v1 - The separation of concerns

A refacturing

Inspired by Jorin's "qvl.io/httpsyet/httpsyet", which I stumbled upon (via GoLangWeekly).

So, as a "real life" example, the original got refactored with focus solely on the aspects of conncurrency, and intentionally and respectfully each and all code related to the actual crawl functionality was left as untouched as possible.

Please feel free to compare the refactored crawler.go with the untouched original crawler.go.ori.304 (304 LoC) or see crawler.go.mini.224 where the parts which became obsolete are removed entirely (and not only commented out).

tl;dr

Using functions generated from pipe/s (see genny.go below), the actual concurrent network to process the site traffic becomes:

	sites, seen := siteForkSeenAttr(c.sites, site.Attr)
	for _, inp := range siteStrew(sites, size) {
		siteDoneFunc(inp, c.crawl) // sites leave inside crawler's crawl
	}
	siteDoneLeave(seen, c) // seen leave without further processing

Simple, is it not? ;-)

Please note: no sync-WaitGroup is needed around the parallel processes (as did the original), Also Done...'s results may be safely discarded. (The traffic congestion is monitored another way.)

So, how to get there?

Overview

The original Crawler "is used as configuration for Run." only, and this limited/focused purpose deserves respect.

crawling

In order to give a home to the data structures relevant during crawling, a new type crawling struct (in new crawling.go - see below) represents a crawling Crawler.

Crawler (the config) and traffic are embedded anonymously; thus crawling inherits their respective methods.

Note: The original implementation uses four hand-made channels and very cleverly orchestrates their handling. Too clever, may be.

Two channels become obsolete:

• queue becomes obsolete as feed-back is sent directly into c.sites.
• wait becomes a *sync-WaitGroup (to keep track of the traffic inside the circular net)

The remainig two channels sites and results got a new home in crawling:

• crawling.add registers entering urls (synchonously, and parallal!)
• crawling.goWaitAndClose patiently awaits it's Wait()
• crawling.crawl decrements every crawled site
• sitePipeLeave decrements the "I've seen your url before"-sites

Some remarks regarding source files follow:

crawling.go

• defines type crawling to represent a crawling Crawler.

• Crawler.crawling instatiates a new crawling, and calls it crawling.crawling (please forgive the pun), which

  • builds the process network (see above)
  • feeds the initial urls (using the original func queueURLs)
  • launches the closer (who simply does a crawling.Wait() before he closes the channels owned by crawling)

crawler_test.go

As we feed sites back into the crawling in parallel (which did not happen originally due to the use of channel queue) the visited map needs to become a guarded map (defined at the end of the source file) in order pass the tests.

Feel free to compare with crawler_test.go.ori.

genny.go

Just contains the go:generate directives for genny to generate what we need from the generic pipe/s function library.

Changes to crawler.go

• func (c Crawler) Run() error

  • typo corrected: "Run the cralwer." => "Run the crawler."
  • ca 30 LoC after initial validation removed,
  • finish with <-c.crawling(urls) instead - wait for crawling to finish

• func makeQueue()

  • completely removed - no need
  • ca 35 LoC

• func (c Crawler) worker

  • remove the for s := range sites loop
  • the loop body becomes a method of crawling: func (c *crawling) crawlSite(s site) (urls []*url.URL)
  • send results (now typed) into c.results (instead of old argument results)

• func queueURLs

  • is now launched in func (c *crawling) add

Thus, ca 80 LoC are removed / deactivated, and:

• no channel is created
• no go routine in launched
• only two send's remain:
  • c.results <- ... from crawlSite(s site)
  • queue <- site from queueURLs, now called with c.sites as argument queue (from func (c *crawling) add).

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