cURL sometimes returning blank string for a valid URL

I'm using the rolling-curl [https://github.com/LionsAd/rolling-curl] library to asynchronously retrieve content from a large amount of web resources as part of a scheduled task. The library allows you to set the maximum number of concurrent CURL connections, and I started out at 20 but later moved up to 50 to increase speed.

It seems that every time I run it, arbitrary urls out of the several thousand being processed just fail and return a blank string. It seems the more concurrent connections I have, the more failed requests I get. The same url that failed one time may work the next time I at开发者_运维问答tempt to run the function. What could be causing this, and how can I avoid it?

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Everything Luc Franken wrote is accurate and his answer lead me to the solution to my version of the questioner's problem, which is:

Remote servers respond according to their own, highly variable, schedules. To give them enough time to respond, it's important to set two cURL parameters to provide a liberal amount of time. They are:

    CURLOPT_CONNECTTIMEOUT => 30
    CURLOPT_TIMEOUT => 30

You can try longer and shorter amounts of time until you find something that minimizes errors. But if you're getting intermittent non-responses with curl/multi-curl/rollingcurl, you can likely solve most of the issue this way.

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In general you assume that this should not happen.

In the case of accessing external servers that is just not the case. Your code should be totally aware of servers which might not respond, don't respond in time or respond wrong. It is allowed in the HTTP process that things can go wrong. If you reach the server you should get notified by an HTTP error code (although that not always happens) but also network issues can create no or useless responses.

Don't trust external input. That's the root of the issue.

In your concrete case you increase the amount of requests consistently. That will create more requests, open sockets and other uses. To find the solution to your exact issue you need advanced access to the server so you can see the logfiles and monitor open connections and other concerns. Preferably you test this on a test server without any other software creating connections so you can isolate the issue.

But how well tested you make it, you have just uncertainties. For example you might get blocked by external servers because you make too many requests. You might be get stuck in some security filters like DDOS filters etc. Monitoring and customization of the amount of requests (automated or by hand) will generate the most stable solution for you. You could also just accept these lost requests and just handle a stable queue which makes sure you get the contents in at a certain moment in time.