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• Mount drive when symbolic link is accessed

  - by Danilo Campos

  I have a MacBook Pro with two internal drives: an SSD and a slow rotational drive. On the rotational drive I keep heavy, rarely accessed files like movies, photos, etc. These are symlinked from the SSD, so applications like iPhoto and iTunes will still find everything where they expect. I don't usually have the rotational drive mounted because it's loud and mostly unused. Is there a way to mount it when the system tries to access data behind a symlink, then unmount it automatically later? (Intermediate *nix user, here, feel free to tell me I am asking for magic.) Thanks for your help!

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• How do I create my own programming language and a compiler for it

  - by Dave

  I am thorough with programming and have come across languages including BASIC, FORTRAN, COBOL, LISP, LOGO, Java, C++, C, MATLAB, Mathematica, Python, Ruby, Perl, JavaScript, Assembly and so on. I can't understand how people create programming languages and devise compilers for it. I also couldn't understand how people create OS like Windows, Mac, UNIX, DOS and so on. The other thing that is mysterious to me is how people create libraries like OpenGL, OpenCL, OpenCV, Cocoa, MFC and so on. The last thing I am unable to figure out is how scientists devise an assembly language and an assembler for a microprocessor. I would really like to learn all of these stuff and I am 15 years old. I always wanted to be a computer scientist someone like Babbage, Turing, Shannon, or Dennis Ritchie. I have already read Aho's Compiler Design and Tanenbaum's OS concepts book and they all only discuss concepts and code in a high level. They don't go into the details and nuances and how to devise a compiler or operating system. I want a concrete understanding so that I can create one myself and not just an understanding of what a thread, semaphore, process, or parsing is. I asked my brother about all this. He is a SB student in EECS at MIT and hasn't got a clue of how to actually create all these stuff in the real world. All he knows is just an understanding of Compiler Design and OS concepts like the ones that you guys have mentioned (i.e. like Thread, Synchronization, Concurrency, memory management, Lexical Analysis, Intermediate code generation and so on)

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• What the Hekaton?

  - by Tony Davis

  Hekaton, the power behind SQL Server 2014′s In-Memory OLTP technology, is intended to make data operations run orders of magnitude faster on SQL Server. This works its magic partly by serving database workloads entirely from main memory, using memory-optimized table structures. It replaces the relational engine’s standard locking model with an optimistic concurrency model based on time-stamped row versions. Deeper down the Hekaton engine uses new, ‘latch free’ data structures. So far, so good, but performance improvements on this scale require a compromise, and the compromise is that these aren’t tables as we understand them. For the database developer, these differences are painful because they involve sacrificing some very important bits of the relational model. Most importantly, Hekaton tables don’t currently support FOREIGN KEY constraints or CHECK constraints, and you can’t put the checks in triggers because there aren’t any DML triggers either. Constraints allow a relational designer to enforce relational integrity and data integrity. Without them, of course, ‘bad data’ can get into our Hekaton tables. There is no easy way of preventing it. For several classes of database and data, this is a show-stopper. One may regard all these restrictions regretfully, seeing limited opportunity to try out Hekaton with current databases, but perhaps there is also a sudden glow of recognition. Isn’t this how we all originally imagined table variables were going to be, back in SQL 2005? And they have much the same restrictions. Maybe, instead of pretending that a currently-designed database can be ‘Hekatonized’ with a few mouse clicks, we should redesign databases for SQL 2014 to replace table variables with Hekaton tables, exploiting this technology for fast intermediate processing, and for the most part forget, for now, the idea of trying to convert our base relational tables into Hekaton tables. Few database developers would be averse to having their working tables running an order of magnitude faster, as long as it didn’t compromise the integrity of the data in the base tables.

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• Book recommendation for learning server management and Apache

  - by japancheese

  Hello, I'm currently managing a site that I feel could be optimized and utilized to be much faster, however, I'm having difficulty finding reliable information to do it. I find the Apache documentation to be a hard read, and too technical about things I don't have a strong grasp on. I'm just looking for a good beginner/intermediate book about server administration to learn as much as possible about Apache, as well as how to create a nice secure, robust server that doesn't crash at the first hint of unusual traffic surges. Thanks to anyone who can point me in the right direction.

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• Are there videocamera which geotag individual frames?

  - by Grzegorz Adam Hankiewicz

  I'm looking for a way to record live video with the specific requirement of having each frame georeferenced with GPS. Right now I'm using a normal video camera with a PDA+GPS that records the position, but it's difficult to sync both of these plus sometimes I've forgotten to turn the PDA+GPS or it has failed for some reason and all my video has been useless. Using google I found that about two years ago a company named Seero produced such video cameras and software, but apparently the domain doesn't exist any more and I only find references of other pages mentioning it. Does somebody know of any other product? I need to record this video in HD and have some way to export to Google Maps or other GIS software the positions of the frames in a way that I can click on the map and see what was being recorded in the video at that point. The precission of the GPS tracking is good enough as one position per second, intermediate frames of the video stream can be interpolated.

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• O'Reilly deals to April 5, 2012 14:00 PT on books on "where"

  - by TATWORTH

  At http://shop.oreilly.com/category/deals/where-conference.do, O'Reilly are offering a series of books on geo-location at 50% off until April 5, 2012 14:00 PT. HTML5 Geolocation Truly revolutionary: now you can write geolocation applications directly in the browser, rather than develop native apps for particular devices. This concise book demonstrates the W3C Geolocation API in action, with code and examples to help you build HTML5 apps using the "write once, deploy everywhere" model. Along the way, you get a crash course in geolocation, browser support, and ways to integrate the API with common geo tools like Google Maps. HTML5 Cookbook With scores of practical recipes you can use in your projects right away, this cookbook helps you gain hands-on experience with HTML5’s versatile collection of elements. You get clear solutions for handling issues with everything from markup semantics, web forms, and audio and video elements to related technologies such as geolocation and rich JavaScript APIs. Each informative recipe includes sample code and a detailed discussion on why and how the solution works. Perfect for intermediate to advanced web and mobile web developers, this handy book lets you choose the HTML5 features that work for you—and helps you experiment with the rest. HTML5 Applications HTML5 is not just a replacement for plugins. It also makes the Web a first-class development environment by giving JavaScript programmers a solid foundation for building industrial-strength applications. This practical guide takes you beyond simple site creation and shows you how to build self-contained HTML5 applications that can run on mobile devices and compete with desktop apps. You’ll learn powerful JavaScript tools for exploiting HTML5 elements, and discover new methods for working with data, such as offline storage and multi-threaded processing. Complete with code samples, this book is ideal for experienced JavaScript and mobile developers alike. There are also other books being offered at a discount at http://shop.oreilly.com/category/deals/where-conference.do

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• Sun Java keytool importing EV certificates into a single keystore

  - by ss0

  At my current job we are using tomcat, customers have custom web portals setup on their own local machines. EV certs are new to me, they have 2 part intermediary and a primary certificate. For our product to work it appears I need to get all three parts installed under a single keystore entry. How can I roll all three parts into a single x.509 compliant file for import? They syntax I am using is as follows: /blah/system/j2sdk/bin/keytool -import -alias foo -keystore /zix/system/jdk1.5.0_06/jre/lib/security/cacerts -file certname.pem -trustcacerts where foo = the keystore name and certname.pem is the main cert. I have tried importing the intermediate certs under their own names into the keystore and I don't know if it's just the product I have to work with (not vanilla tomcat) or what but it doesn't see those. I have seen a working system and all three certs were under the single keystore alias. Anyone have any ideas?

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• MVC and individual elements of the model under a common base class

  - by Stewart

  Admittedly my experience of using the MVC pattern is limited. It might be argued that I don't really separate the V from the C, though I keep the M separate from the VC to the extent I can manage. I'm considering the scenario in which the application's model includes a number of elements that have a common base class. For example, enemy characters in a video game, or shape types in a vector graphics app. The view wants to render these elements. Of course, the different subclasses call for different rendering. The problem is that the elements are part of the model. Rendering them is conceptually part of the view. But how they are to be rendered depends on parameters of both: Attributes and state of the element are parameters of the model User settings are parameters of the view - and to support multiple platforms and/or view modes, different views may be used What's your preferred way of dealing with this? Put the rendering code in the model classes, passing in any view parameters? Put the rendering code in the view, using a switch or similar to select the right rendering for the model element type? Have some intermediate classes as a model-view interface, of which the model will create objects on demand and the view will then render them? Something else?

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• what the best simple video editing program? [closed]

  - by Itay

  Possible Duplicate: What is the easiest video editing program to use on Windows hi, i want to edit videos, i'm looking for a good, premiere like software. but it seems premiere it self is too much for me... frankly i can see how those settings can be helpful for any one.. but it doesn't matter. i've just imported an HD video from my new camera (CS4), first, it cut most of the video, and second, it actually made the quality worse... i've tried a few free, programs, but as soon as i've seen the GUI of those, i closed them... i really like premier, but i honestly don't understand who needs all this PAL/DV stuff... is there any similar application that intend to more intermediate users? tahnks.

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• Importing Omnigraffle drawings into a NeoOffice document

  - by Greg Harman

  What is the best way to import an Omnigraffle drawing into a NeoOffice document? I expect the document to change over time, and I may need to update the drawing later, so I'd like a way to make that happen painlessly. Currently, I export the drawing to a .jpg and then import that in NeoOffice with Insert-Picture-From File... but that's an extra step and extra file to maintain (the intermediate .jpg) and the frame that NeoOffice puts the image in always gives me trouble when I try to update the image with a new one.

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• SSL Certificate Stops Working after Server Reboot on IIS7, W2K8

  - by Zac

  We recently upgraded from W2K3/IIS6 to W2K8/IIS7 and have been having problems with our SSL Certificate (Thawte 123 SSL certificate) ceasing to work after rebooting. Initially, the intermediate certificates would stop working and we could repair the problem by reinstalling all of them after the reboot (annoying, but not the end of the world). Unfortunately, this is no longer working. The certificate chain has been doublechecked by several tools and people with decent knowledge but no one has been able to identify the cause of the problem. The bindings in IIS have been checked as well The cert itself is also still valid. NOTE 1: I have seen THIS question which seems to be very similar, but there is no satisfactory answer in that post and it's a year old so not likely to get one any time soon. NOTE 2: I'm asking this on behalf of a co-worker so won't be able to provide instant feedback to any questions/suggestions but I will pass it on. The url is: http://www.flirtalike.com / https://www.flirtalike.com Screenshots:

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• SQL Server tempdb size seems large, is this normal?

  - by Abe Miessler

  From what I understand the system database is used to hold temporary tables, intermediate results and other temporary information. On one of my database instances I have a tempdb that is seems very large (30GB). This database has not been modified (as in "last modified date" on the mdf file) in over a week. Is it normal to have the temp db remain that large for that long of a period? It seems to me that it should be updating fairly often and returning space that it is using fairly quickly... Am I way off here or is SQL Server doing something weird? FYI: This is a SharePoint 2010 database, not sure if that makes a difference.

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• Installing SSL certs with nginx on Amazon EC2

  - by Ethan

  I finally got a cert from an authority and am struggling to get things working. I've created the appropriate combined certificate (personal + intermediate + root) and nginx is pointing to it. I got an elastic IP and connected it to my EC2 instance. My DNS records point to that IP. But when I point the browser to the hostname, I get the standard "Connection Untrusted" bit, with ssl_error_bad_cert_domain. Port 443 is open - I can get to the site over https if I ignore the warning. Weird thing is, under technical details, it lists the domain I tried to access as valid! When I try and diagnose with ssl testing sites, they don't even detect a certificate! What am I missing here? domain is yanlj.coinculture.info. Note I've got coinculture.info running on a home server without a dedicated IP and have the same problem, but I'll be moving that to the same EC2 instance as soon as I figure this thing out. I thought the elastic IP would solve things but it hasn't

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• Dovecot and StartSSL problems with issuer

  - by knoim

  I am using dovecot (1) and trying to get my StartSSL certificate running. ssl_key_file points to my private key I tried pointing ssl_cert_file to my public key, with and without using the class1 certificate from http://www.startssl.com/certs/sub.class1.server.ca.pem as ssl_ca_file aswell as combing them with cat publickey sub.class1.server.ca.pem chained My mail client keeps telling me the certificate has no issuer, but doing openssl x509 on my public certificate tells me it is C=IL, O=StartCom Ltd., OU=Secure Digital Certificate Signing, CN=StartCom Class 1 Primary Intermediate Server CA My option for the CSR were: openssl req -new -newkey rsa:4096 -nodes Dovecot's log doesn't mention any problems. EDIT: Doesn't seem to be a problem with dovecot. I am having the same problem with postfix. openssl verify gives me the same error.

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• How much knowledge do I need to begin a project in Django

  - by Smock

  I started learning django about a month ago. I have an intermediate C, Java programming experience. I read the first 8 chapters of the django book . Afterwards, I picked up Practical Django Projects by James Bennett and did the first two projects: CMS & Web Blog. Although, I started getting lost when he got to the generic views part. I know that's important but I'm not sure how important that is when trying to implement a project. Anyway, I have a project in mind that I'd like to start; however, I'm nervous as to where to begin. I'm overwhelmed with the number of things that I'd like my project to do but no knowledge or minimal knowledge as to how e.g. how do i implement css and javascript in my project. Moreover, I am aware that some django packages exists to ease development but I don't know if I should use them or not. Anyway, I apologize for my length message. I just want some advice/encouragement. I have a project in mind but do you think I need to read more materials/tutorials or is it smart to just start working on my project based on the minimal knowledge i've gained from those books? Any information that can be provided is much appreciated. I really want to get good at this but I just need some direction.

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• Microsoft CA certificate templates expires sooner than expected

  - by Tim Brigham

  The certificates my Microsoft CA is generating do not match the time period indicated in the template used. How can I resolve this? I recently created a new certificate template for use on my Linux boxes on my Microsoft CA (2008 R2 Enterprise). This template is approved for server and client authentication purposes with a validity period of 10 years - the expected lifetime of our Linux boxes - and the subject name supplied in the request. I have checked both the intermediate and offline CA - both have more than 10 years of life listed. Is there some kind of hard limit I'm hitting here?

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• How do I create my own programming language and a compiler for it

  - by Dave

  I am thorough with programming and have come across languages including BASIC, FORTRAN, COBOL, LISP, LOGO, Java, C++, C, MATLAB, Mathematica, Python, Ruby, Perl, Javascript, Assembly and so on. I can't understand how people create programming languages and devise compilers for it. I also couldn't understand how people create OS like Windows, Mac, UNIX, DOS and so on. The other thing that is mysterious to me is how people create libraries like OpenGL, OpenCL, OpenCV, Cocoa, MFC and so on. The last thing I am unable to figure out is how scientists devise an assembly language and an assembler for a microprocessor. I would really like to learn all of these stuff and I am 15 years old. I always wanted to be a computer scientist some one like Babbage, Turing, Shannon, or Dennis Ritchie. I have already read Aho's Compiler Design and Tanenbaum's OS concepts book and they all only discuss concepts and code in a high level. They don't go into the details and nuances and how to devise a compiler or operating system. I want a concrete understanding so that I can create one myself and not just an understanding of what a thread, semaphore, process, or parsing is. I asked my brother about all this. He is a SB student in EECS at MIT and hasn't got a clue of how to actually create all these stuff in the real world. All he knows is just an understanding of Compiler Design and OS concepts like the ones that you guys have mentioned (ie like Thread, Synchronisation, Concurrency, memory management, Lexical Analysis, Intermediate code generation and so on)

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• scp vs netatalk, samba, and/or vsftpd with External USB drive

  - by KitsuneYMG

  I set up a ubuntu server machine to share an ext2 formatted external usb drive. When attempting to copy a single 275MB files from said device through netatalk, I get estimated download rates at around 45 min. With samba and ftp (using vsftpd) I get 1+ hours! Using scp to copy the file results in complete download within 5 minutes. Another option, ssh+cp from external device to ~ and then using netatalk to grab it from there results in a total time of arounf 7 minutes. Does anyone have a clue what is misconfigured? Assuming that nothing is, is there any fs/pseudo-fs that would use the internal hdd as an intermediate location/onion-layer for the external hdd (for reads only)? Details: AppleVolumes.default: /mnt/ext USB allow:username cnidscheme:cdb options:usedots,upriv

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• Dealing with the node.js callback pyramid

  - by thecoop

  I've just started using node, and one thing I've quickly noticed is how quickly callbacks can build up to a silly level of indentation: doStuff(arg1, arg2, function(err, result) { doMoreStuff(arg3, arg4, function(err, result) { doEvenMoreStuff(arg5, arg6, function(err, result) { omgHowDidIGetHere(); }); }); }); The official style guide says to put each callback in a separate function, but that seems overly restrictive on the use of closures, and making a single object declared in the top level available several layers down, as the object has to be passed through all the intermediate callbacks. Is it ok to use function scope to help here? Put all the callback functions that need access to a global-ish object inside a function that declares that object, so it goes into a closure? function topLevelFunction(globalishObject, callback) { function doMoreStuffImpl(err, result) { doMoreStuff(arg5, arg6, function(err, result) { callback(null, globalishObject); }); } doStuff(arg1, arg2, doMoreStuffImpl); } and so on for several more layers... Or are there frameworks etc to help reduce the levels of indentation without declaring a named function for every single callback? How do you deal with the callback pyramid?

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• Implementation Specialist OPN Exam for OBI Suite 11g is Now LIVE

  - by Mike.Hallett(at)Oracle-BI&EPM

  The OPN specialisation Exam for implementation consultants of OBI Suite 11g is now live and ready for all partners.  You can now update your specialisation certification to the latest product version 11g for OBI: until recently, the accreditation had examined skills for OBI 10g. For more details see Oracle Business Intelligence Foundation Suite 11g Essentials (1Z1-591) where you can apply for this Oracle Implementation Specialist credential. This exam is primarily intended for consultants that are skilled in implementing solutions based on Oracle Business Intelligence Foundation Suite 11g. The certification covers skills such as: installing OBIEE, building the BI Server metadata repository, building BI dashboards, constructing ad hoc queries, defining security settings and configuring and managing cache files. The exam targets the intermediate-level implementation team member. Up-to-date training and field experience are recommended.   Also Note the New OPN on-line Sales & Pre-sales Assessment Tests are available @ Oracle Business Intelligence Foundation Suite 11g Sales Specialist Oracle Business Intelligence Foundation Suite 11g PreSales Specialist Oracle Business Intelligence Foundation Suite 11g Support Specialist FREE Certification Testing at OpenWorld ·       Are you attending OPN Exchange @ OpenWorld ? Then join us at OPN Specialist Test Fest! October 1st - 4th 2012, Marriott Marquis Hotel :  Pre-register here now!   For More Information OPN Certified Specialist exams OPN Certified Specialist FAQ Enablement 2.0 Get Specialized!

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• Parameterized Django models

  - by mgibsonbr

  In principle, a single Django application can be reused in two or more projects, providing functionality relevent to both. That implies that the same database structure (tables and relations) will be re-created identically in different databases, and most times this is not a problem (assuming the projects/databases are unrelated - for instance when someone downloads a complete app to use in their own projects). Sometimes, however, the models must be "tweaked" a little to better fit the problem needs. This can be accomplished by forking the app, but I wondered if there wouldn't be a better option in cases where the app designer can anticipate the most common customizations. For instance, if I have a model that could relate to another as one-to-one or one-to-many, I could specify the unique property as a parameter, that can be specified in the project's settings: class This(models.Model): other = models.ForeignKey(Other, unique=settings.OTHER_TO_THIS) Or if a model can relate to many others, I could create an intermediate table for each of them (thus enforcing referential integrity) instead of using generic fks: for related in settings.MODELS_RELATED_TO_OTHER: model_name = '%s_Other' % related globals()[model_name] = type(model_name, (models.Model,) { me:models.ForeignKey(find_model_class(related)), other:models.ForeignKey(Other), # Some other properties all intersection tables must have }) Etc. Let me stress out that I'm not proposing to change the models at runtime nor anything like that; once the parameters were defined and syncdb called for the first time, those parameters are not to be changed again (unless you're doing a schema migration). Is this a good design? Are there better ways to accomplish the same thing, or maybe drawbacks I coulnd't anticipate? This technique is meant to be used sparingly (only on apps meant to be reused in wildly different contexts, and only when a specific need of customization can be detected while the app model is being designed).

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• Dealing with the node callback pyramid

  - by thecoop

  I've just started using node, and one thing I've quickly noticed is how quickly callbacks can build up to a silly level of indentation: doStuff(arg1, arg2, function(err, result) { doMoreStuff(arg3, arg4, function(err, result) { doEvenMoreStuff(arg5, arg6, function(err, result) { omgHowDidIGetHere(); }); }); }); The official style guide says to put each callback in a separate function, but that seems overly restrictive on the use of closures, and making a single object declared in the top level available several layers down, as the object has to be passed through all the intermediate callbacks. Is it ok to use function scope to help here? Put all the callback functions that need access to a global-ish object inside a function that declares that object, so it goes into a closure? function topLevelFunction(globalishObject, callback) { function doMoreStuffImpl(err, result) { doMoreStuff(arg5, arg6, function(err, result) { callback(null, globalishObject); }); } doStuff(arg1, arg2, doMoreStuffImpl); } and so on for several more layers... Or are there frameworks etc to help reduce the levels of indentation without declaring a named function for every single callback? How do you deal with the callback pyramid?

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• Coordinate spaces and transformation matrices

  - by Belgin

  I'm trying to get an object from object space, into projected space using these intermediate matrices: The first matrix (I) is the one that transforms from object space into inertial space, but since my object is not rotated or translated in any way inside the object space, this matrix is the 4x4 identity matrix. The second matrix (W) is the one that transforms from inertial space into world space, which is just a scale transform matrix of factor a = 14.1 on all coordinates, since the inertial space origin coincides with the world space origin. /a 0 0 0\ W = |0 a 0 0| |0 0 a 0| \0 0 0 1/ The third matrix (C) is the one that transforms from world space, into camera space. This matrix is a translation matrix with a translation of (0, 0, 10), because I want the camera to be located behind the object, so the object must be positioned 10 units into the z axis. /1 0 0 0\ C = |0 1 0 0| |0 0 1 10| \0 0 0 1/ And finally, the fourth matrix is the projection matrix (P). Bearing in mind that the eye is at the origin of the world space and the projection plane is defined by z = 1, the projection matrix is: /1 0 0 0\ P = |0 1 0 0| |0 0 1 0| \0 0 1/d 0/ where d is the distance from the eye to the projection plane, so d = 1. I'm multiplying them like this: (((P x C) x W) x I) x V, where V is the vertex' coordinates in column vector form: /x\ V = |y| |z| \1/ After I get the result, I divide x and y coordinates by w to get the actual screen coordinates. Apparenly, I'm doing something wrong or missing something completely here, because it's not rendering properly. Here's a picture of what is supposed to be the bottom side of the Stanford Dragon: Also, I should add that this is a software renderer so no DirectX or OpenGL stuff here.

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• Structure of a .NET Assembly

  - by Om Talsania

  Assembly is the smallest unit of deployment in .NET Framework.When you compile your C# code, it will get converted into a managed module. A managed module is a standard EXE or DLL. This managed module will have the IL (Microsoft Intermediate Language) code and the metadata. Apart from this it will also have header information.The following table describes parts of a managed module.PartDescriptionPE HeaderPE32 Header for 32-bit PE32+ Header for 64-bit This is a standard Windows PE header which indicates the type of the file, i.e. whether it is an EXE or a DLL. It also contains the timestamp of the file creation date and time. It also contains some other fields which might be needed for an unmanaged PE (Portable Executable), but not important for a managed one. For managed PE, the next header i.e. CLR header is more importantCLR HeaderContains the version of the CLR required, some flags, token of the entry point method (Main), size and location of the metadata, resources, strong name, etc.MetadataThere can be many metadata tables. They can be categorized into 2 major categories.1. Tables that describe the types and members defined in your code2. Tables that describe the types and members referenced by your codeIL CodeMSIL representation of the C# code. At runtime, the CLR converts it into native instructions

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• How are objects modelled in a functional programming language?

  - by Giorgio

  In an answer to this question (written by Pete) there are some considerations about OOP versus FP. In particular, it is suggested that FP languages are not very suitable for modelling (persistent) objects that have an identity and a mutable state. I was wondering if this is true or, in other words, how one would model objects in a functional programming language. From my basic knowledge of Haskell I thought that one could use monads in some way, but I really do not know enough on this topic to come up with a clear answer. So, how are entities with an identity and a mutable persistent state normally modelled in a functional language? EDIT Here are some further details to clarify what I have in mind. Take a typical Java application in which I can (1) read a record from a database table into a Java object, (2) modify the object in different ways, (3) save the modified object to the database. How would this be implemented e.g. in Haskell? I would initially read the record into a record value (defined by a data definition), perform different transformations by applying functions to this initial value (each intermediate value is a new, modified copy of the original record) and then write the final record value to the database. Is this all there is to it? How can I ensure that at each moment in time only one copy of the record is valid / accessible? One does not want to have different immutable values representing different snapshots of the same object to be accessible at the same time.

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