Karussell's questions

Assume we have two servers A and B with 'real' and external IPs and we can switch the so called 'failover ip' (W.X.Y.Z) to point to a specific external IP of A or B. This works from the 'outside' and was easily done. As a background: the failover ip is configured as a new entry in /etc/network/interfaces :

auto eth0:0  
iface eth0:0 inet static
  address W.X.Y.Z
  netmask 255.255.255.224 

Now let us assume W.X.Y.Z is configured dynamically to use hardware A. Now I call 'curl domain.com' from B and it uses the correct failover ip W.X.Y.Z but then resolves somehow to the wrong external IP B (or localhost?) instead of using the configured one A:

Trying W.X.Y.Z ...
* connect to W.X.Y.Z port 443 failed: Connection refused
* Failed to connect to domain.com port 443: Connection refused
* Closing connection 0
curl: (7) Failed to connect to domain.com port 443: Connection refused

When I start the local nginx it can successfully curl domain.com

Do I need to configure DNS locally somehow? How can I find out more about the DNS chain?

Using mtr just prints domain.com if trying this from server B

Is this related to this question?

The failover IP is W.X.Y.Z and is also the A record of domain.com

The /etc/hosts file for both nodes serverA and serverB looks like:

    127.0.0.1       localhost
    127.0.1.1       luminarhost            
    xxx    serverA
    xxx    serverB        

The /etc/network/interfaces of serverA

    ### Hetzner Online AG - installimage
    # Loopback device:
    auto lo
    iface lo inet loopback

    # device: eth0
    auto  eth0
    iface eth0 inet static
      address   xxx
      broadcast xxx
      netmask   xxx
      gateway   xxx
      # default route to access subnet
      up route add -net xxx netmask 255.255.255.224 gw xxx eth0

    iface eth0 inet6 static
      address xxx
      netmask xxx
      gateway xxx

    # failover ip
    auto eth0:0
    iface eth0:0 inet static
      address W.X.Y.Z
      netmask 255.255.255.224

and of serverB it is:

    ### Hetzner Online AG - installimage
    # Loopback device:
    auto lo
    iface lo inet loopback

    # device: eth0
    auto  eth0
    iface eth0 inet static
      address   xxx
      broadcast xxx
      netmask   xxx
      gateway   xxx
      # default route to access subnet
      up route add -net xxx netmask 255.255.255.192 gw xxx eth0

    iface eth0 inet6 static
      address xxx
      netmask xxx
      gateway xxx

    # failover ip
    auto eth0:0
    iface eth0:0 inet static
      address W.X.Y.Z
      netmask 255.255.255.224

I've read 'youtube-architecture' but still do not understand one point.

Assume I have a bit bigger content to deliver and one load balancer (nginx) points via reverse proxying to e.g. 10 back-end servers (java/jetty). Now assume 1 GBit/s for every server.

When the applications gets more users I have some options:

• reduce the maximum download rate per connection. So a few heavy users won't make the connection worse for the others. (limited scalability)
• redirect to the back-end server so that the clients directly download there and the bandwidth multiplies. (the 'youtube' style?)
• use DNS load balancing, here also the bandwidth multiplies. Problem: I do not have much control over the load balancing selection process

Is there another option?

E.g. the 2nd option without the redirection? Or is it this normal that clients just load a thin skin of the application like 'youtube.com' and then download the videos from different domains like 'xy.googlevideo.com'?

I have some jetty servers with a loadbalancer before them. Now I want to update my application without downtime. When one jetty is going down and no longer reachable the loadbalancer automatically removes it from the list, so this is not the issue.

The main problem is to avoid down time: so, I have to make sure only one jetty is restarting at a time - or make sure that at least N jettys are online!

At the moment I'm using a simple bash script where I need to wait manually for one jetty to came back online again before restarting the next jetty and so on.

Now bash is not very optimal for this kind of stuff and I hope there are tools better suited to make the whole task automated. E.g. I could use a simple ping URL http://jetty-number-n.com/ping which responds OK (200) if the n-th jetty is online.

How could I solve this task and with which tool?

Thanks to @ceejayoz I found rolling update for ansible. But it is still suboptimal as I would need to set a fixed timeout.

I cannot find definite information about how a debian package should behave whilst being removed. Should it remove the data? E.g. for mysql I read data is kept which sounds logical to me (using purge will remove all stuff).

But here I found the text:

The difference between remove and purge is that while remove only deletes data and executables, purge also deletes all configuration files in addition.

So, what is true?

Update As I want to package one for ElasticSearch I need to know what is expected.

I know that there are possible duplicates on serverfault, but I read a lot (not only the last days) but I couldn't make it working 100%.

I have the domain http://jetwick.de bought via Strato and then I have a server with a static IP. I can configure the DNS settings for jetwick.de at Strato: the IP directly points to the IP of our server via A-Record. This works.

Now I want that my application which is available from tomcat under localhost:8080/jetwick is shown under jetwick.de - nothing more :)

I have the following apache config on our server /etc/apache2/vhosts/jetwick

<Virtualhost *:80>
      ServerName http://jetwick.de
      ServerAlias jetwick.de www.jetwick.de jetwick.com www.jetwick.com

      ProxyRequests Off
      ProxyPreserveHost On   

      ProxyPass / http://localhost:8080/jetwick/
      ProxyPassReverse / http://localhost:8080/jetwick/

</Virtualhost>

This partially works. But I have problems when I query:

1. jsessionid should NOT be shown - only for clients which do not allow cookies. I guess the cookie info gets lost somewhere like the favicon ...?
2. an additional directory jetwick.de/jetwick will be shown (in the following request the jsessionid disappears)

How would you fix that? You can also point me to a site/duplicate where this case is described, of course!

Do I need mod_jk to make cookies properly working like the answer here suggests?

Update The following settings solved the first (cookie) problem:

ProxyPassReverseCookiePath /jetwick /

I regularly have to deploy stuff via scp. The problem is that the user on the remote machine (e.g. tomcat) does not match the user i am logging in with (e.g. mylogin). And almost always I cannot login directly with the remote user (disabled from the admin)

Assume the following: I want to deploy a file into tomcats' webapps folder but can do this only via 'sudo -U tomcat' on the remote machine before.

At the moment I am doing sth. like this:

scp file.war [email protected]:/home/mylogin/tmp/
ssh [email protected] 'sudo -U tomcat cp /home/mylogin/tmp/file.war /tomcat/webapps/'

Is there a better solution without the temporary folder? I mean, is it possible with only one copying with a sudo "in-between"? Can this be done with portforwarding?

This question is related to this

We have 4GB on our linux server but we can currently use only ~1.8GB for our java-server, which is the first java process listed below. (200 MB are free so we can maximal use 1.6GB + 0.2GB)

The machine crashes when we are using more. So we specified -Xmx1600m and -XX:MaxPermSize=200m to limit the server RAM to 1.8GB. But we need more RAM! Where is the remaining RAM gone?

Here is the program output of top sorted against memory usage (via big 'M'):

Mem:   4083952k total,  3857856k used,   226096k free,   169320k buffers
Swap:  2104504k total,      176k used,  2104328k free,  1939080k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND                                                                                           
28155 root      16   0 1835m 1.6g 7848 S    2 40.5  47:36.26 java                                                                                              
19609 root      16   0 45996 7052 3148 S    0  0.2  14:35.97 httpd2-prefork                                                                                    
 6802 root      16   0 46132 5916 1932 S    0  0.1   0:00.09 httpd2-prefork                                                                                    
 6866 root      15   0 46132 5916 1932 S    0  0.1   0:00.07 httpd2-prefork    

As you can see there are a lot (and even more) httpd2-prefork processes. But even if I accumulate the bytes (25* ~46KB = ~1MB) it will never get that big.

free -m prints:

             total       used       free     shared    buffers     cached
Mem:          3988       3768        219          0        165       1894
-/+ buffers/cache:       1708       2279
Swap:         2055          0       2055

Where is my mistake? Can I tune the server to give the java process more RAM?

BTW: we are not using a virtual machine like here

Update

As it was pointed out in the comments: it is a 32bit kernel only :-( (via uname -a). So I can use only 3GB at least? But the server itself seems to be 64 bit? Because of the lm (long mode) in the output?

grep flags /proc/cpuinfo
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe lm constant_tsc pni monitor ds_cpl cid cx16 xtpr
...

Here is another very similar question.