I have designed database tables (normalised, on an MS SQL server) and created a standalone windows front end for an application that will be used by a handful of users to add and edit information. We will add a web interface to allow searching accross our production area at a later date.
I am concerned that if two users start editing the same record then the last to commit the update would be the 'winner' and important information may be lost. A number of solutions come to mind but I'm not sure if I am going to create a bigger headache.
Do nothing and hope that two users are never going to be editing the same record at the same time. - Might never happed but what if it does?
Editing routine could store a copy of the original data as well as the updates and then compare when the user has finished editing. If they differ show user and comfirm update - Would require two copies of data to be stored.
Add last updated DATETIME column and check it matches when we update, if not then show differences. - requires new column in each of the relevant tables.
Create an editing table that registers when users start editing a record that will be checked and prevent other users from editing same record. - would require carful thought of program flow to prevent deadlocks and records becoming locked if a user crashes out of the program.
Are there any better solutions or should I go for one of these?
If you expect infrequent collisions, Optimistic Concurrency is probably your best bet.
Scott Mitchell wrote a comprehensive tutorial on implementing that pattern:
Implementing Optimistic Concurrency
A classic approach is as follows:
add a boolean field , "locked" to each table.
set this to false by default.
when a user starts editing, you do this:
lock the row (or the whole table if you can't lock the row)
check the flag on the row you want to edit
if the flag is true then
inform the user that they cannot edit that row at the moment
else
set the flag to true
release the lock
when saving the record, set the flag back to false
# Mark Harrison : SQL Server does not support that syntax (SELECT ... FOR UPDATE).
The SQL Server equivalent is the SELECT statement hint UPDLOCK.
See SQL Server Books Online for more information.
-first create filed (update time) to store last update record
-when any user select record save select time,
compare between select time and update time field if( update time) > (select time) that mean another user update this record after select record
SELECT FOR UPDATE and equivalents are good providing you hold the lock for a microscopic amount of time, but for a macroscopic amount (e.g. the user has the data loaded and hasn't pressed 'save' you should use optimistic concurrency as above. (Which I always think is misnamed - it's more pessimistic than 'last writer wins', which is usually the only other alternative considered.)
Another option is to test that the values in the record that you are changing are the still the same as they were when you started:
SELECT
customer_nm,
customer_nm AS customer_nm_orig
FROM demo_customer
WHERE customer_id = #p_customer_id
(display the customer_nm field and the user changes it)
UPDATE demo_customer
SET customer_nm = #p_customer_name_new
WHERE customer_id = #p_customer_id
AND customer_name = #p_customer_nm_old
IF ##ROWCOUNT = 0
RAISERROR( 'Update failed: Data changed' );
You don't have to add a new column to your table (and keep it up to date), but you do have to create more verbose SQL statements and pass new and old fields to the stored procedure.
It also has the advantage that you are not locking the records - because we all know that records will end up staying locked when they should not be...
The database will do this for you. Look at "select ... for update", which is designed just for this kind of thing. It will give you a write lock on the selected rows, which you can then commit or roll back.
With me, the best way i have a column lastupdate (timetamp datatype).
when select and update just compare this value
another advance of this solution is that you can use this column to track down the time data has change.
I think it is not good if you just create a colum like isLock for check update.
Related
Okay, just to clarify: I have a SQL Table (contains ID, School, Student ID, Name, Fee $, Fee Type, and Paid (as the columns)) that needs to be posted on a Grid that will uploaded on a website. The Grid shows everything correctly and shows what Fees need to be Paid. The Paid column has a bit data type for 1 or 0 (basically a checklist.) I am being asked to add two more columns: User and DateChanged. The reason why is to log which staff changed the "Paid" column. It would only capture the Username of the staff who changed it in the SQL Table and also the time. So to clarify even more, I need to create 2 columns: "User, DateChanged" and the columns would log when someone changed the "Paid" column.
For example: User:Bob checks the Paid column for X student on 5/2/17 at 10pm.
In the same row of X student's info, under User column Tom would appear there. Under DateChanged it would show 2017-05-02 10pm.
What steps would I take to make this possible.
I'm currently IT Intern and all this SQL stuff is new to me. Let me know if you need more clarification. FYI The two new columns: User, DateChanged will not be on the grid.
The way to do this as you've described is to use a trigger. I have an example of some code below but be warned as triggers can have unexpected side-effects, depending on how the database and app interface are set up.
If it is possible for you to change the application code that sends SQL queries to the database instead, that would be much safer than using a trigger. You can still add the new fields, you would just be relying on the app to keep them updated instead of doing it all in SQL.
Things to keep in mind about this code:
If any background processes or procedures make updates to the table, it will overwrite the timestamp and username automatically, because this is triggered on any update to the row(s) in question.
If the users don't have any direct access to SQL Server (in other words, the app is the only thing connecting to the database), then it is possible that the app will only be using one database login username for everyone, and in that case you will not be able to figure out which user made the update unless you can change the application code.
If anyone changes something by accident and then changes it back, it will overwrite your timestamp and make it look like the wrong person made the update.
Triggers can potentially bog down the database system if there are a very large number of rows and/or a high number of updates being made to the table constantly, because the trigger code will be executed every time an update is made to a row in the table.
But if you don't have access to change the application code, and you want to give triggers a try, here's some example code that should do what you are needing:
create trigger TG_Payments_Update on Payments
after update
as
begin
update Payments
set DateChanged = GetDate(), UserChanged = USER_NAME()
from Payments, inserted
where Payments.ID = inserted.ID
end
The web app already knows the current user working on the system, so your update would just include that user's ID and the current system time for when the action took place.
I would not rely on SQL Server triggers since that hides what's going on within the system. Plus, as others have said, they have side effects to deal with too.
I have a table that has several fields, 2 of these fields are "startdate" and "enddate", which mark the validity of the record. If i insert 1 new record, the new record cannot overlap with other records in terms of start date and end date.
Hence on insertion of new record i may need to adjust the value of "startdate" and "enddate" of pre-existing records so they don't overlap with the new record. Similarly, any preexisting records that have 100% overlap with the new record, will need to be deleted.
My table is an InnoDB table, which i know supports such transactions.
Are there any examples which show use of insert / update / delete using transactions (all must succeed in order for any one of them to succeed and be commited) ?
I don't know how to do this. Most examples only show the use of saveAssociated() which i'm not sure is capable of catering for delete operations?
Thanks
Kevin
Perhaps you could use the beforeSave callback to search for the preexisting records and delete them before saving your new record.
from the docs:
Place any pre-save logic in this function. This function executes immediately after model data has been successfully validated, but just before the data is saved. This function should also return true if you want the save operation to continue.
I think you're looking to do Transactions: http://book.cakephp.org/2.0/en/models/transactions.html
That should allow you to run your queries - you start a transaction, perform any required actions, and then commit or rollback based on the outcome. Although, given your description I'd think doing some reads and adjusting your data before committing anything might be a better approach. Either way, transactions aren't a bad idea though!
We want to know what rows in a certain table is used frequently, and which are never used. We could add an extra column for this, but then we'd get an UPDATE for every SELECT, which sounds expensive? (The table contains 80k+ rows, some of which are used very often.)
Is there a better and perhaps faster way to do this? We're using some old version of Microsoft's SQL Server.
This kind of logging/tracking is the classical application server's task. If you want to realize your own architecture (there tracking architecture) do it on your own layer.
And in any case you will need application server there. You are not going to update tracking field it in the same transaction with select, isn't it? what about rollbacks? so you have some manager who first run select than write track information. And what is the point to save tracking information together with entity info sending it back to DB? Save it into application server file.
You could either update the column in the table as you suggested, but if it was me I'd log the event to another table, i.e. id of the record, datetime, userid (maybe ip address etc, browser version etc), just about anything else I could capture and that was even possibly relevant. (For example, 6 months from now your manager decides not only does s/he want to know which records were used the most, s/he wants to know which users are using the most records, or what time of day that usage pattern is etc).
This type of information can be useful for things you've never even thought of down the road, and if it starts to grow large you can always roll-up and prune the table to a smaller one if performance becomes an issue. When possible, I log everything I can. You may never use some of this information, but you'll never wish you didn't have it available down the road and will be impossible to re-create historically.
In terms of making sure the application doesn't slow down, you may want to 'select' the data from within a stored procedure, that also issues the logging command, so that the client is not doing two roundtrips (one for the select, one for the update/insert).
Alternatively, if this is a web application, you could use an async ajax call to issue the logging action which wouldn't slow down the users experience at all.
Adding new column to track SELECT is not a practice, because it may affect database performance, and the database performance is one of major critical issue as per Database Server Administration.
So here you can use one very good feature of database called Auditing, this is very easy and put less stress on Database.
Find more info: Here or From Here
Or Search for Database Auditing For Select Statement
Use another table as a key/value pair with two columns(e.g. id_selected, times) for storing the ids of the records you select in your standard table, and increment the times value by 1 every time the records are selected.
To do this you'd have to do a mass insert/update of the selected ids from your select query in the counting table. E.g. as a quick example:
SELECT id, stuff1, stuff2 FROM myTable WHERE stuff1='somevalue';
INSERT INTO countTable(id_selected, times)
SELECT id, 1 FROM myTable mt WHERE mt.stuff1='somevalue' # or just build a list of ids as values from your last result
ON DUPLICATE KEY
UPDATE times=times+1
The ON DUPLICATE KEY is right from the top of my head in MySQL. For conditionally inserting or updating in MSSQL you would need to use MERGE instead
Let's say I have a table (let's call it myTable) on a database server, and I'm writing a desktop client application that allows you to modify records in myTable. I don't want two users to be able to edit the same record, i.e., if user A edits record 1 and user B tries to do the same, he gets notified that the record is currently "locked" by user A.
I'm sure that this is a common problem, so I'm wondering if there is a canonical solution to it.
There is an obvious solution, but it also has an obvious drawback:
Add a field inUseBy to myTable, set it to the user's name once the user starts editing the record, and clear it once the user finishes, e.g.
function editRecord(recordId):
begin transaction
if (select lockedBy from myTable where id = recordId) is not empty:
commit
show "Sorry, record already in use by ..."
else
update myTable set lockedBy = current_user() where id = recordId
commit
show UI window to let user edit and update record
update myTable set lockedBy = empty where id = recordId
Drawback: If user A's application crashes, the record stays locked.
The following approach might seem suitable at a first glance, but won't solve the problem:
Use database locks to lock record 1. This will just cause user B to run into a timeout. I need to lock the record on the application level not on the database level.
A common approach is to use ROWVERSION for optimistic concurrency.
Rowversion is a datatype (Which you'd add as a new column) that updates when the row is updated.
So, you select your row, including the rowversion column. When you send your update back, you make sure the rowversions match - By doing your update and adding "WHERE Rowversion = TheRowversionIGotEarlier" and seeing if ##ROWCOUNT is not 0 - If it is zero, then you can assume someone has modified the row since you read it, and you can return that message to the user.
http://www.mssqltips.com/sqlservertip/1501/optimistic-locking-in-sql-server-using-the-rowversion-data-type/
You're asking about pessimistic concurrency, and as this answer and the comment on your question say - Do consider optimistic concurrency. Consider your model, how would it handle someone starting to edit a record then going to lunch? Or if they never came back from lunch? What if a record has to be edited in the meantime, for a critical business reason?
I'm trying to create a LINQ to SQL class that represents the "latest" version of itself.
Right now, the table that this entity represents has a single auto-incrementing ID, and I was thinking that I would add a version number to the primary key. I've never done anything like this, so I'm not sure how to proceed. I would like to be able to abstract the idea of the object's version away from whoever is using it. In other words, you have an instance of this entity that represents the most current version, and whenever any changes are submitted, a new copy of the object is stored with an incremented version number.
How should I proceed with this?
If you can avoid keeping a history, do. It's a pain.
If a complete history is unavoidable (regulated financial and medical data or the like), consider adding history tables. Use a trigger to 'version' into the history tables. That way, you're not dependent on your application to ensure a version is recorded - all inserts/updates/deletes are captured regardless of the source.
If your app needs to interact with historical data, make sure it's readonly. There's no sense capturing transaction histories if someone can simply change them.
If your concern is concurrent updates, consider using a record change timestamp. When both User A and User B view a record at noon, they fetch the record's timestamp. When User A updates the record, her timestamp matches the record's so the update goes through and the timestamp is updated as well. When User B updates the record five minutes later, his timestamp doesn't match the record's so he's warned that the record has changed since he last viewed it. Maybe it's automatically reloaded...
Whatever you decide, I would avoid inter-mingling current and historic data.
Trigger resources per comments:
MSDN
A SQL Team Introduction
Stackoverflow's Jon Galloway describes a general data-change logging trigger
The keys to an auditing trigger are the virtual tables 'inserted' and 'deleted'. These tables contain the rows effected by an INSERT, UPDATE, or DELETE. You can use them to audit changes. Something like:
CREATE TRIGGER tr_TheTrigger
ON [YourTable]
FOR INSERT, UPDATE, DELETE
AS
IF EXISTS(SELECT * FROM inserted)
BEGIN
--this is an insert or update
--your actual action will vary but something like this
INSERT INTO [YourTable_Audit]
SELECT * FROM inserted
END
IF EXISTS(SELECT * FROM deleted)
BEGIN
--this is a delete, mark [YourTable_Audit] as required
END
GO
The best way to proceed is to stop and seriously rethink your approach.
If you are going to keep different versions of the "object" around, then you are better off serializing it into an xml format and storing that in an XML column with a field for the version number.
There are serious considerations when trying to maintain versioned data in sql server revolving around application maintenance.
UPDATE per comment:
Those considerations include: the inability to remove a field or change the data type of a field in future "versions". New fields are required to be nullable or, at the very least, have a default value stored in the DB for them. As such you will not be able to use them in a unique index or as part of the primary keys.
In short, the only thing your application can do is expand. Provided the expansion can be ignored by previous layers of code.
This is the classic problem of Backwards Compatibility which desktop software makers have struggled with for years. And is the reason you might want to stay away from it.