Question

There are a lot of posts saying that SingleInstance is a bad design. But I think it is the best choice in my situation.
In my service I have to return a list of currently logged-in users (with additional data). This list is identical for all clients. I want to retrieve this list from database every 5 seconds (for example) and return a copy of it to the client, when needed.
If I use PerCall instancing mode, I will retrieve this list from database every single time. This list is supposed to contain ~200-500 records, but can grow up to 10 000 in the future. Every record is complex and contains about 10 fields.

So what about performance? Is it better to use “bad design” and get list once or to use “good approach” and get list from database on every call?

Answer 1

So what about performance? Is it better to use “bad design” and get
list once or to use “good approach” and get list from database on
every call?

Performance and good design are NOT mutually exclusive. The problem with using a single instance is that it can only service a single request at a time. So all other requests are waiting on it to finish doing it’s thing.

Alternatively you could just leverage a caching layer to hold the results of your query instead of coupling that to your service.

Then your code might look something like this:

public IEnumerable<BigDataRecord> GetBigExpensiveQuery(){

   //Double checked locking pattern is necessary to prevent
   // filling the cache multiple times in a multi-threaded
   // environment
   if(Cache["BigQuery"] == null){
      lock(_bigQueryLock){
         if(Cache["BigQuery"] == null){
            var data = DoBigQuery();
            Cache.AddCacheItem(data, TimeSpan.FromSeconds(5));
         }
      }
   }

   return Cache["BigQuery"];

}

Now you can have as many instances as you want all accessing the same Cache.