473,324 Members | 2,257 Online
Bytes | Software Development & Data Engineering Community
Post Job

Home Posts Topics Members FAQ

Join Bytes to post your question to a community of 473,324 software developers and data experts.

Atomicity problem / question

Hi,

I need a way to read a numeric field and then increment it by one in such a
way that even if two users did this at the exact same time, they would still
each get their own unique value. I would prefer a method not requiring me
to lock the whole table.

What is the best way to accomplish this?

I am using MySQL 3.23.58 and I use an ISAM type database.

Thanks,
Lars
Nov 5 '05 #1
10 1630
On Fri, 4 Nov 2005 16:58:28 -0800, "Lars" <no****@dev.null> wrote:
Hi,

I need a way to read a numeric field and then increment it by one in such a
way that even if two users did this at the exact same time, they would still
each get their own unique value. I would prefer a method not requiring me
to lock the whole table.

What is the best way to accomplish this?

I am using MySQL 3.23.58 and I use an ISAM type database.

One way: (There are surely many)
Make sure you have an auto-incrementing field. Insert an empty
record, return the new incremented value to the user.

Next user inserts next record, gets next key.

Once they've completed their input, do an update to the empty record
using he auto-incremented number.

--
gburnore at DataBasix dot Com
---------------------------------------------------------------------------
How you look depends on where you go.
---------------------------------------------------------------------------
Gary L. Burnore | ÝÛ³ºÝ³Þ³ºÝ³³Ýۺݳ޳ºÝ³Ý³Þ³ºÝ³ÝÝÛ³
| ÝÛ³ºÝ³Þ³ºÝ³³Ýۺݳ޳ºÝ³Ý³Þ³ºÝ³ÝÝÛ³
Official .sig, Accept no substitutes. | ÝÛ³ºÝ³Þ³ºÝ³³Ýۺݳ޳ºÝ³Ý³Þ³ºÝ³ÝÝÛ³
| ÝÛ 0 1 7 2 3 / Ý³Þ 3 7 4 9 3 0 Û³
Black Helicopter Repair Services, Ltd.| Official Proof of Purchase
================================================== =========================
Nov 5 '05 #2
> Make sure you have an auto-incrementing field. Insert an empty
record, return the new incremented value to the user.

Next user inserts next record, gets next key.

Once they've completed their input, do an update to the empty record
using he auto-incremented number.


Thanks for your reply.

Your method would probably work, but I was not totally clear in my original
post: I am looking for is a cleaner method. I would prefer not having to
have a separate table just for generating a unique value (or, rather, one
table for each type of unique value).

Something like this would work if I could lock the record:

UPDATE table SET field = field + 1 WHERE whatever
SELECT field from table WHERE whatever

Short of using a table lock or switching to InnoDB, what is the best way to
accomplish this?

Thanks,
Lars
Nov 5 '05 #3
On Fri, 4 Nov 2005 18:09:10 -0800, "Lars" <no****@dev.null> wrote:
Make sure you have an auto-incrementing field. Insert an empty
record, return the new incremented value to the user.

Next user inserts next record, gets next key.

Once they've completed their input, do an update to the empty record
using he auto-incremented number.


Thanks for your reply.

Your method would probably work, but I was not totally clear in my original
post: I am looking for is a cleaner method. I would prefer not having to
have a separate table just for generating a unique value (or, rather, one
table for each type of unique value).

Something like this would work if I could lock the record:

UPDATE table SET field = field + 1 WHERE whatever
SELECT field from table WHERE whatever

Short of using a table lock or switching to InnoDB, what is the best way to
accomplish this?


I didn't say anything at all about a separate table.

The method I mentioned is quite clean. One table, you simply insert
the entire row with nothing in the other fields to get it to give you
the auto-incremented value then use said value to key the update.

--
gburnore at DataBasix dot Com
---------------------------------------------------------------------------
How you look depends on where you go.
---------------------------------------------------------------------------
Gary L. Burnore | ÝÛ³ºÝ³Þ³ºÝ³³Ýۺݳ޳ºÝ³Ý³Þ³ºÝ³ÝÝÛ³
| ÝÛ³ºÝ³Þ³ºÝ³³Ýۺݳ޳ºÝ³Ý³Þ³ºÝ³ÝÝÛ³
Official .sig, Accept no substitutes. | ÝÛ³ºÝ³Þ³ºÝ³³Ýۺݳ޳ºÝ³Ý³Þ³ºÝ³ÝÝÛ³
| ÝÛ 0 1 7 2 3 / Ý³Þ 3 7 4 9 3 0 Û³
Black Helicopter Repair Services, Ltd.| Official Proof of Purchase
================================================== =========================
Nov 5 '05 #4
> I didn't say anything at all about a separate table.

The method I mentioned is quite clean. One table, you simply insert
the entire row with nothing in the other fields to get it to give you
the auto-incremented value then use said value to key the update.


Ok. In my case, the table is for storing many different settings &
configuration options. It has the following columns: Category, Item &
Value. Most of the records in the table have nothing to do with unique
values, but a few of them do. E.g. Category = ControlNumbers, Item =
LastPONumber, Value = 1000

If user A and user B simultaneously request the next invoice number, one
should get 1001, the other 1002 and the LastPONumber field will be 1002
afterwards.

If I understand your approach correctly, I would either have to add an
auto-incremented field to this table, or have an extra table just for this
purpose.

Lars


Nov 5 '05 #5
On Fri, 4 Nov 2005 21:40:15 -0800, "Lars" <no****@dev.null> wrote:
I didn't say anything at all about a separate table.

The method I mentioned is quite clean. One table, you simply insert
the entire row with nothing in the other fields to get it to give you
the auto-incremented value then use said value to key the update.


Ok. In my case, the table is for storing many different settings &
configuration options. It has the following columns: Category, Item &
Value. Most of the records in the table have nothing to do with unique
values, but a few of them do. E.g. Category = ControlNumbers, Item =
LastPONumber, Value = 1000

If user A and user B simultaneously request the next invoice number, one
should get 1001, the other 1002 and the LastPONumber field will be 1002
afterwards.

If I understand your approach correctly, I would either have to add an
auto-incremented field to this table, or have an extra table just for this
purpose.


Right now you do a select for the "LastPONumber", add one to it and
insert it. CHange the LastPONumber to autoincrementing. Problem
solved.
--
gburnore at DataBasix dot Com
---------------------------------------------------------------------------
How you look depends on where you go.
---------------------------------------------------------------------------
Gary L. Burnore | ÝÛ³ºÝ³Þ³ºÝ³³Ýۺݳ޳ºÝ³Ý³Þ³ºÝ³ÝÝÛ³
| ÝÛ³ºÝ³Þ³ºÝ³³Ýۺݳ޳ºÝ³Ý³Þ³ºÝ³ÝÝÛ³
Official .sig, Accept no substitutes. | ÝÛ³ºÝ³Þ³ºÝ³³Ýۺݳ޳ºÝ³Ý³Þ³ºÝ³ÝÝÛ³
| ÝÛ 0 1 7 2 3 / Ý³Þ 3 7 4 9 3 0 Û³
Black Helicopter Repair Services, Ltd.| Official Proof of Purchase
================================================== =========================
Nov 5 '05 #6
>>> I didn't say anything at all about a separate table.

The method I mentioned is quite clean. One table, you simply insert
the entire row with nothing in the other fields to get it to give you
the auto-incremented value then use said value to key the update.
Ok. In my case, the table is for storing many different settings &
configuration options. It has the following columns: Category, Item &
Value. Most of the records in the table have nothing to do with unique
values, but a few of them do. E.g. Category = ControlNumbers, Item =
LastPONumber, Value = 1000

If user A and user B simultaneously request the next invoice number, one
should get 1001, the other 1002 and the LastPONumber field will be 1002
afterwards.
1. lock tables configuration_settings write;
2. update configuration_settings set Value=Value+1
where Category = 'ControlNumbers' and Item = 'LastPONumber';
3. select Value from configuraton_settings
where Category = 'ControlNumbers' and Item = 'LastPONumber';
/* Use Value as your PO number */
4. unlock tables;

If User A is in query 2 or 3, User B will wait at query 1 until User A
finishes query 4.
If I understand your approach correctly, I would either have to add an
auto-incremented field to this table, or have an extra table just for this
purpose.


Right now you do a select for the "LastPONumber",


No, right now he does a select for Value where Category = 'ControlNumbers' and
Item = 'LastPONumber'.
add one to it and
insert it. CHange the LastPONumber to autoincrementing.
You can't change Value in only one row to autoincrementing.
Problem
solved.


There's a lot of utility in a Keyword = Value approach. For instance,
for an address book application, you might have a table: PersonID,
Attribute, and Value. Attribute might be things like 'First Name',
'Birth Date', 'Home Fax Number', 'Work Email', 'Home Street Address',
etc. You can add more without having to change the schema.
The trouble is, what type is Value? It's got dates, names,
telephone numbers, email addresses, etc. in it. You also end up
doing a lot of joins to get specific attributes (if present) in
specific variables.

Gordon L. Burditt
Nov 5 '05 #7
> 1. lock tables configuration_settings write;
2. update configuration_settings set Value=Value+1
where Category = 'ControlNumbers' and Item = 'LastPONumber';
3. select Value from configuraton_settings
where Category = 'ControlNumbers' and Item = 'LastPONumber';
/* Use Value as your PO number */
4. unlock tables; If User A is in query 2 or 3, User B will wait at query 1 until User A
finishes query 4.
Thanks. That is more along the lines I was thinking, but I was not too happy
about a table-level locking since the table is accessed very heavily by many
users. In real life I don't think it would be a problem, though, since it
is only a write lock and only for a split second at a time.
There's a lot of utility in a Keyword = Value approach. For instance,
for an address book application, you might have a table: PersonID,
Attribute, and Value. Attribute might be things like 'First Name',
'Birth Date', 'Home Fax Number', 'Work Email', 'Home Street Address',
etc. You can add more without having to change the schema.
The trouble is, what type is Value? It's got dates, names,
telephone numbers, email addresses, etc. in it.
I find the approach quite flexible for my purposes, since a text field can
store most any type of data. The job of ensuring proper format of dates,
numbers, etc. lie with the business objects of my apps.
You also end up doing a lot of joins to get specific attributes
(if present) in specific variables.

Out of curiosity, could you elaborate on this?

Thanks for your help,
Lars

Nov 5 '05 #8
>Thanks. That is more along the lines I was thinking, but I was not too happy
about a table-level locking since the table is accessed very heavily by many
users. In real life I don't think it would be a problem, though, since it
is only a write lock and only for a split second at a time.


"only a write lock" is a little like "only an atomic bomb".
A write lock prevents others from accessing AT ALL; a read
lock prevents others from writing. But what you need here is
a write lock.
There's a lot of utility in a Keyword = Value approach. For instance,
for an address book application, you might have a table: PersonID,
Attribute, and Value. Attribute might be things like 'First Name',
'Birth Date', 'Home Fax Number', 'Work Email', 'Home Street Address',
etc. You can add more without having to change the schema.
The trouble is, what type is Value? It's got dates, names,
telephone numbers, email addresses, etc. in it.


I find the approach quite flexible for my purposes, since a text field can
store most any type of data. The job of ensuring proper format of dates,
numbers, etc. lie with the business objects of my apps.
You also end up doing a lot of joins to get specific attributes
(if present) in specific variables.

Out of curiosity, could you elaborate on this?


A query for printing business cards with the keyword = value
organization might look like:

SELECT a.value, b.value, c.value, d.value, e.value, f.value, g.value
FROM addrbook a
LEFT JOIN addrbook b on a.personid = b.personid and b.attribute = 'Last Name'
LEFT JOIN addrbook c on a.personid = c.personid and c.attribute = 'Work Street Address'
LEFT JOIN addrbook d on a.personid = d.personid and d.attribute = 'Work City'
LEFT JOIN addrbook e on a.personid = e.personid and e.attribute = 'Work State'
LEFT JOIN addrbook f on a.personid = f.personid and f.attribute = 'Work Zip'
LEFT JOIN addrbook g on a.personid = g.personid and g.attribute = 'Work Phone'
WHERE a.attribute = 'First Name';

(personid, attribute) is a primary key so lookups should be fast,
but it still joins 7 copies of the table. Attribute is a good
candidate for an enum if you're willing to have to change the schema
to add new ones. The corresponding query with a table with a column
for each attribute might be:

SELECT firstname, lastname, workstreetaddress, workcity, workstate,
workzip, workphone
FROM addrbook;

Gordon L. Burditt
Nov 5 '05 #9
On Sat, 05 Nov 2005 16:11:48 -0000, go***********@burditt.org (Gordon
Burditt) wrote:

You can't change Value in only one row to autoincrementing.


Where did you get that idea?
--
gburnore at DataBasix dot Com
---------------------------------------------------------------------------
How you look depends on where you go.
---------------------------------------------------------------------------
Gary L. Burnore | ÝÛ³ºÝ³Þ³ºÝ³³Ýۺݳ޳ºÝ³Ý³Þ³ºÝ³ÝÝÛ³
| ÝÛ³ºÝ³Þ³ºÝ³³Ýۺݳ޳ºÝ³Ý³Þ³ºÝ³ÝÝÛ³
Official .sig, Accept no substitutes. | ÝÛ³ºÝ³Þ³ºÝ³³Ýۺݳ޳ºÝ³Ý³Þ³ºÝ³ÝÝÛ³
| ÝÛ 0 1 7 2 3 / Ý³Þ 3 7 4 9 3 0 Û³
Black Helicopter Repair Services, Ltd.| Official Proof of Purchase
================================================== =========================
Nov 5 '05 #10
> "only a write lock" is a little like "only an atomic bomb".
A write lock prevents others from accessing AT ALL; a read
lock prevents others from writing. But what you need here is
a write lock.


Oh, I had that backwards. That may not work too well, then. Probably a
separate table to hold these unique numbers would be a better approach after
all, since it would not interfere with the access to the rest of the
settings in the table.
You also end up doing a lot of joins to get specific attributes
(if present) in specific variables.

Out of curiosity, could you elaborate on this?


A query for printing business cards with the keyword = value
organization might look like:

SELECT a.value, b.value, c.value, d.value, e.value, f.value, g.value
FROM addrbook a
LEFT JOIN addrbook b on a.personid = b.personid and b.attribute = 'Last
Name'
LEFT JOIN addrbook c on a.personid = c.personid and c.attribute = 'Work
Street Address'
LEFT JOIN addrbook d on a.personid = d.personid and d.attribute = 'Work
City'
LEFT JOIN addrbook e on a.personid = e.personid and e.attribute = 'Work
State'
LEFT JOIN addrbook f on a.personid = f.personid and f.attribute = 'Work
Zip'
LEFT JOIN addrbook g on a.personid = g.personid and g.attribute = 'Work
Phone'
WHERE a.attribute = 'First Name';


I got it. No, this is not at all what I use that kind of tables for. The
Item, Value approach I use only for storing settings & resources, etc.

Thanks,
Lars
Nov 6 '05 #11

This thread has been closed and replies have been disabled. Please start a new discussion.

Similar topics

3
by: Stevey | last post by:
I have the following XML file... <?xml version="1.0"?> <animals> <animal> <name>Tiger</name> <questions> <question index="0">true</question> <question index="1">true</question> </questions>
7
by: nospam | last post by:
Ok, 3rd or is it the 4th time I have asked this question on Partial Types, so, since it seems to me that Partial Types is still in the design or development stages at Microsoft, I am going to ask...
3
by: Ekqvist Marko | last post by:
Hi, I have one Access database table including questions and answers. Now I need to give answer id automatically to questionID column. But I don't know how it is best (fastest) to do? table...
10
by: glenn | last post by:
I am use to programming in php and the way session and post vars are past from fields on one page through to the post page automatically where I can get to their values easily to write to a...
10
by: Rider | last post by:
Hi, simple(?) question about asp.net configuration.. I've installed ASP.NET 2.0 QuickStart Sample successfully. But, When I'm first start application the follow message shown. ========= Server...
5
by: John Bokma | last post by:
Hi, I am working on a simple chat program and want to use AJAX. In order to get the new lines added to the chat I need to call a server-side script, say every x seconds. A second way to get...
53
by: Jeff | last post by:
In the function below, can size ever be 0 (zero)? char *clc_strdup(const char * CLC_RESTRICT s) { size_t size; char *p; clc_assert_not_null(clc_strdup, s); size = strlen(s) + 1;
56
by: spibou | last post by:
In the statement "a *= expression" is expression assumed to be parenthesized ? For example if I write "a *= b+c" is this the same as "a = a * (b+c)" or "a = a * b+c" ?
2
by: Allan Ebdrup | last post by:
Hi, I'm trying to render a Matrix question in my ASP.Net 2.0 page, A matrix question is a question where you have several options that can all be rated according to several possible ratings (from...
3
by: kirk | last post by:
I have a class with a custom event that starts a timer when it's holding subscriptions and stops the timer otherwise. The timer repeatedly calls an API which takes a string. For each call the...
0
by: DolphinDB | last post by:
Tired of spending countless mintues downsampling your data? Look no further! In this article, you’ll learn how to efficiently downsample 6.48 billion high-frequency records to 61 million...
0
by: ryjfgjl | last post by:
ExcelToDatabase: batch import excel into database automatically...
0
isladogs
by: isladogs | last post by:
The next Access Europe meeting will be on Wednesday 6 Mar 2024 starting at 18:00 UK time (6PM UTC) and finishing at about 19:15 (7.15PM). In this month's session, we are pleased to welcome back...
0
by: Vimpel783 | last post by:
Hello! Guys, I found this code on the Internet, but I need to modify it a little. It works well, the problem is this: Data is sent from only one cell, in this case B5, but it is necessary that data...
1
by: CloudSolutions | last post by:
Introduction: For many beginners and individual users, requiring a credit card and email registration may pose a barrier when starting to use cloud servers. However, some cloud server providers now...
1
by: Defcon1945 | last post by:
I'm trying to learn Python using Pycharm but import shutil doesn't work
1
by: Shællîpôpï 09 | last post by:
If u are using a keypad phone, how do u turn on JavaScript, to access features like WhatsApp, Facebook, Instagram....
0
by: af34tf | last post by:
Hi Guys, I have a domain whose name is BytesLimited.com, and I want to sell it. Does anyone know about platforms that allow me to list my domain in auction for free. Thank you
0
isladogs
by: isladogs | last post by:
The next Access Europe User Group meeting will be on Wednesday 3 Apr 2024 starting at 18:00 UK time (6PM UTC+1) and finishing by 19:30 (7.30PM). In this session, we are pleased to welcome former...

By using Bytes.com and it's services, you agree to our Privacy Policy and Terms of Use.

To disable or enable advertisements and analytics tracking please visit the manage ads & tracking page.