Add docs on how to set alias after initial data completion (#223)

docs/docs/connectors/elasticsearch.md

@@ -40,4 +40,49 @@ sqlBuilder.Sql(@"
 factory.AddElasticsearchSink("*", elasticSearchConnectionSettings);
 
 ...
-```
+```
+
+### Set alias on initial data completion
+
+One way to integrate with elasticsearch is to create a new index for each new stream version and change an alias to point to the new index.
+This is possible by using the *GetIndexNameFunc* and *OnInitialDataSent* functions in the options.
+
+Example:
+
+```csharp
+factory.AddElasticsearchSink("*", new FlowtideDotNet.Connector.ElasticSearch.FlowtideElasticsearchOptions()
+{
+    ConnectionSettings = connectionSettings,
+    CustomMappings = (props) =>
+    {
+        // Add cusotm mappings
+    },
+    GetIndexNameFunc = (writeRelation) =>
+    {
+        // Set an index name that will be unique for this run
+        // The index name must be possible to be recovered between crashes to write to the same index
+        return $"{writeRelation.NamedObject.DotSeperated}-{tagVersion}";
+    },
+    OnInitialDataSent = async (client, writeRelation, indexName) =>
+    {
+        var aliasName = writeRelation.NamedObject.DotSeperated;
+        // Get indices that the alias already points to.
+        var oldIndices = await client.GetIndicesPointingToAliasAsync(aliasName);
+        // Add the index to the alias
+        var putAliasResponse = await client.Indices.PutAliasAsync(indexName, aliasName);
+
+        if (putAliasResponse.IsValid)
+        {
+            // Remove all old indices that existed on the alias
+            foreach (var oldIndex in oldIndices)
+            {
+                await client.Indices.DeleteAsync(oldIndex);
+            }
+        }
+        else
+        {
+            throw new InvalidOperationException(putAliasResponse.ServerError.Error.StackTrace);
+        }
+    },
+});
+```