.NET/ASP.NET/C#/VB.NET PDF Document SDK

async { // Check the limit if visited.Count < limit then // Wait for a URL... let! url = self.Receive() if not (visited.Contains(url)) then // Spawn off a new task for the new url. Each collects // links and posts them back to the urlCollector. do! Async.SpawnChild (async { let! links = collectLinks url for link in links do do self <-- link }) // Recurse into the waiting state return! waitForUrl(visited.Add(url)) } // This is the initial state. waitForUrl(Set.empty)) We can initiate a web crawl from a particular URL as follows: > urlCollector <-- "http://news.google.com";; finished reading http://news.google.com, got 191 links finished reading http://news.google.com/ output=rss, got 0 links finished reading http://www.ktvu.com/politics/13732578/detail.html, got 14 links finished reading http://www.washingtonpost.com/wp-dyn/content/art..., got 218 links finished reading http://www.newsobserver.com/politics/story/646..., got 56 links finished reading http://www.foxnews.com/story/0,2933,290307,0...l, got 22 links ... The key techniques shown in Listing 13-13 are as follows: The type RequestGate encapsulates the logic needed to ensure that we place a global limit on the number of active web requests occurring at any one point in time. This is instantiated to the particular instance webRequestGate with limit 5. This uses a System. Threading.Semaphore object to coordinate access to this shared resource. Semaphores are discussed in more detail in the section Using Shared-Memory Concurrency. The RequestGate type ensures that web requests sitting in the request queue do not block threads but rather wait asynchronously as callback items in the thread pool until a slot in the webRequestGate becomes available. The collectLinks function is a regular asynchronous computation. It first enters the RequestGate (that is, acquires one of the available entries in the Semaphore). Once a response has been received, it reads off the HTML from the resulting reader, scrapes the HTML for links using regular expressions, and returns the generated set of links.

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USERNAME SID ID1 ID2 LMODE REQUEST BLOCK TY --------- ---------- ---------- ---------- ---------- ---------- ---------- -OPS$TKYTE 5 100 0 4 0 0 AE OPS$TKYTE 5 89786 0 3 0 0 TM OPS$TKYTE 5 89787 0 3 0 0 TM OPS$TKYTE 5 1507344 13 6 0 0 TX ops$tkyte%ORA11GR2> select object_name, object_id 2 from user_objects 3 where object_id in (89756,89787))) 4 / OBJECT_NAME OBJECT_ID ------------------------------ ---------T1 89786 T2 89787

Note The AE lock is an edition lock, new in Oracle Database 11g. It is part of the Edition Based Redefinition

The urlCollector is the only message-processing program. It is written using a MailboxProcessor. In its main state it waits for a fresh URL and spawns a new asynchronous computation to call collectLinks once one is received. For each collected link a new message is sent back to the urlCollector s mailbox. Finally, we recurse to the waiting state, having added the fresh URL to the overall set of URLs we have traversed so far. The operator <-- is used as shorthand for posting a message to an agent. This is a recommended abbreviation in F# asynchronous programming. The AcquireAsync method of the RequestGate type uses a design pattern called a holder. The object returned by this method is an IDisposable object that represents the acquisition of a resource. This holder object is bound using use, and this ensures the resource is released when the computation completes or when the computation ends with an exception. Listing 13-13 shows that it is relatively easy to create sophisticated, scalable asynchronous programs using a mix of message passing and asynchronous I/O techniques. Modern web crawlers have thousands of outstanding open connections, indicating the importance of using asynchronous techniques in modern scalable web-based programming.

feature (not covered in this particular book). ID1 is the object id of the edition that SID is using currently. This edition lock protects the referenced edition from modification (dropping of the edition, for example) in much the same way the TM locks protect the tables they point to from structural modification.