Data modeling is a method used to define and analyze data requirements needed to support the business processes of an organization. The data requirements are recorded as a conceptual data model with associated data definitions. Actual implementation of the conceptual model is called a logical data model.
Referential integrity
Referential integrity is a property of data which, when satisfied, requires every value of one attribute (column) of a relation (table) to exist as a value of another attribute in a different (or the same) relation (table).[1]
For referential integrity to hold in a relational database, any field in a table that is declared a foreign key can contain only values from a parent table’s primary key or a candidate key. For instance, deleting a record that contains a value referred to by a foreign key in another table would break referential integrity. Some relational database management systems (RDBMS) can enforce referential integrity, normally either by deleting the foreign key rows as well to maintain integrity, or by returning an error and not performing the delete. Which method is used may be determined by a referential integrity constraint defined in a data dictionary.
Via Referential integrity – Wikipedia, the free encyclopedia.
Foreign key
A Foreign Key is a field in a relational table that matches a candidate key of another table. The foreign key can be used to cross-reference tables.
For example, say we have two tables, a CUSTOMER table that includes all customer data, and an ORDER table that includes all customer orders. The intention here is that all orders must be associated with a customer that is already in the CUSTOMER table. To do this, we will place a foreign key in the ORDER table and have it relate to the primary key of the CUSTOMER table.
Phone Systems To Power Your Business
Seamless Communications at your Desk or on the Road
- A unified view of you from any phone.
- Tools that let your calls find you anytime, anywhere.
- Easy call transfers to and from all your phones
via Phone Systems To Power Your Business – Mobility – Digium® The Asterisk Company.
Adding MySQL Cluster Data Nodes Online
The redistribution for NDBCLUSTER tables already existing before the new data nodes were added is not automatic, but can be accomplished using simple SQL statements in mysql or another MySQL client application. However, all data and indexes added to tables created after a new node group has been added are distributed automatically among all cluster data nodes, including those added as part of the new node group.
via MySQL :: MySQL Cluster :: 5.12.1 Adding MySQL Cluster Data Nodes Online: General Issues.
Public key authentication with ssh
Introduction
This LinuxAnswer describes how to set up public key authentication with ssh{1}.
It will include all sub components like scp too.
I will refer to the client machine{2} as the “client” and the server machine{3} as “server”
via Public key authentication with ssh – LinuxQuestions.org.
Dell Loses Orders as Facebook Do-It-Yourself Servers Gain
Hewlett-Packard, Dell and companies that sell the computers off the shelf are losing sales in a key market because Facebook and larger rival Google Inc. (GOOG) are leading a switch among Internet companies to do-it-yourself servers. These customized machines now account for 20 percent of the U.S. market for servers, which generated $31.9 billion globally in last year, said Jeffrey Hewitt, an analyst at Stamford, Connecticut-based Gartner Inc.
via Dell Loses Orders as Facebook Do-It-Yourself Servers Gain: Tech – Bloomberg.
ClusterLabs
Pacemaker keeps your applications running when they or the machines they’re running on fail. However it can’t do this without connectivity to the other machines in the cluster – a significant problem in its own right.
Rather than re-implement the wheel, Pacemaker supports existing implimentations such as Heartbeat. Heartbeat provides:
- a mechansigm to reliably send messages between nodes,
- notifications when machines appear and disappear
- a list of machines that are up that is consistent throughout the cluster
Heartbeat was also the first stack supported by the Pacemaker codebase.
via FAQ – ClusterLabs.
rsnapshot
rsnapshot is a filesystem snapshot utility for making backups of local and remote systems.
Using rsync and hard links, it is possible to keep multiple, full backups instantly available. The disk space required is just a little more than the space of one full backup, plus incrementals.
Depending on your configuration, it is quite possible to set up in just a few minutes. Files can be restored by the users who own them, without the root user getting involved.
Via rsnapshot.
Xen: How to Convert An Image-Based Guest To An LVM-Based Guest
This short article explains how you can move/convert a Xen guest that uses disk images to LVM volumes. Virtual machines that use disk images are very slow and heavy on disk IO, therefore it’s often better to use LVM. Also, LVM-based guests are easier to back up (using LVM snapshots).
I do not issue any guarantee that this will work for you!