How can I handle pagination in web scraping with Pholcus?

Pholcus is a distributed, high-concurrency, and powerful web crawler software written in Go, which is designed for handling complex web scraping tasks including pagination. To handle pagination in Pholcus, you need to understand how the target website's pagination works and then implement the logic within your Pholcus spider.

Pagination on websites can be implemented in various ways; the most common ones are:

1. Incremental page numbers in the URL (e.g., ?page=1, ?page=2, etc.).
2. "Next" button with a link to the next page.
3. Infinite scrolling, which dynamically loads content as the user scrolls down.

Below is a conceptual example of how you might handle incremental page numbers in the URL using Pholcus. It assumes you are already familiar with the basics of creating a Pholcus spider.

package main

import (
    "github.com/henrylee2cn/pholcus/exec"
    _ "github.com/henrylee2cn/pholcus_lib" // Import the default libraries (you can create your custom ones)
    "log"
    "strconv"
)

func main() {
    exec.DefaultRun("web")
}

// Create a new spider
var MySpider = &Spider{
    Name:        "MyPaginationSpider",
    Description: "An example spider to handle pagination",
    Pausetime:   300,
    // Other spider configuration options
    // ...
    RuleTree: &RuleTree{
        Root: func(ctx *Context) {
            ctx.AddQueue(&request.Request{
                Url:    "http://example.com/startpage",
                Rule:   "ParsePage",
                Method: "GET",
            })
        },
        Trunk: map[string]*Rule{
            "ParsePage": {
                ItemFields: []string{
                    "Title",
                    "URL",
                    // Other fields you want to scrape
                },
                ParseFunc: func(ctx *Context) {
                    // Parse the current page
                    // ...

                    // Determine the URL for the next page
                    // (you would need to extract or calculate this from the page)
                    curPage := ctx.GetTemp("curPage", 1).(int)
                    nextPage := curPage + 1

                    // Check if there's a next page
                    if hasNextPage(nextPage) {
                        nextURL := "http://example.com/startpage?page=" + strconv.Itoa(nextPage)
                        ctx.AddQueue(&request.Request{
                            Url:    nextURL,
                            Rule:   "ParsePage",
                            Method: "GET",
                            Temp: map[string]interface{}{
                                "curPage": nextPage,
                            },
                        })
                    }
                },
            },
        },
    },
}

func hasNextPage(pageNumber int) bool {
    // Implement logic to determine if there is a next page
    // This could be based on a fixed number of pages, a condition, or by checking the existence of a "next" link on the page
    // For example:
    return pageNumber <= 10 // Let's say there are 10 pages in total
}

In the example above, the hasNextPage function is a placeholder where you would implement the actual logic to determine if there's a next page. The spider starts by requesting the initial page ("http://example.com/startpage") and parsing it in the "ParsePage" rule.

Inside the "ParsePage" rule, after processing the current page, you would typically check for a "next" link or increment the page number to construct the URL for the next page. The spider then queues up the next page by adding it with ctx.AddQueue.

If pagination on your target website is done with a "Next" button, you would need to extract the URL of that button and queue it up similarly.

If the website uses infinite scrolling, handling it might be more complex. You would likely need to simulate the AJAX requests the website makes to fetch more content as the user scrolls down.

Remember to respect the website's robots.txt and terms of service when scraping, and consider the ethical implications and legality of your actions. Additionally, always use web scraping responsibly by not overloading the website's servers and by providing appropriate pauses between requests (Pausetime in the spider configuration).

Always test your spiders thoroughly to ensure they work correctly and handle edge cases, such as the end of the pagination.