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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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In an interesting article at Linux Magazine, Sam Ockman discusses the origin of the term "Open Source". Check it out here.
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Recital implements SQL-92 and most of the SQL-99 standard for SQL, but also provides lower level navigational data access for performing high transaction throughput. It is the choice of the application developer whether to use SQL, navigational data access, or a combination of both depending upon the type of application being developed.
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Use iptables to restrict access to Recital Web only from localhost.

iptables -I INPUT -j ACCEPT -p tcp --destination-port 8001 -i lo
iptables -A INPUT -j DROP -p tcp --destination-port 8001 -i eth0


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Each Recital table can have one or more data dictionaries to provide a central repository for constraints and other metadata. 

Here's how to set up field validation for a field with a small static number of acceptable values.

Using the example.dbf table from the southwind sample database, validation can be added to the title field to ensure it matches one of a list values.
open database southwind
alter table example add constraint;
(title set check inlist(alltrim(title),"Miss","Mr","Mrs","Ms"))
The inlist() function checks whether the specified expression exists in the comma-separated list which follows.  An attempt to update title with a value not in the list will give an error: Validation on field 'TITLE' failed.

If you have access to the Recital Workbench, you can use the modify structure worksurface to add and alter your dictionary entries, including a customized error message if required.

validation


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Usually, you do not need to setup an email server under Linux. Most GUI email clients support Gmail POP3 and IMAP configurations. But, how do you send mail via the standard /usr/bin/mail user agents in a shell script? Programs such as sendmail / postfix / exim can be configured as a gmail smarthost but they are largely overkill for this use. The ssmtp program is a neat utility that does just that for you via gmail.

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Platforms supported

  • Intel® / AMD™ 32 bit Linux
  • Intel® / AMD™ 64 bit Linux
  • HP PA-RISC HP-UX® 10.20 and above
  • Sun® SPARC Solaris™ 8 and above
  • HP Alpha OpenVMS 7.2-1 and above
  • SCO® OpenServer 5.0.5 and above
  • Sun® Intel® Solaris™ 10 and above
  • IBM AIX® 4.3 and above
  • HP Integrity OpenVMS 8.2-1 and above
  • HP Intel® Itanium® HP-UX® 11.23 and above
  • Mac OS X leopard 10.5 and above

Large File Support is available for Windows, Itanium HP-UX and Linux. 
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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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Recital's version of Samba allows application data to be shared and locked correctly across these systems, allowing a truly integrated heterogeneous data environment. For example Recital applications running on a UNIX/Linux server can read and update FoxPro databases residing on a Microsoft Windows NT server through the use of Samba.
You can view the modified changes by downloading the following files and patching these into your current Samba installation.
open.c (file opening and share modes)
version.h (versioning information)
The variable CPPFLAGS in the file Makefile will require the define -DRECITAL added to it.
In order to make the locking compatible between UNIX/Linux and Windows the following environment variable must be placed in the profile.db for Unix/Linux Developer and in profile.uas for the Universal Application Server:
DB_SAMBA=YES ;export DB_SAMBA 
The following settings need to be added to the smb.conf file to ensure that file names are always converted to lower case:
preserve case = no 
default case = lower 
mangle case = yes 
The following settings need to be added to the smb.conf file for locking to operate correctly:
oplocks = False
share modes = no
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All temporary files created by Recital are stored in the directory specified by the environment variable DB_TMPDIR.

 
In order to have these files stored in memory first create a temporary directory
mkdir /opt/recital/tmp
 
Then mount the directory with the tmpfs command
mount -t tmpfs -o size=1g recitaltmpfs /usr/recital/tmp
 
Then change the DB_TMPDIR variable in the recital.conf to point to the newly created temporary directory.
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