Glossary - WANdisco Subversion MultiSite Plus

Glossary A

Access Control: A set of functionality implemented as a proxy, providing more granular authentication and authorization rules for Subversion than can be implemented with Apache alone.
Acceptor: The recipient node for an incoming Agreement message initiated by a Proposer.
Active Node: A Node / Site that is active (both accepting and creating proposals) but is not part of the agreement engine that determines the correct ordering.
Active Voter Node: A Node / Site that is active (both accepting and creating proposals) and also part of DConE2's agreement engine that determines the correct ordering.
Admin Console: In Subversion MultiSite the admin console is a web user interface for managing WANdisco's MultiSite products. By default access to the admin console / user interface can be gained by pointing a browser at the machine hosting the WANdisco product:

Subversion MultiSite Plus:
http://[HOST MACHINE's IP]:8080/multisite-local/

Subversion MultiSite 4.0 or earlier:
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.

Glossary B

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.
bypassable transaction: Is bypassable if all nodes in Replication Group fail on this exact transaction.

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 client?s 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.

DConE: WANdisco's Distributed Coordinated engine, the software engine underlying replication. Read the Whitepaper on our Dcone technology.
DConE 2: In 2013 WANdisco released a brand new, fully re-written Distributed Coordination engine, capable of delivering active-active replication with greater flexibility and efficiency.
Designated Node: The High Availability node with priority order 1 (as listed in the prefs.xml file). This is the High Availability node with which the Failover Agent expects to communicate under normal operation.
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Failover Agent: is the intermediary that acts as an application proxy/gateway between the Subversion client and the High Availability replicators. The Failover Agent keeps track of which High Availability replicas are available, and issues the Subversion 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. WANdisco provides MultiSite and Clustering products that give JIRA enterprise class scalability and reliability. JIRA isn't an acronym, it's a contraction of 'Gojira'.

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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: applies to nodes that are part of a replication group. The term is no longer used for MultiSite products from release 3.7.
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Node: See Site

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.

Port 6444: The default port used to access the the Admin Console. This port is configurable during setup.
Port 8080: The default port used by the replicator to proxy Apache. The replicator listens on port 8080, and forwards requests to the Apache web-dav module on port 8181. This port is configurable during setup.
prefs.xml: The preferences file contains configuration information. Replication Group configuration is stored in the Membership Configuration Registry. The files are specific to each node. The preference files are located in (svn or cvs)-replicator/config.
prefs.xml: The preferences file contains configuration information. Replication Group configuration is stored in the Membership Configuration Registry. The files are specific to each node. The preference files are located in (svn or cvs)-replicator/config.
Provider Id: Another name for the GUID.

proxy

Prior to the release of Subversion MultiSite Plus, WANdisco's replicator operated as a proxy. The chief distinguishing characteristic of DConE 2 is that it operates at the file system level and no longer needs to be run as a proxy between users and the SCM server.

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: This is a term which is used primarily for Subversion MultSite 4.0 and earlier. The do not apply to Subversion MultiSite Enterprise Edition which has a much more flexible approach to replication.
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. Quorum is defined in the MembershipConfigurationRegistry? and with single node configurations (e.g. after initial setup and for quarantined nodes) in prefs.xml file. MultiSite by default has Singleton Response quorum.

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 Subversion/CVS instance that is an exact equivalent or copy of another Subversion/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 replicas Subversion/CVS repository in sync.
Replicator: The intermediary that acts as an application proxy/gateway between Subversion clients and a given Subversion 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 . 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 ocurred.
RTO: 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; Subversion and CVS.
Site: In Subversion MultiSite Enterprise Edition a Site is the term used both for the location where replication is to be handled and also the name specifically for the machine on which subversion repository data is hosted.
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.)
Subversion: An open-source revision control system, which aims to be a compelling replacement for CVS.
SVN: Abbreviation used for Subversion.
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 tyically 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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Tie-breaker

A Site/Node is an Active Node that is also called upon to cast a deciding vote should the other Active Voters come to a deadlock. Tie-breaker nodes are called upon if there are an even number of Active Voters, and therefore the posibilty that no agreement may be formed over ordering.
There can only be a single tie-breaker per replication group - if you make a site into a tie-breaker, it will remove that role from an existing tie-breaker node. [top]

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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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