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Recital is a dynamic programming language particularly suited to the development of database applications. While Recital still supports standard procedural programming, new extensions to the language give you the power and flexibility of object-oriented programming. Object-oriented design and object-oriented programming represent a change in focus from standard procedural programming. This short primer will give you a good understanding of how to program object-oriented Recital.
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A quick tip for optimizing TCP performance on linux.

edit /etc/sysctl.conf add the lines:

If using gigabit ethernet:

net.ipv4.tcp_mem= 98304 131072 196608
net.ipv4.tcp_window_scaling=1
net.core.wmem_default = 65536
net.core.rmem_default = 65536
net.core.wmem_max=8388608

To reload these use:

# sysctl -p

If using infiniband:

net.ipv4.tcp_window_scaling=1
net.ipv4.tcp_timestamps=0
net.ipv4.tcp_sack=0
net.ipv4.tcp_rmem=10000000 10000000 10000000
net.ipv4.tcp_wmem=10000000 10000000 10000000
net.ipv4.tcp_mem=10000000 10000000 10000000
net.core.rmem_max=524287
net.core.wmem_max=524287
net.core.rmem_default=524287
net.core.wmem_default=524287
net.core.optmem_max=524287
net.core.netdev_max_backlog=300000

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Occasionally as a Linux administrator you will be in the situation where working on a remote server and you are left with no option but to force a reboot the system. This may be for a number of reasons, but where I have found it most frequent is when working on Linux clusters in a remote location.

When the "reboot" or "shutdown" commands are executed daemons are gracefully stopped and storage volumes unmounted.
This is usually accomplished via scripts in the /etc/init.d directory which will wait for each daemon to shut down gracefully before proceeding on to the next one. This is where a situation can develop where your Linux server fails to shutdown cleanly leaving you unable to administer the system until it is inspected locally. This is obviously not ideal so the answer is to force a reboot on the system where you can guarantee that the system will power cycle and come back up. The method will not unmount file systems nor sync delayed disk writes, so use this at your own discretion.

To force the kernel to reboot the system we will be making use of the magic SysRq key.

The magic_SysRq_key provides a means to send low level instructions directly to the kernel via the /proc virtual file system.


To enable the use of  the magic SysRq option type the following at the command prompt:

 

    echo 1 > /proc/sys/kernel/sysrq

 

Then to reboot the machine simply enter the following:

 

    echo b > /proc/sysrq-trigger


Voilà! Your system will instantly reboot.
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This website runs in a  virtual machine under vmware server. It is clustered between two servers using heartbeat and DRBD.
 
When VMware server starts up a virtual machine it generates a uuid (unique id) based on the machine it is running on and stores this in the .vmx file.
 
When heartbeat switches from slave to master, it will start VMware server (which is setup as a resource in the haresources file).
 
Virtual machines that you want started automatically when you start VMware server will not start because the uuid changes between the master and backup systems. To get around this problem always do the following:
  • edit the .vmx file and add the following line

    uuid.action = "keep"
If this is not done then everytime you try to run the virtual machine on the backup system in your cluster, VMware server will complain that the virtual machine has been copied or moved and it will not start it.
  • set the virtual machine to power off when vmware is stopped. Do not set this to "suspend" or it will not restart on the backup machine.
This will allow the virtual machine to start properly on the backup machine.
 
 
 
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Recital 10 introduced the DIE( )and EXIT( ) functions. These functions operates in the same way as the PHP DIE( ) and EXIT( ) functions. They output a message and terminate the current session in both Recital and Recital Web.
try
  open database southwind
catch
  die("Cannot open database, please try later.")
endtry
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I Just added a few new functions to the latest 10.0.1 patch release which should be generally available next week.
  1. New MD5( expC ) function to calculate an MD5 crypto key from any character expression
  2. New MD5FILE( expC ) function calculates an MD5 crypto key for a given filename. If the filename is a pattern e.g *.* it will calculate the key across all files matching the pattern
  3. New mod_recital.so available for using Recital Web on linux x86_32
  4. New mod_recital64.so available for using Recital Web on linux x86_64 
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When using Recital on linux you can integrate your favorite linux shell commands and use then directly inside Recital using the alias command. This can be particularly useful when you ssh into a remote system and run recital. You can then issue linux commands without having to open another terminal session. Several aliased shell commands are predefined in /opt/recital/conf/config.db. You can add others to suit your needs.
 
On my system i have these commands aliased.
alias pwd  "? default()"
alias cp   "copy file "
alias mv   "rename "
alias rm   "erase "
alias ls   "run('ls $0')"
alias ps   "run('ps $0')"
alias grep "run('grep $0')"
alias cd   "set default to $1"
alias cls  "clear screen"
These commands can now be used inside the Recital command window just as you would use them at the linux prompt, including the ability to pipe commands together.
ls -l | grep .prg
ps -elf | grep db.exe
The run() function that is used to run the shell command as specified in the alias command will capture output and display it in a text viewer. If you want to run the command and display the contents full screen, then specify true as the third parameter to the run().
run("command", true, true) 
The arguments to run() are as follows.
Argument Description
1 the command line to run
2 True if output should be displayed in a text area (default True)
3 True if the output should be displayed full screen (default False)
 
The alias command handles parameter substitition.
Macro Description
$0 the command line following the command name
$1..$n the arguments given to the command
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By default Recital uses PAM to authenticate users.  It is also possible to tell PAM to use Kerberos.  Simply replace the existing entries in the /etc/pam.d/recital file with the ones below:

auth       sufficient   pam_krb5.so try_first_pass
auth       sufficient   pam_unix.so shadow nullok try_first_pass
account    required     pam_unix.so broken_shadow
account    [default=bad success=ok user_unknown=ignore] pam_krb5.so
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The SET RELATION Recital Navigational Data Command can be used to link two (or more) tables based on the master index key of the child table.  With a relation active, as you move through the parent table, the record pointer also moves in the child table, automatically selecting the first related record or moving to the end of file if no related record exists.    

open database southwind
// open child table
use order_details order orderid in 0
// open parent table
use orders order orderid in 0
set relation to orderid into order_details
do while not eof()
? orders.orderid, order_details.productid
skip
enddo

The code above will display the productid from the first related record, but you will often want to display information from all the related records in the child or detail table as in an SQL Left Outer Join.

open database southwind
select orders.orderid, order_details.productid;
from orders left outer join order_details;
on orders.orderid = order_details.orderid

In this case, we can use a second nested DO WHILE loop, for example:

open database southwind
use order_details order orderid in 0
use orders order orderid in 0
set relation to orderid into order_details
do while not eof()
// Display first or 0 child record
? orders.orderid, order_details.productid
// Display any additional child records
do while not eof(order_details)
? orders.orderid, order_details.productid
skip in order_details
enddo
skip
enddo

Or we can use the SET SKIP command.  The SET SKIP command can be used with DISPLAY, LIST and REPORT and automatically skips through all the related records in the child table.

open database southwind
use order_details order orderid in 0
use orders order orderid in 0
set relation to orderid into order_details
set skip on
set skip to order_details
list orders.orderid, order_details.productid

LIST and DISPLAY offer a number of keyword options to allow you to configure the display output.  REPORT offers full column based report design.
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When you start the loadbalancer.org appliance you will see the following:

Default login:
Username: root
Password: loadbalancer

Access to webclient from an external client is:
http://192.168.1.129:9080
http://192.168.1.129:9443

You can access the web administrator using the IP and ports described onscreen.

For the sri lanka porject we are looking for performance and the network diagram indicates we are happy to have the cluster on the same subnet as the rest of the network.

Direct routing is the fasted performance possible, it has the advantage over NAT that the Loadbalancer does not become a bottleneck for incoming and outgoing packets. With DR the loadbalancer simply examines incoming packets and the servers to route the packets directly back to the requesting user.

The web interfaceis the only way to fully configure the loadbalancer vm. The console tool lbwizard will get it initiallised and any further configurations can then be done via the webinterface.

Using lbwizard for the Sri lanka configuration follow these steps.

On the first Loadbalancer:

//Start

Is this unit part for a HA Pair?
YES

Have you already setup the Slave?
NO

Is this a one-armed configuration?
YES

Enter the IP Address for the interface eth0?
Enter IP address you wish to be assigned to the SLAVE loadbalancer.

Enter the netmask for interface eth0?
Enter netmask for the subnet.

Enter the Floating IP adrress?
Enter the IP address that will be IP assosiacted the the HA-pair of loadbalancers.

//Finish

On the 2nd loadbalancer VM, run the lbwizard.

//Start

Is this unit part of an HA-Pair?
YES

Have you already set up the Slave?
YES

What is the slave units UP address?
Enter the IP which you entered when configuring the other loadbalancer VM.

Is this a one-armed configuration?
YES

Enter the IP Address for the interface eth0?
Enter the IP that will be assigned to the MASTER loadbalancer

Enter the netmask for interface eth0?
Enter the subnet netmask.

Enter the Floating IP address?
Enter the IP address that will be IP assosiacted the the HA-pair of loadbalancers.

Enter the address of the default gateway?
Enter the deafult gateway for the subnet.

Enter the IP of the nameserver?
Enter the dns server.

Enter the port for the first Virtual server?
Enter 22 for ssh

Enter the IP address of the first real server?
Enter the real IP of the first appserver

//Finish

Now this is complete we need to go to the web admin interface to configure the 2nd Real Server. As the lbwizard program will only allow you to configure 1 real server.

Now login to the web admin using the default password:

username: loadbalancer
password: loadbalancer

Note: Connect to the IP you have now set for your master loadbalancer

Goto the edit configuration tab

Now click add a real server:

Enter a label
IP address of the server plus the port of the service i.e. 192.168.1.125:22


Edit Configuration -> Virtual Servers

persistancte -> NO

Scheduler-> LC
LC - Least-Connection: assign more jobs to real servers with
fewer active jobs.

Service to check -> custom1

Check port -> 22

Forwarding Method -> DR

Feedback Method -> Agent

Arp Problem when using DR

Every real server must be configured to respond to the VIP address as well as the RIP
address.

You can use iptables (netfilter) on the real server to re-direct incoming packets destined for the virtual
server IP address.

This is a simple case of adding the following command to your start up script (rc.local):

//replace 10.0.0.21 with the Virtual Server IP
iptables -t nat -A PREROUTING -p tcp -d 10.0.0.21 -j REDIRECT

chkconfig iptables on

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