Colly – Scraping Framework for Golang | Hacker News Reader

Colly – Scraping Framework for Golang

github.com

116 pointstampo931 comments

Some discussion from two months ago:

https://news.ycombinator.com/item?id=15408784

Thanks! Missed that earlier.

I'm always surprised by how many web scraping frameworks/libraries I see sprout here on HN on a regular basis. Is web scraping something people are doing, or is web scraping the new high-concurrency version of the "to do list" utility everyone used to write as an exercise?

This is an honest question, I'm not trying to take a dig at anyone in particular.

i worked at a startup for 6 months (fintech) whose entire value prop was built on the back of scraping.

Oh, I know there are very niche cases of companies making money out of it (one of the two jobs in my life that required web scraping was a freelance job for a company doing sector-specific news aggregation) I just find the stream of new libraries for doing this kind of fascinating.

Just yesterday I wrote a program to scrape some Amazon search and product pages [0].

Why? Because Amazon's search is outright broken. The number of results changes when you change sorting mode, and sometimes, sorting by a different criteria will just serve you a "no products found" error page.

I'll generally write a personal product comparison program when it becomes clear that I can't be certain that I can find the best product by hand. Often, even specialized websites that should have parametrized product search/filtering don't have their data properly indexed, so you have to scrape and parse it yourself. Another reason is to cross-reference with data from other sources. E.g. what laptop can I buy that has the best single-threaded CPU performance (within some other restrictions) [1]?

[0]: https://github.com/CyberShadow/choose-product/blob/master/am...

[1]: https://github.com/CyberShadow/choose-product/blob/master/le...

Dammit, I see D code and I want to learn the language so badly. I just need to find the right project for it (I might have something in the pipeline, as there's some stuff I need to do with libav and I really hate the idea of using C++.)

I find that I frequently want to dump a table from some webpage into a sqlite db file and run queries on it. Most sites don't offer a CSV or XLS download option, so a simple script to scrape the data usually ends up being the simplest option.

I wrote web scrapers as my daily work. For so many times, people(coworkers) just came to me saying "Hi, can you scrape this site for me?", however, what they want might just be `extracting` some table out of a login-protected page. It made no sense to simulate the login in a script or something and extract it by xpath or css. So I just came up with a Chrome Extension(100 SLOC) that tries to extracting the main table from a web page, then you can copy the table to Excel or anything you like. It's kinda like Readability, but only for tables, not articles.

[1] https://github.com/yifeikong/listify/

This framework has been posted before.

The solutions I write using homegrown utilities are both more elegant and faster than any framework or library I have ever seen posted to HN or recommended elsewhere. Not to mention smaller and more agile. IMHO.

All frameworks and libraries that I have seen will all fail given the right input i.e. fuzzing.

I think parsing and transforming the content from webpages is just viewed as work that no one wants to do because, for whatever reason, webpages are still unpredictable.

>Lightning Fast and Elegant Scraping Framework for Gophers

the bottle neck in scraping is never the parsing/DOM representation/traversal.

Good performance matters if you have decent networking infrastructure or your server has limited resources.

Bandwidth and IP limits are the most common bottle necks, but these can be solved using multiple proxies and ssh tunnels. Colly has built in support for switching proxies [1].

[1] http://go-colly.org/docs/best_practices/distributed/

the limitation is usually rate throttling on the page

>but these can be solved using multiple proxies and ssh tunnels. Colly has built in support for switching proxies

interesting

>your server has limited resources.

possibly.

Tell that to the project I migrated from scrapy to go six months back. Granted, scrapy might be doing other "fun" things to eat into performance, but it was really night and day. Immediately went from CPU bottleneck to network.

i don't understand? your insuperable bottleneck was always network then

It has always been scheduling and anti-blocking

The obvious question - why would I use this over Scrapy?

Because you're using Golang and not Python.

Here here. The right tool for the right job. And I can't think of a "righter" tool for this kind of job.

Edit - not picking on you, but given the quality and ecosystem of libraries and ancillary tools for scrapy, I don't even consider alternatives at this point. Good on anyone who does it to learn but for actual workloads I won't consider anything else.

For DOM parsing I cannot imagine that there could anything better than https://github.com/PuerkitoBio/goquery.

Which is funny because if you look at the code this is using goquery. Which then makes you wonder why would I use this when I can just use goquery?

Because goquery is only a DOM parser/manipulator and not a full-fledged (or even half-fledged) "scraping framework"?

Please breakup your main `colly.go` file into separate parts. If possible you shouldn't have a 30 line imports definition covering everything from cookies and regex to html and sync access.

Make sure to use DNS caching on the box else add it in Go.

Colly only supports a single machine via map of visited URL's. Would be great if you replace with a queue like redis or beanastalkd.

    visitedURLs map[uint64]bool

Please don't follow this suggestion. It's very helpful and healthy to have everything in a single file if you consider that manageable, so no problem at all.

Yes, this looks like very common Go code. No need to increase complexity by breaking it up.

It would be one thing if he just wanted a single go file for one reason, but the repo already contains multiple files. I'm recommending he break up the main file according to concerns. (common in go)

Well, in Go all files in one directory are part of one package (best practice) and files within the same package do not have to import each other to have access to other functions. Therefore, breaking the package into several files is common practice.

A sane approach to this is for example to create a separate file for each type (Collector, HTMlElement, Request, Response, ...) and its attached functions/methods.

> […] It's very helpful and healthy to have everything in a single file if you consider that manageable.

SQLite has a different opinion — https://www.sqlite.org/amalgamation.html

I don't think that's a good comparison to justify large code files:

a) it's automatically generated, so you can do dev work on the "easier" split version

b) Go comes with package management that helps solve the deployment issue

c) Question: Does the concept of a "compilation unit" affect the Go compiler the same way?

> SQLite would like to disagree with you... ...Over 100 separate source files

Um, how does this disagree?

How does this go with running the JS on the SPAs that make up a large portion of the web today?

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