Redundancy and Replication Help Make Your Systems Stress-Free

Fuente: ERIC Institute of Education Sciences

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Bibliographic Details
Main Author:	Mitchell, Erik
Format:	Recurso educativo Open Access
Language:	en
Published:	2011
Subjects:	Human Factors Engineering Computer System Design Online Systems Gateway Systems Systems Analysis Duplication Database Management Systems Information Technology Redundancy
Online Access:	Acceder al recurso
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author	Mitchell, Erik
author_facet	Mitchell, Erik Mitchell, Erik
collection	Education Resources Information Center
contents	Redundancy and Replication Help Make Your Systems Stress-Free Mitchell, Erik Human Factors Engineering Computer System Design Online Systems Gateway Systems Systems Analysis Duplication Database Management Systems Information Technology Redundancy In mid-April, Amazon EC2 services had a small problem. Apparently, a large swath of its cloud computing environment had such substantial trouble that a number of customers had server issues. A number of high-profile sites, including Reddit, Evite, and Foursquare, went down when Amazon experienced issues in their US East 1a region (Justinb 2011). The author's library was heavily invested in the affected availability zone, and although they were unable to launch new instances, they managed to get through without downtime. To design a sufficiently redundant system in a cloud environment, one should focus on three primary areas: (1) system configuration; (2) data replication; and (3) service fail-over. This article briefly examines each of these areas.
format	Recurso educativo Open Access
id	eric_EJ948372
institution	ERIC Institute of Education Sciences
language	en
publishDate	2011
record_format	eric
spellingShingle	Redundancy and Replication Help Make Your Systems Stress-Free Mitchell, Erik Human Factors Engineering Computer System Design Online Systems Gateway Systems Systems Analysis Duplication Database Management Systems Information Technology Redundancy Redundancy and Replication Help Make Your Systems Stress-Free Mitchell, Erik Human Factors Engineering Computer System Design Online Systems Gateway Systems Systems Analysis Duplication Database Management Systems Information Technology Redundancy In mid-April, Amazon EC2 services had a small problem. Apparently, a large swath of its cloud computing environment had such substantial trouble that a number of customers had server issues. A number of high-profile sites, including Reddit, Evite, and Foursquare, went down when Amazon experienced issues in their US East 1a region (Justinb 2011). The author's library was heavily invested in the affected availability zone, and although they were unable to launch new instances, they managed to get through without downtime. To design a sufficiently redundant system in a cloud environment, one should focus on three primary areas: (1) system configuration; (2) data replication; and (3) service fail-over. This article briefly examines each of these areas.
title	Redundancy and Replication Help Make Your Systems Stress-Free
topic	Human Factors Engineering Computer System Design Online Systems Gateway Systems Systems Analysis Duplication Database Management Systems Information Technology Redundancy
url	https://eric.ed.gov/?id=EJ948372

if (filemtime($filename) + $maxlifetime < time()) { unlink($filename); $count++; } } return $count; } /** * A function that is called internally when session data is to be saved. * * @param string $sessId The current session ID * @param string $data The session data to write * * @return bool */ protected function saveSession($sessId, $data): bool { $sessFile = $this->path . '/sess_' . $sessId; if ($handle = fopen($sessFile, 'w')) { $return = false; // Lock the file for exclusive access to avoid issues with multiple // processes writing session simultaneously: if (flock($handle, LOCK_EX)) { $return = fwrite($handle, $data); // Make sure that there's no trailing data by truncating the file to // the correct length: ftruncate($handle, strlen($data)); fflush($handle); flock($handle, LOCK_UN); } fclose($handle); if ($return !== false) { return true; } } // If we got this far, something went wrong with the file output... // It is tempting to throw an exception here, but this code is called // outside of the context of exception handling, so all we can do is // echo a message.

// Anecdotal testing Today and Yesterday seems to indicate destroy() // is called by the garbage collector and everything is good. // Something to keep in mind though. return 0; } /** * Write function that is called when session data is to be saved. * * @param string $sessId The current session ID * @param string $data The session data to write * * @return bool */ public function write($sessId, $data): bool { if ($this->writesDisabled) { return true; } return $this->saveSession($sessId, $data); } /** * A function that is called internally when session data is to be saved. * * @param string $sessId The current session ID * @param string $data The session data to write * * @return bool */ abstract protected function saveSession($sessId, $data): bool; }

* * @return void */ public function writeClose() { // The assumption is that we're using PHP's ext/session. // session_write_close() will actually overwrite $_SESSION with an // empty array on completion -- which leads to a mismatch between what // is in the storage object and $_SESSION. To get around this, we // temporarily reset $_SESSION to an array, and then re-link it to // the storage object. // // Additionally, while you _can_ write to $_SESSION following a // session_write_close() operation, no changes made to it will be // flushed to the session handler. As such, we now mark the storage // object isImmutable. $storage = $this->getStorage(); if (! $storage->isImmutable()) { $_SESSION = $storage->toArray(true); session_write_close(); $storage->fromArray($_SESSION); $storage->markImmutable(); } } /** * Attempt to set the session name * * If the session has already been started, or if the name provided fails * validation, an exception will be raised. * * @param string $name * @return SessionManager * @throws Exception\InvalidArgumentException */ public function setName($name) { if ($this->sessionExists()) { throw new Exception\InvalidArgumentException( 'Cannot set session name after a session has already started'

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