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• Developing Schema Compare for Oracle (Part 5): Query Snapshots

  - by Simon Cooper

  If you've emailed us about a bug you've encountered with the EAP or beta versions of Schema Compare for Oracle, we probably asked you to send us a query snapshot of your databases. Here, I explain what a query snapshot is, and how it helps us fix your bug. Problem 1: Debugging users' bug reports When we started the Schema Compare project, we knew we were going to get problems with users' databases - configurations we hadn't considered, features that weren't installed, unicode issues, wierd dependencies... With SQL Compare, users are generally happy to send us a database backup that we can restore using a single RESTORE DATABASE command on our test servers and immediately reproduce the problem. Oracle, on the other hand, would be a lot more tricky. As Oracle generally has a 1-to-1 mapping between instances and databases, any databases users sent would have to be restored to their own instance. Furthermore, the number of steps required to get a properly working database, and the size of most oracle databases, made it infeasible to ask every customer who came across a bug during our beta program to send us their databases. We also knew that there would be lots of issues with data security that would make it hard to get backups. So we needed an easier way to be able to debug customers issues and sort out what strange schema data Oracle was returning. Problem 2: Test execution time Another issue we knew we would have to solve was the execution time of the tests we would produce for the Schema Compare engine. Our initial prototype showed that querying the data dictionary for schema information was going to be slow (at least 15 seconds per database), and this is generally proportional to the size of the database. If you're running thousands of tests on the same databases, each one registering separate schemas, not only would the tests would take hours and hours to run, but the test servers would be hammered senseless. The solution To solve these, we needed to be able to populate the schema of a database without actually connecting to it. Well, the IDataReader interface is the primary way we read data from an Oracle server. The data dictionary queries we use return their data in terms of simple strings and numbers, which we then process and reconstruct into an object model, and the results of these queries are identical for identical schemas. So, we can record the raw results of the queries once, and then replay these results to construct the same object model as many times as required without needing to actually connect to the original database. This is what query snapshots do. They are binary files containing the raw unprocessed data we get back from the oracle server for all the queries we run on the data dictionary to get schema information. The core of the query snapshot generation takes the results of the IDataReader we get from running queries on Oracle, and passes the row data to a BinaryWriter that writes it straight to a file. The query snapshot can then be replayed to create the same object model; when the results of a specific query is needed by the population code, we can simply read the binary data stored in the file on disk and present it through an IDataReader wrapper. This is far faster than querying the server over the network, and allows us to run tests in a reasonable time. They also allow us to easily debug a customers problem; using a simple snapshot generation program, users can generate a query snapshot that could be sent along with a bug report that we can immediately replay on our machines to let us debug the issue, rather than having to obtain database backups and restore databases to test systems. There are also far fewer problems with data security; query snapshots only contain schema information, which is generally less sensitive than table data. Query snapshots implementation However, actually implementing such a feature did have a couple of 'gotchas' to it. My second blog post detailed the development of the dependencies algorithm we use to ensure we get all the dependencies in the database, and that algorithm uses data from both databases to find all the needed objects - what database you're comparing to affects what objects get populated from both databases. We get information on these additional objects using an appropriate WHERE clause on all the population queries. So, in order to accurately replay the results of querying the live database, the query snapshot needs to be a snapshot of a comparison of two databases, not just populating a single database. Furthermore, although the code population queries (eg querying all_tab_cols to get column information) can simply be passed straight from the IDataReader to the BinaryWriter, we need to hook into and run the live dependencies algorithm while we're creating the snapshot to ensure we get the same WHERE clauses, and the same query results, as if we were populating straight from a live system. We also need to store the results of the dependencies queries themselves, as the resulting dependency graph is stored within the OracleDatabase object that is produced, and is later used to help order actions in synchronization scripts. This is significantly helped by the dependencies algorithm being a deterministic algorithm - given the same input, it will always return the same output. Therefore, when we're replaying a query snapshot, and processing dependency information, we simply have to return the results of the queries in the order we got them from the live database, rather than trying to calculate the contents of all_dependencies on the fly. Query snapshots are a significant feature in Schema Compare that really helps us to debug problems with the tool, as well as making our testers happier. Although not really user-visible, they are very useful to the development team to help us fix bugs in the product much faster than we otherwise would be able to.

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• GDM locale problems

  - by Simón

  I have two problems with GDM on Ubuntu 10.04. The first is with locales. In my system I have defined: $ cat /etc/environment PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games" LANG="es_ES.UTF-8" LANGUAGE="es_ES:es:en_US:en" $ cat /etc/default/locale LANG="es_ES.UTF-8" LANGUAGE="es_ES:es:en_US:en" $ cat /var/lib/locales/supported.d/local es_ES UTF-8 es_ES.UTF-8 UTF-8 en_US UTF-8 en_US.UTF-8 UTF-8 But when I enter in gnome desktop: $ locale LANG=es_ES LANGUAGE=es_ES:es:en_US:en LC_CTYPE="es_ES" LC_NUMERIC="es_ES" LC_TIME="es_ES" LC_COLLATE="es_ES" LC_MONETARY="es_ES" LC_MESSAGES="es_ES" LC_PAPER="es_ES" LC_NAME="es_ES" LC_ADDRESS="es_ES" LC_TELEPHONE="es_ES" LC_MEASUREMENT="es_ES" LC_IDENTIFICATION="es_ES" LC_ALL= I have deleted ~/.dmrc and I have restarted the system but nothing. GDM login screen also doesn't permit change this setting. However, in the text terminals (tty1,...): $ locale LANG=es_ES.UTF-8 LANGUAGE=es_ES:es:en_US:en LC_CTYPE="es_ES.UTF-8" LC_NUMERIC="es_ES.UTF-8" LC_TIME="es_ES.UTF-8" LC_COLLATE="es_ES.UTF-8" LC_MONETARY="es_ES.UTF-8" LC_MESSAGES="es_ES.UTF-8" LC_PAPER="es_ES.UTF-8" LC_NAME="es_ES.UTF-8" LC_ADDRESS="es_ES.UTF-8" LC_TELEPHONE="es_ES.UTF-8" LC_MEASUREMENT="es_ES.UTF-8" LC_IDENTIFICATION="es_ES.UTF-8" LC_ALL= The solution to problem is to edit .drmc file, but I think this isn't the right way. Why doesn't GDM read/apply the system locales? Why don't I see, in GDM login screen, the box to change the locale?

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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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• Group Matchmaking

  - by Simon Kérouack

  Consider different groups(1 or more players) queuing together, we want to make 2 opposing teams containing each the same amount of players while keeping the groups together. At the same time we want to make both teams' average ranking as close as possible. Now also consider we have as a working set the subset of groups currently queuing within a given ranking range. For an example, let's say we have the following groups, ordered by queuing time: Id, playerCount, totalRank, avgRank 0, 3, 126, 42 1, 2, 60, 30 2, 1, 25, 25 3, 2, 80, 40 4, 1, 40, 40 5, 1, 20, 20 6, 3, 150, 50 for this specific subset, the expected output should ideally be: team1: 0, 1 (total: 186) team2: 2, 5, 6 (total: 195) up to now the solution I have been using is to balance out each team by making each team pick the group with highest ranking within the subset turn by turn. The team who picks is the one with the currently lowest average rank unless one is already full. If one team is already full the other team tries to complete itself with groups that would make the rank gap as small as possible. This solution turns out to have issues with frequent edge cases and I'm looking for a better solution, or some fine-tuning that could be made. In most cases, players seems to want teams of 5 people and queue in group of 2. Our average subset when 2 teams of 5 are chosen is made of about 14 players if that may be of any help.

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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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• Mp3s synced on Ubuntu One not showing up under artist or album

  - by Simon

  I have synced approximately 10GB of music with U1 over the last 48 hours. Nearly everything appears OK: the songs are tagged correctly, I can queue and play them. However, with the exception of a few that I uploaded first, all of them don't appear under Artist or Album and I can't use the search to find them. I can only find them under Song. Am I just being impatient and U1 just needs time to index these mp3s, or is something wrong?

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• How to follow object on CatmullRomSplines at constant speed (e.g. train and train carriage)?

  - by Simon

  I have a CatmullRomSpline, and using the very good example at https://github.com/libgdx/libgdx/wiki/Path-interface-%26-Splines I have my object moving at an even pace over the spline. Using a simple train and carriage example, I now want to have the carriage follow the train at the same speed as the train (not jolting along as it does with my code below). This leads into my main questions: How can I make the carriage have the same constant speed as the train and make it non jerky (it has something to do with the derivative I think, I don't understand how that part works)? Why do I need to divide by the line length to convert to metres per second, and is that correct? It wasn't done in the linked examples? I have used the example I linked to above, and modified for my specific example: private void process(CatmullRomSpline catmullRomSpline) { // Render path with precision of 1000 points renderPath(catmullRomSpline, 1000); float length = catmullRomSpline.approxLength(catmullRomSpline.spanCount * 1000); // Render the "train" Vector2 trainDerivative = new Vector2(); Vector2 trainLocation = new Vector2(); catmullRomSpline.derivativeAt(trainDerivative, current); // For some reason need to divide by length to convert from pixel speed to metres per second but I do not // really understand why I need it, it wasn't done in the examples??????? current += (Gdx.graphics.getDeltaTime() * speed / length) / trainDerivative.len(); catmullRomSpline.valueAt(trainLocation, current); renderCircleAtLocation(trainLocation); if (current >= 1) { current -= 1; } // Render the "carriage" Vector2 carriageLocation = new Vector2(); float carriagePercentageCovered = (((current * length) - 1f) / length); // I would like it to follow at 1 metre behind carriagePercentageCovered = Math.max(carriagePercentageCovered, 0); catmullRomSpline.valueAt(carriageLocation, carriagePercentageCovered); renderCircleAtLocation(carriageLocation); } private void renderPath(CatmullRomSpline catmullRomSpline, int k) { // catMulPoints would normally be cached when initialising, but for sake of example... Vector2[] catMulPoints = new Vector2[k]; for (int i = 0; i < k; ++i) { catMulPoints[i] = new Vector2(); catmullRomSpline.valueAt(catMulPoints[i], ((float) i) / ((float) k - 1)); } SHAPE_RENDERER.begin(ShapeRenderer.ShapeType.Line); SHAPE_RENDERER.setColor(Color.NAVY); for (int i = 0; i < k - 1; ++i) { SHAPE_RENDERER.line((Vector2) catMulPoints[i], (Vector2) catMulPoints[i + 1]); } SHAPE_RENDERER.end(); } private void renderCircleAtLocation(Vector2 location) { SHAPE_RENDERER.begin(ShapeRenderer.ShapeType.Filled); SHAPE_RENDERER.setColor(Color.YELLOW); SHAPE_RENDERER.circle(location.x, location.y, .5f); SHAPE_RENDERER.end(); } To create a decent sized CatmullRomSpline for testing this out: Vector2[] controlPoints = makeControlPointsArray(); CatmullRomSpline myCatmull = new CatmullRomSpline(controlPoints, false); .... private Vector2[] makeControlPointsArray() { Vector2[] pointsArray = new Vector2[78]; pointsArray[0] = new Vector2(1.681817f, 10.379999f); pointsArray[1] = new Vector2(2.045455f, 10.379999f); pointsArray[2] = new Vector2(2.663636f, 10.479999f); pointsArray[3] = new Vector2(3.027272f, 10.700000f); pointsArray[4] = new Vector2(3.663636f, 10.939999f); pointsArray[5] = new Vector2(4.245455f, 10.899999f); pointsArray[6] = new Vector2(4.736363f, 10.720000f); pointsArray[7] = new Vector2(4.754545f, 10.339999f); pointsArray[8] = new Vector2(4.518181f, 9.860000f); pointsArray[9] = new Vector2(3.790908f, 9.340000f); pointsArray[10] = new Vector2(3.172727f, 8.739999f); pointsArray[11] = new Vector2(3.300000f, 8.340000f); pointsArray[12] = new Vector2(3.700000f, 8.159999f); pointsArray[13] = new Vector2(4.227272f, 8.520000f); pointsArray[14] = new Vector2(4.681818f, 8.819999f); pointsArray[15] = new Vector2(5.081817f, 9.200000f); pointsArray[16] = new Vector2(5.463636f, 9.460000f); pointsArray[17] = new Vector2(5.972727f, 9.300000f); pointsArray[18] = new Vector2(6.063636f, 8.780000f); pointsArray[19] = new Vector2(6.027272f, 8.259999f); pointsArray[20] = new Vector2(5.700000f, 7.739999f); pointsArray[21] = new Vector2(5.300000f, 7.440000f); pointsArray[22] = new Vector2(4.645454f, 7.179999f); pointsArray[23] = new Vector2(4.136363f, 6.940000f); pointsArray[24] = new Vector2(3.427272f, 6.720000f); pointsArray[25] = new Vector2(2.572727f, 6.559999f); pointsArray[26] = new Vector2(1.900000f, 7.100000f); pointsArray[27] = new Vector2(2.336362f, 7.440000f); pointsArray[28] = new Vector2(2.590908f, 7.940000f); pointsArray[29] = new Vector2(2.318181f, 8.500000f); pointsArray[30] = new Vector2(1.663636f, 8.599999f); pointsArray[31] = new Vector2(1.209090f, 8.299999f); pointsArray[32] = new Vector2(1.118181f, 7.700000f); pointsArray[33] = new Vector2(1.045455f, 6.880000f); pointsArray[34] = new Vector2(1.154545f, 6.100000f); pointsArray[35] = new Vector2(1.281817f, 5.580000f); pointsArray[36] = new Vector2(1.700000f, 5.320000f); pointsArray[37] = new Vector2(2.190908f, 5.199999f); pointsArray[38] = new Vector2(2.900000f, 5.100000f); pointsArray[39] = new Vector2(3.700000f, 5.100000f); pointsArray[40] = new Vector2(4.372727f, 5.220000f); pointsArray[41] = new Vector2(4.827272f, 5.220000f); pointsArray[42] = new Vector2(5.463636f, 5.160000f); pointsArray[43] = new Vector2(5.554545f, 4.700000f); pointsArray[44] = new Vector2(5.245453f, 4.340000f); pointsArray[45] = new Vector2(4.445455f, 4.280000f); pointsArray[46] = new Vector2(3.609091f, 4.260000f); pointsArray[47] = new Vector2(2.718181f, 4.160000f); pointsArray[48] = new Vector2(1.990908f, 4.140000f); pointsArray[49] = new Vector2(1.427272f, 3.980000f); pointsArray[50] = new Vector2(1.609090f, 3.580000f); pointsArray[51] = new Vector2(2.136363f, 3.440000f); pointsArray[52] = new Vector2(3.227272f, 3.280000f); pointsArray[53] = new Vector2(3.972727f, 3.340000f); pointsArray[54] = new Vector2(5.027272f, 3.360000f); pointsArray[55] = new Vector2(5.718181f, 3.460000f); pointsArray[56] = new Vector2(6.100000f, 4.240000f); pointsArray[57] = new Vector2(6.209091f, 4.500000f); pointsArray[58] = new Vector2(6.118181f, 5.320000f); pointsArray[59] = new Vector2(5.772727f, 5.920000f); pointsArray[60] = new Vector2(4.881817f, 6.140000f); pointsArray[61] = new Vector2(5.318181f, 6.580000f); pointsArray[62] = new Vector2(6.263636f, 7.020000f); pointsArray[63] = new Vector2(6.645453f, 7.420000f); pointsArray[64] = new Vector2(6.681817f, 8.179999f); pointsArray[65] = new Vector2(6.627272f, 9.080000f); pointsArray[66] = new Vector2(6.572727f, 9.699999f); pointsArray[67] = new Vector2(6.263636f, 10.820000f); pointsArray[68] = new Vector2(5.754546f, 11.479999f); pointsArray[69] = new Vector2(4.536363f, 11.599998f); pointsArray[70] = new Vector2(3.572727f, 11.700000f); pointsArray[71] = new Vector2(2.809090f, 11.660000f); pointsArray[72] = new Vector2(1.445455f, 11.559999f); pointsArray[73] = new Vector2(0.936363f, 11.280000f); pointsArray[74] = new Vector2(0.754545f, 10.879999f); pointsArray[75] = new Vector2(0.700000f, 9.939999f); pointsArray[76] = new Vector2(0.918181f, 9.620000f); pointsArray[77] = new Vector2(1.463636f, 9.600000f); return pointsArray; } Disclaimer: My math is very rusty, so please explain in lay mans terms....

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• How would I balance a multiplayer competitive game

  - by Simon

  I'm looking at my first foray into developing a game, and would love to know whether you guys have any thoughts on game balancing on limited multiplayer games. The game I have in mind involves a neutral player that has to achieve a goal, with two supporting "deity" players who are one of 'good' and 'evil' - One of the deity players would try to help the player achieve their goal, while the other would try to thwart them. Any thoughts or pointers on how I can ensure the deities are balanced? If you want me to expand, I will, just didn't want to give away too much of the game play before I finish it.

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• How to check that I have recovered from Penguin 2.0?

  - by Simon Walker

  I have 3 year old website which has been hit by Penguin 2.0 in May. The website traffic dropped almost 30%. I have been working hard from last 2.5 months on the website and my website's traffic recovered in last week of August. In fact, I am receiving more traffic then ever. When I look at the stats, I find my website's search engine visibility has been improved. It is now appearing for more search queries. My website's impressions have also increased. What I am worried about is that my website is nowhere in top 5 pages for keywords having high competition and carrying the highest search volume. They are few in number but important. Should I consider my current situation as recovery or it's just the partial recovery? If it is only partial, then how come traffic is more then it was before penguin 2.0?

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• Reach Local Proxy Page - Duplicate content?

  - by Simon Bennett

  We have a client who has instructed Reach Local to manage their paid SEO work etc. RL have created a proxy version of the page at http://example-px.rtrk.co.uk which mirrors the existing site completely. Would I be correct in assuming that this would count as duplicate content and one or both of the sites would be penalized because of this? And would the addition of a rel="canonical" meta-tag on the proxy site assist with this? Many thanks in advance.

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• Best strategy (tried and tested) for using Box2D in a real-time multiplayer game?

  - by Simon Grey

  I am currently tackling real-time multiplayer physics updates for a game engine I am writing. My question is how best to use Box2D for networked physics. If I run the simulation on the server, should I send position, velocity etc to every client on every tick? Should I send it every few ticks? Maybe there is another way that I am missing? How has this problem been solved using Box2D before? Anyone with some ideas would be greatly appreciated!

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• Inside the DLR – Invoking methods

  - by Simon Cooper

  So, we’ve looked at how a dynamic call is represented in a compiled assembly, and how the dynamic lookup is performed at runtime. The last piece of the puzzle is how the resolved method gets invoked, and that is the subject of this post. Invoking methods As discussed in my previous posts, doing a full lookup and bind at runtime each and every single time the callsite gets invoked would be far too slow to be usable. The results obtained from the callsite binder must to be cached, along with a series of conditions to determine whether the cached result can be reused. So, firstly, how are the conditions represented? These conditions can be anything; they are determined entirely by the semantics of the language the binder is representing. The binder has to be able to return arbitary code that is then executed to determine whether the conditions apply or not. Fortunately, .NET 4 has a neat way of representing arbitary code that can be easily combined with other code – expression trees. All the callsite binder has to return is an expression (called a ‘restriction’) that evaluates to a boolean, returning true when the restriction passes (indicating the corresponding method invocation can be used) and false when it does’t. If the bind result is also represented in an expression tree, these can be combined easily like so: if ([restriction is true]) { [invoke cached method] } Take my example from my previous post: public class ClassA { public static void TestDynamic() { CallDynamic(new ClassA(), 10); CallDynamic(new ClassA(), "foo"); } public static void CallDynamic(dynamic d, object o) { d.Method(o); } public void Method(int i) {} public void Method(string s) {} } When the Method(int) method is first bound, along with an expression representing the result of the bind lookup, the C# binder will return the restrictions under which that bind can be reused. In this case, it can be reused if the types of the parameters are the same: if (thisArg.GetType() == typeof(ClassA) && arg1.GetType() == typeof(int)) { thisClassA.Method(i); } Caching callsite results So, now, it’s up to the callsite to link these expressions returned from the binder together in such a way that it can determine which one from the many it has cached it should use. This caching logic is all located in the System.Dynamic.UpdateDelegates class. It’ll help if you’ve got this type open in a decompiler to have a look yourself. For each callsite, there are 3 layers of caching involved: The last method invoked on the callsite. All methods that have ever been invoked on the callsite. All methods that have ever been invoked on any callsite of the same type. We’ll cover each of these layers in order Level 1 cache: the last method called on the callsite When a CallSite<T> object is first instantiated, the Target delegate field (containing the delegate that is called when the callsite is invoked) is set to one of the UpdateAndExecute generic methods in UpdateDelegates, corresponding to the number of parameters to the callsite, and the existance of any return value. These methods contain most of the caching, invoke, and binding logic for the callsite. The first time this method is invoked, the UpdateAndExecute method finds there aren’t any entries in the caches to reuse, and invokes the binder to resolve a new method. Once the callsite has the result from the binder, along with any restrictions, it stitches some extra expressions in, and replaces the Target field in the callsite with a compiled expression tree similar to this (in this example I’m assuming there’s no return value): if ([restriction is true]) { [invoke cached method] return; } if (callSite._match) { _match = false; return; } else { UpdateAndExecute(callSite, arg0, arg1, ...); } Woah. What’s going on here? Well, this resulting expression tree is actually the first level of caching. The Target field in the callsite, which contains the delegate to call when the callsite is invoked, is set to the above code compiled from the expression tree into IL, and then into native code by the JIT. This code checks whether the restrictions of the last method that was invoked on the callsite (the ‘primary’ method) match, and if so, executes that method straight away. This means that, the next time the callsite is invoked, the first code that executes is the restriction check, executing as native code! This makes this restriction check on the primary cached delegate very fast. But what if the restrictions don’t match? In that case, the second part of the stitched expression tree is executed. What this section should be doing is calling back into the UpdateAndExecute method again to resolve a new method. But it’s slightly more complicated than that. To understand why, we need to understand the second and third level caches. Level 2 cache: all methods that have ever been invoked on the callsite When a binder has returned the result of a lookup, as well as updating the Target field with a compiled expression tree, stitched together as above, the callsite puts the same compiled expression tree in an internal list of delegates, called the rules list. This list acts as the level 2 cache. Why use the same delegate? Stitching together expression trees is an expensive operation. You don’t want to do it every time the callsite is invoked. Ideally, you would create one expression tree from the binder’s result, compile it, and then use the resulting delegate everywhere in the callsite. But, if the same delegate is used to invoke the callsite in the first place, and in the caches, that means each delegate needs two modes of operation. An ‘invoke’ mode, for when the delegate is set as the value of the Target field, and a ‘match’ mode, used when UpdateAndExecute is searching for a method in the callsite’s cache. Only in the invoke mode would the delegate call back into UpdateAndExecute. In match mode, it would simply return without doing anything. This mode is controlled by the _match field in CallSite<T>. The first time the callsite is invoked, _match is false, and so the Target delegate is called in invoke mode. Then, if the initial restriction check fails, the Target delegate calls back into UpdateAndExecute. This method sets _match to true, then calls all the cached delegates in the rules list in match mode to try and find one that passes its restrictions, and invokes it. However, there needs to be some way for each cached delegate to inform UpdateAndExecute whether it passed its restrictions or not. To do this, as you can see above, it simply re-uses _match, and sets it to false if it did not pass the restrictions. This allows the code within each UpdateAndExecute method to check for cache matches like so: foreach (T cachedDelegate in Rules) { callSite._match = true; cachedDelegate(); // sets _match to false if restrictions do not pass if (callSite._match) { // passed restrictions, and the cached method was invoked // set this delegate as the primary target to invoke next time callSite.Target = cachedDelegate; return; } // no luck, try the next one... } Level 3 cache: all methods that have ever been invoked on any callsite with the same signature The reason for this cache should be clear – if a method has been invoked through a callsite in one place, then it is likely to be invoked on other callsites in the codebase with the same signature. Rather than living in the callsite, the ‘global’ cache for callsite delegates lives in the CallSiteBinder class, in the Cache field. This is a dictionary, typed on the callsite delegate signature, providing a RuleCache<T> instance for each delegate signature. This is accessed in the same way as the level 2 callsite cache, by the UpdateAndExecute methods. When a method is matched in the global cache, it is copied into the callsite and Target cache before being executed. Putting it all together So, how does this all fit together? Like so (I’ve omitted some implementation & performance details): That, in essence, is how the DLR performs its dynamic calls nearly as fast as statically compiled IL code. Extensive use of expression trees, compiled to IL and then into native code. Multiple levels of caching, the first of which executes immediately when the dynamic callsite is invoked. And a clever re-use of compiled expression trees that can be used in completely different contexts without being recompiled. All in all, a very fast and very clever reflection caching mechanism.

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• How do i make a minecraft server mod? [closed]

  - by Simon

  Possible Duplicate: Mods for Minecraft Server - how does it work? I have made some minecraft client mods, but i've started a server a mounth ago and i want to make a mod for it, but i cant find any tutorial on the internet. How can then the other guys making those mods for minecraft server know how they are going to do? Do they try forward as i tryed or are they doing something else. I would be glad if someone could tell me how to do or find tutorials for me, couse I have tryed to find them in nearly a week of searching. But i guess im searching at the wrong spot of internet, what do i know :o

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• Frequent GUI pauses in Ubuntu 13.04 / Unity / Intel HD4000

  - by Simon

  I'm experiencing very frequent (and regular) GUI pauses on my system. Every 30 seconds (pretty much exactly) the GUI will freeze for maybe .25 to .5 seconds. The mouse stops moving, keys stop echoing and a stopwatch timer briefly pauses. I'm using the Intel Graphics driver available from: https://download.01.org/gfx/ubuntu/13.04/main I've looked in a few places and tried a few things for a solution: I've checked cron and anacron for scheduled processes. I've disabled background processes (eg mysql, postgres, apache) not that these were doing anything anyway I've checked the following posts and tried the suggestions there: Unity GUI pauses/freezes for less than a few seconds How to go about troubleshooting frequent system pauses I've watched the system using top and System Monitor and there are no spikes (or even blips) of cpu usage when the pauses occur. There are no obvious error messages in dmesg or syslog There is loads of free RAM (8GB+) and no swap usage If it helps it's a ZooStorm i5 laptop with a HD4000 GPU, 16GB Ram and an SSD. Any help / suggestions would be very gratefully received.

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• Why enumerator structs are a really bad idea (redux)

  - by Simon Cooper

  My previous blog post went into some detail as to why calling MoveNext on a BCL generic collection enumerator didn't quite do what you thought it would. This post covers the Reset method. To recap, here's the simple wrapper around a linked list enumerator struct from my previous post (minus the readonly on the enumerator variable): sealed class EnumeratorWrapper : IEnumerator<int> { private LinkedList<int>.Enumerator m_Enumerator; public EnumeratorWrapper(LinkedList<int> linkedList) { m_Enumerator = linkedList.GetEnumerator(); } public int Current { get { return m_Enumerator.Current; } } object System.Collections.IEnumerator.Current { get { return Current; } } public bool MoveNext() { return m_Enumerator.MoveNext(); } public void Reset() { ((System.Collections.IEnumerator)m_Enumerator).Reset(); } public void Dispose() { m_Enumerator.Dispose(); } } If you have a look at the Reset method, you'll notice I'm having to cast to IEnumerator to be able to call Reset on m_Enumerator. This is because the implementation of LinkedList<int>.Enumerator.Reset, and indeed of all the other Reset methods on the BCL generic collection enumerators, is an explicit interface implementation. However, IEnumerator is a reference type. LinkedList<int>.Enumerator is a value type. That means, in order to call the reset method at all, the enumerator has to be boxed. And the IL confirms this: .method public hidebysig newslot virtual final instance void Reset() cil managed { .maxstack 8 L_0000: nop L_0001: ldarg.0 L_0002: ldfld valuetype [System]System.Collections.Generic.LinkedList`1/Enumerator<int32> EnumeratorWrapper::m_Enumerator L_0007: box [System]System.Collections.Generic.LinkedList`1/Enumerator<int32> L_000c: callvirt instance void [mscorlib]System.Collections.IEnumerator::Reset() L_0011: nop L_0012: ret } On line 0007, we're doing a box operation, which copies the enumerator to a reference object on the heap, then on line 000c calling Reset on this boxed object. So m_Enumerator in the wrapper class is not modified by the call the Reset. And this is the only way to call the Reset method on this variable (without using reflection). Therefore, the only way that the collection enumerator struct can be used safely is to store them as a boxed IEnumerator<T>, and not use them as value types at all.

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• Oh no! My padding's invalid!

  - by Simon Cooper

  Recently, I've been doing some work involving cryptography, and encountered the standard .NET CryptographicException: 'Padding is invalid and cannot be removed.' Searching on StackOverflow produces 57 questions concerning this exception; it's a very common problem encountered. So I decided to have a closer look. To test this, I created a simple project that decrypts and encrypts a byte array: // create some random data byte[] data = new byte[100]; new Random().NextBytes(data); // use the Rijndael symmetric algorithm RijndaelManaged rij = new RijndaelManaged(); byte[] encrypted; // encrypt the data using a CryptoStream using (var encryptor = rij.CreateEncryptor()) using (MemoryStream encryptedStream = new MemoryStream()) using (CryptoStream crypto = new CryptoStream( encryptedStream, encryptor, CryptoStreamMode.Write)) { crypto.Write(data, 0, data.Length); encrypted = encryptedStream.ToArray(); } byte[] decrypted; // and decrypt it again using (var decryptor = rij.CreateDecryptor()) using (CryptoStream crypto = new CryptoStream( new MemoryStream(encrypted), decryptor, CryptoStreamMode.Read)) { byte[] decrypted = new byte[data.Length]; crypto.Read(decrypted, 0, decrypted.Length); } Sure enough, I got exactly the same CryptographicException when trying to decrypt the data even in this simple example. Well, I'm obviously missing something, if I can't even get this single method right! What does the exception message actually mean? What am I missing? Well, after playing around a bit, I discovered the problem was fixed by changing the encryption step to this: // encrypt the data using a CryptoStream using (var encryptor = rij.CreateEncryptor()) using (MemoryStream encryptedStream = new MemoryStream()) { using (CryptoStream crypto = new CryptoStream( encryptedStream, encryptor, CryptoStreamMode.Write)) { crypto.Write(data, 0, data.Length); } encrypted = encryptedStream.ToArray(); } Aaaah, so that's what the problem was. The CryptoStream wasn't flushing all it's data to the MemoryStream before it was being read, and closing the stream causes it to flush everything to the backing stream. But why does this cause an error in padding? Cryptographic padding All symmetric encryption algorithms (of which Rijndael is one) operates on fixed block sizes. For Rijndael, the default block size is 16 bytes. This means the input needs to be a multiple of 16 bytes long. If it isn't, then the input is padded to 16 bytes using one of the padding modes. This is only done to the final block of data to be encrypted. CryptoStream has a special method to flush this final block of data - FlushFinalBlock. Calling Stream.Flush() does not flush the final block, as you might expect. Only by closing the stream or explicitly calling FlushFinalBlock is the final block, with any padding, encrypted and written to the backing stream. Without this call, the encrypted data is 16 bytes shorter than it should be. If this final block wasn't written, then the decryption gets to the final 16 bytes of the encrypted data and tries to decrypt it as the final block with padding. The end bytes don't match the padding scheme it's been told to use, therefore it throws an exception stating what is wrong - what the decryptor expects to be padding actually isn't, and so can't be removed from the stream. So, as well as closing the stream before reading the result, an alternative fix to my encryption code is the following: // encrypt the data using a CryptoStream using (var encryptor = rij.CreateEncryptor()) using (MemoryStream encryptedStream = new MemoryStream()) using (CryptoStream crypto = new CryptoStream( encryptedStream, encryptor, CryptoStreamMode.Write)) { crypto.Write(data, 0, data.Length); // explicitly flush the final block of data crypto.FlushFinalBlock(); encrypted = encryptedStream.ToArray(); } Conclusion So, if your padding is invalid, make sure that you close or call FlushFinalBlock on any CryptoStream performing encryption before you access the encrypted data. Flush isn't enough. Only then will the final block be present in the encrypted data, allowing it to be decrypted successfully.

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• 2D management game [on hold]

  - by Simon Bull

  Very newbie question but I have a game idea in mind. It will be 2d and data centric, like football manager. However I am struggling to find a platform that would suit. I am an experienced line of business developer so am happy to write code, but I would like a platform that does some of the leg work for me so was avoiding OpenGL. I would also like to be able deploy to iOS, android, windows and OS X. What are the options? To be more clear, the game is not a normal platform or shooter type game, so game maker is likely to be way too basic and unity seems a little over the top (though I am not sure if the GUI options would fit?). The majority of the game is more like business screens just displaying data and having buttons to click. Are there options for this type of game (May help to look at football manager)?

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• How to Prevent Site For Penality [on hold]

  - by Simon Walker

  I have been working hard to recover site from Penguin 2.0 and I was successful in it. A week before Penguin 2.1 one of my competitor created thousands of spam backlinks in just 2 days. The timing is more than perfect to make my website victim of Penguin 2.1. I have disavowed all those dirty links and regularly updating the disavow sheet for poor backlinks. Is there else I can do with the site to recover faster. http://bit.ly/fvbyLg

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• Dual Boot Ubuntu and Windows 7: BOOTMGR is missing when I tried to boot in Windows

  - by Simon Polak

  So, I don't know what exactly how I managed to delete the MBR record on windows partition. But let me explain what I did next, I ran the ubuntu boot repair tool and now Windows is not even listed in my grub loader. So I went and booted with windows cd and choose repair. Then I ran ubuntu boot repair again via live cd. Here is the log http://paste.ubuntu.com/1426181/. Still no luck. Looks like osprobe can't detect windows on my /dev/sda2 partition. Any clues ? Here is how my partitions look like: Disk /dev/sda: 500.1 GB, 500107862016 bytes 255 heads, 63 sectors/track, 60801 cylinders, total 976773168 sectors Units = sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x525400d1 Device Boot Start End Blocks Id System /dev/sda1 * 2048 206847 102400 7 HPFS/NTFS/exFAT /dev/sda2 206848 509620669 254706911 7 HPFS/NTFS/exFAT /dev/sda3 509622270 976773119 233575425 5 Extended /dev/sda5 509622272 957757439 224067584 83 Linux /dev/sda6 957759488 976773119 9506816 82 Linux swap / Solaris

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• Application development : method to manage backgound process

  - by Simon Dubois

  I am developing an application with different behavior depending on the arguments : - "-config" starts a Gtk window to change options, start and close the daemon. - "-daemon" starts a background process that does something every X minutes. I already know how to use fork/system/exec etc... But I would like to know the main logic of such application to : - restart or refresh the daemon when configuration change. - keep only one instance of the daemon. I have red that killing the daemon to restart it is not a clean way to do. How other applications do ? (ubuntuone, weather forecast, rss feed working with notification area) Thanks for your help. PS : I don't want to create a system-wide daemon, just a user application with a background process.

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• How to use data mining principles in this project?

  - by Simon

  I'm getting a Data Mining class this semester and we are free for the final project. For a few months I'm working on procedural planets rendering (something like this http://www.youtube.com/watch?v=rL8zDgTlXso). Do you have any idea of which data mining principles I could use to keep working this project ? Maybe I could try to generate interesting terrains from a set of real maps ? Any publications on that subject ? Any other ideas ?

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• Best way to cache apt downloads on a LAN?

  - by Ken Simon

  I have multiple Ubuntu machines at home and a pretty slow internet connection, and sometimes multiple machines need to be updated at once (especially during new Ubuntu releases.) Is there a way where only one of my machines needs to download the packages, and the other machines can use the first machine to get the debs? Does it involve setting up my own local mirror? Or a proxy server? Or can it be made simpler?

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• High resolution graphical representation of the Earth's surface

  - by Simon

  I've got a library, which I inherited, which presents a zoomable representation of the Earth. It's a Mercator projection and is constructed from triangles, the properties of which are stored in binary files. The surface is built up, for any given view port, by drawing these triangles in an overlapping fashion to produce the image. The definition of each triangle is the lat/long of the vertices. It looks OK at low values of zoom but looks progressively more ragged as the user zooms in. The view ports are primarily referenced though a rectangle of lat/long co-ordinates. I'd like to replace it with a better quality approach. The problem is, I don't know where to begin researching the options as I am not familiar either with the projections needed nor the graphics techniques used to render them. For example, I imagine that I could acquire high resolution images, say Mercator projections although I'm open to anything, break them into tiles and somehow wrap them onto a graphical representation of a sphere. I'm not asking for "how do I", more where should I begin to understand what might be involved and the techniques I will need to learn. I am most grateful for any "Earth rendering 101" pointers folks might have.

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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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• Searching for an online shop accessible via API

  - by Simon A. Eugster

  I need an online shop with a custom interface (customizing items with Ajax, with a preview included). Writing it myself does not make too much sense (implementing all the payment options etc.), so I would like to use an existing online shop (OpenSource). I would like to build my own UI which, for example, tells the shop to add an item to its cart -- i.e. without using the online shop's native UI. More precisely, it should be an online gallery where the user can directly order an image if he likes it. The final checkout/payment page can be native again. Is there a shop system that supports this? Or is it still faster to write it on my own? Or are there better options?

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