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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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  • 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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  • 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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  • PeopleSoft at Alliance 2012 Executive Forum

    - by John Webb
    Guest Posting From Rebekah Jackson This week I jointed over 4,800 Higher Ed and Public Sector customers and partners in Nashville at our annual Alliance conference.   I got lost easily in the hallways of the sprawling Gaylord Opryland Hotel. I carried the resort map with me, and I would still stand for several minutes at a very confusing junction, studying the map and the signage on the walls. Hallways led off in many directions, some with elevators going down here and stairs going up there. When I took a wrong turn I would instantly feel stuck, lose my bearings, and occasionally even have to send out a call for help.    It strikes me that the theme for the Executive Forum this year outlines a less tangible but equally disorienting set of challenges that our higher education customer’s CIOs are facing: Making Decisions at the Intersection of Business Value, Strategic Investment, and Enterprise Technology. The forces acting upon higher education institutions today are not neat, straight-forward decision points, where one can glance to the right, glance to the left, and then quickly choose the best course of action. The operational, technological, and strategic factors that must be considered are complex, interrelated, messy…and the stakes are high. Michael Horn, co-author of “Disrupting Class: How Disruptive Innovation Will Change the Way the World Learns”, set the tone for the day. He introduced the model of disruptive innovation, which grew out of the research he and his colleagues have done on ‘Why Successful Organizations Fail’. Highly simplified, the pattern he shared is that things start out decentralized, take a leap to extreme centralization, and then experience progressive decentralization. Using computers as an example, we started with a slide rule, then developed the computer which centralized in the form of mainframes, and gradually decentralized to mini-computers, desktop computers, laptops, and now mobile devices. According to Michael, you have more computing power in your cell phone than existed on the planet 60 years ago, or was on the first rocket that went to the moon. Applying this pattern to Higher Education means the introduction of expensive and prestigious private universities, followed by the advent of state schools, then by community colleges, and now online education. Michael shared statistics that indicate 50% of students will be taking at least one on line course by 2014…and by some measures, that’s already the case today. The implication is that technology moves from being the backbone of the campus, the IT department’s domain, and pushes into the academic core of the institution. Innovative programs are underway at many schools like Bellevue and BYU Idaho, joined by startups and disruptive new players like the Khan Academy.   This presents both threat and opportunity for higher education institutions, and means that IT decisions cannot afford to be disconnected from the institution’s strategic plan. Subsequent sessions explored this theme.    Theo Bosnak, from Attain, discussed the model they use for assessing the complete picture of an institution’s financial health. Compounding the issue are the dramatic trends occurring in technology and the vendors that provide it. Ovum analyst Nicole Engelbert, shared her insights next and suggested that incremental changes are no longer an option, instead fundamental changes are affecting the landscape of enterprise technology in higher ed.    Nicole closed with her recommendation that institutions focus on the trends in higher education with an eye towards the strategic requirements and business value first. Technology then is the enabler.   The last presentation of the day was from Tom Fisher, Sr. Vice President of Cloud Services at Oracle. Tom runs the delivery arm of the Cloud Services group, and shared his thoughts candidly about his experiences with cloud deployments as well as key issues around managing costs and security in cloud deployments. Okay, we’ve covered a lot of ground at this point, from financials planning, business strategy, and cloud computing, with the possibility that half of the institutions in the US might not be around in their current form 10 years from now. Did I forget to mention that was raised in the morning session? Seems a little hard to believe, and yet Michael Horn made a compelling point. Apparently 100 years ago, 8 of the top 10 education institutions in the world were German. Today, the leading German school is ranked somewhere in the 40’s or 50’s. What will the landscape be 100 years from now? Will there be an institution from China, India, or Brazil in the top 10? As Nicole suggested, maybe US parents will be sending their children to schools overseas much sooner, faced with the ever-increasing costs of a US based education. Will corporations begin to view skill-based certification from an online provider as a viable alternative to a 4 year degree from an accredited institution, fundamentally altering the education industry as we know it?

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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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  • Apache2: Limit simultaneous requests & throttle bandwidth per IP/client?

    - by xentek
    I want to limit simultaneous requests & throttle bandwidth per IP/Client on a single apache vhost. In other words, I want to ensure that this site, which hosts large media files, doesn't get hammered by someone trying to download everything all at once (just happened the other night). I'd like to limit the outgoing transfer speed overall for this site, as well as limit the number of connections a single IP can make to the server to a sane default (i.e. within normal browser limits for multiple requests so page loads aren't effected too much). Bonus points if I can actually scope it to file types (i.e. leave web files alone, but apply these rules to just the media files). We're running Ubuntu 9.04 on all the servers, and have two apache/php servers being load balanced via Round Robin by a squid proxy server. MySQL is running on its own box as well. We've got plenty of bandwidth to give them, so I don't really want overall caps, but just want to throttle the amount of memory/CPU it takes to serve this site. There other sites on these servers that we don't want to apply these rules too, just want to keep this one from hogging all the resources. Let me know if you need more info! Thanks in advance for your suggestions!

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  • How browsers handle multiple IPs

    - by Sandman4
    Can someone direct me to information on exact browsers behavior when browser gets multiple A records for a given hostname (say ip1 and ip2), and one of them is not accessible. I interested in EXACT details, like (but not limited to): Will browser get 2 IPs from OS, or it will get only one ? Which ip will browser try first (random or always the first one) ? Now, let's say browser started with the failed ip1 For how long will browser try ip1 ? If user hits "stop" while it waits for ip1, and then clicks refresh which IP will browser try ? What will happen when it times-out - will it start trying ip2 or give error ? (And if error, which ip will browser try when user clicks refresh). When user clicks refresh, will any browser attempt new DNS lookup ? Now let's assume browser tried working ip2 first. For the next page request, will browser still use ip2, or it may randomly switch ips ? For how long browsers keep IPs in their cache ? When browsers sends a new DNS request, and get SAME ips, will it CONTINUE to use the same known-to-be-working IP, or the process starts from scratch and it may try any of the two ? Of course it all may be browser dependent, and may also vary between versions and platforms, I'd be happy to have maximum of details. The purpose of this - I'm trying to understand what exactly users will experience when round-robin DNS based used and one of the hosts fails. Please, I'm NOT asking about how bad DNS load balancing is, and please refrain from answering "don't do it", "it's a bad idea", "you need heartbeat/proxy/BGP/whatever" and so on.

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  • Squid reverse proxy array - siblings not communicating with each other

    - by V. Romanov
    I want to set up 2 squid servers to act as reverse proxy and cache for a webserver on our intranet. The load balancing will be done with DNS round robin or just different mappings for different clients. The thing is, I want both servers to try and contact each other to see if they have the object required in cache before contacting the webserver for it (the network that servers the webserver is the bottleneck and I'm trying to eliminate it) Both squids are configured the same, here are the relevant config lines : acl dvr1_cache_it_best_tv_com dstdomain dvr1.cache.it.best-tv.com acl squid1_it_best_tv_com dstdomain squid1.it.best-tv.com acl squid2_it_best_tv_com dstdomain squid2.it.best-tv.com http_access allow dvr1_cache_it_best_tv_com http_access allow squid1_it_best_tv_com http_access allow squid2_it_best_tv_com http_access allow all http_port 8081 accel defaultsite=dvr1.cache.it.best-tv.com cache_peer dvr1.origin.it.best-tv.com parent 80 0 no-query originserver name=Proxy_dvr1_origin_it_best_tv_com cache_peer squid1.it.best-tv.com sibling 8081 3130 weight=10 name=Proxy_Squid1_it_best_tv_com cache_peer squid2.it.best-tv.com sibling 8081 3130 weight=10 name=Proxy_Squid2_it_best_tv_com cache_peer_access Proxy_dvr1_origin_it_best_tv_com allow dvr1_cache_it_best_tv_com cache_peer_access Proxy_squid1_it_best_tv_com allow squid1_it_best_tv_com cache_peer_access Proxy_squid1_it_best_tv_com allow squid2_it_best_tv_com cache_peer_access Proxy_squid1_it_best_tv_com allow dvr1_cache_it_best_tv_com cache_peer_access Proxy_squid2_it_best_tv_com allow squid1_it_best_tv_com cache_peer_access Proxy_squid2_it_best_tv_com allow squid2_it_best_tv_com cache_peer_access Proxy_squid2_it_best_tv_com allow dvr1_cache_it_best_tv_com just to make it clear - dvr1.cache is the alias for the proxy servers. dvr1.origin is the web server. Both servers work, both serve content and cache it and work fine. However, when I clear the cache on one server and then access it, it gets the content from the parent (DVR1_ORIGIN) instead of going to the sibling squid. What did I configure wrong? Or perhaps I don't understand the architecture correctly? I read the squid manuals but as far as I see i did it all by the book and yet it doesn't work right. Any help will be appreciated!

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  • Good Enough Failover Strategy for DNS / MySQL / Email

    - by IMB
    I've asked and read a lot questions regarding DNS failover but the more I read the more complicated it becomes, some people say it's good enough some say it isn't. No clear answers from what I read. I was wondering if we can set it straight once and for all, at least for the requirements of most websites out there. Right now let's assume the following: We don't need really need load-balancing, what we need is a failover solution. We are running a website based on LAMP on a VPS. We need to make sure that the Web Server, MySQL, Email are always accessible if not 99%. Basically here's my idea and questions about it: Web Server: We need at least one failover server (another VPS on a separate data center). Is DNS Failover via Round Robin good, if not, what's the best? And how do you exactly implement it? How do you make the files you upload/delete on Server A is also on Server B? MySQL: I've only read a brief intro to MySQL replication and I assume that I can replicate Server A to Server B and vice versa on the fly right? So just it case Server A fails and Server B is now running, it will continue to work and replicate to Server A when it becomes available. So in essence Server B is now the primary server, and will later on failover to Server A, should a failure happen again. Email: If we are gonna use DNS Failover, using webmail or relying on emails stored on the server is probably not a good idea right? Since some emails might be on Server A while some might be on Server B? I assume a basic email forwarder to a 3rdparty is good enough (like Gmail for example) to ensure all emails are kept in one place. Here's a basic diagram for a better picture: http://i.stack.imgur.com/KWSIi.png

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