Frequently Asked Questions

A simple lightweight WordPress plugin which adds Open Graph metadata to WordPress powered sites.

Web crawler
Note, however, that "No problem should ever have to be solved twice. To improve freshness, the crawler should penalize the elements that change too often. As an American and native English-speaker myself, I have previously been reluctant to suggest this, lest it be taken as a sort of cultural imperialism. Don't try to learn it first. But before Linux, most hacking was done on either proprietary operating systems or a handful of quasi-experimental homegrown systems like MIT's ITS that were never deployed outside of their original academic niches. When you're ready to start programming, I would recommend starting with Python. The five-dots-in-nine-squares diagram that decorates this document is called a glider.

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The parse tree doesn't contain any information about the original layout. Tidy then pretty prints the parse tree using the current layout options. Trying to preserve the original layout would interact badly with the repair operations needed to build a clean parse tree and considerably complicate the code. Some browsers can screw up the right alignment of text depending on how you layout headings. As an example, consider:. Both of these should be rendered the same.

Sadly a common browser bug fails to trim trailing whitespace and misaligns the first heading. HTML Tidy will protect you from this bug, except when you set the indent option to "yes". Tidy offers you a choice of character encodings: The full set of HTML 4. Cleaned up output uses HTML entity names for characters when appropriate. Otherwise characters outside the normal range are output as numeric character entities. Tidy doesn't yet recognize the use of the HTML meta element for specifying the character encoding.

Tidy offers advice on accessibility problems for people using non-graphical browsers. The most common thing you will see is the suggestion you add a summary attribute to table elements. The idea is to provide a summary of the table's role and structure suitable for use with aural browsers.

Tidy's -clean option will replace them by style properties and rules using CSS. This makes the markup easier to read and maintain as well as reducing the file size!

Tidy is expected to get smarter at this in the future. Tidy deletes empty paragraph and heading elements etc. The use of empty paragraph elements is not recommended for adding vertical whitespace.

These declarations can be combined to define an a new empty inline or empty block element, but you are not advised to declare tags as being both inline and block! Note that the new tags can only appear where Tidy expects inline or block-level tags respectively. This means you can't yet place new tags within the document head or other contexts with restricted content models.

So far the most popular use of this feature is to allow Tidy to be applied to Cold Fusion files. ASP is normally interpreted by the web server before delivery to the browser. Note that Tidy doesn't understand the scripting language used within pseudo elements and attributes, and can easily get confused. Tidy may report missing attributes when these are hidden within preprocessor code.

Tidy can also get things wrong if the code includes quote marks, e. Tidy will now see the quote mark preceding ID as ending the attribute value, and proceed to complain about what follows.

Note you can choose whether to allow line wrapping on spaces within pseudo elements or not using the wrap-asp option. Tidy supports another preprocessing syntax called "Tango", but only for attribute values. A setting of zero causes PHP to behave as before. It is turned on by default. Left undefined, PHP turns this on by default. You can turn it off at your own risk.

When using IIS this option must be turned off. Setting this variable may cause security issues, know what you are doing first. This allows IIS to define the security context that the request runs under. Default is to enable logging. The temporary directory used for storing files when doing file upload.

Must be writable by whatever user PHP is running as. If not specified PHP will use the system's default. If the directory specified here is not writable, PHP falls back to the system default temporary directory.

The maximum number of files allowed to be uploaded simultaneously. Starting with PHP 5. For details on the default values, see the documentation for the relevant connection functions. These warnings were displayed by default until PHP 5. Edit Report a Bug. Description of core php. Here's a short explanation of the configuration directives. It takes on a comma-delimited list of class names.

When set to 0 , assertion code will be generated but it will be skipped not executed at runtime. When set to -1 , assertion code will not be generated, making the assertions zero-cost production mode. When an integer is used, the value is measured in bytes. Shorthand notation, as described in this FAQ , may also be used. The size required for the cache entry data is system dependent. Starting with PHP 4. Next, if your php. But if you also have this in php. For example, change the directive to: This is a possible solution for a problem which seems to be a php-ini-problem but is not.

This problem may be caused by an apache-module SecFilter. Adding the following lines to the. Note that on some Unix systems i. Many people advise to disable such potentially-insecure functions like system , exec , passthru , eval and so on in php. This might help in case someone happens to maintain old applications with a charset other than utf Please note that the latter includes not only size of uploaded file plus post data but also multipart sequences.

This one had me goin' for a while. This can be a significant performance hit. On Windows you use ; as the seperator. They can also be used for web scraping see also data-driven programming. A web crawler is also known as a spider , [1] an ant , an automatic indexer , [2] or in the FOAF software context a Web scutter. A Web crawler starts with a list of URLs to visit, called the seeds. As the crawler visits these URLs, it identifies all the hyperlinks in the page and adds them to the list of URLs to visit, called the crawl frontier.

URLs from the frontier are recursively visited according to a set of policies. If the crawler is performing archiving of websites it copies and saves the information as it goes. The archive is known as the repository and is designed to store and manage the collection of web pages.

The repository only stores HTML pages and these pages are stored as distinct files. A repository is similar to any other system that stores data, like a modern day database. The only difference is that a repository does not need all the functionality offered by a database system. The repository stores the most recent version of the web page retrieved by the crawler. The large volume implies the crawler can only download a limited number of the Web pages within a given time, so it needs to prioritize its downloads.

The high rate of change can imply the pages might have already been updated or even deleted. The number of possible URLs crawled being generated by server-side software has also made it difficult for web crawlers to avoid retrieving duplicate content. If there exist four ways to sort images, three choices of thumbnail size, two file formats, and an option to disable user-provided content, then the same set of content can be accessed with 48 different URLs, all of which may be linked on the site.

This mathematical combination creates a problem for crawlers, as they must sort through endless combinations of relatively minor scripted changes in order to retrieve unique content. As Edwards et al. The behavior of a Web crawler is the outcome of a combination of policies: Given the current size of the Web, even large search engines cover only a portion of the publicly available part.

This requires a metric of importance for prioritizing Web pages. The importance of a page is a function of its intrinsic quality, its popularity in terms of links or visits, and even of its URL the latter is the case of vertical search engines restricted to a single top-level domain , or search engines restricted to a fixed Web site. Designing a good selection policy has an added difficulty: Junghoo Cho et al.

Their data set was a ,pages crawl from the stanford. One of the conclusions was that if the crawler wants to download pages with high Pagerank early during the crawling process, then the partial Pagerank strategy is the better, followed by breadth-first and backlink-count.

However, these results are for just a single domain. Cho also wrote his Ph. Najork and Wiener performed an actual crawl on million pages, using breadth-first ordering. The explanation given by the authors for this result is that "the most important pages have many links to them from numerous hosts, and those links will be found early, regardless of on which host or page the crawl originates. It is similar to a PageRank computation, but it is faster and is only done in one step.

An OPIC-driven crawler downloads first the pages in the crawling frontier with higher amounts of "cash". Experiments were carried in a ,pages synthetic graph with a power-law distribution of in-links. However, there was no comparison with other strategies nor experiments in the real Web. The comparison was based on how well PageRank computed on a partial crawl approximates the true PageRank value.

Surprisingly, some visits that accumulate PageRank very quickly most notably, breadth-first and the omniscient visit provide very poor progressive approximations. One can extract good seed from a previously-crawled-Web graph using this new method. Using these seeds, a new crawl can be very effective. Some crawlers may also avoid requesting any resources that have a "? Crawlers usually perform some type of URL normalization in order to avoid crawling the same resource more than once.

There are several types of normalization that may be performed including conversion of URLs to lowercase, removal of ". Some crawlers intend to download as many resources as possible from a particular web site. So path-ascending crawler was introduced that would ascend to every path in each URL that it intends to crawl.

Cothey found that a path-ascending crawler was very effective in finding isolated resources, or resources for which no inbound link would have been found in regular crawling.

The importance of a page for a crawler can also be expressed as a function of the similarity of a page to a given query. Web crawlers that attempt to download pages that are similar to each other are called focused crawler or topical crawlers. The concepts of topical and focused crawling were first introduced by Filippo Menczer [20] [21] and by Soumen Chakrabarti et al.

The main problem in focused crawling is that in the context of a Web crawler, we would like to be able to predict the similarity of the text of a given page to the query before actually downloading the page. A possible predictor is the anchor text of links; this was the approach taken by Pinkerton [23] in the first web crawler of the early days of the Web.

The performance of a focused crawling depends mostly on the richness of links in the specific topic being searched, and a focused crawling usually relies on a general Web search engine for providing starting points.

An example of the focused crawlers are academic crawlers, which crawls free-access academic related documents, such as the citeseerxbot , which is the crawler of CiteSeer X search engine.

Because most academic papers are published in PDF formats, such kind of crawler is particularly interested in crawling PDF , PostScript files, Microsoft Word including their zipped formats. Because of this, general open source crawlers, such as Heritrix , must be customized to filter out other MIME types , or a middleware is used to extract these documents out and import them to the focused crawl database and repository.

These academic documents are usually obtained from home pages of faculties and students or from publication page of research institutes. Because academic documents takes only a small fraction in the entire web pages, a good seed selection are important in boosting the efficiencies of these web crawlers. This increases the overall number of papers, but a significant fraction may not provide free PDF downloads.

The Web has a very dynamic nature, and crawling a fraction of the Web can take weeks or months. By the time a Web crawler has finished its crawl, many events could have happened, including creations, updates, and deletions. From the search engine's point of view, there is a cost associated with not detecting an event, and thus having an outdated copy of a resource. The most-used cost functions are freshness and age. This is a binary measure that indicates whether the local copy is accurate or not.

The freshness of a page p in the repository at time t is defined as:. This is a measure that indicates how outdated the local copy is.

September is for Seniors

As editor of the Jargon File and author of a few other well-known documents of similar nature, I often get email requests from enthusiastic network newbies asking (in effect) "how can I learn to be a wizardly hacker?". Back in I noticed that there didn't seem to be any other FAQs or web documents that addressed this vital question, so I started . A Web crawler, sometimes called a spider or spiderbot and often shortened to crawler, is an Internet bot that systematically browses the World Wide Web, typically for the purpose of Web indexing (web spidering).. Web search engines and some other sites use Web crawling or spidering software to update their web content or indices of others sites' web . Help Welcome to SparkNotes! We’re your brilliant, book-smart best friend, and we’re here to help you ace that test, transform that paper into pure gold, and understand even the most intricately-plotted Shakespeare plays.