I have a schema of a database and a web application. I want to have the web application be able to select, insert and remove rows to a table, but the table may not exist, maybe in a testing environment, and the table may be missing columns, most likely because the web application has updated.
I want to be able to make sure that the table is ready to accept the data that the web application sends to it during the time the application is alive.
The idea I had is the application (written in Java) will have a table structure embedded into it, and when the application starts, just copy all of the data in the table (if it exists) to a temporary table, delete the old table and make a new one with the temporary table's data, and then drop the temporary table. As you can tell, it's nowhere near innovative.
Another idea I had is use the SHOW COLUMNS command to correct any missing columns parallel with the SHOW TABLES LIKE to check if it exists, but I feel like Stack Overflow would've had a better solution. Is that all I can do?
There are many ways to solve the problem of consistency of the database version and the version of the application.
However, in the production database, this situation is unacceptable.
I think that the simplest ways are the best.
To ensure such compliance, it is enough to execute a script that updates the database before performing the testing.
START TRANSACTION;
DROP TABLE ... IF EXISTS;
CREATE TABLE ...
COMMIT;
Remember about IF EXISTS and having DROP grant!
Such a script can be easily managed by placing it in RCS and controlling the version number needed in the application.
You can also save this version number in some table in the database itself and check when the application starts, whether the number is compatible with the assumed one and if you do not call the database update script.
Have a look at JPA an Hibernate. There is hbm2ddl.auto property. Looks like "update" option does what you want.
For more details
What are the possible values of the Hibernate hbm2ddl.auto configuration and what do they do
Related
I'm wondering if there is any better way than going through the tables one by one adding the columns missing when some fields/tables needs to be added because of the most recent changes in the app?
For example, I'm working at the localhost and when I finish doing the new version of my app, I will put all the files into my FTP and, sometimes, I have done, in my local database, changes and so it means that I also need to update my database at my server.
There's any better way to add/edit the columns/tables without changing the info? Some of the columns are also deleted, etc.
Hopefully you've thought your database design through so that making changes to the structure is a rare occurrence. If you're making regular changes to the number of columns or adding tables, it's likely a sign that you haven't normalized your database structure sufficiently.
Anyway, I'd script it as an SQL file that you deploy (which you can then run through phpMyAdmin or the command line or any other means you prefer to execute SQL queries). This has the added advantage of being something you can easily duplicate across your development and production databases, send to customers, and if you wish store in version control so you know when exactly you made the changes to the database.
This way, you'll end up with an SQL file that has a couple of statements like
ALTER TABLE `foo` ADD `new` INT NOT NULL ;
or something similar.
As for how you'd make the file, probably the easiest way is just copying and pasting the generated SQL statement from phpMyAdmin after modifying the table -- the SQL code used to make the change is shown near the top of the screen on the next page. You can copy and paste that to a new text file to create your SQL file. You may wish to add the first line
use `baz`;
using your database name instead of "baz". That way you don't have to specify on import which database the changes are meant for.
Hope this helps.
I am looking for a solution to the following:
Database: A
Table: InvoiceLines
Database: B
Table: MyLog
Every time lines are added to InvoiceLines in database A, I want to run a query that updates the table MyLog in database B. And I want it instantly.
Normally I would create a trigger in database A on INSERT in InvoiceLines. The problem is that database A belongs to a ERP program where I don't want to make any changes at all (updates, unknown functionality in 3-layer program, etc)
Any hints to help me in the right direction...?
You can use transactional replication to send changes from your table in database A to a copy in DB B, then create your triggers on the copy. It's not "instant," but it's usually considered "near real time."
You might be able to use DB mirroring to do this somehow, but you'd have to do some testing to see if you could get it to work right (maybe set up triggers in the mirror that don't exist in the original?)
One possible solution to replicate trigger's functionality without database update is to poll the table by an external application (i.e. java) which on finding new insert would fire required query.
In SQLServer2008, something similar can be done via C# assembly but again this needs to be installed which requires database update.
I would like to implement a custom database initialization strategy so that I can:
generate the database if not exists
if model change create only new tables
if model change create only new fields without dropping the table and losing the data.
Thanks in advance
You need to implement IDatabaseInitializer interface.
Eg
public class MyInitializer : IDatabaseInitializer<MyDbContext>
{
public void InitializeDatabase(MyDbContext context)
{
//your logic here
}
}
And then set your initializer at your application startup
Database.SetInitializer<ProductCatalog>(new MyInitializer());
Here's an example
You will have to manually execute commands to alter the database.
context.ObjectContext.ExecuteStoreCommand("ALTER TABLE dbo.MyTable ADD NewColumn VARCHAR(20) NULL");
You can use a tool like SQL Compare to script changes.
There is a reason why this doesn't exist yet. It is very complex and moreover IDatabaseInitializer interface is not very prepared for such that (there is no way to make such initialization database agnostic). Your question is "too broad" to be answered to your satisfaction. With your reaction to #Eranga's correct answer you simply expect that somebody will tell you step by step how to do that but we will not - that would mean we will write the initializer for you.
What you need to do what you want?
You must have very good knowledge of SQL Server. You must know how does SQL server store information about database, tables, columns and relations = you must understand sys views and you must know how to query them to get data about current database structure.
You must have very good knowledge of EF. You must know how does EF store mapping information. You must be able to explore metadata get information about expected tables, columns and relations.
Once you have old database description and new database description you must be able to write a code which will correctly explore changes and create SQL DDL commands for changing your database. Even this look like the simplest part of the whole process this is actually the hardest one because there are many other internal rules in SQL server which cannot be violated by your commands. Sometimes you really need to drop table to make your changes and if you don't want to lose data you must first push them to temporary table and after recreating table you must push them back. Sometimes you are doing changes in constraints which can require temporarily turning constrains off, etc. There is good reason why tools which do this on SQL level (comparing two databases) are probably all commercial.
Even ADO.NET team doesn't implemented this and they will not implement it in the future. Instead they are working on something called migrations.
Edit:
That is true that ObjectContext can return you script for database creation - that is exactly what default initializers are using. But how it could help you? Are you going to parse that script to see what changed? Are you going to execute that script in another connection to use the same code as for current database to see its structure?
Yes you can create a new database, move data from the old database to a new one, delete the old one and rename a new one but that is the most stupid solution you can ever imagine and no database administrator will ever allow that. Even this solution still requires analysis of changes to create correct data transfer scripts.
Automatic upgrade is a wrong way. You should always prepare upgrade script manually with help of some tools, test it and after that execute it manually or as part of some installation script / package. You must also backup your database before you are going to do any changes.
The best way to achieve this is probably with migrations:
http://nuget.org/List/Packages/EntityFramework.SqlMigrations
Good blog posts here and here.
Is there any way to automatically create a trigger on creation of new table in MySQL?
As I've pointed out in your other question, I think a process and security review is in order here. It's an audited database, so nobody (especially third-party service providers) should be creating tables in your database without your knowledge.
The issue you've got is, as well as the new table being created, you will also need to have another table created to store the audited/changed records, which will have an identical structure as the original table with possibly a time/date and user column. If a third-party provider is creating this table, they won't know to create the auditing table, therefore even if you could generate your triggers dynamically, they wouldn't work.
It's impossible to create a single table that will hold all changes record for all other tables in your database because the structure between tables inevitably differs.
Therefore: make all change requests (e.g. providers wants to create TableX, they submit a change request (including the SQL script) explaining the reason for the change) to yourself and/or your team.
You execute the SQL on a test copy of your database, and use the same structure to create another table to hold the modified records.
You then create and test the necessary triggers, generate a new SQL script to create the two tables and your triggers and execute that on your live database. You give your provider permissions to use the new table and away they go.
Everyone's happy. Yes, it may take a little while longer, and yes you'll have more work to do, but that's a hell of a lot less work than is required to try and parse query logs to re-create records that have already been changed/deleted, or parse the binary log and keep up-to-date with every change, and modify your code when the format of the log file changes etc etc.
I'm trying to use a simple mysql database but tweak it so that every field is backed up up to an indefinite number of versions. The best way I can illustrate this is by replacing each and every field of every table with a stack of all the values this field has ever had (each of these values should be timestamped). I guess it's kind of like having customized version control for all my data..
Any ideas on how to do this?
The usual method for "tracking any changes" to a table is to add insert/update/delete trigger procedures on the table and have those records saved in a history table.
For example, if your main data table is "ItemInfo" then you would also have an ItemInfo_History table that got a copy of the new record every time anything changed (via the triggers).
This keeps the performance of your primary table consistent, yet gives you access to the history of any changes if you need it.
Here are some examples, they are for SQL Server but they demonstrate the logic:
My Repository table
My Repository History table
My Repository Insert trigger procedure
My Repository Update trigger procedure
Hmm, what you're talking about sounds similar to Slowly Changing Dimension.
Be aware that version control on arbitrary database structures is officially a rather Hard Problem. :-)
A simple solution would be to add a version/revision field to the tables, and whenever a record is updated, instead of updating it in place, insert a copy with the changes applied and the version number incremented. Then when selecting, always choose the record with the latest version. That's roughly how most such schemes are implemented (e.g. Wikimedia does it pretty much this exact way).
Maybe a tool can help you to do that for you. Have a look at nextep designer :
https://github.com/christophefondacci/nextep-designer
With this IDE you will be able to take snapshots of your database structure and data and put it under version control. After this you can compute the differences between any 2 versions and generate the appropriate SQL that can insert / update / delete your data.
Maybe this is an alternative way to achieve what you wanted.