SQL Server Express equivalent for EXTERNAL DATA SOURCE - sql-server

Question

As per https://azure.microsoft.com/en-us/documentation/articles/sql-database-elastic-query-getting-started-vertical/, it is now possible for one database in Azure SQL to query other Azure SQL databases. For my use case, I plan to have one database serving reference data for other databases, which fits nicely to Topology 1 (vertical sharding).
This is great for a deployed environment, but for local development I typically develop using SQL Server Express. As of SQL Server 2012 Express, the CREATE EXTERNAL DATA SOURCE is not a valid syntax.
Is it possible to also reap the benefit of external data source for local development?

Answer 1

After weighing the feature sets, I decided to differentiate the setup of my local database and Azure SQL.
When local SQL Server database wants to reference an Azure SQL database, it can do so using Linked Server
When a fellow Azure SQL database wants to reference another Azure SQL database, only then it uses external data source
i.e. locally
-- Make a link to the cloud
EXEC sp_addlinkedserver
#server=N'MyExternalServer',
#srvproduct=N'Azure SQL Db',
#provider=N'SQLNCLI',
#datasrc=N'<server address>',
#catalog='<database name>';
GO
EXEC sp_addlinkedsrvlogin
#rmtsrvname = '<server address>',
#useself = 'FALSE',
#locallogin=NULL,
#rmtuser = '<username>',
#rmtpassword = '<password>'
GO
select * from [MyExternalServer].[<database name>].[<schema>].[<table name>]
Whereas for Azure SQL:
CREATE MASTER KEY ENCRYPTION BY PASSWORD = '<password>';
CREATE DATABASE SCOPED CREDENTIAL ElasticDBQueryCred
WITH IDENTITY = '<username>',
SECRET = '<password>';
CREATE EXTERNAL DATA SOURCE MyElasticDBQueryDataSrc WITH
(TYPE = RDBMS,
LOCATION = '<server>',
DATABASE_NAME = '<database name>',
CREDENTIAL = ElasticDBQueryCred,
) ;
create schema <internalschema>
CREATE EXTERNAL TABLE <internalschema>.<internaltablename>
(
... // list of columns
WITH
( DATA_SOURCE = MyElasticDBQueryDataSrc,
SCHEMA_NAME = <schema>,
OBJECT_NAME = <table name>
)
select * from <internalschema>.<internaltablename>
The challenge now is to make the database scripts be common using both approaches. To reference a table using Linked Server, it has to be addressed using four-part identifier [server].[database].[schema].[tablename]. Contrast this with external data source where it can be addressed just by using [schema].[tablename].
Using inspiration from this question: https://dba.stackexchange.com/questions/74566/sql-server-using-4-part-identifiers-when-database-may-be-on-the-same-server, my approach is to create a synonym on my local database that redirects [schema].[tablename] to [externalserver].[externaldatabase].[externalschema].[tablename].
i.e. locally:
create schema <internalschema>
CREATE SYNONYM <internalschema>.<internaltablename> FOR [MyExternalServer].[<database name>].[<schema>].[<table name>]
After which, the same statement would work for both cases:
select * from <internalschema>.<internaltablename>
EDIT:
One big problem with this approach is that you cannot use wrap your script under distributed transaction, because Azure SQL does not allow DTC.

Answer 2

if we are talking about SQL Server, external data source/table/file format are supported starting from SQL Server 2016 - https://msdn.microsoft.com/en-us/library/dn935022.aspx .