Monday, October 17, 2005

Planning for failure - embedded systems

Here's a short story about total system design, and planning for failure.

Last friday night, I was filling up my truck with gas, and I did the "pay at the pump" thing, using my credit card. After I filled my tank, the pump's display helpfully asked "Do you want a receipt? Yes / No". I usually say no, but I wanted to remember the price per gallon, so I said "Yes." Within 2 seconds, it said "Printer Not Available." Why did they offer to print a receipt if the printer is not working? A more helpful system would have simply stated "See attendant if you want a receipt," knowing that the printer was down.

To make this system more effective, they would have to:
a) know when the attendant had replaced the paper roll
b) remember that last time they tried to print, it didn't work.
c) reset the printer state when the attendant did a test print, or closed the access panel.

Perhaps cost was the prohibitive factor.

Friday, October 14, 2005

SSPI connection for Rails SQLServer connection adapter

Recently, I added support for SSPI (a connection that uses your local user account to authenticate you on the database server without supplying another password). To use an SSPI connection, do not supply a username or a password.

sqlserver_adapter.rb:

username = config[:username] ? config[:username].to_s : ''
...
# Use integrated security
if config[:integrated] or (username == '' and password == '')
conn = DBI.connect("DBI:ADO:Provider=SQLOLEDB;Data Source=#{host};Initial Catalog=#{database};Integrated Security=SSPI;")
else
conn = DBI.connect("DBI:ADO:Provider=SQLOLEDB;Data Source=#{host};Initial Catalog=#{database};User Id=#{username};Password=#{password};")
end

Rails connection_adapter SqlServer fix for boolean fields

I tried to use Ruby on Rails with Microsoft SQLServer recently, and I kept having problems with my "bit" fields. Looking on the web, the Rails repository claims to have fixed this, so I downloaded the new adapter from svn. It didn't work for me, so I started hacking. I really need to send this as a diff to the ActiveRecord maintainers, with test code to prove that it works. By the way, I am using ADO mode ... maybe their original code works for ODBC mode. I suspect that tests should be run for both modes. Anyway, enough chatter, here is the fix!

In file sqlserver_adapter.rb, add the #cast_to_bit and modify #quote as following:


def cast_to_bit(value)
case value
when nil then nil
when '1' then 1
when '0' then 0
when 1 then 1
when 0 then 0
when 'true' then 1
when 'false' then 0
else
# puts "Invalid Boolean #{value}"
0
end
end

def quote(value, column = nil)
case value
when String
if column && column.type == :binary
"'#{quote_string(column.string_to_binary(value))}'"
elsif column && column.type == :boolean
"#{column.cast_to_bit(value)}"
else
"'#{quote_string(value)}'"
end
when NilClass then "NULL"
when TrueClass then '1'
when FalseClass then '0'
when Float, Fixnum, Bignum then value.to_s
when Date then "'#{value.to_s}'"
when Time, DateTime then "'#{value.strftime("%Y-%m-%d %H:%M:%S")}'"
else "'#{quote_string(value.to_yaml)}'"
end
end


Update: Check my new blog entry for more on booleans. How does Rails scaffolding select HTML input tags?

Update2: As of Dec 8, 2005, the Rails 0.14.3 has changed the SqlServer adapter. My boolean fix may not be required (I haven't tested it yet).

Thursday, October 13, 2005

.Net Strings - LastIndexOf in c#

Today, I tried to use string methods IndexOf and LastIndexOf to parse a directory entry string that I received across an SSH channel. I guess I should have read the method description closer, but I expected LastIndexOf to find the last space character in a string. The signature I wanted to use is:

public int LastIndexOf(char, int, int);

Reports the index position of the last occurrence of a specified String within this instance. The search starts at a specified character position and examines a specified number of character positions.

It turns out that LastIndex will decrement it's starting index for "count" characters. Here was my incorrect code:

private void method () {
string dirEntry = "-rwxr-xr-x 1 abcde fghij 44458 Feb 22 1987 prt01.txt";
int m, n;

// I thought this would scan from 32 to 44 and find the last space char
m = dirEntry.LastIndexOf(' ', 32, 12);
// it returned 32

// code that does what I wanted:
m = 32;
for(int i=32; i<44; i++)
{
if (ca[i] != ' ')
{
m = i;
break;
}
}

// find first space after the numeric string
n = dirEntry.IndexOf(' ', m+1, 12);

// if m and n are correct, get the file size
string fsize = dirEntry.Substring(m, n-m);
}

Design Note: I must admit that there are alot of "magic numbers" in here. I normally do not write code with magic numbers embedded in it. I prefer to assign class constants, or use INI, XML or database initialization as appropriate.

Saturday, October 08, 2005

Upgrading Ubuntu from Hoary to Breezy

For those of you still running Ubuntu Hoary, I have upgraded 2 computers to Breezy now, and I like the improvements just fine. There is a more up-to-date software selection, including PHP5, Ruby, Rails, Mono 1.8.x and MonoDevelop. If you want to pull from Packages.DotDeb.Org, you can even try out MySQL 5.x. I haven't done that yet, but I have found some instructions on the Ubuntu Forums that I plan to use as a guideline.

When you upgrade, you may have to stop your email server. I had to on both computers. If you want an easier installation, there are Breezy Colony CDs that you can download with BitTorrent. I haven't done that yet, but if I install on another PC before Breezy goes live, I will try it.

Best of luck to my fellow ubuntu-ites.

Friday, October 07, 2005

Ruby On Rails (Resources)

Here are some links that I consider essential to working with Ruby on Rails.

Ruby On Rails
RubyForge.org
Ruby Code Snippets
What is Ruby on Rails

I have Rails running on Win32 and Ubuntu Linux (Breezy) so far, with MS SqlServer and MySQL as back-end databases. I just wish Ruby and Debian developers could come to a conclusion about package management between apt-get and gem. The debian team wants to package up each gem so they can control the location of C interface libraries. Of course, we sacrifice timely access to updated gems, but that is no different than any other package. Personally, I have un-installed the apt-get managed versions of all ruby packages including rails and rake, and installed them through gem install. Someday I need to

If you want to try out the bleeding edge of Ruby on Rails, download and install the Subversion source control system, then pull down the rails source into your application's vendor directory. Speaking of Edge Rails, I noticed that it supports a plugin system, so we can write mixins that add functionality to ActiveRecords, etc.

Thursday, October 06, 2005

SQLServer 2000 (TSQL) Resources

They have an awesome collection of tips at ...
BigBold.com SQL Snippets

I check their ruby and rails sections as well.

(This page will be updated as I dig through my links.)

Monday, October 03, 2005

Using a spreadsheet to simplify data entry

I've used a spreadsheet for data entry on a number of occasions. It is quite handy! All it requires is a string concatenate function, and a bit of cleverness. This is great for "quick-n-dirty" data entry, or for searches. The comma separated list is usually used for "SELECT * FROM TBL WHERE X IN ( ## excel text goes here ## )".

# to build a comma separated list
=CONCATENATE("'", A2, "',")

# insert new records
=CONCATENATE("INSERT INTO #t (Nbr, Code, Descr) VALUES ('", A4, "', '", B4, "', '", C4,"');")

SQLServer 2000: Concatenate fields in a table

Here's a handy way to create a comma-separated list from a small lookup table. I put the most interesting statement in bold.

-- BEGIN CODE

-- silence the "rows affected" message
SET NOCOUNT ON ;

-- create a temp table
CREATE TABLE #temp(id int identity(1,1) not null, state varchar(50) not null);
-- insert a few states for our example
INSERT INTO #temp(state) values('AK');
INSERT INTO #temp(state) values('CA');
INSERT INTO #temp(state) values('HI');
INSERT INTO #temp(state) values('OK');
INSERT INTO #temp(state) values('TX');

-- declare a variable large enough to hold the needed results
DECLARE @strg varchar(1000) ;

-- collect fields (ordered any way you want)
-- you can also do "top 10"
SELECT @strg = COALESCE(@strg + ',', '') + State
FROM #temp t
ORDER BY State DESC

PRINT 'My States: ' + @strg ;

DROP TABLE #temp ;
-- END CODE

Capture output from SqlServer 2000

Here is a method of capturing the output of a command executed by the system shell. It is typically called from the SA account, and used to automate maintenance tasks. I do not recommend that you allow other users to send statements to a command shell.

--- BEGIN CODE
DECLARE @s varchar(255);

-- our command
SET @s = 'DIR C:\TEMP\*.EXE' ;

-- the temporary table
CREATE TABLE #tCmdOutput( output varchar(1024) NULL ) ;

-- capturing output from the command
INSERT INTO #tCmdOutput( output )
EXEC Master..XP_CmdShell @s ;

-- delete blank lines
DELETE FROM #tCmdOutput WHERE output is null ;

-- show output as a result set
SELECT * FROM #tCmdOutput ;

-- clean up our session
DROP TABLE #tCmdOutput ;
--- END CODE

Introduction

Hi! I am a computer programmer, working for the last few years with C# and Vb on .Net 1.x and Windows. At prior jobs, I have written Java, FoxPro for Windows, MFC, C, Visual Basic and Fortran. I have more experience with SQLServer 7 and 2000 than with any other database, writing huge stored procedures for invoicing, etc. Recently, I have started looking at Ruby and Rails, and I intend to comment on my experiences as they occur.

I am starting a blog so I can "give some back" to the various communities that I am learning from.
I will mostly write about Ruby, Ruby on Rails, MySQL 4x and 5.x, Ubuntu Linux, Mono.net, and more, but I am always studying on some new topic, so who knows!.

Since this is my first post, it will probably change over time. I am always studying on a new hobby or a new programming topic!

Update
I've decided to split my blog posts, keeping this one for programming topics only.
Check out Bees In Oklahoma or Forging Fun (my blacksmithing blog) if you're interested in my hobbies.