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  • How to scale out image hosting/serving?

    - by Continuation
    I asked this question on stackoverflow and it was suggested that I try it here: I'm building a website where users can upload photos and I'd also convert uploaded photos into thumbnails. Planning ahead, if the website gets popular, how do I scale it out so that the images (both original and thumbnails) will be stored in and served from multiple servers? Maybe a cluster? Is there any open source software that would help me in this? Thanks.

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  • Linux: How to rename old mysqld when upgrading MySQL?

    - by Continuation
    I'm upgrading MySQL from MySQL 5.0 to Percona Server 5.1. I'm planning to just use yum remove and yum install to do the upgrade. However, I read in the documentation that it's a good idea to rename the old mysqld to mysqld-5.0. And if the upgrade doesn't work, I could just revert back to the old version. How exactly does this work? If I use yum remove, doesn't that mean the old mysqld is removed? So how do I rename it? Where is mysqld located? How do I find it? Thanks.

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  • How does virtualization improve server utilization?

    - by Continuation
    The biggest benefit of virtualization is usually said to be improved server utilization. But why do I need virtualization for that? Say I got N physical servers that are lightly used. Why don't I just combine all the apps on those N servers into 1 physical server? This way I don't incur the performance penalty of virtualization. What does virtualization buy me in this case?

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  • Does Tomcat or Jetty cache dynamic content?

    - by Continuation
    I'm working on a Servlet app with contents that are updated periodically. Hence, between updates any dynamic pages generated by the Servlet can be cached. Does Tomcat or Jetty (or any Servlet container) offer a way to cache dynamically generated pages? Or would I need to use a caching reverse proxy like Squid to accomplish that?

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  • How do you handle the task of changing the schema of a production MySQL database?

    - by Continuation
    One of the biggest complaints I have heard about MySQL is that it locks up a table if you try to change its schema like adding a column or adding an index. By "locking up the table" does it mean I can neither read nor write to the table? Sometimes for hours? That seems a pretty severe limitations. I was going to use MySQL for my new project but this gives me pause. Is there a workaround for this? How do you handle the task of changing the schema of your production MySQL database? By the way someone told me Postgresql doesn't have this problem. Is that true - I can both read and write to a Postgresql table while changing its schema? Is there any performance penalty incurred? Would love to hear your experiences.

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  • How to tell if any MySQL connections has been dropped or timed out?

    - by Continuation
    A client is using PHP to connect to MySQL. The PHP scripts and the MySQL database are located on 2 different Linux servers. He complained that database connections were being dropped or timed out and asked me to take a look. Is there any place in MySQL that can show me what and how many connections have been dropped or timed out? I looked into slow query log and didn't see anything. Any suggestions on how to diagnose this dropped/timed out database connection problem? Thanks EDIT: Slow query log is enabled in my.cnf: log-slow-queries=/var/log/mysql-slow-queries.log And when I do a mysql> show global status; I got: | Slow_queries | 11402347 | So there are a lot of slow queries. But the file /var/log/mysql-slow-queries.log doesn't exist. Why is that?

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  • How to set up Nginx as a caching reverse proxy?

    - by Continuation
    I heard recently that Nginx has added caching to its reverse proxy feature. I looked around but couldn't find much info about it. I want to set up Nginx as a caching reverse proxy in front of Apache/Django: to have Nginx proxy requests for some (but not all) dynamic pages to Apache, then cache the generated pages and serve subsequent requests for those pages from cache. Ideally I'd want to invalidate cache in 2 ways: Set an expiration date on the cached item To explicitly invalidate the cached item. E.g. if my Django backend has updated certain data, I'd want to tell Nginx to invalidate the cache of the affected pages Is it possible to set Nginx to do that? How?

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  • MySQL: how to convert many MyISAM tables to InnoDB in a production database?

    - by Continuation
    We have a production database that is made up entirely of MyISAM tables. We are considering converting them to InnoDB to gain better concurrency & reliability. Can I just alter the myISAM tables to InnoDB without shutting down MySQL? What are the recommend procedures here? How long will such a conversion take? All the tables have a total size of about 700MB There are quite a large number of tables. Is there any way to apply ALTER TABLE to all the MyISAM tables at once instead of doing it one by one? Any pitfalls I need to be aware of? Thank you

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  • Linux: How do I use Munin in cPanel to monitor MySQL?

    - by Continuation
    I have a cPanel server running CentOS 5.5. I want to use Munin to monitor MySQL. I went to: Main >> cPanel >> Manage Plugins and selected "Install and keep updated" for Munin and clicked "Save". I got the usual bunch of status updates about the install. At the end I got: Going to read '/home/.cpan/sources/modules/02packages.details.txt.gz' Database was generated on Wed, 02 Mar 2011 18:28:33 GMT ..........................................................................DONE Going to read '/home/.cpan/sources/modules/03modlist.data.gz' Out of memory! Callback called exit. Done Done Done Process Complete As you can see I got an "Out of memory!" message. But after that it said "Process Complete". Was Munin installed? When I went back to "Manage Plugins" it Munin has a check that against "Install and keep updated". So is everything alright? And how do I use Munin now? How do i configure it to monitor MySQL? Where can I see the results? Thanks.

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  • Is it safe to set MySQL isolation to "Read Uncommitted" (dirty reads) for typical Web usage? Even with replication?

    - by Continuation
    I'm working on a website with typical CRUD web usage pattern: similar to blogs or forums where users create/update contents and other users read the content. Seems like it's OK to set the database's isolation level to "Read Uncommitted" (dirty reads) in this case. My understanding of the general drawback of "Read Uncommitted" is that a reader may read uncommitted data that will later be rollbacked. In a CRUD blog/forum usage pattern, will there ever be any rollback? And even if there is, is there any major problem with reading uncommitted data? Right now I'm not using any replication, but in the future if I want to use replication (row-based, not statement-based) will a "Read Uncommitted" isolation level prevent me from doing so? What do you think? Has anyone tried using "Read Uncommitted" on their RDBMS?

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  • How to set up Nginx as a caching reverse proxy?

    - by Continuation
    I heard recently that Nginx has added caching to its reverse proxy feature. I looked around but couldn't find much info about it. I want to set up Nginx as a caching reverse proxy in front of Apache/Django: to have Nginx proxy requests for some (but not all) dynamic pages to Apache, then cache the generated pages and serve subsequent requests for those pages from cache. Ideally I'd want to invalidate cache in 2 ways: Set an expiration date on the cached item To explicitly invalidate the cached item. E.g. if my Django backend has updated certain data, I'd want to tell Nginx to invalidate the cache of the affected pages Is it possible to set Nginx to do that? How?

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  • What are the pros & cons of these MySQL engines for OLTP -- XtraDB, PBXT, or TokuDB?

    - by Continuation
    I'm working on a social website with an approximate read/write split of 90/10. Trying to decide on a MySQL engine. The ones I'm interested in are: XtraDB PBXT TokuDB What are the pros and cons of them for my use case? A few specific questions: PBXT uses log-based structure that avoids double-writes. It sounds very elegant, but the benchmark I've seen doesn't show any/much advantages over XtraDB. Do you have any experience with PBXT/XtraDB you can share? TokuDB sounds VERY interesting. But all the benchmarks I've seen are about single-threaded bulk inserts - inserting 100M rows for example. that's not very relevant for OLTP. What about its performance with large number of concurrent threads writing and reading at the same time? Anyone has tried that?

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  • What do desktop users use SSD for? [closed]

    - by Continuation
    I keep hearing people say how much faster their desktops/laptops are after switching to SSD. But then when I ask them what do they use SSD for, the answers are always "booting". Not trying to start a flame war here. I use SSD for my database server and it makes a huge difference. A single SSD can replace a 10 drives RAID 10 and is actually both much faster and MUCH CHEAPER. But I just can't think of a usecase for desktops where SSD could make a similar impact. Sure it's kinda nice to cut boot time down from 40 seconds to 10 seconds. But is it that big a deal? Would love to hear how SSD improves your desktop performance.

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  • How to create a map-like (clouds) texture [duplicate]

    - by user16547
    This question already has an answer here: How do you generate tileable Perlin noise? 9 answers If you place a map of the world on a sphere, it will look like the image is continuous. Basically the left end of the image is sort of a continuation of the right end. You won't be able to see any cuts. I'm trying to create a clouds texture to add to my planet such that it will seem it has clouds. I managed to create the clouds in GIMP, however, I can't figure out how to make sure the left end of my image is a smooth continuation of the right end. For example if you were to map the below image to your sphere (I removed transparency to make it clearer), there would be a very obvious transition from the right end of the image back to the left end on your sphere. How would I create a texture such that I get rid of that? Sorry for my lack of terminology.

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  • Linux HA / cluster: what are the differences between Pacemaker, Heartbeat, Corosync, wackamole?

    - by Continuation
    Can you help me understand Linux HA? Pacemaker, Heartbeat, Corosync seem to be part of a whole HA stack, but how do they fit together? How does wackamole differ from Pacemaker/Heartbeat/Corosync? I've seen opinions that wackamole is better than Heartbeat because it's peer-based. Is that valid? The last release of wackamole was 2.5 years ago. Is it still being maintained or active? What would you recommend for HA setup for web/application/database servers?

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  • Jetty 7 NIO issue?

    - by Continuation
    I saw this test showing Jetty 7's performance drops drastically when switched from blocking IO to NIO (95% drop): http://wiki.apache.org/HttpComponents/HttpCoreBenchmark Is this a known issue? Have you experienced it first hand? Should I be avoiding NIO on Jetty?

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  • How do you deal with denormalization / secondary indexes in database sharding?

    - by Continuation
    Say I have a "message" table with 2 secondary indexes: "recipient_id" "sender_id" I want to shard the "message" table by "recipient_id". That way to retrieve all messages sent to a certain recipient I only need to query one shard. But at the same time, I want to be able to make a query that ask for all messages sent by a certain sender. Now I don't want to send that query to every single shard of the "message" table. One way to do this is to duplicate the data and have a "message_by_sender" table sharded by "sender_id". The problem with that approach is that every time a message has been sent, I need to insert the message into both "message" and "message_by_sender" tables. But what if after inserting into "message" the insertion into "message_by_sender" fail? In that case the message exists in "message" but not in "message_by_sender". How do I make sure that if a message exists in "message" then it also exists in "message_by_sender" without resorting to 2 phase commit? This must be a very common issue for anyone who shards their databases. How do you deal woth it?

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  • How well does Solr scale over large number of facet values?

    - by Continuation
    I'm using Solr and I want to facet over a field "group". Since "group" is created by users, potentially there can be a huge number of values for "group". Would Solr be able to handle a use case like this? Or is Solr not really appropriate for facet fields with a large number of values? I understand that I can set facet.limit to restrict the number of values returned for a facet field. Would this help in my case? Say there are 100,000 matching values for "group" in a search, if I set facet.limit to 50. would that speed up the query, or would the query still be slow because Solr still needs to process and sort through all the facet values and return the top 50 ones? Any tips on how to tune Solr for large number of facet values? Thanks.

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  • How to make Django work with unsupported MySQL drivers such as gevent-mysql or Concurrence's MySQL d

    - by Continuation
    I'm interested in running Django on an async framework like Concurrence or gevent. Both frameworks come with its own async MySQL driver. Problem is Django only officially supports MySQLdb. What do I need to do to make Django work with the MySQL drivers that come with gevent or Concurrence? Is there a step-by-step guide somewhere that I can follow? Is this a major undertaking? Thanks.

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  • Python: why can't descriptors be instance variables?

    - by Continuation
    Say I define this descriptor: class MyDescriptor(object): def __get__(self, instance, owner): return self._value def __set__(self, instance, value): self._value = value def __delete__(self, instance): del(self._value) And I use it in this: class MyClass1(object): value = MyDescriptor() >>> m1 = MyClass1() >>> m1.value = 1 >>> m2 = MyClass1() >>> m2.value = 2 >>> m1.value 2 So value is a class attribute and is shared by all instances. Now if I define this: class MyClass2(object) value = 1 >>> y1 = MyClass2() >>> y1.value=1 >>> y2 = MyClass2() >>> y2.value=2 >>> y1.value 1 In this case value is an instance attribute and is not shared by the instances. Why is it that when value is a descriptor it can only be a class attribute, but when value is a simple integer it becomes an instance attribute?

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  • How the yin-yang puzzle works?

    - by Hrundik
    I'm trying to grasp the semantics of call/cc in Scheme, and the Wikipedia page on continuations shows the yin-yang puzzle as an example: (let* ((yin ((lambda (cc) (display #\@) cc) (call-with-current-continuation (lambda (c) c)))) (yang ((lambda (cc) (display #\*) cc) (call-with-current-continuation (lambda (c) c)))) ) (yin yang)) It should output @\*@\**@\**\*@\**\**@..., but I don't understand why; I'd expect it to output @\*@\**\**\**\**\*... Can somebody explain in detail why the yin-yang puzzle works the way it works?

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  • MVC HTML.RenderAction – Error: Duration must be a positive number

    - by BarDev
    On my website I want the user to have the ability to login/logout from any page. When the user select login button a modal dialog will be present to the user for him to enter in his credentials. Since login will be on every page, I thought I would create a partial view for the login and add it to the layout page. But when I did this I got the following error: Exception Details: System.InvalidOperationException: Duration must be a positive number. There are other ways to work around this that would not using partial views, but I believe this should work. So to test this, I decided to make everything simple with the following code: Created a layout page with the following code @{Html.RenderAction("_Login", "Account");} In the AccountController: public ActionResult _Login() { return PartialView("_Login"); } Partial View _Login <a id="signin">Login</a> But when I run this simple version this I still get this error: Exception Details: System.InvalidOperationException: Duration must be a positive number. Source of error points to "@{Html.RenderAction("_Login", "Account");}" There are some conversations on the web that are similar to my problem, which identifies this as bug with MVC (see links below). But the links pertain to Caching, and I'm not doing any caching. OuputCache Cache Profile does not work for child actions http://aspnet.codeplex.com/workitem/7923 Asp.Net MVC 3 Partial Page Output Caching Not Honoring Config Settings Asp.Net MVC 3 Partial Page Output Caching Not Honoring Config Settings Caching ChildActions using cache profiles won't work? Caching ChildActions using cache profiles won't work? I'm not sure if this makes a difference, but I'll go ahead and add it here. I'm using MVC 3 with Razor. Update Stack Trace [InvalidOperationException: Duration must be a positive number.] System.Web.Mvc.OutputCacheAttribute.ValidateChildActionConfiguration() +624394 System.Web.Mvc.OutputCacheAttribute.OnActionExecuting(ActionExecutingContext filterContext) +127 System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func1 continuation) +72 System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func1 continuation) +784922 System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodWithFilters(ControllerContext controllerContext, IList1 filters, ActionDescriptor actionDescriptor, IDictionary2 parameters) +314 System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) +784976 System.Web.Mvc.Controller.ExecuteCore() +159 System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext) +335 System.Web.Mvc.<c_DisplayClassb.b_5() +62 System.Web.Mvc.Async.<c_DisplayClass1.b_0() +20 System.Web.Mvc.<c_DisplayClasse.b_d() +54 System.Web.Mvc.<c_DisplayClass4.b_3() +15 System.Web.Mvc.ServerExecuteHttpHandlerWrapper.Wrap(Func`1 func) +41 System.Web.HttpServerUtility.ExecuteInternal(IHttpHandler handler, TextWriter writer, Boolean preserveForm, Boolean setPreviousPage, VirtualPath path, VirtualPath filePath, String physPath, Exception error, String queryStringOverride) +1363 [HttpException (0x80004005): Error executing child request for handler 'System.Web.Mvc.HttpHandlerUtil+ServerExecuteHttpHandlerAsyncWrapper'.] System.Web.HttpServerUtility.ExecuteInternal(IHttpHandler handler, TextWriter writer, Boolean preserveForm, Boolean setPreviousPage, VirtualPath path, VirtualPath filePath, String physPath, Exception error, String queryStringOverride) +2419 System.Web.HttpServerUtility.Execute(IHttpHandler handler, TextWriter writer, Boolean preserveForm, Boolean setPreviousPage) +275 System.Web.HttpServerUtilityWrapper.Execute(IHttpHandler handler, TextWriter writer, Boolean preserveForm) +94 System.Web.Mvc.Html.ChildActionExtensions.ActionHelper(HtmlHelper htmlHelper, String actionName, String controllerName, RouteValueDictionary routeValues, TextWriter textWriter) +838 System.Web.Mvc.Html.ChildActionExtensions.RenderAction(HtmlHelper htmlHelper, String actionName, String controllerName, RouteValueDictionary routeValues) +56 ASP._Page_Views_Shared_SiteLayout_cshtml.Execute() in c:\Projects\Odat Projects\Odat\Source\Presentation\Odat.PublicWebSite\Views\Shared\SiteLayout.cshtml:80 System.Web.WebPages.WebPageBase.ExecutePageHierarchy() +280 System.Web.Mvc.WebViewPage.ExecutePageHierarchy() +104 System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) +173 System.Web.WebPages.WebPageBase.Write(HelperResult result) +89 System.Web.WebPages.WebPageBase.RenderSurrounding(String partialViewName, Action1 body) +234 System.Web.WebPages.WebPageBase.PopContext() +234 System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context) +384 System.Web.Mvc.<>c__DisplayClass1c.<InvokeActionResultWithFilters>b__19() +33 System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func1 continuation) +784900 System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func1 continuation) +784900 System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext controllerContext, IList1 filters, ActionResult actionResult) +265 System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) +784976 System.Web.Mvc.Controller.ExecuteCore() +159 System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext) +335 System.Web.Mvc.<c_DisplayClassb.b_5() +62 System.Web.Mvc.Async.<c_DisplayClass1.b_0() +20 System.Web.Mvc.<c_DisplayClasse.b_d() +54 System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +453 System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +371 Update When I Break in Code, it errors at @{Html.RenderAction("_Login", "Account");} with the following exception. The inner exception Error executing child request for handler 'System.Web.Mvc.HttpHandlerUtil+ServerExecuteHttpHandlerAsyncWrapper'. at System.Web.HttpServerUtility.ExecuteInternal(IHttpHandler handler, TextWriter writer, Boolean preserveForm, Boolean setPreviousPage, VirtualPath path, VirtualPath filePath, String physPath, Exception error, String queryStringOverride) at System.Web.HttpServerUtility.Execute(IHttpHandler handler, TextWriter writer, Boolean preserveForm, Boolean setPreviousPage) at System.Web.HttpServerUtilityWrapper.Execute(IHttpHandler handler, TextWriter writer, Boolean preserveForm) at System.Web.Mvc.Html.ChildActionExtensions.ActionHelper(HtmlHelper htmlHelper, String actionName, String controllerName, RouteValueDictionary routeValues, TextWriter textWriter) at System.Web.Mvc.Html.ChildActionExtensions.RenderAction(HtmlHelper htmlHelper, String actionName, String controllerName, RouteValueDictionary routeValues) at ASP._Page_Views_Shared_SiteLayout_cshtml.Execute() in c:\Projects\Odat Projects\Odat\Source\Presentation\Odat.PublicWebSite\Views\Shared\SiteLayout.cshtml:line 80 at System.Web.WebPages.WebPageBase.ExecutePageHierarchy() at System.Web.Mvc.WebViewPage.ExecutePageHierarchy() at System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage) at System.Web.WebPages.WebPageBase.Write(HelperResult result) at System.Web.WebPages.WebPageBase.RenderSurrounding(String partialViewName, Action1 body) at System.Web.WebPages.WebPageBase.PopContext() at System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context) at System.Web.Mvc.ControllerActionInvoker.<>c__DisplayClass1c.<InvokeActionResultWithFilters>b__19() at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func1 continuation) Answer Thanks Darin Dimitrov Come to find out, my AccountController had the following attribute [System.Web.Mvc.OutputCache(NoStore =true, Duration = 0, VaryByParam = "*")]. I don't believe this should caused a problem, but when I removed the attribute everything worked. BarDev

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  • Practical Scheme Programming

    - by Ixmatus
    It's been a few months since I've touched Scheme and decided to implement a command line income partitioner using Scheme. My initial implementation used plain recursion over the continuation, but I figured a continuation would be more appropriate to this type of program. I would appreciate it if anyone (more skilled with Scheme than I) could take a look at this and suggest improvements. I'm that the multiple (display... lines is an ideal opportunity to use a macro as well (I just haven't gotten to macros yet). (define (ab-income) (call/cc (lambda (cc) (let ((out (display "Income: ")) (income (string->number (read-line)))) (cond ((<= income 600) (display (format "Please enter an amount greater than $600.00~n~n")) (cc (ab-income))) (else (let ((bills (* (/ 30 100) income)) (taxes (* (/ 20 100) income)) (savings (* (/ 10 100) income)) (checking (* (/ 40 100) income))) (display (format "~nDeduct for bills:---------------------- $~a~n" (real->decimal-string bills 2))) (display (format "Deduct for taxes:---------------------- $~a~n" (real->decimal-string taxes 2))) (display (format "Deduct for savings:-------------------- $~a~n" (real->decimal-string savings 2))) (display (format "Remainder for checking:---------------- $~a~n" (real->decimal-string checking 2)))))))))) Invoking (ab-income) asks for input and if anything below 600 is provided it (from my understanding) returns (ab-income) at the current-continuation. My first implementation (as I said earlier) used plain-jane recursion. It wasn't bad at all either but I figured every return call to (ab-income) if the value was below 600 kept expanding the function. (please correct me if that apprehension is incorrect!)

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