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• How to calculate a RTOS task's time

  - by Adnan

  Hello all, I have written a code in c for Arm7 using RTOS. There are multiple tasks who's priority is set to same level. So the tasks executes on round-robin base. There is an exception that one task (Default) has set to lower priority then the other task in rtos. So that if no task is running, the default or lower priority task runs. Now i want to calculate the exact total timing (time duration) for that default task runs. Can any one give some idea what to do .... and how to do in code.. Regards Dani

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• How does a VxWorks scheduler get executed?

  - by Ashwin

  Hello All, Would like to know how the scheduler gets called so that it can switch tasks. As in even if its preemptive scheduling or round robin scheduling - the scheduler should come in to picture to do any kind of task switching. Supposing a low priority task has an infinite loop - when does the scheduler intervene and switch to a higher priority task? Query is: 1. Who calls the scheduler? [in VxWorks] 2. If it gets called at regular intervals - how is that mechanism implemented? Thanks in advance. --Ashwin

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• How to properly siglongjmp out of signal handler?

  - by EpsilonVector

  Suppose I have the following code: In order to implement a context switch I activate ualarm and when it jumps to the handler it setjmp's the current context, and longjmps to the next, expecting to eventually return to the alarm handler and longjmped back into this context (the contexts are cycled through in a Round Robin). For this I need to keep SIGALRM unblocked in between alarm_handlers. I came up with the following code, which doesn't seem to work. What's wrong with it and what is the right way to do this? void alarm_handler(){ if(sigsetjmp(toc->threads[toc->RR_pointer].env, 0)){ ualarm(200, 0); signal(SIGALRM, alarm_handler); return; } get_next_context_number(toc->RR_pointer); //is a macro for (j=0; j<10; j++) printf("ALARM HANDLER\n"); siglongjmp(toc->threads[toc->RR_pointer].env, 1); }

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• language to create flowcharts

  - by robintw

  Hi, This seems like something which must have been answered before, but I can't find anything appropriate in the question archives. Basically, I'm looking for a little Domain Specific Language to create flowcharts. I'm terrible at graphic design and making things look nice, and I'd really like a langauge where I could write something in code and it would produce a pretty flowchart. I've come across GraphViz, but it seems more suited to creating things like Finite State Machine diagrams, rather than process flowcharts. It also doesn't have the simple DSL-style front-end that would allow me to easily work it. Any ideas? I'm sure this must have been done before... Robin

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• How to indefinitely pause a thread in Java and later resume it?

  - by Carlos Torres

  Maybe this question has been asked many times before, but I never found a satisfying answer. The problem: I have to simulate a process scheduler, using the round robin strategy. I'm using threads to simulate processes and multiprogramming; everything works fine with the JVM managing the threads. But the thing is that now I want to have control of all the threads so that I can run each thread alone by a certain quantum (or time), just like real OS processes schedulers. What I'm thinking to do: I want have a list of all threads, as I iterate the list I want to execute each thread for their corresponding quantum, but as soon the time's up I want to pause that thread indefinitely until all threads in the list are executed and then when I reach the same thread again resume it and so on. The question: So is their a way, without using deprecated methods stop(), suspend(), or resume(), to have this control over threads?

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• Map the physical file path in asp.net mvc

  - by rmassart

  Hi, I am trying to read an XSLT file from disk in my ASP.Net MVC controller. What I am doing is the following: string filepath = HttpContext.Request.PhysicalApplicationPath; filepath += "/Content/Xsl/pubmed.xslt"; string xsl = System.IO.File.ReadAllText(filepath); However, half way down this thread on forums.asp.net there is the following quote HttpContext.Current is evil and if you use it anywhere in your mvc app you are doing something wrong because you do not need it. Whilst I am not using "Current", I am wondering what is the best way to determine the absolute physical path of a file in MVC? For some reason (I don't know why!) HttpContext doesn't feel right for me. Is there a better (or recommended/best practice) way of reading files from disk in ASP.Net MVC? Thanks for your help, Robin

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• How to limit the wordpress tagcloud by date?

  - by Nordin

  Hello, I've been searching for quite a while now to find a way to limit wordpress tags by date and order them by the amount of times they appeared in the selected timeframe. But I've been rather unsuccesful. What I'm trying to achieve is something like the trending topics on Twitter. But in this case, 'trending tags'. By default the wordpress tagcloud displays the most popular tags of all time. Which makes no sense in my case, since I want to track current trends. Ideally it would be something like: Most popular tags of today Obama (18 mentions) New York (15 mentions) Iron Man (11 mentions) Robin Hood (7 mentions) And then multiplied for 'most popular this week' and 'most popular this month'. Does anyone know of a way to achieve this?

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• Load balancer - how to write one for a custom application?

  - by Poni

  Hi! I've written a simple server application which will run distributed on several machines. My question is how does a network load balancer works, in general? I've heard of round-robin and other algorithms, but what I haven't got answer to is how does the process really goes? In socket terms. The client connects to one of the load balancer machines, asks for a "free-to-connect-to" server and simply connects to it? That's the simpliest way I can think of. .. or, does it use the load balancer as a proxy (that implies that all the NBs must be always connected to the application servers, and data is transferred through them)? It's more of a general question. How would you do this? Thank you all!

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• Horizontal Scaling of Tomcat in Microsoft Azure

  - by Fabe

  Hey everyone, I am working on this quiet a while, but still no conclustion. I want to do horizontal scaling of Tomcat instances in Microsoft Azure (1,2,3,... Tomcat instances for one service). I read lots of articles about session replication, clustering,... with Tomcat. Since Azure does not support Multicasts, there is no easy way to cluster Tomcat. Also sticky sessions is no options, because Azure does round robin load balancing. Setting up two services - one with Terracotta or Apache mod_jk - and the other with Tomcat instances seems overkill for me (if even doable)... Is this even possible? Thanks in advance for reading and answering my question. Every comment/idea is highly appreciated.

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• TDD approach for complex function

  - by jamie

  I have a method in a class for which they are a few different outcomes (based upon event responses etc). But this is a single atomic function which is to used by other applications. I have broken down the main blocks of the functionality that comprise this function into different functions and successfully taken a Test Driven Development approach to the functionality of each of these elements. These elements however aren't exposed for other applications would use. And so my question is how can/should i easily approach a TDD style solution to verifying that the single method that should be called does function correctly without a lot of duplication in testing or lots of setup required for each test? I have considered / looked at moving the blocks of functionality into a different class and use Mocking to simulate the responses of the functions used but it doesn't feel right and the individual methods need to write to variables within the main class (it felt really heath robinson). The code roughly looks like this (i have removed a lot of parameters to make things clearer along with a fair bit of irrelevant code). public void MethodToTest(string parameter) { IResponse x = null; if (function1(parameter)) { if (!function2(parameter,out x)) { function3(parameter, out x); } } // ... // more bits of code here // ... if (x != null) { x.Success(); } }

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• Programmatically determine the relative "popularities" of a list of items (books, songs, movies, etc

  - by Horace Loeb

  Given a list of (say) songs, what's the best way to determine their relative "popularity"? My first thought is to use Google Trends. This list of songs: Subterranean Homesick Blues Empire State of Mind California Gurls produces the following Google Trends report: (to find out what's popular now, I restricted the report to the last 30 days) Empire State of Mind is marginally more popular than California Gurls, and Subterranean Homesick Blues is far less popular than either. So this works pretty well, but what happens when your list is 100 or 1000 songs long? Google Trends only allows you to compare 5 terms at once, so absent a huge round-robin, what's the right approach? Another option is to just do a Google Search for each song and see which has the most results, but this doesn't really measure the same thing

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• Java Dynamic Binding

  - by Chris Okyen

  I am having trouble understanding the OOP Polymorphic principl of Dynamic Binding ( Late Binding ) in Java. I looked for question pertaining to java, and wasn't sure if a overall answer to how dynamic binding works would pertain to Java Dynamic Binding, I wrote this question. Given: class Person { private String name; Person(intitialName) { name = initialName; } // irrelevant methods is here. // Overides Objects method public void writeOutput() { println(name); } } class Student extends Person { private int studentNumber; Student(String intitialName, int initialStudentNumber) { super(intitialName); studentNumber = initialStudentNumber; } // irrellevant methods here... // overides Person, Student and Objects method public void writeOutput() { super.writeOutput(); println(studentNumber); } } class Undergaraduate extends Student { private int level; Undergraduate(String intitialName, int initialStudentNumber,int initialLevel) { super(intitialName,initialStudentNumber); level = initialLevel; } // irrelevant methods is here. // overides Person, Student and Objects method public void writeOutput() { super.writeOutput(); println(level); } } I am wondering. if I had an array called person declared to contain objects of type Person: Person[] people = new Person[2]; person[0] = new Undergraduate("Cotty, Manny",4910,1); person[1] = new Student("DeBanque, Robin", 8812); Given that person[] is declared to be of type Person, you would expect, for example, in the third line where person[0] is initialized to a new Undergraduate object,to only gain the instance variable from Person and Persons Methods since doesn't the assignment to a new Undergraduate to it's ancestor denote the Undergraduate object to access Person - it's Ancestors, methods and isntance variables... Thus ...with the following code I would expect person[0].writeOutput(); // calls Undergraduate::writeOutput() person[1].writeOutput(); // calls Student::writeOutput() person[0] to not have Undergraduate's writeOutput() overidden method, nor have person[1] to have Student's overidden method - writeOutput(). If I had Person mikeJones = new Student("Who?,MikeJones",44,4); mikeJones.writeOutput(); The Person::writeOutput() method would be called. Why is this not so? Does it have to do with something I don't understand about relating to arrays? Does the declaration Person[] people = new Person[2] not bind the method like the previous code would?

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• It's All In The Cloud

  - by Natalia Rachelson

  People turned out in droves for Steve Miranda's Apps Cloud General Session. Steve, as engaging as ever, covered our Apps strategy in the cloud and reinforced that Oracle has a complete set of cloud services including: •    Human Capital Management•    Talent Management•    Sales and Marketing•    Customer Service and Support•    Financial Management•    Procurement, Sourcing, and Inventory•    Project Portfolio Management•    Governance, Risk, and Compliance... all delivered on top of the Social, Platform, and Common Infrastructure.Steve talked about Fusion being the centerpiece of our Cloud Services. The fact that Fusion is 100 percent standards based is a big, big deal! In addition, our ERP Cloud Service is the most complete cloud service on the market. And email marketing is dead -- social marketing is where the action is. It's also where Oracle is investing heavily from a Sales & Marketing Cloud perspective. Steve covered the strategic acquisitions Oracle has made to enhance our organic Cloud offering. Specifically, Oracle bought RightNow to make our Customer Service and Support Cloud service complete. We also bought Taleo to add Recruiting and Learning capabilities to our Talent Management Cloud. Steve talked about our customers and how they are benefiting from the use of a variety of our Cloud Services. Red Robin is driving lower labor and food costs with Oracle ERP Cloud Service. He used Elizabeth Arden as the profile customer for HCM and Talent Management Service, UBS for HCM and Talent Management Service, and Brocade for Talent Management. All these customers are benefiting from a comprehensive and fully integrated HR platform that aligns compensation with performance and enhances workforce motivation and retention. At the same time, Hitachi Data Systems is using Oracle Taleo Performance Management Cloud to recruit the right competencies, pinpoint areas of improvement, and develop and monitor employee goals to support the global account organization. KLM and Overstock.com are gaining the benefits of Oracle's Customer Service and Support Service from RightNow by better engaging and serving customer needs online and through call centers. And last but not least, Graco and Key Energy are leveraging mobility features and sales forecasting and territory management capabilities within the Oracle Sales and Marketing Service. They expect to gain better visibility to sales information and drive more efficient sales campaigns and empower their sales force with data they need to make sales. Overall, Oracle Apps Cloud Services are enjoying a significant momentum in the marketplace. Steve projected an air of confidence and enthusiasm highlighting Oracle's latest successes with Cloud services.

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• Understanding Linux SCSI queue depths

  - by Troels Arvin

  I'm experimenting with the effects of different SCSI queue depth values on a Dell server running CentOS Linux 5.4 (x86_64). The server has two QLogic QLE2560 FC HBAs connected via multipathing to a storage system. The storage system has allocated two LUNs to the server, each connected through four paths in an active-active-active-active round-robin configuration. All in all, the two LUNs exist as eight /dev/sdX devices, represented by two devices in /dev/mpath. I currently adjust the queue depth values in /etc/modprobe.conf and check the result (after rebooting) by looking in the seventh column of /proc/scsi/sg/devices. Two questions related to that: Is there a way to adjust queue depths without rebooting or unloading the qla2xxx kernel module? E.g., can I echo a new queue depth value into some /proc or /sys-like file to update the queue depth? If I set the queue depth to 128, is that 128 in total for all devices handled by the qla2xxx module?, or 128 for each HBA? (256 in total), or 128 for each of the eight /dev/sdX devices (1024 in toal)?, or 128 for each of the two /dev/mpath/... devices (256 in total)? This is important for me to know so that my server doesn't flood the storage system, affecting other servers connected to it.

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• Mounting Replicated Gluster Multi-AZ Storage

  - by Roman Newaza

  I have Replicated Gluster Storage which is used by Auto scaling Servers. Both, Auto scaling and Storage are allocated in two Availability zones. Gluster: Number of Bricks: 4 x 2 = 8 Transport-type: tcp Bricks: Brick1: gluster01:/storage/1a # Zone A Brick2: gluster02:/storage/1b # Zone B Brick3: gluster03:/storage/2a # Zone A Brick4: gluster04:/storage/2b # Zone B Brick5: gluster01:/storage/3a # Zone A Brick6: gluster02:/storage/3b # Zone B Brick7: gluster03:/storage/4a # Zone A Brick8: gluster04:/storage/4b # Zone B I used Round Robin DNS for Gluster entry point, so DNS name resolves to all of the storage server addresses which are returned in different order all the time: # host storage.domain.com storage.domain.com has address xx.xx.xx.x1 storage.domain.com has address xx.xx.xx.x2 storage.domain.com has address xx.xx.xx.x3 storage.domain.com has address xx.xx.xx.x4 The Storage is mounted with Native Gluster Client: # grep storage /etc/fstab storage.domain.com:/storage /storage glusterfs defaults,log-level=WARNING,log-file=/var/log/gluster.log 0 0 I have heard Gluster might be mounted with the first Server IP and after that it will fetch its configuration with the rest of Servers. Personally, I never tested single Server mount setup and I don't know how Gluster handles this. On EC2, traffic among single Availability zone is free and between different zones is not. When Client in zone A writes to storage and IP of Storage in zone B is returned, it will cost me twice more for data transfer: Client (Zone A) - Storage Server (Zone B) - Replication to Storage Server (Zone A). Question: Would it be better to mount Storage Server of the same zone, so that data transfer charges apply only for replication (A - A - B)?

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• BGP Multihomed/Multi-location best practice

  - by Tom O'Connor

  We're in the process of designing a new iteration of our network where we improve resilliency by adding a second datacentre. We'll be adding a second datacentre, with an identical configuration of servers as our primary location. To achieve network connectivity, we're looking into a couple of possible methods. See earlier questions http://serverfault.com/questions/86736/best-way-to-improve-resilience and http://serverfault.com/questions/101582/dns-round-robin-failover-and-load-balancing I'm pretty convinced that BGP is the right way to go about this, and this question is not about RRDNS. 1) If we have 2 locations, do we announce the same IP address block from both locations? 2) If we did this, but had a management ssh interface on x.x.x.50 from datacentre A, but it was on x.x.x.150 in datacentre B. What is the best practice mechanism for achieving this? Because if I were nearest to A, then all my traffic would go to x.50, but if i attempted to connect to x.150, I'd not be able to connect, because this address wouldn't be valid at A, but only at B. Is the best solution to announce 2 different netblocks, one at each location, facilitating the need for RRDNS, or to announce a single block, and run some form of VPN between the two sites for managment traffic?

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• Java EE at JavaOne - A Few Picks from a Very Rich Line-up

  - by Janice J. Heiss

  A rich and diverse set of sessions cast a spotlight on Java EE at this year’s JavaOne, ranging from the popular Web Framework Smackdown, to Java EE 6 and Spring, to sessions exploring Java EE 7, and one on the implications of HTML5. Some of the world’s best EE architects and developers will be sharing their insight and expertise. If only I could be at ten places at once!BOF4149 - Web Framework Smackdown 2012    Markus Eisele - Principal IT Architect, msg systems ag    Graeme Rocher - Senior Staff Engineer, VMware    James Ward - Developer Evangelist, Heroku    Ed Burns - Consulting Member of Technical Staff, Oracle    Santiago Pericasgeertsen - Software Engineer, Oracle* Monday, Oct 1, 8:30 PM - 9:15 PM - Parc 55 - Cyril Magnin II/III Much has changed since the first Web framework smackdown, at JavaOne 2005. Or has it? The 2012 edition of this popular panel discussion surveys the current landscape of Web UI frameworks for the Java platform. The 2005 edition featured JSF, Webwork, Struts, Tapestry, and Wicket. The 2012 edition features representatives of the current crop of frameworks, with a special emphasis on frameworks that leverage HTML5 and thin-server architecture. Java Champion Markus Eisele leads the lively discussion with panelists James Ward (Play), Graeme Rocher (Grails), Edward Burns (JSF) and Santiago Pericasgeertsen (Avatar).CON6430 - Java EE and Spring Framework Panel Discussion    Richard Hightower - Developer, InfoQ    Bert Ertman - Fellow, Luminis    Gordon Dickens - Technical Architect, IT101, Inc.    Chris Beams - Senior Technical Staff, VMware    Arun Gupta - Technology Evangelist, Oracle* Tuesday, Oct 2, 10:00 AM - 11:00 AM - Parc 55 - Cyril Magnin II/III In the age of Java EE 6 and Spring 3, enterprise Java developers have many architectural choices, including Java EE 6 and Spring, but which one is right for your project? Many of us have heard the debate and seen the flame wars—it’s a topic with passionate community members, and it’s a vibrant debate. If you are looking for some level-headed discussion, grounded in real experience, by developers who have tried both, then come join this discussion. InfoQ’s Java editors moderate the discussion, and they are joined by independent consultants and representatives from both Java EE and VMWare/SpringSource.BOF4213 - Meet the Java EE 7 Specification Leads   Linda Demichiel - Consulting Member of Technical Staff, Oracle   Bill Shannon - Architect, Oracle* Tuesday, Oct 2, 5:30 PM - 6:15 PM – Parc 55 - Cyril Magnin II/III This is your chance to meet face-to-face with the engineers who are developing the next version of the Java EE platform. In this session, the specification leads for the leading technologies that are part of the Java EE 7 platform discuss new and upcoming features and answer your questions. Come prepared with your questions, your feedback, and your suggestions for new features in Java EE 7 and beyond.CON10656 - JavaEE.Next(): Java EE 7, 8, and Beyond    Ian Robinson - IBM Distinguished Engineer, IBM    Mark Little - JBoss CTO, NA    Scott Ferguson - Developer, Caucho Technology    Cameron Purdy - VP Development, Oracle*Wednesday, Oct 3, 4:30 PM - 5:30 PM - Parc 55 - Cyril Magnin II/IIIIn this session, hear from a distinguished panel of industry and open source luminaries regarding where they believe the Java EE community is headed, starting with Java EE 7. The focus of Java EE 7 and 8 is mostly on the cloud, specifically aiming to bring platform as a service (PaaS) providers and application developers together so that portable applications can be deployed on any cloud infrastructure and reap all its benefits in terms of scalability, elasticity, multitenancy, and so on. Most importantly, Java EE will leverage the modularization work in the underlying Java SE platform. Java EE will, of course, also update itself for trends such as HTML5, caching, NoSQL, ployglot programming, map/reduce, JSON, REST, and improvements to existing core APIs.CON7001 - HTML5 WebSocket and Java    Danny Coward - Java, Oracle*Wednesday, Oct 3, 4:30 PM - 5:30 PM - Parc 55 - Cyril Magnin IThe family of HTML5 technologies has pushed the pendulum away from rich client technologies and toward ever-more-capable Web clients running on today’s browsers. In particular, WebSocket brings new opportunities for efficient peer-to-peer communication, providing the basis for a new generation of interactive and “live” Web applications. This session examines the efforts under way to support WebSocket in the Java programming model, from its base-level integration in the Java Servlet and Java EE containers to a new, easy-to-use API and toolset that are destined to become part of the standard Java platform.

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• how to design pound -> varnish -> jboss for ha + loadbalancing

  - by andreash

  Hello, I'm planning a new infrastructure for our web application. We have two JBossAS5 servers, running in a cluster. Session state will be replicated via JBoss Cache. In front of that, there should be some cache, to speed up delivery of static elements. However, most of the traffic to our app will be via HTTPS. So far, I had been thinking of two Varnish caches in front of the JBossASs, each being configured for loadbalancing to the two JBossASs via round-robin. Since Varnish doesn't handle HTTPS, then there would need to be two pound proxies in front of the Varnishs, dealing with the HTTPS. The two pounds would be made high-available with Heartbeat/LinuxHA. The traffic to www.example.com would then be going through our firewall, from there to the virtual IP of the pounds, from there to the Varnishs, and from there to the JBossASs. Question 1: Does this make sense? Or is it overly complicated, and the same goal can be reached with simpler methods? Question 2: If my layout is fine, how do I configure the pound - Varnish step? Should I a) make the Varnish service high-available through Heartbeat/LinuxHA as well and direct traffic from pound to the virtual IP of the Varnishs, or should I rather b) Configure two independent Varnishs and use load-balancing in pound to address the different Varnishs? Thanks a lot for your insight! Andreas.

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• Multi-IP address zimbra server DNS PTR records and spam

  - by David Fraser

  We have a mail server running Zimbra (ZCS 6.0.8). The server has 5 active public IP addresses in the same subnet. (.226-.230). I currently have A records for each of these (host0.domain.com..host4.domain.com), with the main host.domain.com of the machine pointing to .226. Our host has ended up being listed on the SORBS DUHL list (even though it's in a server farm). According to them you can get removed quickly by checking that your host has an MX record, an A record, and a PTR record that points back to the hostname given in the MX record. I tried setting the PTR records so that each of these addresses resolved back to their A record (i.e. .228 had a PTR to host2.domain.com). However, I then got mail being rejected from other servers because when Postfix (under Zimbra control) sends out mail, it uses the main hostname for the HELO - there doesn't seem to be any way to override it. So the PTR records currently say host.domain.com for all 5 IP addresses. What's the correct way to handle this? Should I have an A record for the domain that points to all the IP addresses (for round-robin handling)? I'm nervous of changes that could cause problems, so I'm wondering what the standard way to handle a multiple-IP-address mail server is.

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• Managing multiple Apache proxies simultaneously (mod_proxy_balancer)

  - by Hank

  The frontend of my web application is formed by currently two Apache reverse proxies, using mod_proxy_balancer to distribute traffic over a number of backend application servers. Both frontend reverse proxies, running on separate hosts, are accessible from the internet. DNS round robin distributes traffic over both. In the future, the number of reverse proxies is likely to grow, since the webapplication is very bandwidth-heavy. My question is: how do I keep the state of both reverse balancers / proxies in sync? For example, for maintenance purposes, I might want to reduce the load on one of the backend appservers. Currently I can do that by accessing the Balancer-Manager web form on each proxy, and change the distribution rules. But I have to do that on each proxy manually and make sure I enter the same stuff. Is it possible to "link" multiple instances of mod_proxy_balancer? Or is there a tool out there that connects to a number of instances, and updates all with the same information? Update: The tool should retrieve the runtime status and make runtime changes, just like the existing Balancer-Manager, only for a number of proxies - not just for one. Modification of configuration files is not what I'm interested in (as there are plenty tools for that).

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• How many reverse proxies (nginx, haproxy) is too many?

  - by Alysum

  I'm setting up a HA (high availability) cluster using nginx, haproxy & apache. I've been reading great things about nginx and haproxy. People tend to choose one or the other but I like both. Haproxy is more flexible for load balancing than nginx's simple round robin (even with the upstream-fair patch). But I'd like to keep nginx for redirecting non-https to https among other things right at the point of entry to the cluster. On the other hand, nginx is a lot faster for serving static contents and would reduce the load on the powerful apache which loves to eat a lot of RAM! Here is my planned setup: Load balancer: nginx listens on port 80/443 and proxy_forwards to haproxy on 8080 on the same server to load balance between the multiple nodes. Nodes: nginx on the node listens to requests coming from haproxy on 8080, if the content is static, serve it. But if it's a backend script (in my case PHP), proxy forward to apache2 on the same node server listenning on a different port number. Technically this setup works but my concerns are whether having the requests going through several proxies is going to slow down requests? Most of the requests will be PHP requests as the backends are services (which means groing from nginx - haproxy - nginx - apache). Thoughts? Cheers

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• Can a pool of memcache daemons be used to share sessions more efficiently?

  - by Tom

  We are moving from a 1 webserver setup to a two webserver setup and I need to start sharing PHP sessions between the two load balanced machines. We already have memcached installed (and started) and so I was pleasantly surprized that I could accomplish sharing sessions between the new servers by changing only 3 lines in the php.ini file (the session.save_handler and session.save_path): I replaced: session.save_handler = files with: session.save_handler = memcache Then on the master webserver I set the session.save_path to point to localhost: session.save_path="tcp://localhost:11211" and on the slave webserver I set the session.save_path to point to the master: session.save_path="tcp://192.168.0.1:11211" Job done, I tested it and it works. But... Obviously using memcache means the sessions are in RAM and will be lost if a machine is rebooted or the memcache daemon crashes - I'm a little concerned by this but I am a bit more worried about the network traffic between the two webservers (especially as we scale up) because whenever someone is load balanced to the slave webserver their sessions will be fetched across the network from the master webserver. I was wondering if I could define two save_paths so the machines look in their own session storage before using the network. For example: Master: session.save_path="tcp://localhost:11211, tcp://192.168.0.2:11211" Slave: session.save_path="tcp://localhost:11211, tcp://192.168.0.1:11211" Would this successfully share sessions across the servers AND help performance? i.e save network traffic 50% of the time. Or is this technique only for failovers (e.g. when one memcache daemon is unreachable)? Note: I'm not really asking specifically about memcache replication - more about whether the PHP memcache client can peak inside each memcache daemon in a pool, return a session if it finds one and only create a new session if it doesn't find one in all the stores. As I'm writing this I'm thinking I'm asking a bit much from PHP, lol... Assume: no sticky-sessions, round-robin load balancing, LAMP servers.

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• What's the best way to run Drupal and Django sites behind the same Varnish server?

  - by Alexis Bellido

  I have a high traffic website running with Drupal and Apache, five web servers behind a Varnish server load balancing. Let's say this site is example.com. I'm using five backends and a director like this in my default.vcl: director balancer round-robin { { .backend = web1; } { .backend = web2; } { .backend = web3; } { .backend = web4; } { .backend = web5; } } Now I'm working on a new Django project that will be a new section of this site running on example.com/new-section. After checking the documentation I found I can do something like this: sub vcl_recv { if (req.url ~ "^/new-section/") { set req.backend = newbackend; } else { set req.backend = default; } } That is, using a different backend for a subdirectory /new-section under the same domain. My question is, how do I make something like this work with my director and load balancing setup? I'm probably going to run two or more web servers (backends) with my new Django project, each one with a mix of Gunicorn, Nginx, and a few Python packages, and would like to put all of those in their own Varnish director to load balance. Is it possible to do use the above approach to decide which director to use?, like this: sub vcl_recv { if (req.url ~ "^/new-section/") { set req.director = newdirector; } else { set req.director = balancer; } } All suggestions welcome. Thanks!

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• What's the piece of hardware listening on Facebook's or Wikipedia's IP address?

  - by Igor Ostrovsky

  I am trying to understand how massive sites like Facebook or Wikipedia work, for my intellectual curiosity. I read about various techniques for building scalable sites, but I am still puzzled about one particular detail. The part that confuses me is that ultimately, the DNS will map the entire domain to a single IP address, or a handful of IP addresses in the case of round-robin DNS. For example, wikipedia.org has only one type-A DNS record. So, people from all over the world visiting Wikipedia have to send a request to the one IP address specified in DNS. What is the piece of hardware that listens on the IP address for a massive site, and how can it possibly handle all the load coming from the requests for users all over the world? Edit 1: Thanks for all the responses! Anycast seems like a feasible answer... Does anyone know of a way to check whether a particular IP address is anycast-routed, so that I could verify that this really is the trick used in practice by large sites? Edit 2: After more reading on the topic, it appears that anycast is not typically used for dynamic web content. Anycast is usually used for UDP (e.g., DNS lookups), or sometimes for static content. One interesting thing to note is that Facebook uses profile.ak.fbcdn.net to host static content like style sheets and javascript libraries. Each time I ping this name, I get a response from a different IP address. However, I can't tell whether this is anycast in action, or a completely different technique. Back to my original question: as far as I can tell, even a large site will have a single expensive piece of load-balancing hardware listening on its handful of public IP addresses.

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• Insufficient channel capacity of 1GBit

  - by Roman S

  There is a Caching Server (Varnish): it receives data from Amazon S3 on request, saves it for some time and gives it to the client. We have encountered the problem of insufficient channel capacity of 1GBit. Peak load within 4 hours completely chokes the channel. Server performance is sufficient for now. Approximately 4.5TB of data are transmitted per day. More than 100TB are accumulated per month. The first thought that comes to mind is simply to add one more 1GBit port and sleep peacefully until 2GBit are not enough (it may happen quite quickly) or one server is not able to handle it. And then we just need to add new Caching Servers. But now we need a Load Balancer, which will send requests on one and the same URL, always on one and the same server (to avoid multiple copies of the same cached objects). Here are the questions: Does a Balancer need a band equal to sum of all bands of Caching Servers? What shall we do in case there are no ports in a Balancer? Should we add more Balancers or solve the problem by means of Round robin DNS? What are the standard approaches to such problems? Can anyone advise hosting-companies, which can solve this problem? We are interested in American and European markets.

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