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Day: April 25, 2019

Testing an Event-Driven System

Published 2019-04-25 by Kevin Feasel

Andy Chambers takes us through how to test an event-driven system:

Each distinct service has a nice, pure data model with extensive unit tests, but now with new clients (and consequently new requirements) coming thick and fast, the number of these services is rapidly increasing. The testing guardian angel who sometimes visits your thoughts during your morning commute has noticed an increase in the release of bugs that could have been prevented with better integration tests.

Finally after a few incidents in production, and with velocity slowing down due to the deployment pipeline frequently being clogged up by flaky integration tests, you start to think about what you want from your test suite. You set off looking for ideas to make really solid end-to-end tests. You wonder if it’s possible to make them fast. You think about all the things you could do with the time freed up by not having to apply manual data fixes that correct for deploying bad code.

At the end of it all, hopefully you’ll arrive here and learn about the Test Machine.

Check it out. Testing these types of system is certainly possible, but can be a bit difficult because of the additional layers of complexity.

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Monads and Monoids and Functors

Published 2019-04-25 by Kevin Feasel

Anmol Sarna explains the concept of a monad:

In functional programming, a monad is a design pattern that allows structuring programs generically while automating away boilerplate code needed by the program logic.

To simplify the above definition a bit more, We can think of monads as wrappers. You just take an object and wrap it with a monad.

Let’s just be clear on one thing: A Monad is not a class or a trait; Neither is it only dedicated to the Scala language. It is a concept related to functional programming.

This also includes a few examples in Scala.

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CosmosDB Continuation Tokens

Published 2019-04-25 by Kevin Feasel

Hasan Savran walks us through the idea of a continuation token in CosmosDB:

In CosmosDB, TOP option is required and its default value is 100. You can change the default value by sending a different value using the request header “x-ms-max-item-count“. If you have 40000 rows in your Orders table, and run the same query in CosmosDB, you will get 100 rows(documents) rather than 40000 rows(documents). CosmosDB returns all kind of metadata with the data. You can find this metadata in the response headers. One of those responses is, “x-ms-continuation” and it is responsible to display the rest of the rows of your query. If you like to get the next set of results, you can take “x-ms-continuation” value from the response headers and attach it to your next request to get the next set of rows. CosmosDB SDK does this automatically for you. SDK checks for the x-ms-continuation value when you check HasMoreResults property. If this property is true, that means CosmosDB returned a continuation token.

I have fanciful notions of SQL Server offering something similar—think of a grid built from a query. Get the first 50 rows from the result set and store that off in tempdb somewhere, using the “continuation token” (which might just be the full name in tempdb) and auto-trashing after a certain amount of time.

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Window Functions with IGNORE NULLs

Published 2019-04-25 by Kevin Feasel

Lukas Eder walks us through a bit of functionality I wish we had in SQL Server:

On each row, the VALUE column should either contain the actual value, or the “last_value” preceding the current row, ignoring all the nulls. Note that I specifically wrote this requirement using specific English language. We can now translate that sentence directly to SQL:

last_value (t.value) ignore nulls over (order by d.value_date)

Since we have added an ORDER BY clause to the window function, the default frame RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW applies, which colloquially means “all the preceding rows”. (Technically, that’s not accurate. It means all rows with values less than or equal to the value of the current row – see Kim Berg Hansen’s comment)

Only a few database products have this and SQL Server is not one of them.

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Aggregate Pushdown with GROUP BY

Published 2019-04-25 by Kevin Feasel

Paul White takes us through several performance improvements around aggregate pushdown:

SQL Server 2016 introduced serial batch mode processing and aggregate pushdown. When pushdown is successful, aggregation is performed within the Columnstore Scan operator itself, possibly operating directly on compressed data, and taking advantage of SIMD CPU instructions.

The performance improvements possible with aggregate pushdown can be very substantial. The documentation lists some of the conditions required to achieve pushdown, but there are cases where the lack of ‘locally aggregated rows’ cannot be fully explained from those details alone.

This article covers additional factors that affect aggregate pushdown for GROUP BY queries only. Scalar aggregate pushdown (aggregation without a GROUP BY clause), filter pushdown, and expression pushdown may be covered in a future post.

Read the whole thing.

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Orphaned Users in SQL Server

Published 2019-04-25 by Kevin Feasel

Dave Bland walks us through one way to fix an orphaned user:

In my many years of working as a DBA, I have encountered many disabled logins.  However, I have never really encountered what looks to be a disabled database user account.  I didn’t even think it was possible to disable a user account in a SQL Server database.  I checked the user account properties just to makes sure I was correct.  Sure enough, no option to disable a user account. This finally turned out to be a simple case of looks can be deceiving.

You can also use the sp_change_users_login procedure to fix orphaned users.

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Controlling Power BI Visual Visibility

Published 2019-04-25 by Kevin Feasel

Matt Allington shows how we can take one Power BI visual and use it to control the visibility status of another visual:

I have written a few articles in the past that toy with the ideas of changing visibility and text colour based on selection.  I started to wonder if it was possible to make a visual appear (or not) based on a selection from the user.  There is no out of the box way to do that today. It is possible to use bookmarks to show an hide an object, but the user must click a specific button to do this. I want the user to be able to interact with a report and see (or not see) a chart based on some valid selection across the report.  Microsoft is already working on building expression based formatting across the breadth of Power BI however as of now the only item you can change is the header in a chart.

Hopefully this gets better over time.

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