a repository can be transferred (rsync'ed etc) to the damaged node.

High Availability provides continuous hot backup, while making failover and disaster recovery automatic and transparent for both developers and administrators. One or more backup servers can be placed on the same local area network (LAN) as the primary server, or located remotely over a wide area network (WAN).
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Install node The node where you run the MultiSite install program. This is the first node in your replication group.
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JIRA Browser-based bug, issue, task and defect tracking system, and project management software solution used for open source and enterprise projects. JIRA isn't an acronym, it's a contraction of 'Gojira'. When listing product features and bug fixes we'll usually add a reference code for the corresponding JIRA issue.

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LDAP Lightweight Directory Access Protocol, is an Internet protocol that email and other programs use to look up information from a server.

Learner Any node that wants to learn the outcome of an Agreement step. Typically after a crash, a node becomes a Learner for all the agreements that occurred while it was down.
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Membership is a term that used to describe nodes that are connected to each other as part of a replication group.
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Node A server on which is installed a replicator. The Replica (repository) may also reside on the node, though for CVS/SVN products it can be located on another server.
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One copy equivalence All replicas are functionally equivalent copies of each other. The theory is that if two or more nodes start with identical copies of data and all changes to that data are applied in the same order to each copy, at the end of X transactions, the copies will all remain equivalent.

Owl A fan of Sheffield Wednesday Football Club.
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Port 8082 The default port used to access the the Admin Console. This port is configurable during setup.

prefs.xml The preferences file is used in SVN MultiSite 4.2 or earlier. prefs.xml contains configuration information and is specific to each node. The file is located in svn-replicator/config or cvs-replicator/config. See the knowledgebase article "Prefs.xml Basic Syntax"

Provider Id Another name for the GUID.

Quarantined Mode When activating a membership or Replication Group, new nodes start in quarantined mode. This means they will not participate in replication until they gain membership. This is to allow for testing of the new node's SCM configuration before executing replicating transactions.

Quorum A set of nodes that can reach agreement to determine transaction order. In the case of an even number of nodes, the distinguished node settles a conflict. Replicator products based on the first version of DConE (MultiSite 3.x and 4.x) work with discrete roles and use the concept of the 'distinguished node' as a defense against a 'split brain'. The following Quorum styles chiefly apply to the earlier DConE replication protocol. In contrast, SVN MultiSite Plus and DConE² offer greater flexibility and support the concept of a "tie-breaker" node that does away with the need for a distinguished node.

Read more about SVN MultiSite Plus's more flexible approach to building quorums - 3. Replication Strategy.

Quorum (Singleton) Singleton Response quorum - only one of the nodes in the membership decides on the transaction order. With Singleton Response quorum, the node that decides transaction ordering is called the distinguished node. The Singleton quorum offers the fastest response time for those users working at the distinguished node, because as soon as the distinguished node determines that a transaction can be processed in the correct order, the transaction is sent to SVN. Any replicator except the distinguished node can go down, but the replication group continues. The replication group replays the missing transactions when that node rejoins the group. However, the Singleton quorum also represents a single point of failure, since replication halts if the distinguished node fails.

Quorum (Majority) Majority Response is another quorum option, whereby you specify that a majority of the sites must agree on transaction order before any transaction is committed. Having a majority quorum ensures that if one site goes down in a replication group, even the distinguished node, the other sites can continue uninterrupted, as long as a majority of the sites remain available. The replication group replays the missing transactions when that site rejoins the group.

In a majority quorum, the distinguished node's role is that of a tie-breaker. For example, in a four node replication group, three sites make the quorum (three sites must agree about transaction ordering). If two nodes want one transaction first, and the other two want another transaction first, then the distinguished node gets a weighted vote. The group with the distinguished node determines the transaction ordering. With an even number of nodes with majority quorum, you can schedule the distinguished node to rotate to different nodes around the world, to 'follow the sun'.

Quorum (Unanimous) The last quorum option is unanimous response, which requires that all replicators must be reachable to accomplish transaction ordering.

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read-only mode As a result of a failed write operation a node can go into read-only mode. Write operations cannot proceed until the problematic transaction has been fixed

Replica a SVN/CVS instance that is an exact equivalent or copy of another SVN/CVS instance. In WANdisco's MultiSite product, a replica is also called a node.

Replication Group a collection of replicators that work together to keep SVN/CVS repositories in sync.

Replicator The intermediary that acts as an application proxy/gateway between SVN clients and a given SVN server. Each Replica has an associated Replicator. It coordinates with other peer replicators to ensure that all replicas of the SCM repositories stay in sync with each other.

RPO Recovery Point Objective. In disaster recovery the RPO is the time before a system failure at which the recovered system will restart. An RPO of 2 hours means that, in the event of a failure, the system would be brought back to the point that is no more than 2 hours before the failure occurred.

RTO Recovery Time Objective. In disaster recovery the RTO is the time in which the system should be recovered from a failure. With an RTO of 2 hours, the system should be recovered no more than two hours after the failure occured. High Availability offers an RTO of 0, meaning that recovery is immediate.
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SCM Software Configuration Management is the field dedicated to tracking and controlling changes in the software. WANdisco provides products that support two popular SCM solutions; SVN and CVS.

Site A location that has one or more nodes.

sub group Any High Availability group, consisting of at least two nodes and one Failover Agent. (The UI calls this a sub group, whether it is stand-alone or within a MultiSite deployment.)

SVN An open-source revision control system, which aims to be a compelling replacement for CVS.

SVN Abbreviation used for SVN.

Synchronized Stop Is a special transaction that will prevent further write transactions from happening. A replicator will process transactions normally up to this special transaction and the node will enter a "stopping" state. This causes all nodes to stop after processing up through the exact same GSN. A Synchronized Stop is typically actioned prior to administrative tasks to a server in a Replication Group. For example, when adding a node a synchronized stop must be completed first to ensure that all nodes are in sync. This transaction requires Unanimous Agreement in order to complete. Even if one of the nodes is not available, the other nodes will enter the stopping state and prevent write transactions. Unanimous Agreement means that all nodes need to be available for a synchronized stop to complete.
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WANdisco Wide Area Network Distributed computing. WANdisco is a leading provider of distributed software development solutions. By using WANdisco's unique replication technology, software development occurs anywhere without the constraints associated with far-flung distribution.

WebDAV Web-based Distributed Authoring and Versioning, or WebDAV is a set of extensions to the Hypertext Transfer Protocol (HTTP) that allows computer-users to edit and manage files collaboratively on remote web servers.
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Yeturu Aahlad Dr Aahlad is Wandisco's chief scientist and is recognised as a authority on distributed computing in which he currently holds 3 patents.

Prior to WANdisco, Dr. Aahlad served as the distributed systems architect for iPlanet (Sun/Netscape Alliance) Application Server. At Netscape, Dr. Aahlad joined the elite team in charge of creating a new server platform based on the CORBA distributed object framework.

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