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I have an application with multiple Azure SQL databases sharded by a key. Every time we open a shard database connection, we retrieve the ShardMapManager and the ListShardMap:

public static ShardMapManager CreateOrGetShardMapManager(string shardMapManagerConnectionString)
{
    // Get shard map manager database connection string
    // Try to get a reference to the Shard Map Manager in the Shard Map Manager database. If it doesn't already exist, then create it.
    ShardMapManager shardMapManager = default(ShardMapManager);
    bool shardMapManagerExists = ShardMapManagerFactory.TryGetSqlShardMapManager(shardMapManagerConnectionString, ShardMapManagerLoadPolicy.Lazy, out shardMapManager);

    if (!shardMapManagerExists)
    {
        // The Shard Map Manager does not exist, so create it
        shardMapManager = ShardMapManagerFactory.CreateSqlShardMapManager(shardMapManagerConnectionString);
    }

    return shardMapManager;
}

public static ListShardMap<T> CreateOrGetListMap<T>(ShardMapManager shardMapManager, string shardMapName) 
{
    // Try to get a reference to the Shard Map.
    ListShardMap<T> shardMap = null;
    bool shardMapExists = shardMapManager.TryGetListShardMap(shardMapName, out shardMap);

    if (!shardMapExists)
    {
        // The Shard Map does not exist, so create it
        shardMap = shardMapManager.CreateListShardMap<T>(shardMapName);
    }

    return shardMap;
}

And then we open a connection by passing the shard key to OpenConnectionForKey():

public static DbConnection CreateShardConnection(int shardingKey, string connectionStr, string shardMapName)
{
    var shardMapManager = CreateOrGetShardMapManager(connectionStr);
    var shardMap = CreateOrGetListMap<int>(shardMapManager, shardMapName);
    SqlConnection conn = shardMap.OpenConnectionForKey<int>(shardingKey, GenerateCredentialsConnString(connectionStr), ConnectionOptions.Validate);
    return conn;
}

We only use one shard map and one list map for all the shards, so my question is - can we cache these objects in a global variable? Are they designed to be thread safe, and will it help performance?

Update

I found here https://learn.microsoft.com/en-us/azure/sql-database/sql-database-elastic-scale-shard-map-management#constructing-a-shardmapmanager that the ShardMapManager should indeed be created once and stored in a global variable.

But what about the ListShardMap? If we only have one, can we store that in a global variable too?

Code Slinger
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1 Answers1

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ShardMapManager contains a cache of mappings, and if we had multiple caches then (a) we would be consuming more memory due to duplicates, and (b) we would overall get more cache misses, both of which would result in worse performance. So we want to have a singleton ShardMapManager in order to have only one copy of the cache.

ListShardMap contains an internal reference to the ShardMapManager who created it, so that it can reference this cache. If you create multiple ListShardMap instances from the same ShardMapManager, they all refer to the same cache (inside that SMM) so it's totally fine.

Jared Moore
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