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Conservative two-phase locking

In computer science, conservative two-phase locking (C2PL) is a locking method used in DBMS and relational databases.

Conservative 2PL prevents deadlocks.

The difference between 2PL and C2PL is that C2PL’s transactions obtain all the locks they need before the transactions begin. This is to ensure that a transaction that already holds some locks will not block waiting for other locks.

In heavy lock contention, C2PL reduces the time locks are held on average, relative to 2PL and Strict 2PL, because transactions that hold locks are never blocked.

In light lock contention, C2PL holds more locks than is necessary, because it is hard to tell what locks will be needed in the future, thus leads to higher overhead.

Also, a transaction will not even obtain any locks if it cannot obtain all the locks it needs in its initial request. Furthermore, each transaction needs to declare its read and write set (data items to be read/written during transaction), which is not always possible. Because of these limitations, C2PL is not used very frequently.

Hold Ya Head

Hold Ya Head is the title of the second single by Notorious B.I.G from his Duets: The Final Chapter album, a remixed album of Biggie Smalls’ work. The single is a double A-side with “Spit Your Game”, a collaboration with Twista and Bone Thugs-N-Harmony. “Hold Ya Head” features a sample from Bob Marley. The song was originally featured as the B-side to “Nasty Girl” in the UK and Australia but the song is now being released as a single in its own right.

The single was released on 24th April in the UK [1] and reached #64 in the UK charts [2].


Video

The video featured Voletta Wallace, B.I.G.’s mother, Biggie was projected on a wall doing a concert from before his death.


Remixes

The song is a remix of “Suicidal Thoughts”, the outro to Biggie’s first album, Ready to Die. The Bob Marley sample is taken from his song “Johnny Was”.


Track listing


UK - CD: 1

  1. Spit Your Game [Remix Edit] (Featuring Twista, Eightball & MJG & Bone Thugs-N-Harmony)
  2. Hold Ya Head [Main Version] (Featuring Bob Marley)


UK - CD: 2

  1. Spit Your Game [Remix] (Featuring Twista, Eightball & MJG & Bone Thugs-N-Harmony)
  2. Hold Ya Head [Main Version] (Featuring Bob Marley)
  3. Spit Your Game [MyTone - Personalized Ringtone]


UK - 12″ Vinyl

  1. Spit Your Game [Remix] (Featuring Twista, Eightball & MJG & Bone Thugs-N-Harmony)
  2. Spit Your Game [Instrumental Remix]
  3. Hold Ya Head [Main Version] (Featuring Bob Marley)
  4. Hold Ya Head [Instrumental]


Promo CD

  1. Spit Your Game [Remix Edit] (Featuring Twista, Eightball & MJG & Bone Thugs-N-Harmony)
  2. Spit Your Game [Remix Instrumental]
  3. Hold Ya Head [Amended Album Version] (Featuring Bob Marley)
  4. Hold Ya Head [Instrumental]

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Attorney

An attorney is someone who represents someone else in the transaction of business:

  • For attorney-at-law, see attorney at law, advocate, lawyer, solicitor, barrister or civil law notary.
  • For attorney-in-fact, see power of attorney.

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Concurrency control

In computer science, especially in the fields of computer programming (see also concurrent programming, parallel programming), operating systems (see also parallel computing) , multiprocessors, and databases, concurrency control ensures that correct results for concurrent operations are generated, while getting those results as quickly as possible.


Concurrency control in databases

Concurrency control in database management systems (DBMS) ensures that database transactions are performed concurrently without the concurrency violating the data integrity of a database. Transactions should be executed safely and follow the ACID rules, as described below. The DBMS must guarantee that only serializable (unless Serializability is relaxed), recoverable schedules are generated, and also that no committed actions are lost while undoing aborted transactions.


Transaction ACID rules

  • Atomicity - Either the effects of all or none of its operations remain when a transaction is completed - in other words, to the outside world the transaction appears to be indivisible, atomic.
  • Consistency - Every transaction must leave the database in a consistent state.
  • Isolation - Transactions cannot interfere with each other. Providing isolation is the main goal of concurrency control.
  • Durability - Successful transactions must persist through crashes.


Concurrency control mechanism

The main categories of concurrency control mechanisms are:

  • Optimistic - Delay the synchronization for a transaction until its end without blocking (read, write) operations, and then abort transactions that violate desired synchronization rules.
  • Pessimistic - Block operations of transaction that would cause violation of synchronization rules.

There are several methods for concurrency control. Among them:

  • Two-phase locking
  • Strict two-phase locking
  • Conservative two-phase locking
  • Index locking
  • Multiple granularity locking

A Lock is a database system object associated with a database object (typically a data item) that prevents undesired (typically synchronization rule violating) operations of other transactions by blocking them. Database system operations check for lock existence, and halt when noticing a lock type that is intended to block them.

There are also non-lock concurrency control methods, among them:

  • Conflict (serializability, precedence) graph checking
  • Timestamp ordering
  • commitment ordering

Almost all currently implemented lock-based and non-lock-based concurrency control mechanisms guarantee schedules that are conflict serializable (unless relaxed forms of serializability are needed). However, there are many research texts encouraging view serializable schedules for possible gains in performance, especially when not too many conflicts exist (and not too many aborts of completely executed transactions occur), due to reducing the considerable overhead of blocking mechanisms.


See also

  • Isolation (computer science)
  • Serializability
  • Schedule
  • Multiversion concurrency control
  • Global concurrency control
  • Concurrent programming


External links

  • Portland Pattern Repository: Synchronization Strategies
  • Portland Pattern Repository: Category Concurrency

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Cayuga-Seneca Canal

The Cayuga-Seneca Canal is a canal in New York, USA. It is now part of the New York State Canal System.

The Cayuga-Seneca Canal connects the Erie Canal to Cayuga Lake and Seneca Lake. It is approximately 20 miles long.


History

The Seneca River, now the Cayuga-Seneca Canal, always has been an economic engine for the communities of Waterloo and Seneca Falls. The Seneca Lock Navigation Co., a private enterprise formed in 1813, dammed three sets of rapids and installed locks to allow goods to be transported to the Erie Canal. In 1818, a canal was opened between Cayuga and Seneca Lakes. By 1823, an average of eight boats a day were passing through the lock at Waterloo, carrying flour, potash, pork, whiskey, lumber and wool and returning with other products and merchandise. Job Smith, Seneca Falls’ first businessman, opened a portage company on the eastern end of the river in 1787. The company transported travelers, boats and goods around a mile-long series
of rapids with a 2-foot drop known as “the Falls.”

The locks at Seneca Falls were completed in 1818. Improvements between the lakes, completed in 1821, made eight stone locks and nearly two miles of dug canal in addition to sections of the river. To further develop industry, the rapids were dammed to form the Upper, Middle and Lower Falls, and rapid industrialization began in 1825. Through Seneca Falls, there was a lock for every important mill site, raising or lowering the boats a total of 42 feet.

In 1825, a canal was begun to connect Seneca Lake with the newly constructed Erie Canal at Montezuma and the Cayuga-Seneca Canal was put into use in 1828. The Canal was enlarged in 1862, and the Flats, an area adjacent to and east of the village center, grew into a major industrial area, producing fire engines, hose carts and other firefighting equipment, pumps and other iron goods including stove parts, bootjacks, corn shellers, meat choppers, sausage stuffers, flatirons and bells. A knitting mill made socks and once produced 85,000 pairs of socks for the Army and Navy.

C-S Canal locks were modified in 1918 when New York State created the New York Barge Canal System. State engineers replaced five locks with a large two-flight lock - a 49 foot lift that required a great pool of water for their operation. This spelled the demise of the Flats. Buildings were destroyed or moved to create Van Cleef Lake. Flooding was completed on August 20, 1915.


Locks

The following list of locks are provided for the current canal, from east to west:

Lock # Location Lift
CS1 Cayuga n/a
CS2 Seneca Falls n/a
CS3 Seneca Falls n/a
CS4 Waterloo n/a


External Links

  • Information and Boater’s Guide to the New York State Canal System
  • New York State Canals


References

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John C. Stennis Lock and Dam

The John C. Stennis Lock and Dam (formerly named Columbus Lock and Dam) is part of the Tennessee-Tombigbee Waterway. It is located close to Columbus, Mississippi, and it impounds Columbus Lake. It is named for the longtime U.S. Senator from Mississippi, John C. Stennis.


References


External links

  • Tennessee-Tombigbee Waterway (U.S. Army Corps of Engineers)
  • Tennessee-Tombigbee Waterway – Pictures and stories

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Transaction processing

For other meanings, see the disambiguation page at Transaction.

In computer science, transaction processing is information processing that is divided into individual, indivisible operations, called transactions. Each transaction must succeed or fail as a complete unit; it cannot remain in an intermediate state.


Description

Transaction processing is designed to maintain a database in a known, consistent state, by ensuring that any operations carried out on the database that are interdependent are either all completed successfully or all cancelled successfully.

For example, consider a typical banking transaction that involves moving £500 from a customer’s savings account to a customer’s checking account. This transaction is a single operation in the eyes of the bank, but it involves at least two separate operations in computer terms: debiting the savings account by £500, and crediting the checking account by £500. If the debit operation succeeds but the credit does not (or vice versa), the books of the bank will not balance at the end of the day. There must therefore be a way to ensure that either both operations succeed or both fail, so that there is never any inconsistency in the bank’s database as a whole. Transaction processing is designed to provide this.

Transaction processing allows multiple individual operations on a database to be linked together automatically as a single, indivisible transaction. The transaction-processing system ensures that either all operations in a transaction are completed without error, or none of them are. If some of the operations are completed but errors occur when the others are attempted, the transaction-processing system “rolls back” all of the operations of the transaction (including the successful ones), thereby erasing all traces of the transaction and restoring the database to the consistent, known state that it was in before processing of the transaction began. If all operations of a transaction are completed successfully, the transaction is “committed” by the system, and all changes to the database are made permanent; the transaction cannot be rolled back once this is done.

Transaction processing guards against hardware and software errors that might leave a transaction partially completed, with a database left in an unknown, inconsistent state. If the computer system crashes in the middle of a transaction, the transaction processing system guarantees that all operations in any uncommitted (i.e., not completely processed) transactions are cancelled.

Transactions are processed in a strict chronological order. If transaction n+1 touches the same portion of the database as transaction n, transaction n+1 does not begin until transaction n is committed. Before any transaction is committed, all other transactions affecting the same part of the database must also be committed; there can be no “holes” in the sequence of preceding transactions.


Methodology

The basic principles of all transaction-processing systems are the same. However, the terminology may vary from one transaction-processing system to another, and the terms used below are not necessarily universal.


Rollback

Transaction-processing systems ensure database integrity by recording intermediate states of the database as it is modified, then using these records to restore the database to a known state if a transaction cannot be committed. For example, copies of information on the database prior to its modification by a transaction are set aside by the system before the transaction can make any modifications (this is sometimes called a before image). If any part of the transaction fails before it is committed, these copies are used to restore the database to the state it was in before the transaction began (rollback).


Rollforward

It is also possible to keep a separate journal of all modifications to a database (sometimes called after images); this is not required for rollback of failed transactions, but it is useful for updating the database in the event of a database failure, so some transaction-processing systems provide it. If the database fails entirely, it must be restored from the most recent back-up. The back-up will not reflect transactions committed since the back-up was made. However, once the database is restored, the journal of after images can be applied to the database (rollforward) to bring the database up to date. Any transactions in progress at the time of the failure can then be rolled back. The result is a database in a consistent, known state that includes the results of all transactions committed up to the moment of failure.


Deadlocks

In some cases, two transactions may, in the course of their processing, attempt to access the same portion of a database at the same time, in a way that prevents them from proceeding. For example, transaction A may access portion X of the database, and transaction B may access portion Y of the database. If, at that point, transaction A then tries to access portion Y of the database while transaction B tries to access portion X, a deadlock occurs, and neither transaction can move forward. Transaction-processing systems are designed to detect these deadlocks when they occur. Typically both transactions will be cancelled and rolled back, and then they will be started again in a different order, automatically, so that the deadlock doesn’t occur again. Or sometimes, just one of the deadlocked transactions will be cancelled, rolled back, and automatically re-started after a short delay.

Deadlocks can also occur between three or more transactions. The more transactions involved, the more difficult they are to
detect, to the point that transaction processing systems find there is a practical limit to the deadlocks they can detect.


ACID criteria

Transaction processing has these benefits:

  • It allows sharing of computer resources among many users
  • It shifts the time of job processing to when the computing resources are less busy
  • It avoids idling the computing resources without minute-by-minute human interaction and supervision
  • It is used on expensive classes of computers to help amortize the cost by keeping high rates of utilization of those expensive resources


Implementations

Standard transaction-processing software, notably IBM’s Information Management System, was first developed in the 1960s, and was often closely coupled to particular database management systems. Client-server computing implemented similar principles in the 1980s with mixed success. However, in more recent years, the distributed client-server model has become considerably more difficult to maintain. As the number of transactions grew in response to various online services (especially the Web), a single distributed database was not a practical solution. In addition, most online systems consist of a whole suite of programs operating together, as opposed to a strict client-server model where the single server could handle the transaction processing. Today a number of transaction processing systems are available that work at the inter-program level and which scale to large systems, including mainframes.

An important open industry standard is the X/Open Distributed Transaction Processing (DTP) (see JTA). However, proprietary transaction-processing environments such as IBM’s CICS are still very popular, although CICS has evolved to include open industry standards as well.


See also

  • ACID
  • ACMS
  • Audit trail
  • CICS
  • Database transaction
  • IMS
  • Java EE (e.g. WebSphere Application Server)
  • Java Transaction API (JTA)
  • Two-phase commit
  • Transaction Processing Facility


Books

  • Jim Gray, Andreas Reuter, Transaction Processing - Concepts and Techniques, 1993, Morgan Kaufmann, ISBN 1-55860-190-2
  • Philip A. Bernstein, Eric Newcomer, Principles of Transaction Processing, 1997, Morgan Kaufmann, ISBN 1-55860-415-4
  • Ahmed K. Elmagarmid (Editor), Transaction Models for Advanced Database Applications, Morgan-Kaufmann, 1992, ISBN 1-55860-214-3

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Cylinder lock

A cylinder lock is a lock in constructed with a cylinder that a locksmith can easily unscrew to facilitate rekeying<ref>
</ref>. The cylinder may contain any of a variety of locking mechanisms, including the pin tumbler lock, the wafer tumbler lock and the disc tumbler lock.

The first main advantage to a cylinder lock is that the cylinder may be changed without altering the boltwork. Removing the cylinder requires only loosening a set screw, then unscrewing the cylinder from the boltwork. The second is that it is usually possible to obtain, from a lock manufacturer, cylinders in different formats that can all be used with the same type of key. This allows the user to have keyed-alike, and master-keyed systems that incorporate a wide variety of different types of lock, such as nightlatches, deadbolts and roller door locks. Typically, padlocks can also be included, although these rarely have removable cylinders.

Standardised types of cylinder include key-in-knobset cylinders, rim (also known as nightlatch) cylinders, Ingersoll format cylinders, American, and Scandinavian round mortise cylinders, and Scandinavian oval cylinders. There are also standardised cross-sectional profiles for lock cylinders that may vary in length; for example to suit different door thicknesses. These profiles include the europrofile (or DIN standard), the British oval profile and the Swiss profile.

Cruciform pin-tumbler locks may also use interchangeable cylinders, as do a few sophisticated lever locks.

Individually Keyed System (KD)
With an individually keyed system, each cylinder can be opened by its individual key.

Keyed Alike (KA)
This system allows for a number of cylinders to be operated by the same key. It is ideally suited to residential applications such as front and back doors.

Master Keyed (MK)
A master-keyed system involves each lock having its own individual key which will not operate any other lock in the system, but where all locks can be operated by a single master-key.

Grand Master Keyed (GMK)
This is an extension of the master-keyed system where each lock has its own individual key and the locks are divided into 2 or more groups. Each lock group is operated by a master-key and the entire system is operated by one grand master-key.

Common Entrance Suite (CES)
This system is widely used in apartments, office blocks and hotels. Each apartment (for example) has its own individual key which will not open the doors to any other apartments, but will open common entrance doors and communal service areas.


References

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Bray Lock

Bray Lock is a lock and weir situated on the northern bank of the River Thames near Dorney, just above the M4 Motorway crossing of the Thames. It is a manned lock, and the lock keepers cottage is on an island between the lock and the weir. It is owned and managed by the Environment Agency. Bray itself and Maidenhead, Berkshire, England are on the opposite side of the river but can be reached from the lock.


Reach above the lock

Along the reach is Maidenhead, preceded by Brunel’s famous railway bridge. The Maidenhead bank is lined with large Edwardian houses. The Thames Path follows the Bucks (Eastern) bank to Maidenhead Bridge, which it crosses, and then proceeds on the Berkshire side to Boulter’s Lock.


See also

  • Locks on the River Thames

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Whitchurch Lock

Whitchurch Lock is a lock and weir situated on the River Thames in England. The lock is located in the Oxfordshire village of Whitchurch-on-Thames but the weir crosses the river to the Berkshire village of Pangbourne. Both lock and weir are owned and managed by the Environment Agency.

The long serving lock keeper is Brian Butcher who has worked on the river for more than forty years starting as an apprentice engineer for Bert Bushnell’s hire fleet based in Maidenhead in the 1960s.


Access to the lock

Whitchurch Lock is one of the few locks on the River Thames which has no public access other than by boat.


Reach above the lock

The reach passes along the Chiltern Hills, culminating in Goring Gap. On the Oxfordshire side are Hartslock beech woods, named after a lock that was removed in 1910. On the Berksire side is Child Beale Wildlife Park. The Thames Path crosses Whitchurch Bridge into Oxfordshire and continues through Whitchurch away from the river as it goes round Coombe Park, returning to the river at Hartslock. It continues on the Oxfordshire river bank to Goring.


See also

  • Locks on the River Thames

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Omega Lock

The Omega Lock (called the Chip Square in Galaxy Force) is a device in the Transformers: Cybertron cartoon series that, along with the Cyber Planet Keys, is needed to stop the black hole which threatens to consume Cybertron and the entire universe.

The Omega Lock was part of the space bridge project, an attempt made in ancient times to connect all worlds inhabited by Transformers. It was located aboard the starship that sought to establish a colony on Earth. This starship floated in the air, but eventually crashed and sank into the ocean, becoming the source for the human legend of Atlantis. Contact between the Earth colony — as well as those on Velocitron, Jungle Planet and Giant Planet — and Cybertron was lost many thousands of years ago. The Omega Lock also contains a map to the remaining Cyber Planet Keys.

The Omega Lock emits a sound, referred to as a “trans-sonic signal,” called the Omega Frequency which humans can hear but Transformers cannot. (This fact is only present in Cybertron and is not present in the original Japanese dialogue of Galaxy Force.)

When Vector Prime saw the Atlantis Pattern on a t-shirt worn by Lori, he realized that the Atlantis starship, and with it the Omega Lock, might be located on Earth.

In “Retreat”, Starscream managed to steal the Omega Lock and the three Cyber Planet Keys so he could become a giant possibly the size of a Giant Planet transformer. He lost the Omega Lock to Sonic Wing Mode Optimus who placed in the center of Cybertron which was revealed to be the transformer god Primus.

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Okeechobee Waterway

The Okeechobee Waterway is a man-made waterway stretching from Fort Myers on the west coast to Stuart on the east coast of Florida. It was built/finished in 1937 to provide a water route across Florida, allowing boats to pass east–west across the state rather than travelling the long route around the southern end of the state.


External links

  • USACE Okeechobee Waterway Website

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Power door lock

  1. REDIRECTPower door locks

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Whitchurch Lock

Whitchurch Lock is a lock and weir situated on the River Thames in England. The lock is located in the Oxfordshire village of Whitchurch-on-Thames but the weir crosses the river to the Berkshire village of Pangbourne. Both lock and weir are owned and managed by the Environment Agency.

The long serving lock keeper is Brian Butcher who has worked on the river for more than forty years starting as an apprentice engineer for Bert Bushnell’s hire fleet based in Maidenhead in the 1960s.


Access to the lock

Whitchurch Lock is one of the few locks on the River Thames which has no public access other than by boat.


Reach above the lock

The reach passes along the Chiltern Hills, culminating in Goring Gap. On the Oxfordshire side are Hartslock beech woods, named after a lock that was removed in 1910. On the Berksire side is Child Beale Wildlife Park. The Thames Path crosses Whitchurch Bridge into Oxfordshire and continues through Whitchurch away from the river as it goes round Coombe Park, returning to the river at Hartslock. It continues on the Oxfordshire river bank to Goring.


See also

  • Locks on the River Thames

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