Sunday, 3 May 2020

Use Managed Service Identity for Synapse PolyBase

I a previous PolyBase example, to read data from the storage account, we stored the secret in Synapse. Is it possible to use Managed Identity instead of storing secrets in Synapse? However I can not find the Managed Identity of  my SQL Server.
Azure Synapse Analytics with PolyBase reading Azure Storage Account

You can use a Managed Identity, but there are two requirements. First this only works with 'StorageV2 (general purpose v2)'. 'BlobStorage' or 'Storage (general purpose v1)' will not work! Secondly you need to register your SQL Server that hosts Synapse in your Active Directory. This will allow you to select your SQL Server within the Access control (IAM).

1) Create Storage Account
Create an Azure Storage Account and make sure the type is StorageV2 (general purpose v2). The storage account of this example is called 'bitoolsstorage' and it has a container called 'mycontainer'. You can choose your own names, but these names will be used in the example code.
Bijschrift toevoegen

2) Create Synapse
Create a Synapse Data Warehouse including a SQL Server to host it. Our SQL Server is called 'bitoolssynapseserver' and our Synapse SQL Pool (data warehouse) is called 'synapsedwh'. Again choose your own names and change those in the example code below.
Synapse SQL Pool (data warehouse)

3) Register SQL Server in AD
Next step is to register the SQL Server that hosts your Synapse DWH in the Active Directory. This will allow you to find your SQL Server in the next step as a Managed Identity. At the moment of writing this needs to be done via PowerShell and cannot be done via the portal.

We will be using Cloud Shell (PowerShell in the portal), but you can also use PowerShell (ISE) on your Windows device but then you have to execute two extra commands (login and select subscription).

  • Click on the Cloud Shell icon in the upper right corner (next to the searchbox). This will start PowerShell in the portal. If this is the first time using it you first need to connect it to an Azure Storage Account.
  • Then execute the Set-AzSqlServer command. The first parameter is the resource group where SQL Server is located. The second parameter is the name of SQL Server (without and the last parameter will assign the Managed Identity.
# PowerShell
Set-AzSqlServer -ResourceGroupName "Joost_van_Rossum" -ServerName "bitoolssynapseserver" -AssignIdentity
Register SQL Server as Managed Identity

If you are using PowerShell on your Windows device instead of Cloud Shell then use this code
# PowerShell
# Login to Azure (popup will appear)

# Select your subscription
Select-AzSubscription -SubscriptionId "2c67b23a-4ba2-4273-bc82-274a743b43af"

# Assign Managed Identity
Set-AzSqlServer -ResourceGroupName "Joost_van_Rossum" -ServerName "bitoolssynapseserver" -AssignIdentity

4) Storage Blob Data Contributor
Now it's time to give your SQL Server access to the Azure Storage Account. The role we need for this according the documentation is 'Storage Blob Data Contributor', but I also tested it with 'Storage Blob Data Reader' and that works fine as well (since we are only reading data). Note: You need to be owner of the resource (group) to delegate access to others.
  • Go to your Storage Account from step 1
  • Click on Access control (IAM) in the left menu
  • Click on the + Add icon and choose Add role assignment
  • In the Role drop down select 'Storage Blob Data Contributor'
  • Leave the Assign access to drop down unchanged
  • In the Select box start typing the name of your SQL Server
  • Select your SQL Server and click on the Save button
Deligate Access to Managed Identity of SQL Server

5) Master Key
We are finished in the Azure portal and now its time to start with the actual PolyBase code. Start SQL Server Managed Studio (SSMS), but make sure your Synapse is not paused.

First step is to create a master key to encrypt any secrets, but only if you do not already have one (although we will not use any secrets). You can check that in the table sys.symmetric_keys. If a row exists where the symmetric_key_id column is 101 (or the name column is '##MS_DatabaseMasterKey##') then you already have a master key. Otherwise we need to create one. For Synapse a masterkey password is optional. For this example we will not use the password.
--Master key
IF NOT EXISTS (SELECT * FROM sys.symmetric_keys WHERE symmetric_key_id = 101)
    PRINT 'Creating Master Key'
    PRINT 'Master Key already exists'

6) Credentials
Next step is to create a credential which will be used to access the Storage Account. For a Managed Identity you don't use secrets:
    IDENTITY = 'Managed Service Identity'

Give the credential a descriptive name so that you know where it is used for. You can find all credentials in the table sys.database_credentials:
--Find all credential
SELECT * FROM sys.database_credentials

7) External data source
With the credential from the previous step we will create an External data source that points to the Storage Account and container where your file is located. Execute the code below where:
  • TYPE = HADOOP (because PolyBase uses the Hadoop APIs to access the container)
  • LOCATION = the connection string to the container in your Storage Account starting with abfss.
  • CREDENTIAL = the name of the credentials created in the previous step.
--Create External Data Source
CREATE EXTERNAL DATA SOURCE bitoolsstorage_abfss
    LOCATION = 'abfss://',
    CREDENTIAL = bitools_msi

Give the external source a descriptive name so that you know where it is used for. You can find all external data sources in the table sys.external_data_sources:
--Find all external sources
SELECT * FROM sys.external_data_sources

Notice that the filename or subfolder is not mentioned in the External Data Source. This is done in the External Table. This allows you to use multiple files from the same folder as External Tables.

8) External File format
Now we need to describe the format used in the source file. In our case we have a comma delimited file. You can also use this file format to supply the date format, compression type or encoding.
--Create External Data Source
    FORMAT_TYPE = DelimitedText,

Give the format a descriptive name so that you know where it is used for. You can find all external file formats in the table sys.external_file_formats:
--Find all external file formats
SELECT * FROM sys.external_file_formats

9) External Table
The last step before we can start quering, is creating the external table. In this create table script you need to specify all columns, datatypes and the filename that you want to read. The filename starts with a forward slash. You also need the datasource from step 7 and the file format from step 8.
--Create External table
CREATE EXTERNAL TABLE dbo.sensordata (
    [Date] nvarchar(50) NOT NULL,
    [temp] INT NOT NULL,
    [hmdt] INT NOT NULL,
    [location] nvarchar(50) NOT NULL
    DATA_SOURCE=bitoolsstorage_abfss, -- from step 7
    FILE_FORMAT=TextFile              -- from step 8
PolyBase does not like columnname headers. It will handle it like a regular data row and throw an error when the datatype doesn't match. There is a little workaround for this with REJECT_TYPE and REJECT_VALUE. However this only works when the datatype of the header is different than the datatypes of the actual rows. Otherwise you have to filter the header row in a subsequent step.
--Create External table with header
CREATE EXTERNAL TABLE dbo.sensordata2 (
    [Date] DateTime2(7) NOT NULL,
    [temp] INT NOT NULL,
    [hmdt] INT NOT NULL,
    [location] nvarchar(50) NOT NULL
    REJECT_TYPE = VALUE, -- Reject rows with wrong datatypes
    REJECT_VALUE = 1     -- Allow 1 failure (the header)
You can find all external tables in the table sys.external_tables.
--Find all external tables
SELECT * FROM sys.external_tables
However you can also find the External Table (/the External Data Source/the External File Format) in the Object Explorer of SSMS.
SSMS Object Explorer

10) Query external table
Now you can query the external table like any other regular table. However the table is read-only so you can not delete, update or insert records. If you update the source file then the data in this external table also changes instantly because the file is used to get the data.
SELECT count(*) FROM dbo.sensordata;
SELECT * FROM dbo.sensordata;
Quering an external table

In this post you learned how to give the Managed Identity of SQL Server access to your Storage Account. This saves you some maintenance for the secrets. And you learned how to use PolyBase to read files from that Storage Account using the Managed Identity.

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