验证Web服务中的Solr连接

Question

I am developing a web service which is supposed to get some data, use them for a query, search in Solr and returns the appropriate result! It works fine but I need it to initialize Solr only once so far I've got this:

 private static bool initialized = false;

    [WebMethod]
    public XmlDocument getContributor(string name,string email)
    {
        if (!initialized)
        {
            Startup.Init<SolrSearchResult>("http://Host:44416/solr");
            initialized = true;
        }
        if (string.IsNullOrEmpty(email))
        {
            return SolrSearchResult.SearchData(name);
        }
        return SolrSearchResult.SearchDataWithEmail(name, email);
    }

But I think when multiple users using, it wouldn't work! I need a smarter way to fix that! I would appreciate any suggestions!

PS: I've seen SampleSolrApp, there startup.init is used in Application_Start but I don't know what would be the equivalent here.

Answer 1

One way to make sure that Startup.Init is never called more than once when perhaps multiple concurrent calls are made to your getContributor method is by introducing a mutual-exclusion lock to synchronize access to that block of code.

In your case, I would start by first introducing a static object to lock on:

private static readonly object syncRoot = new object();

Then enclose that part of your code in a lock-statement:

lock (syncRoot)
{
    // only 1 thread ever enters here at any time.

    if (!initialized)
    {
        Startup.Init<SolrSearchResult>("http://Host:44416/solr");
        initialized = true;
        // no more threads can ever enter here.
    }
}

The lock keyword ensures that one thread does not enter a critical section of code while another thread is in the critical section. If another thread tries to enter a block of code that is locked, it will wait until the object is released.

As a side-note; there's a little technique you can use to optimize this code further called double-checked locking which avoids incurring a small performance cost to acquire the lock each time getContributor is called:

// check to see if its worth locking in the first place.
if (!initialized)
{
    lock (syncRoot)
    {
        // only 1 thread ever enters here at any time.

        if (!initialized)
        {
            Startup.Init<SolrSearchResult>("http://Host:44416/solr");
            initialized = true;
            // no more threads can ever enter here.
        }
    }
}

This works when initialized is never required to become false and you don't need Startup.Init to run again at a later stage, for whatever reason. Otherwize, you could run into problems using this code as-is.