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Awk is an powerful text processing language that allows you to manipulate files containing columns of data and strings. Awk is extremely useful, both for general operation of Unix commands, and for data transformation.
 
Introduced in Recital 10 is the PIPETOSTR() function (as well as backticks `` inside strings) which can be used  in conjunction with awk to transform recital data.
 
The following links provide good tutorials on awk:
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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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In this article Yvonne Milne looks at the use of the Recital Remote Data Connectivity Functions with Recital Database Gateways.

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If when your attempt to create device meta-data fails this is drbd preventing you from corrupting a file system present on the target partition.

$ drbdadm create-md drbd0

v08 Magic number not found
md_offset 30005817344
al_offset 30005784576
bm_offset 30004867072

Found ext2 filesystem which uses 190804004 kB
current configuration leaves usable 29301628 kB

Device size would be truncated, which
would corrupt data and result in
'access beyond end of device' errors.
You need to either
* use external meta data (recommended)
* shrink that filesystem first
* zero out the device (destroy the filesystem)
Operation refused.

Command 'drbdmeta /dev/drbd0 v08 /dev/sda4 internal create-md' terminated with exit code 40
drbdadm aborting

Once you have confirmed that the file system present on the target partition is no longer required at the prompt type the following:

Replace /dev/sdaX with the block device you are targeting.

dd if=/dev/zero of=/dev/sdaX bs=1M count=128

Once this has completed the drbdadm create-md drbd0 command will complete with a "success."

$ drbdadm create-md drbd0
v08 Magic number not found
v07 Magic number not found
v07 Magic number not found
v08 Magic number not found
Writing meta data...
initialising activity log
NOT initialized bitmap
New drbd meta data block successfully created.
success
$

 

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It would appear that gigabit LAN is not! In fact it often runs at the same speed as 100Mbps LAN. Let's look at why exactly.

After configuring your network you can use the ifconfig command to see what speeds the LAN is connected. Even though 1000Mbps is reported by the card, the reality is that the overall throughtput may well be ~100Mpbs. You can try copying a large file using scp to demonstrate this.

As it turns out, in order to use a gigabit LAN you need to use CAT6 cables. CAT5 and CAT5E are not good enough. End result, the ethernet cards throttle back the speed to reduce dropped packets and errors.

You can find a good article here titled Squeeze Your Gigabit NIC for Top Performance. After tuning up the TCP parameters i found that it made no dfifference. The principal reasons behind low gigabit ethernet performance can be summed up as follows.

  1. Need to use CAT6 cables
  2. Slow Disk speed
  3. Limitations of the PCI bus which the gigabit ethernet cards use

You can get an idea about the disk speed using the hdparm command:

Display the disk partitions and choose the main linux partition which has the / filesystem.

# fdisk -l


Then get disk cache and disk read statistics:

# hdparm -Tt /dev/sda0


On my desktop system the sata disk perfomance is a limiting factor. These were the results:

/dev/sda1:
Timing cached reads:   9984 MB in  2.00 seconds = 4996.41 MB/sec
Timing buffered disk reads:   84 MB in  3.13 seconds =  58.49 MB/sec

Well, that equates to a raw disk read speed of 58.49 * 8 = 467Mbps which is half the speed of a gigabit LAN.

So.. NAS storage with lots of memory looks to be the way to go... If you use the right cables!


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In this article Barry Mavin, CEO and Chief Software Architect for Recital, details how to work with Triggers in the Recital Database Server.

Overview

A trigger is a special kind of stored procedure that runs when you modify data in a specified table using one or more of the data modification operations: UPDATE, INSERT, or DELETE.

Triggers can query other tables and can include complex SQL statements. They are primarily useful for enforcing complex business rules or requirements. For example, you can control whether to allow a new order to be inserted based on a customer's current account status.

Triggers are also useful for enforcing referential and data integrity.

Triggers can be used with any data source that is handled natively by the Recital Database Engine. This includes Recital, FoxPro, FoxBASE, Clipper, dBase, CISAM, and RMS data,

Creating and Editing Triggers

To create a new Trigger,  right-click the Procedures node in the Databases tree of the Project Explorer and choose Create. To modify an existing Trigger select the Trigger in the Databases Tree in the Project Explorer by double-clicking on it, or select Modify from the context menu. By convertion we recommend that you name your Stored Procedures beginning with "sp_xxx_", user-defined functions with "f_xxx_", and Triggers with "dt_xxx_", where xxx is the name of the table that they are associated with.

Associating Triggers with a Table

Once you have written your Triggers as detailed above you can associate them with the operations performed on a Table by selecting the Table tab.

The Tables tab allows you to select a Trigger procedure by clicking on the small button at the right of the Text field.

Types of Triggers

As can be seen from the Tables tab detailed below, The Recital Database Server handles 6 distinct types of Triggers.

Open Trigger

The Open Trigger is called after is a table is opened but before any operations are performed on it. You can use this trigger to record a log of table usage or provide a programmable means of checing security. If the Trigger procedure returns .F. (false), then the table is not opened. You can use a TRY...CATCH block around the associated command to inform the user.

Close Trigger

The Close Trigger is called just prior to a table being closed. In this trigger you may find it useful to get transaction counts by using the IOSTATS() built-in 4GL function, and record these values in a transaction log.

Update Trigger

The Update Trigger is called prior to a record update operation being performed. You can use this trigger to perform complex application or data specific validation. If the Trigger procedure returns .F. (false), then the record is not updated. You can use inform the user from within the Trigger procedure the reason that the data cannot be updated.

Delete Trigger

The Delete Trigger is called prior to a record delete operation being performed. You can use this trigger to perform complex application or data specific validation such as cross-table lookups e.g. attempting to delete a customer recortd when there are still open orders for that specific customer. If the Trigger procedure returns .F. (false), then the record is not deleted.

Insert Trigger

The Insert Trigger is called prior to a record insert (append) operation being performed. You can use this trigger to perform such tasks as setting up default values of columns within the record. If the Trigger procedure returns .F. (false), then the record is not inserted.

Rollback Trigger

The RollbackTrigger is called prior to a rollback operation being performed from within a form. If the Trigger procedure returns .F. (false), then the record is not rolled back to its original state.

Testing the Trigger

To test run the Trigger, select the Trigger in the Databases Tree in the Project Explorer by double-clicking on it. Once the Database Administrator is displayed, click the Run button to run the Trigger.

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The getUIComponentBitmapData method can create bitmapdata for a given IUIComponent. Pass any UIcomponent to get its respective bitmapdata.

public static function getUIComponentBitmapData(target:IUIComponent):BitmapData {      
    var resultBitmapData:BitmapData = new BitmapData(target.width, target.height);     
    var m:Matrix = new Matrix();     
    resultBitmapData.draw(target, m);     
    return resultBitmapData; 
}

Now convert the bitmapdata to a jpeg bytearray.

private static function encodeToJPEG(data:BitmapData, quality:Number = 75):ByteArray {     
    var encoder:JPGEncoder = new JPGEncoder(quality);     
    return encoder.encode(data); 
}

Now encode the ByteArray into Base64.

public static function base64Encode(data:ByteArray):String {     
    var encoder:Base64Encoder = new Base64Encoder();     
    encoder.encodeBytes(data);     
    return encoder.flush(); 
}

Upload the base64 encoded ByteArray to the server.

public static uploadData():void {     
    var url:String = "saveFile.php";     
    var urlRequest:URLRequest = new URLRequest(url);     
    urlRequest.method = URLRequestMethod.POST;     
    var urlLoader:URLLoader = new URLLoader();     
    var urlVariables:URLVariables = new URLVariables();     
    urlVariables.file = jpgEncodedFile;    // as returned from base64Encode()     
    urlLoader.data = urlVariables;     
    urlLoader.load(urlRequest); 
}

The saveFile.php file on the server.


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Here is a simple shell script to copy your ssh authorization key to a remote machine so that you can run ssh and scp without having to repeatedly login.

#!/bin/sh
# save in file ssh_copykeyto.sh then chmod +x ssh_copykeyto.sh
KEY="$HOME/.ssh/id_rsa.pub"
if [ ! -f ~/.ssh/id_rsa.pub ];then
echo "private key not found at $KEY"
echo "create it with "ssh-keygen -t rsa" before running this script
exit
fi
if [ -z $1 ];then
echo "Bad args: specify user@host as the first argument to this script"
exit
fi
echo "Copying ssh authorization key to $1... "
KEYCODE=`cat $KEY`
ssh -q $1 "mkdir ~/.ssh 2>/dev/null; chmod 700 ~/.ssh; echo "$KEYCODE" >> ~/.ssh/authorized_keys; \ chmod 644 ~/.ssh/authorized_keys"
echo "done!"
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MQCURMSGS()

Syntax

MQCURMSGS( <expN> )

Description

The MQCURMSGS() function returns the current number of unread messages in the queue specified by <expN>

Example

mqdes=mqopen("/myqueue")
if (mqdes < 0)
   messagebox(strerror()+",errno="+alltrim(str(error())))
   return
endif
do while (mqcurmsgs(mqdes) > 0)
   mstr=mqreceive(mqdes)
   if (empty(mstr))
      messagebox(strerror()+",errno="+alltrim(str(error())))
      return
   endif
   messagebox(mstr)
end do
mqclose(mqdes)

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After installing nomachine, if you get an error connecting whereby nomachine errors out after  "Negotiating link parameters"
 

When installing nomachine on redhat 5.3 64-bit be sure to:

  1. Make sure you have installed the 64-bit packages as the 32-bit ones will not work.
  2. add the hostname to /etc/hosts
  3. Check "Disable encryption of all traffic" (in configuration / advanced tab)
On Centos 32-bit:
  1. add the hostname to /etc/hosts
  2. make sure the host IP is not specified as 127.0.0.1 line
  3. Uncheck "Disable encryption of all traffic" (in configuration / advanced tab)
 
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