Persistent/keepalive HTTP with the PHP Curl library?

Question

I'm using a simple PHP library to add documents to a SOLR index, via HTTP.

There are 3 servers involved, currently:

1. The PHP box running the indexing job
2. A database box holding the data being indexed
3. The solr box.

At 80 documents/sec (out of 1 million docs), I'm noticing an unusually high interrupt rate on the network interfaces on the PHP and solr boxes (2000/sec; what's more, the graphs are nearly identical -- when the interrupt rate on the PHP box spikes, it also spikes on the Solr box), but much less so on the database box (300/sec). I imagine this is simply because I open and reuse a single connection to the database server, but every single Solr request is currently opening a new HTTP connection via cURL, thanks to the way the Solr client library is written.

So, my question is:

1. Can cURL be made to open a keepalive session?
2. What does it take to reuse a connection? -- is it as simple as reusing the cURL handle resource?
3. Do I need to set any special cURL options? (e.g. force HTTP 1.1?)
4. Are there any gotchas with cURL keepalive connections? This script runs for hours at a time; will I be able to use a single connection, or will I need to periodically reconnect?

Answer 1

cURL PHP documentation (curl_setopt) says:

CURLOPT_FORBID_REUSE - TRUE to force the connection to explicitly close when it has finished processing, and not be pooled for reuse.

So:

1. Yes, actually it should re-use connections by default, as long as you re-use the cURL handle.
2. by default, cURL handles persistent connections by itself; should you need some special headers, check CURLOPT_HTTPHEADER
3. the server may send a keep-alive timeout (with default Apache install, it is 15 seconds or 100 requests, whichever comes first) - but cURL will just open another connection when that happens.

Answer 2

Curl sends the keep-alive header by default, but:

1. create a context using curl_init() without any parameters.
2. store the context in a scope where it will survive (not a local var)
3. use CURLOPT_URL option to pass the url to the context
4. execute the request using curl_exec()
5. don't close the connection with curl_close()

very basic example:

function get($url) {
    global $context;
    curl_setopt($context, CURLOPT_URL, $url);
    return curl_exec($context);
}

$context = curl_init();
//multiple calls to get() here
curl_close($context);

Answer 3

1. On the server you are accessing keep-alive must be enabled and maximum keep-alive requests should be reasonable. In the case of Apache, refer to the apache docs.

2. You have to be re-using the same cURL context.

3. When configuring the cURL context, enable keep-alive with timeout in the header:

   curl_setopt($curlHandle, CURLOPT_HTTPHEADER, array(
       'Connection: Keep-Alive',
       'Keep-Alive: 300'
   ));
   

Answer 4

If you don't care about the response from the request, you can do them asynchronously, but you run the risk of overloading your SOLR index. I doubt it though, SOLR is pretty damn quick.

Asynchronous PHP calls?