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• Database Owner Conundrum

  - by Johnm

  Have you ever restored a database from a production environment on Server A into a development environment on Server B and had some items, such as Service Broker, mysteriously cease functioning? You might want to consider reviewing the database owner property of the database. The Scenario Recently, I was developing some messaging functionality that utilized the Service Broker feature of SQL Server in a development environment. Within the instance of the development environment resided two databases: One was a restored version of a production database that we will call "RestoreDB". The second database was a brand new database that has yet to exist in the production environment that we will call "DevDB". The goal is to setup a communication path between RestoreDB and DevDB that will later be implemented into the production database. After implementing all of the Service Broker objects that are required to communicate within a database as well as between two databases on the same instance I found myself a bit confounded. My testing was showing that the communication was successful when it was occurring internally within DevDB; but the communication between RestoreDB and DevDB did not appear to be working. Profiler to the rescue After carefully reviewing my code for any misspellings, missing commas or any other minor items that might be a syntactical cause of failure, I decided to launch Profiler to aid in the troubleshooting. After simulating the cross database messaging, I noticed the following error appearing in Profiler: An exception occurred while enqueueing a message in the target queue. Error: 33009, State: 2. The database owner SID recorded in the master database differs from the database owner SID recorded in database '[Database Name Here]'. You should correct this situation by resetting the owner of database '[Database Name Here]' using the ALTER AUTHORIZATION statement. Now, this error message is a helpful one. Not only does it identify the issue in plain language, it also provides a potential solution. An execution of the following query that utilizes the catalog view sys.transmission_queue revealed the same error message for each communication attempt: SELECT     * FROM        sys.transmission_queue; Seeing the situation as a learning opportunity I dove a bit deeper. Reviewing the database properties  The owner of a specific database can be easily viewed by right-clicking the database in SQL Server Management Studio and selecting the "properties" option. The owner is listed on the "General" page of the properties screen. In my scenario, the database in the production server was created by Frank the DBA; therefore his server login appeared as the owner: "ServerName\Frank". While this is interesting information, it certainly doesn't tell me much in regard to the SID (security identifier) and its existence, or lack thereof, in the master database as the error suggested. I pulled together the following query to gather more interesting information: SELECT     a.name     , a.owner_sid     , b.sid     , b.name     , b.type_desc FROM        master.sys.databases a     LEFT OUTER JOIN master.sys.server_principals b         ON a.owner_sid = b.sid WHERE     a.name not in ('master','tempdb','model','msdb'); This query also helped identify how many other user databases in the instance were experiencing the same issue. In this scenario, I saw that there were no matching SIDs in server_principals to the owner SID for my database. What login should be used as the database owner instead of Frank's? The system stored procedure sp_helplogins will provide a list of the valid logins that can be used. Here is an example of its use, revealing all available logins: EXEC sp_helplogins;  Fixing a hole The error message stated that the recommended solution was to execute the ALTER AUTHORIZATION statement. The full statement for this scenario would appear as follows: ALTER AUTHORIZATION ON DATABASE:: [Database Name Here] TO [Login Name]; Another option is to execute the following statement using the sp_changedbowner system stored procedure; but please keep in mind that this stored procedure has been deprecated and will likely disappear in future versions of SQL Server: EXEC dbo.sp_changedbowner @loginname = [Login Name]; .And They Lived Happily Ever After Upon changing the database owner to an existing login and simulating the inner and cross database messaging the errors have ceased. More importantly, all messages sent through this feature now successfully complete their journey. I have added the ownership change to my restoration script for the development environment.

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• Essential Tools for the WPF Novice

  When Michael sets out to do something, there are no half-measures; So when he set out to learn WPF, we all stand to benefit from the thorough research that he put into the task. He wondered what utility applications could assist programming in WPF. Here are the fruits of all his work.

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• SQL Server Optimizer Malfunction?

  - by Tony Davis

  There was a sharp intake of breath from the audience when Adam Machanic declared the SQL Server optimizer to be essentially "stuck in 1997". It was during his fascinating "Query Tuning Mastery: Manhandling Parallelism" session at the recent PASS SQL Summit. Paraphrasing somewhat, Adam (blog | @AdamMachanic) offered a convincing argument that the optimizer often delivers flawed plans based on assumptions that are no longer valid with today’s hardware. In 1997, when Microsoft engineers re-designed the database engine for SQL Server 7.0, SQL Server got its initial implementation of a cost-based optimizer. Up to SQL Server 2000, the developer often had to deploy a steady stream of hints in SQL statements to combat the occasionally wilful plan choices made by the optimizer. However, with each successive release, the optimizer has evolved and improved in its decision-making. It is still prone to the occasional stumble when we tackle difficult problems, join large numbers of tables, perform complex aggregations, and so on, but for most of us, most of the time, the optimizer purrs along efficiently in the background. Adam, however, challenged further any assumption that the current optimizer is competent at providing the most efficient plans for our more complex analytical queries, and in particular of offering up correctly parallelized plans. He painted a picture of a present where complex analytical queries have become ever more prevalent; where disk IO is ever faster so that reads from disk come into buffer cache faster than ever; where the improving RAM-to-data ratio means that we have a better chance of finding our data in cache. Most importantly, we have more CPUs at our disposal than ever before. To get these queries to perform, we not only need to have the right indexes, but also to be able to split the data up into subsets and spread its processing evenly across all these available CPUs. Improvements such as support for ColumnStore indexes are taking things in the right direction, but, unfortunately, deficiencies in the current Optimizer mean that SQL Server is yet to be able to exploit properly all those extra CPUs. Adam’s contention was that the current optimizer uses essentially the same costing model for many of its core operations as it did back in the days of SQL Server 7, based on assumptions that are no longer valid. One example he gave was a "slow disk" bias that may have been valid back in 1997 but certainly is not on modern disk systems. Essentially, the optimizer assesses the relative cost of serial versus parallel plans based on the assumption that there is no IO cost benefit from parallelization, only CPU. It assumes that a single request will saturate the IO channel, and so a query would not run any faster if we parallelized IO because the disk system simply wouldn’t be able to handle the extra pressure. As such, the optimizer often decides that a serial plan is lower cost, often in cases where a parallel plan would improve performance dramatically. It was challenging and thought provoking stuff, as were his techniques for driving parallelism through query logic based on subsets of rows that define the "grain" of the query. I highly recommend you catch the session if you missed it. I’m interested to hear though, when and how often people feel the force of the optimizer’s shortcomings. Barring mistakes, such as stale statistics, how often do you feel the Optimizer fails to find the plan you think it should, and what are the most common causes? Is it fighting to induce it toward parallelism? Combating unexpected plans, arising from table partitioning? Something altogether more prosaic? Cheers, Tony.

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• Showplan Operator of the Week - Concatenation

  Fabiano continues in his mission to describe, one week at a time, all the major Showplan Operators used by SQL Server's Query Optimiser to build the Query Plan. This week he gets the Concatenation operator .

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• A Deep Dive into Transport Queues (Part 2)

  Johan Veldhuis completes his 'Deep Dive' by plunging even deeper into the mysteries of MS Exchange's Transport queues that are used to temporarily store messages which are waiting until they are passed through to the next stage, and explains how to change the way they work via configuration settings.

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• I'm blogging again, and about time too

  - by fatherjack

  No, seriously, this one is about time. I recently had an issue in a work database where a query was giving random results, sometimes the query would return a row and other times it wouldn't. There was quite a bit of work distilling the query down to find the reason for this and I'll try to explain by demonstrating what was happening by using some sample data in a table with rather a contrived use case. Let's assume we have a table that is designed to have a start and end date for something, maybe...(read more)

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• Microsoft Access as a Weapon of War

  - by Damon Armstrong

  A while ago (probably a decade ago, actually) I saw a report on a tracking system maintained by a U.S. Army artillery control unit.  This system was capable of maintaining a bearing on various units in the field to help avoid friendly fire.  I consider the U.S. Army to be the most technologically advanced fighting force on Earth, but to my terror I saw something on the title bar of an application displayed on a laptop behind one of the soldiers they were interviewing: Tracking.mdb Oh yes.  Microsoft Office Suite had made it onto the battlefield.  My hope is that it was just running as a front-end for a more proficient database (no offense Access people), or that the soldier was tracking something else like KP duty or fantasy football scores.  But I could also see the corporate equivalent of a pointy-haired boss walking into a cube and asking someone who had piddled with Access to build a database for HR forms.  Except this pointy-haired boss would have been a general, the cube would have been a tank, and the HR forms would have been targets that, if something went amiss, would have been hit by a 500lb artillery round. Hope that solider could write a good query

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• Microsoft Access as a Weapon of War

  - by Damon

  A while ago (probably a decade ago, actually) I saw a report on a tracking system maintained by a U.S. Army artillery control unit.  This system was capable of maintaining a bearing on various units in the field to help avoid friendly fire.  I consider the U.S. Army to be the most technologically advanced fighting force on Earth, but to my terror I saw something on the title bar of an application displayed on a laptop behind one of the soldiers they were interviewing: Tracking.mdb Oh yes.  Microsoft Office Suite had made it onto the battlefield.  My hope is that it was just running as a front-end for a more proficient database (no offense Access people), or that the soldier was tracking something else like KP duty or fantasy football scores.  But I could also see the corporate equivalent of a pointy-haired boss walking into a cube and asking someone who had piddled with Access to build a database for HR forms.  Except this pointy-haired boss would have been a general, the cube would have been a tank, and the HR forms would have been targets that, if something went amiss, would have been hit by a 500lb artillery round. Hope that solider could write a good query :)

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• A weekend with the Samsung Galaxy Tab

  - by Richard Mitchell

  This weekend I took one of the Samsung Galaxy Tabs we have lying around the office here home to see how I got on with it as I've been thinking of buying one. Initial impressions The look and feel of the Tab is quite nice. It's a lot smaller than an iPad but that is no bad thing as I imagine they are targeted at different markets. The Tab fits into my inside coat pocket nicely and doesn't feel like it's weighing me down too much. Connecting up the Tab to the network at work was fine, typing in...(read more)

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• No Rest for the Virtuous

  - by Chris Massey

  It has been an impressively brutal month in terms of security breaches, and across a whole range of fronts. The "Cablegate" leaks, courtesy of Wikileaks, appear to be in a league of their own. The "Operation Payback" DDoS attacks against PayPal, MasterCard and Visa (not to mention the less successful attack against Amazon) are equally impressive. Even more recently, the Gawker Media Network was subjected to a relatively sophisticated hack attack by Gnosis, with the hackers gaining access to some...(read more)

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• Implementing Cluster Continuous Replication, Part 3

  Cluster continuous replication (CCR) uses log shipping and failover to provide a more resilient email system with faster recovery. Once it is installed, a clustered server requires different management routines. These are done either with a GUI tool, The Failover Cluster Management Console, or the Exchange Management Shell. You can use Powershell as well for some tasks. Confused? Not for long, since Brien Posey is once more here to help.

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• Online accounts advanced setting with Empathy (13.10)

  - by uruloke

  the new online accounts doesn't have the advanced settings as the empathy accounts had. How do i change the google server to connect to? i read here: https://wiki.gnome.org/Empathy/FAQ I can't connect to my Google Talk account Your router is probably blocking DNS SRV requests. If possible you should try to fix it. If you can't, the easiest work around is to set "talk.google.com" in the "Server" field of the advanced section of the account. So i think this might fix my problem, or maybe just an option to shift the port it connects to. and is there anyone that knows how to use join any IRC channels with Empathy? i have installed the plugin, but i don't know how to join a channel.

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• Antenna Aligner part 1: In the beginning.

  - by Chris George

  Picture the scene, it's 9pm, I'm in my caravan (yes I know, I've heard all the jokes!) with my family and I'm trying to tune the tv by moving the aerial, retuning, moving the aerial again, retuning... 45 mins and much cursing later I succeed. Surely there must be an easier way than this? Aha, an app; there must be an app for that? So I search in the AppStore for such an app, but curiously drew a blank. Then the seeds of the idea started to grow. I can code, I work in a software house with lots of very clever people, surely I can make an app that points to the nearest digital tv transmitter! Not having looked into app development before, I investigated how one goes about making an iPhone app and was quickly greeted by a now familiar answer "Buy a mac!". That was not an option for many reasons, mostly wife related! My dreams were starting to fade until one of my colleagues pointed out that within Red Gate, the very company I work for, there was on-going development on a piece of software that would allow me to write an app using Visual Studio on a Windows machine, Nomad! Once I signed up for the beta program I got to work learning the Jquery mobile / Phonegap framework. Within a couple of hours I had written (in Visual Studio), built in the cloud (using Nomad) and published (via TestFlight) my first iPhone app onto my iPhone ! It didn't do much, but it was a step in the right direction. To be continued...

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• Subterranean IL: Exception handling 1

  - by Simon Cooper

  Today, I'll be starting a look at the Structured Exception Handling mechanism within the CLR. Exception handling is quite a complicated business, and, as a result, the rules governing exception handling clauses in IL are quite strict; you need to be careful when writing exception clauses in IL. Exception handlers Exception handlers are specified using a .try clause within a method definition. .try <TryStartLabel> to <TryEndLabel> <HandlerType> handler <HandlerStartLabel> to <HandlerEndLabel> As an example, a basic try/catch block would be specified like so: TryBlockStart: // ... leave.s CatchBlockEndTryBlockEnd:CatchBlockStart: // at the start of a catch block, the exception thrown is on the stack callvirt instance string [mscorlib]System.Object::ToString() call void [mscorlib]System.Console::WriteLine(string) leave.s CatchBlockEnd CatchBlockEnd: // method code continues... .try TryBlockStart to TryBlockEnd catch [mscorlib]System.Exception handler CatchBlockStart to CatchBlockEnd There are four different types of handler that can be specified: catch <TypeToken> This is the standard exception catch clause; you specify the object type that you want to catch (for example, [mscorlib]System.ArgumentException). Any object can be thrown as an exception, although Microsoft recommend that only classes derived from System.Exception are thrown as exceptions. filter <FilterLabel> A filter block allows you to provide custom logic to determine if a handler block should be run. This functionality is exposed in VB, but not in C#. finally A finally block executes when the try block exits, regardless of whether an exception was thrown or not. fault This is similar to a finally block, but a fault block executes only if an exception was thrown. This is not exposed in VB or C#. You can specify multiple catch or filter handling blocks in each .try, but fault and finally handlers must have their own .try clause. We'll look into why this is in later posts. Scoped exception handlers The .try syntax is quite tricky to use; it requires multiple labels, and you've got to be careful to keep separate the different exception handling sections. However, starting from .NET 2, IL allows you to use scope blocks to specify exception handlers instead. Using this syntax, the example above can be written like so: .try { // ... leave.s EndSEH}catch [mscorlib]System.Exception { callvirt instance string [mscorlib]System.Object::ToString() call void [mscorlib]System.Console::WriteLine(string) leave.s EndSEH}EndSEH:// method code continues... As you can see, this is much easier to write (and read!) than a stand-alone .try clause. Next time, I'll be looking at some of the restrictions imposed by SEH on control flow, and how the C# compiler generated exception handling clauses.

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• TortoiseSVN and Subversion Cookbook Part 3: In, Out, and Around

  Subversion doesn't have to be difficult, especially if you have Michael Sorens's guide at hand. After dealing in previous articles with checkouts and commits in Subversion, and covering the various file-manipulation operations that are required for Subversion, Michael now deals in this article with file macro-management, the operations such as putting things in, and taking things out, that deal with repositories and projects.

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• Simple Query tuning with STATISTICS IO and Execution plans

  A great deal can be gleaned from the use of the STATISTICS IO and the execution plan, when you are checking that a query is performing properly. Josef Richberg, the current holder of the 'Exceptional DBA' award, explains how an apparently draconian IT policy turns out to be a useful ways of ensuring that Stored Procedures are carefully checked for performance before they are released

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• TSQL formatting - a sure fire way to start a conversation.

  - by fatherjack

  There are probably as many opinions on ways to format code as there are people writing code and I am not here to say that any one is better than any other. Well, that isn't true. I am here to say that one way is better than another but this isn't a matter of preference or personal taste, this is an example of where sloppy formatting can cause TSQL to weird and whacky things but following some simple methods can make your code more reliable and more robust when . Take these two pieces of code, ready...(read more)

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• Antenna Aligner Part 3: Kaspersky

  - by Chris George

  Quick one today. Since starting this project, I've been encountering times where Nomad fails to build my app. It would then take repeated attempts at building to then see a build go through successfully. Rob, who works on Nomad at Red Gate, investigated this and it showed that certain parts of the message required to trigger the 'cloud build' were not getting through to the Nomad app, causing the HTTP connection to stall until timeout. After much scratching heads, it turns out that the Kaspersky Internet Security system I have installed on my laptop at home, was being very aggressive and was causing the problem. Perhaps it's trying to protect me from myself? Anyway, we came up with an interim solution why the Nomad guys investigate with Kaspersky by setting Visual Studio to be a trusted application with the Kaspersky settings and setting it to not scan network traffic. Hey presto! This worked and I have not had a single build problem since (other than losing internet connection, or that embarrassing moment when you blame everyone else then realise you've accidentally switched off your wireless on the laptop).

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• AsyncBridge? Async on .NET 4.0 using VS11

  - by Alex.Davies

  I've just found something quite cool. It's a code snippet that lets you use the real VS 11 C#5 compiler to write code that uses the async and await keywords, but to target .NET 4.0. It was published by Daniel Grunwald (from SharpDevelop).That means I can stop using the Async CTP for VS2010, which is not at all supported anymore, and a pain to install if you have windows updates turned on. Obviously I couldn't ask all my users to install .NET 4.5 beta, but .NET Demon is a VS 2010 extension, so we already have .NET 4.0. At the time of writing, VS11 is in beta still, but hopefully it's stable enough for my team to use!I would have written the code myself, but I had the wrong impression that the C# 5 beta compiler only looked in mscorlib for the helper classes it needs to implement async methods. Turns out you can provide them yourself. You can get the code here: https://gist.github.com/1961087You just add it to your project, and the compiler will apparently pick it up and use it to implement async/await. I'm at my parents' place for Easter without access to a machine with VS 11 to try it out. Let me know whether you get it to work!This reminds me of LINQBridge, which let us use C# 3 LINQ, but only require .NET 2. We should stick up a webpage to explain, with a nice easy dll, put it in nuget, and call it AsyncBridge.If you were really enthusiastic, you could re-implement the skeleton of the Task Parallel Library against .NET 2 to use async/await without even requiring .NET 4. Our usage stats suggest that practically everyone that uses Red Gate tools already has .NET 4 installed though, so I don't think I'll go to the effort.

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• Glimpse: Open Source Web Development

  - by Elizabeth Ayer

  We’re delighted to announce that Red Gate will be backing Glimpse! For those of you who aren’t familiar with the project, Glimpse is an open source tool which does for the server what Firebug does for the client. It’s been in beta for the last year, and we’re very excited to give Glimpse the support and dedicated effort needed to take it to a v1 and beyond. Glimpse’s founders (Nik Molnar and Anthony van der Hoorn) have joined Red Gate, and they’re just as excited as we are about the opportunities that active development of Glimpse will bring. They will continue to write code, support the community and drive the project forward (as they’ve done since its inception). With full-time attention on growing Glimpse and its community, users and developers can expect the project to accelerate, with frequent releases of new functionality. Red Gate is excited about its first major involvement with open source. You may well be wondering, though, why Red Gate is doing this. Glimpse dovetails beautifully with Red Gate’s .NET tools, which makes Glimpse an ideal framework for plugging in advanced, paid-for functionality (like performance analysis) the way web developers want to see it. As a means to this end, we will contribute to the Glimpse open source project in order to broaden its adoption and delight web developers. Since bringing in .NET Reflector in 2008, we’ve learnt sharp lessons from the community about the right and wrong ways to engage with developers, not to mention the enduring value of free. Glimpse further shows what the .NET community can achieve through open source collaboration, and we’re looking forward to working with the Glimpse community to make something enduring and awesome. Nik and Anthony, themselves passionate advocates of community-driven software, will continue to control the Glimpse project, steering it to best meet the needs of its users and contributors. If you have any questions or queries about Glimpse, or Red Gate’s involvement in the project, please tweet with the #glimpse hashtag, contact us at Red Gate on [email protected], or post to the Glimpse Development Forum on Google Groups.

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• Know your Data Lineage

  - by Simon Elliston Ball

  An academic paper without the footnotes isn’t an academic paper. Journalists wouldn’t base a news article on facts that they can’t verify. So why would anyone publish reports without being able to say where the data has come from and be confident of its quality, in other words, without knowing its lineage. (sometimes referred to as ‘provenance’ or ‘pedigree’) The number and variety of data sources, both traditional and new, increases inexorably. Data comes clean or dirty, processed or raw, unimpeachable or entirely fabricated. On its journey to our report, from its source, the data can travel through a network of interconnected pipes, passing through numerous distinct systems, each managed by different people. At each point along the pipeline, it can be changed, filtered, aggregated and combined. When the data finally emerges, how can we be sure that it is right? How can we be certain that no part of the data collection was based on incorrect assumptions, that key data points haven’t been left out, or that the sources are good? Even when we’re using data science to give us an approximate or probable answer, we cannot have any confidence in the results without confidence in the data from which it came. You need to know what has been done to your data, where it came from, and who is responsible for each stage of the analysis. This information represents your data lineage; it is your stack-trace. If you’re an analyst, suspicious of a number, it tells you why the number is there and how it got there. If you’re a developer, working on a pipeline, it provides the context you need to track down the bug. If you’re a manager, or an auditor, it lets you know the right things are being done. Lineage tracking is part of good data governance. Most audit and lineage systems require you to buy into their whole structure. If you are using Hadoop for your data storage and processing, then tools like Falcon allow you to track lineage, as long as you are using Falcon to write and run the pipeline. It can mean learning a new way of running your jobs (or using some sort of proxy), and even a distinct way of writing your queries. Other Hadoop tools provide a lot of operational and audit information, spread throughout the many logs produced by Hive, Sqoop, MapReduce and all the various moving parts that make up the eco-system. To get a full picture of what’s going on in your Hadoop system you need to capture both Falcon lineage and the data-exhaust of other tools that Falcon can’t orchestrate. However, the problem is bigger even that that. Often, Hadoop is just one piece in a larger processing workflow. The next step of the challenge is how you bind together the lineage metadata describing what happened before and after Hadoop, where ‘after’ could be  a data analysis environment like R, an application, or even directly into an end-user tool such as Tableau or Excel. One possibility is to push as much as you can of your key analytics into Hadoop, but would you give up the power, and familiarity of your existing tools in return for a reliable way of tracking lineage? Lineage and auditing should work consistently, automatically and quietly, allowing users to access their data with any tool they require to use. The real solution, therefore, is to create a consistent method by which to bring lineage data from these data various disparate sources into the data analysis platform that you use, rather than being forced to use the tool that manages the pipeline for the lineage and a different tool for the data analysis. The key is to keep your logs, keep your audit data, from every source, bring them together and use the data analysis tools to trace the paths from raw data to the answer that data analysis provides.

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• Subterranean IL: The ThreadLocal type

  - by Simon Cooper

  I came across ThreadLocal<T> while I was researching ConcurrentBag. To look at it, it doesn't really make much sense. What's all those extra Cn classes doing in there? Why is there a GenericHolder<T,U,V,W> class? What's going on? However, digging deeper, it's a rather ingenious solution to a tricky problem. Thread statics Declaring that a variable is thread static, that is, values assigned and read from the field is specific to the thread doing the reading, is quite easy in .NET: [ThreadStatic] private static string s_ThreadStaticField; ThreadStaticAttribute is not a pseudo-custom attribute; it is compiled as a normal attribute, but the CLR has in-built magic, activated by that attribute, to redirect accesses to the field based on the executing thread's identity. TheadStaticAttribute provides a simple solution when you want to use a single field as thread-static. What if you want to create an arbitary number of thread static variables at runtime? Thread-static fields can only be declared, and are fixed, at compile time. Prior to .NET 4, you only had one solution - thread local data slots. This is a lesser-known function of Thread that has existed since .NET 1.1: LocalDataStoreSlot threadSlot = Thread.AllocateNamedDataSlot("slot1"); string value = "foo"; Thread.SetData(threadSlot, value); string gettedValue = (string)Thread.GetData(threadSlot); Each instance of LocalStoreDataSlot mediates access to a single slot, and each slot acts like a separate thread-static field. As you can see, using thread data slots is quite cumbersome. You need to keep track of LocalDataStoreSlot objects, it's not obvious how instances of LocalDataStoreSlot correspond to individual thread-static variables, and it's not type safe. It's also relatively slow and complicated; the internal implementation consists of a whole series of classes hanging off a single thread-static field in Thread itself, using various arrays, lists, and locks for synchronization. ThreadLocal<T> is far simpler and easier to use. ThreadLocal ThreadLocal provides an abstraction around thread-static fields that allows it to be used just like any other class; it can be used as a replacement for a thread-static field, it can be used in a List<ThreadLocal<T>>, you can create as many as you need at runtime. So what does it do? It can't just have an instance-specific thread-static field, because thread-static fields have to be declared as static, and so shared between all instances of the declaring type. There's something else going on here. The values stored in instances of ThreadLocal<T> are stored in instantiations of the GenericHolder<T,U,V,W> class, which contains a single ThreadStatic field (s_value) to store the actual value. This class is then instantiated with various combinations of the Cn types for generic arguments. In .NET, each separate instantiation of a generic type has its own static state. For example, GenericHolder<int,C0,C1,C2> has a completely separate s_value field to GenericHolder<int,C1,C14,C1>. This feature is (ab)used by ThreadLocal to emulate instance thread-static fields. Every time an instance of ThreadLocal is constructed, it is assigned a unique number from the static s_currentTypeId field using Interlocked.Increment, in the FindNextTypeIndex method. The hexadecimal representation of that number then defines the specific Cn types that instantiates the GenericHolder class. That instantiation is therefore 'owned' by that instance of ThreadLocal. This gives each instance of ThreadLocal its own ThreadStatic field through a specific unique instantiation of the GenericHolder class. Although GenericHolder has four type variables, the first one is always instantiated to the type stored in the ThreadLocal<T>. This gives three free type variables, each of which can be instantiated to one of 16 types (C0 to C15). This puts an upper limit of 4096 (163) on the number of ThreadLocal<T> instances that can be created for each value of T. That is, there can be a maximum of 4096 instances of ThreadLocal<string>, and separately a maximum of 4096 instances of ThreadLocal<object>, etc. However, there is an upper limit of 16384 enforced on the total number of ThreadLocal instances in the AppDomain. This is to stop too much memory being used by thousands of instantiations of GenericHolder<T,U,V,W>, as once a type is loaded into an AppDomain it cannot be unloaded, and will continue to sit there taking up memory until the AppDomain is unloaded. The total number of ThreadLocal instances created is tracked by the ThreadLocalGlobalCounter class. So what happens when either limit is reached? Firstly, to try and stop this limit being reached, it recycles GenericHolder type indexes of ThreadLocal instances that get disposed using the s_availableIndices concurrent stack. This allows GenericHolder instantiations of disposed ThreadLocal instances to be re-used. But if there aren't any available instantiations, then ThreadLocal falls back on a standard thread local slot using TLSHolder. This makes it very important to dispose of your ThreadLocal instances if you'll be using lots of them, so the type instantiations can be recycled. The previous way of creating arbitary thread-static variables, thread data slots, was slow, clunky, and hard to use. In comparison, ThreadLocal can be used just like any other type, and each instance appears from the outside to be a non-static thread-static variable. It does this by using the CLR type system to assign each instance of ThreadLocal its own instantiated type containing a thread-static field, and so delegating a lot of the bookkeeping that thread data slots had to do to the CLR type system itself! That's a very clever use of the CLR type system.

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• Cheating on Technical Debt

  - by Tony Davis

  One bad practice guaranteed to cause dismay amongst your colleagues is passing on technical debt without full disclosure. There could only be two reasons for this. Either the developer or DBA didn’t know the difference between good and bad practices, or concealed the debt. Neither reflects well on their professional competence. Technical debt, or code debt, is a convenient term to cover all the compromises between the ideal solution and the actual solution, reflecting the reality of the pressures of commercial coding. The one time you’re guaranteed to hear one developer, or DBA, pass judgment on another is when he or she inherits their project, and is surprised by the amount of technical debt left lying around in the form of inelegant architecture, incomplete tests, confusing interface design, no documentation, and so on. It is often expedient for a Project Manager to ignore the build-up of technical debt, the cut corners, not-quite-finished features and rushed designs that mean progress is satisfyingly rapid in the short term. It’s far less satisfying for the poor person who inherits the code. Nothing sends a colder chill down the spine than the dawning realization that you’ve inherited a system crippled with performance and functional issues that will take months of pain to fix before you can even begin to make progress on any of the planned new features. It’s often hard to justify this ‘debt paying’ time to the project owners and managers. It just looks as if you are making no progress, in marked contrast to your predecessor. There can be many good reasons for allowing technical debt to build up, at least in the short term. Often, rapid prototyping is essential, there is a temporary shortfall in test resources, or the domain knowledge is incomplete. It may be necessary to hit a specific deadline with a prototype, or proof-of-concept, to explore a possible market opportunity, with planned iterations and refactoring to follow later. However, it is a crime for a developer to build up technical debt without making this clear to the project participants. He or she needs to record it explicitly. A design compromise made in to order to hit a deadline, be it an outright hack, or a decision made without time for rigorous investigation and testing, needs to be documented with the same rigor that one tracks a bug. What’s the best way to do this? Ideally, we’d have some kind of objective assessment of the level of technical debt in a software project, although that smacks of Science Fiction even as I write it. I’d be interested of hear of any methods you’ve used, but I’m sure most teams have to rely simply on the integrity of their colleagues and the clear perceptions of the project manager… Cheers, Tony.

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• From 0 to MVP in 4 weeks

  - by fatherjack

  You may know from my previous posts that I have just started a local SQL Server User Group. 3 weeks ago there was no such group within 100 miles and then we had a meeting. Now, in eight days time, there is going to be a second meeting and I am very excited to be able to say that we will be having an MVP speaker for one of the sessions. Aaron Nelson (Blog|Twitter) made an incredibly generous offer of speaking for us on using PowerShell with SQL Server and I didn't hang around before I said "Yes...(read more)

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• Just interviewed, turned down, now got an email asking to chat with recruiter. No response. What should I do? [closed]

  - by Lambert

  I was turned down after two interviews by a prominent company for an internship, and only a couple days later, I was asked when I had 10-15 minutes to chat today. Of course, I loved to, so I emailed within just 10 minutes of their email and let them know what times I was available at, and asked them when the best time should be, and if I should go somewhere or expect a phone call. No reply has come from them since yesterday afternoon, the recruiter wanted to talk to me today. I don't want to lose this opportunity, but I have no way to contact the recruiter other than by email, and the recruiter hasn't responded to my emails from yesterday, even though we were supposed to talk today. What's the best thing I can do (preferably within the next few hours!) to get the job? Is that even probably why she emailed me, or was a different reason likely? Any ideas?

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