What is the best way to handle errors and retries in Pholcus?

Pholcus is a distributed, high concurrency and powerful web crawler software written in Go language. Handling errors and retries in Pholcus involves setting up your spider to deal with network issues, parsing errors, or any other exceptions that might occur during the scraping process.

Pholcus provides a mechanism to retry failed requests automatically. Here are some tips on how to handle errors and implement retries effectively:

1. Set Maximum Retry Count

You can set the maximum number of retries for a request by using the TryTimes property of the Request object. If a request fails, Pholcus will automatically retry it until it reaches the maximum number of attempts you have set.

2. Log Errors

Logging errors is crucial for debugging and understanding what went wrong during the scraping process. Pholcus has a logging system that you can use to log errors when they occur.

3. Custom Error Handling

You can implement custom error handling by checking the response status code and deciding whether to retry the request or handle the error in some other way.

4. Use Proxies

Using proxies can help you avoid IP bans and rate limits. You can rotate proxies to keep your scraping process running smoothly, even if some requests fail due to IP-based restrictions.

5. Implement Delays and Timeouts

Implementing delays between requests and setting appropriate timeouts can prevent overloading the server and minimize the chance of errors due to server overload.

Example Implementation in Pholcus

Here is a hypothetical example of setting up retries in a Pholcus spider:

package main

import (
    "github.com/henrylee2cn/pholcus/app"
    "github.com/henrylee2cn/pholcus/app/spider"
    "github.com/henrylee2cn/pholcus/common/goquery"
    "log"
)

func main() {
    sp := spider.NewSpider(MySpider, "MySpider")

    // Set up the spider here
    app.Run(sp)
}

var MySpider = &spider.Spider{
    Name: "MySpider",
    RuleTree: &spider.RuleTree{
        Root: func(ctx *spider.Context) {
            ctx.AddQueue(&spider.Request{
                Url:      "http://example.com", // Target URL
                Rule:     "parsePage",
                TryTimes: 3, // Set the number of retries
            })
        },
        Trunk: map[string]*spider.Rule{
            "parsePage": {
                ParseFunc: func(ctx *spider.Context) {
                    query := ctx.GetDom()

                    // Do something with the DOM here

                    // Log an error if something goes wrong
                    if somethingWentWrong {
                        log.Println("An error occurred:", someError)
                        // Retry the request if necessary
                        ctx.Retry()
                    }
                },
            },
        },
    },
}

In the example above, we define a spider with a root function that adds a request to the queue with a specified TryTimes value, which is the maximum number of retries. In the parsePage rule, we implement the parsing logic and include error handling where we can log errors and retry the request if necessary using ctx.Retry().

Remember to handle errors gracefully and respect the website's terms of service and robots.txt file when scraping. Additionally, always make sure to set reasonable retry intervals and maximum retry counts to avoid causing issues for the servers you are scraping from.