Re: string as lock key?

From: Scott Allen (bitmask_at_[nospam)
Date: 11/05/04 

Date: Fri, 05 Nov 2004 11:21:12 -0500

Hi Lloyd:

I generally avoid locking on stirng types because they are such wierd
beasts, you never know who else might have a reference to the same
string.

Consider:

String s1 = "MyTest";
String s2 = new
       StringBuilder().Append("My").Append("Test").ToString();
String s3 = String.Intern(s2);
Console.WriteLine((Object)s2==(Object)s1); // Different references.
Console.WriteLine((Object)s3==(Object)s1); // The same reference.

I'm still debating if you really need a lock here. The worst that
could happen is you might end up with extra database queries if
multiple requests come in and don't find an entry in the cache. Once
the cache is warm you'll then have the overhead of many find grained
locks. The lock is relatively cheap as locks go, but how many of them
might you have? 

--
Scott
http://www.OdeToCode.com/blogs/scott/
On Fri, 5 Nov 2004 20:58:06 +1100, "Lloyd Dupont"
<ld@NewsAccount.galador.net> wrote:
>in an ASP.NET page I have some static method which get value for the cache 
>or, if the cache is empty, query the database, put the value in the cache 
>and return it.
>
>because ASP.NET is thread intensive I was thinking to lock these method, 
>using a fine grained lock.
>the best lock I was thinking about was the string key in the cache!
>however I have one concern, these unique string might be used in other lock, 
>could they?
>could this be a problem ?
>
>here is my code I wonder about:
>-----------
>        public static string LogsByMonthCacheKey(string blogname)
>        {
>            return string.Intern(BLOG_KEY + ":LogsByMonth:" + blogname);
>        }
>        public static DataTable GetLogsByMonth(string blogname)
>        {
>            string ck = LogsByMonthCacheKey(blogname);
>            lock(ck) // lock with a unique string, is it alright ?
>            {
>                DataTable result = (DataTable) GetCache(ck);
>                if (result != null)
>                    return result;
>
>                SqlConnection conn = DBConnectionPool.Get();
>                try
>                {
>                    SqlCommand select = conn.CreateCommand();
>                    select.CommandText = "dbo.gb_EntryByMonth";
>                    select.CommandType = CommandType.StoredProcedure;
>                    select.Parameters.Add(new SqlParameter("@blogname", 
>blogname));
>
>                    SqlDataAdapter sda = new SqlDataAdapter(select);
>                    result = new DataTable();
>                    sda.Fill(result);
>                }
>                finally { DBConnectionPool.Let(conn); }
>
>                AddCache(ck, result);
>                return result;
>            }
>        }
> 
>

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