I am using Cassandra database integrated into a spring boot application.
My Question is around the schema actions. If I need to make structural changes to the DB, say add a column to a table, the database needs to be recreated, however this means all the existing data gets deleted:
schema-action: CREATE_IF_NOT_EXISTS
The only way I have managed to solve this is by using the RECREATE scheme action, but as mentioned earlier, this results in data-loss.
What would be the best approach to handle this? To add structural changes such as a column name with out having to recreate the database and lose all existing data?
Thanks
Cassandra does allow you to modify the schema of an existing table without recreating it from scratch, using the ALTER TABLE statement via cqlsh. However, as explained in that link, there are some important limitations on the kind of changes you can do. You cannot modify the primary key of the table at all, you can add or delete regular columns, and you can't change the type of a column to a non-compatible one.
The reason for most of these limitations is how Cassandra needs to deal with the old data that already exists in the table. For example, it doesn't make sense to say that a column A that until now contained strings - will now contain integers - how are we supposed to handle all the old values in column A which weren't integers?
As Aaron rightly said in a comment, it is unlikely you'll want to do these schema changes as part of your application. These are usually rare operations which are done manually, or via some management application - not your usual application.
Related
I am trying to track changes made to a database (schema) using a java app. We are trying to track changes for each column/unique-constraint/index and table.
Functionally I know table.column is unique. So, if the datatype of a column changes, we know which column to find and record the change. But what if the name changes? If JDBC's result set is ordered (it asks for index), then I can rely on the order to give me the same column everytime, even if the name changes. Will there be any surprises here, since it is a result 'set'?
However, I learnt that we can change the order of the columns as well. Isn't there any unique ID associated with the columns so that they can be picked up on that basis?
I would mostly not want to use information_schema route, but even though i checked there for mysql, found nothing useful.
I am using hibernate envers for making history of my data, it's working fine as well. The problem here is, it's creating duplicate data in history table i.e. creating data in history table whether there is any change in audited table or not. I want only changed fields stored in my history table. I am new to hibernate envers. What can I do?
If I understand your question correctly, Envers doesn't work that way, at least not out of the box.
Envers is a commit-snapshot auditing solution where just before commit, it examines audited entity state and determines whether any attributes have been modified or not and records a snapshot of all audited fields of that entity at that point in time. This means that the only time an audit entry isn't created is when no attributes have been modified.
But it also uses the snapshot approach because it fits really well with the Query API.
Consider the inefficiency that would occur if a query to find an entity at a given revision had to read all rows from that revision back to the beginning of time, iterating each row and merging the column state captured to just instantiate a single row result-set.
With the snapshot approach, it boils down to the following query, no loops or iterative work.
SELECT e FROM AuditedEntity e WHERE e.revisionNumber = :revisionNumber
This is far more efficient from a I/O perspective both with the database reading the data pages and the network for streaming a single row result-set rather than multi-row result-set to the client.
I'd say in this case, the saying "space is cheap" really holds true when you compare that against the cost and inefficiencies your application would face doing it any other way.
If this is something you'd like Envers to support, perhaps via some user configured strategy then you're welcomed to log a new feature request in JIRA for hibernate-envers and I can take a look at its feasibility.
I had similar problem.
In my case the error was that audited field had higher precision than the database field. Please see my reply to another thread: https://stackoverflow.com/a/65844949/13381019
i have some large data in one table and small data in other table,is there any way to run initial load of golden gate so that same data in both tables wont be changed and rest of the data got transferred from one table to other.
Initial loads are typically for when you are setting up the replication environment; however, you can do this as well on single tables. Everything in the Oracle database is driven by System Change Numbers/Change System Numbers (SCN/CSN).
By using the SCN/CSN, you can identify what the starting point in the table should be and start CDC from there. Any prior to the SCN/CSN will not get captured and would require you to manually move that data in some fashion. That can be done by using Oracle Data Pump (Export/Import).
Oracle GoldenGate also provided a parameter called SQLPredicate that allows you to use a "where" clause against a table. This is handy with initial load extracts because you would do something like TABLE ., SQLPredicate "as of ". Then data before that would be captured and moved to the target side for a replicat to apply into a table. You can reference that here:
https://www.dbasolved.com/2018/05/loading-tables-with-oracle-goldengate-and-rest-apis/
Official Oracle Doc: https://docs.oracle.com/en/middleware/goldengate/core/19.1/admin/loading-data-file-replicat-ma-19.1.html
On the replicat side, you would use HANDLECOLLISIONS to kick out any ducplicates. Then once the load is complete, remove it from the parameter file.
Lots of details, but I'm sure this is a good starting point for you.
That would require programming in java.
1) First you would read your database
2) Decide which data has to be added in which table on the basis of data that was read.
3) Execute update/ data entry queries to submit data to tables.
If you want to run Initial Load using GoldenGate:
Target tables should be empty
Data: Make certain that the target tables are empty. Otherwise, there
may be duplicate-row errors or conflicts between existing rows and
rows that are being loaded. Link to Oracle Documentations
If not empty, you have to treat conflicts. For instance if the row you are inserting already exists in the target table (INSERTROWEXISTS) you should discard it, if that's what you want to do. Link to Oracle Documentation
I made Java/JDBC code which performs simple/basic operations on a database.
I want to add code which helps me to keep a track of when a particular database was accessed, updated, modified etc by this program.
I am thinking of creating another database inside my DBMS where these details or logs will be stored for each database involved.
Is this the best way to do it ? Are there any other ways (preferably simple) to do this ?
EDIT-
For now, I am using MySQL. But, I also want my code to work with at least
Oracle SQL and MS-SQL as well.
It is pretty standard to add a "last_modified" column to a table and then add an update trigger on the table to set it to the db current time. Then your apps don't need to worry about it. Also, a "create_time" is often used as well, populated by an insert trigger.
Update after comment:
Seems you are looking for audit logs. Some write apps where data manipulation only happens through stored procedures and not through inserts and updates. A fixed api. So you want to add an item to a table, you call the stored proc:
addItem(itemName, itemDescription)
Then the proc inserts into the item table and does what ever logging is necessary.
Another technique, if you are using some kind of framework for your jdbc access (say Spring) might be to intercept at that layer.
In almost all tables, I have the following columns:
CreatedBy
CreatedAt
These columns have default values of the current user and current time, respectively. They are populated when a row is added.
This solves only part of your problem. You can start adding triggers, but that gets complicated. Another method is to force modification access to the database through stored procedures, and then log the stored procedures. This has other advantages, in terms of controlling what users can do. But, you might want more flexibility.
A third possibility are auditing tools, that keep track of all queries being run on the database. I think most databases have a way of turning on internal auditing, although these are very specific to the database. There are also third party tools that allow you to see what has happened. Note, though, that these methods will affect performance if your database is doing high volume transactions.
For more information, you should revise your question to specify which database you are using or planning on using.
I need the sample program in Java for keeping the history of table if user inserted, updated and deleted on that table. Can anybody help in this?
Thanks in advance.
If you are working with Hibernate you can use Envers to solve this problem.
You have two options for this:
Let the database handle this automatically using triggers. I don't know what database you're using but all of them support triggers that you can use for this.
Write code in your program that does something similar when inserting, updating and deleting a user.
Personally, I prefer the first option. It probably requires less maintenance. There may be multiple places where you update a user, all those places need the code to update the other table. Besides, in the database you have more options for specifying required values and integrity constraints.
Well, we normally have our own history tables which (mostly) look like the original table. Since most of our tables already have the creation date, modification date and the respective users, all we need to do is copy the dataset from the live table to the history table with a creation date of now().
We're using Hibernate so this could be done in an interceptor, but there may be other options as well, e.g. some database trigger executing a script, etc.
How is this a Java question?
This should be moved in Database section.
You need to create a history table. Then create database triggers on the original table for "create or replace trigger before insert or update or delete on table for each row ...."
I think this can be achieved by creating a trigger in the sql-server.
you can create the TRIGGER as follows:
Syntax:
CREATE TRIGGER trigger_name
{BEFORE | AFTER } {INSERT | UPDATE |
DELETE } ON table_name FOR EACH ROW
triggered_statement
you'll have to create 2 triggers one for before the operation is performed and another after the operation is performed.
otherwise it can be achieved through code also but it would be a bit tedious for the code to handle in case of batch processes.
You should try using triggers. You can have a separate table (exact replica of your table of which you need to maintain history) .
This table will then be updated by trigger after every insert/update/delete on your main table.
Then you can write your java code to get these changes from the second history table.
I think you can use the redo log of your underlying database to keep track of the operation performed. Is there any particular reason to go for the program?
You could try creating say a List of the objects from the table (Assuming you have objects for the data). Which will allow you to loop through the list and compare to the current data in the table? You will then be able to see if any changes occurred.
You can even create another list with a object that contains an enumerator that gives you the action (DELETE, UPDATE, CREATE) along with the new data.
Haven't done this before, just a idea.
Like #Ashish mentioned, triggers can be used to insert into a seperate table - this is commonly referred as Audit-Trail table or audit log table.
Below are columns generally defined in such audit trail table : 'Action' (insert,update,delete) , tablename (table into which it was inserted/deleted/updated), key (primary key of that table on need basis) , timestamp (the time at which this action was done)
It is better to audit-log after the entire transaction is through. If not, in case of exception being passed back to code-side, seperate call to update audit tables will be needed. Hope this helps.
If you are talking about db tables you may use either triggers in db or add some extra code within your application - probably using aspects. If you are using JPA you may use entity listeners or perform some extra logic adding some aspect to your DAO object and apply specific aspect to all DAOs which perform CRUD on entities that needs to sustain historical data. If your DAO object is stateless bean you may use Interceptor to achive that in other case use java proxy functionality, cglib or other lib that may provide aspect functionality for you. If you are using Spring instead of EJB you may advise your DAOs within application context config file.
Triggers are not suggestable, when I stored my audit data in file else I didn't use the database...my suggestion is create table "AUDIT" and write java code with help of servlets and store the data in file or DB or another DB also ...