I am using the Play! framework, and have a difficulty with in the following scenario.
I have a server process which has a 'read-only' transaction. This to prevent any possible database lock due to execution as it is a complicated procedure. There are one or two record to be stored, but I do that as a job, as I found doing them in the main thread could result in a deadlock under higher load.
However, in one occasion I need to create an object and subsequently use it.
However, when I create the object using a Job, wait for the resulting id (with a Promise return) and then search in the database for it, it cannot be found.
Is there an easy way to have the JPA search 'afresh' in the DB at this point? I implemented a 5 sec. pause to test, so I am sue it is not because the procedure hadn't finished yet.
Check if there is a transaction wrapped around your INSERT and if there is one check that the transaction is COMMITed.
Related
Is it possible to create a warning message in a trigger in Firebird 2.5?
I know I can create an exception message which will stop the user from saving the record changes, but in this instance I don't mind if the user continues.
Could I call a procedure that generates the message?
There is no mechanism in Firebird to produce warnings in PSQL code, you can only raise exceptions, which in triggers will result in the effect of the executed statement that fired the trigger to be undone.
In short, this is not possible.
There are workarounds possible, but those would require 'external' protocols, like, for example, inserting the warning message into a global temporary table, requiring the calling code to explicitly select from that temporary table after execution.
SQL model does provide putting query on pause and then waiting for extra input from client to either unfreeze it or fail it. SQL is not user-interactive service and there is no confirmation dialogs. You have to rethink your application design.
One possible avenue, nominally staying withing 2-tier client-server framework, would be creating temporary tabless for all the data you want to save (for example transaction-scope GTTs), and then have TWO stored procedures. One SP would be sanity-checking and returning list of warnings, if any. Another SP then would dump the data from GTTs to main, persistent tables without doing those checks.
Your client app would select warnings from the check-SP first, if it returns any then show them to the user, then either call save-SP and commit, or rollback without calling save-SP.
This is abusing C/S idea, so there would be dragons. First of all, you would have to have several GTTs and two SPs for E-V-E-R-Y pausable data saving in your app. And that can be a lot.
Also, notice, that database data may change after you called check-SP and before you called save-SP. Becuse some OTHER application running elsewhere could be changing and committing data during that pause. Especially if you transaction was of READ COMMMITTED kind. But with SNAPSHOT tx too.
Better approach would be to drop C/S scheme and go to 3-tier model, AKA multi-tier, AKA "Application Server". That way your client app sends the "briefcase" of data to the app-server, it would be app-server (not SQL triggers) doing all the data validation, and then it would be saving it to data storage backend, SQL or any other.
There, of course, still would be that problem, that data could had been changed by other users, why you paused one user and waited him to read and decide. But you would have more flexibility in app-server on data reconcilation, than you would have with plain SQL.
In PostgreSQL, are DEFERRED triggers executed before (within) the completion of the transaction or just after it?
The documentation says:
DEFERRABLE
NOT DEFERRABLE
This controls whether the constraint can be deferred. A constraint
that is not deferrable will be checked immediately after every
command. Checking of constraints that are deferrable can be postponed
until the end of the transaction (using the SET CONSTRAINTS command).
It doesn't specify if it is still inside the transaction or out. My personal experience says that it is inside the transaction and I need it to be outside!
Are DEFERRED (or INITIALLY DEFERRED) triggers executed inside of the transaction? And if they are, how can I postpone their execution to the time when the transaction is completed?
To give you a hint what I'm after, I'm using pg_notify and RabbitMQ (PostgreSQL LISTEN Exchange) to send out messages. I process such messages in an external application. Right now I have a trigger which notifies the external app of the newly inserted records by including the record's id in the message. But in a non-deterministic way, once in a while, when I try to select a record by its id at hand, the record can not be found. That's because the transaction is not complete yet and the record is not actually added to the table. If I can only postpone the execution of the trigger for after the completion of the transaction, everything will work out.
In order to get better answers let me explain the situation even closer to the real world. The actual scenario is a little more complicated than what I explained before. The source code can be found here if anyone's interested. Becuase of reasons that I'm not gonna dig into, I have to send the notification from another database so the notification is actually sent like:
PERFORM * FROM dblink('hq','SELECT pg_notify(''' || channel || ''', ''' || payload || ''')');
Which I'm sure makes the whole situation much more complicated.
Triggers (including all sorts of deferred triggers) fire inside the transaction.
But that is not the problem here, because notifications are delivered between transactions anyway.
The manual on NOTIFY:
NOTIFY interacts with SQL transactions in some important ways.
Firstly, if a NOTIFY is executed inside a transaction, the notify
events are not delivered until and unless the transaction is
committed. This is appropriate, since if the transaction is aborted,
all the commands within it have had no effect, including NOTIFY. But
it can be disconcerting if one is expecting the notification events to
be delivered immediately. Secondly, if a listening session receives a
notification signal while it is within a transaction, the notification
event will not be delivered to its connected client until just after
the transaction is completed (either committed or aborted). Again, the
reasoning is that if a notification were delivered within a
transaction that was later aborted, one would want the notification to
be undone somehow — but the server cannot "take back" a notification
once it has sent it to the client. So notification events are only
delivered between transactions. The upshot of this is that
applications using NOTIFY for real-time signaling should try to keep
their transactions short.
Bold emphasis mine.
pg_notify() is just a convenient wrapper function for the SQL NOTIFY command.
If some rows cannot be found after a notification has been received, there must be a different cause! Go find it. Likely candidates:
Concurrent transactions interfering
Triggers doing something more or different than you think they do.
All sorts of programming errors.
Either way, like the manual suggests, keep transactions that send notifications short.
dblink
Update: Transaction control in a PROCEDURE or DO statement in Postgres 11 or later makes this a lot simpler. Just COMMIT; to (also) send waiting notifications.
Original answer (mostly for Postgres 10 or older):
PERFORM * FROM dblink('hq','SELECT pg_notify(''' || channel || ''', ''' || payload || ''')');
... which should be rewritten with format() to simplify and make the syntax secure:
PRERFORM dblink('hq', format('NOTIFY %I, %L', channel, payload));
dblink is a game-changer here, because it opens a separate transaction in the other database. This is sometimes used to fake autonomous transaction.
Does Postgres support nested or autonomous transactions?
How do I do large non-blocking updates in PostgreSQL?
dblink() waits for the remote command to finish. So the remote transaction will most probably commit first. The manual:
The function returns the row(s) produced by the query.
If you can send notification from the same transaction instead, that would be a clean solution.
Workaround for dblink
If notifications have to be sent from a different transaction, there is a workaround with dblink_send_query():
dblink_send_query sends a query to be executed asynchronously, that is, without immediately waiting for the result.
DO -- or plpgsql function
$$
BEGIN
-- do stuff
PERFORM dblink_connect ('hq', 'your_connstr_or_foreign_server_here');
PERFORM dblink_send_query('con1', format('SELECT pg_sleep(3); NOTIFY %I, %L ', 'Channel', 'payload'));
PERFORM dblink_disconnect('con1');
END
$$;
If you do this right before the end of the transaction, your local transaction gets 3 seconds (pg_sleep(3)) head start to commit. Chose an appropriate number of seconds.
There is an inherent uncertainty to this approach, since you get no error message if anything goes wrong. For a secure solution you need a different design. After successfully sending the command, chances for it to still fail are extremely slim, though. The chance that successful notifications are missed seem much higher, but that's built into your current solution already.
Safe alternative
A safer alternative would be to write to a queue table and poll it like discussed in #Bohemian's answer. This related answer demonstrates how to poll safely:
Postgres UPDATE … LIMIT 1
I'm posting this as an answer, assuming the actual problem you are trying to solve is deferring execution of an external process until after the transaction is completed (rather than the X-Y "problem" you're trying to solve using trigger Kung Fu).
Having the database tell an app to do something is a broken pattern. It's broken because:
There's no fallback if the app doesn't get the message, eg because it's down, network explodes, whatever. Even the app replying with an acknowledgment (which it can't), wouldn't fix this problem (see next point)
There's no sensible way to retry the work if the app gets the message but fails to complete it (for any of lots of reasons)
In contrast, using the database as a persistant queue, and having the app poll it for work, and take the work off the queue when work is complete, has none of the above problems.
There are lots of ways to achieve this. The one I prefer is to have some process (usually trigger on insert, update and delete) put data into a "queue" table. Have another process poll that table for work to do, and delete from the table when work is complete.
It also adds some other benefits:
The production and consumption of work is decoupled, which means you can safely kill and restart your app (which must happen from time to time, eg deploying) - the queue table will happily grow while the app is down, and will drain when the app is back up. You can even replace the app with an entirely new one
If for whatever reason you want to initiate processing of certain items, you can just manually insert rows into the queue table. I used this technique myself to initiate the processing of all items in a database that needed initialising by being put on the queue once. Importantly, I didn't need to do a perfunctory update to every row just to fire the trigger
Getting to your question, a slight delay can be introduced by adding a timestamp column to the queue table and having the poll query only select rows that are older than (say) 1 second, which gives the database time to complete its transaction
You can't overload the app. The app will read only as much work as it can handle. If your queue is growing, you need a faster app, or more apps If multiple consumers are operating, concurrency can be solved by (for example) adding a "token" column to the queue table
Queues that are backed by database tables is the basis of how persistent queues are implemented in commercial grade queue-based platforms, so the pattern is well tested, used and understood.
Leave the database to do what it does best, and the only thing it does well: Manage data. Don't try to make your database server into an app server.
I have a chunk oriented processor in the form "reader / processor / writer" called Job1. I have to execute database EJB operations after this job ends, if possible, in the same transaction. I have others jobs (implemented by Tasklets) that I could do this in a simply manner. I this jobs I simply call this operations in tasklet, before finish exeute method. But in this case I don't know the right way to do. In a first try I implemented it by a step listener (outside transaction). But I cannot, because there are uma architecture rule in my company to don't call database operations in listeners. I could execute it after this step in another step in a tasklet and I will come this way if I don't find a better one, but moreover if it's possible I like to execute this operations in the same transaction of Job1.
A couple notes:
In a chunk based step (reader/processor/writer), typically you'll have multiple transactions. One for each chunk.
Because of 1, you typically can't do a db call in at the end of a step that is in the same transaction as the items were processed in. They were processed in multiple transactions.
That being said, from what it sounds like, the best option would be to put your call in another step after the chunk based one.
According to postgresql docs;
CREATE DATABASE cannot be executed inside a transaction block.
Is there a technical reason for this?
When you try it, you get the error:
ERROR: CREATE DATABASE cannot run inside a transaction block
This comes from src/backend/access/transam/xact.c (line 3023 on my sources, but varies by version), in PreventTransactionChain(...).
The comment there explains that:
This routine is to be called by statements that must not run inside
a transaction block, typically because they have non-rollback-able
side effects or do internal commits.
For CREATE DATABASE it's called from src/backend/tcop/utility.c in standard_ProcessUtility under the case for T_CreatedbStmt, but unfortunately there isn't any informative comment that says why specifically CREATE DATABASE isn't safe to run in a transaction.
Looking at the sources, I can see that for one thing it forces a checkpoint.
Overall, though, I don't see anything that really screams out "we can't do this transactionally". It's more "we haven't implemented the functionality to do this transactionally".
It's conceptual reason: files creation has no relation to DB transaction and there is no guaranty that during the rollback they will be deleted.
In our production org, we have a system of uploading sales data into Salesforce using command line data loader. This data is loaded into a temporary object Temp. We have created a formula field (which combines three fields) to form a unique key. The purpose of the object is to reduce user efforts for creating the key manually.
There is an after insert trigger on Temp which calls an asynchronous method which upserts the data to another object SalesData using the key. The insert/update trigger on SalesData checks the various fields and creates/updates the records in another object SalesRecords. After the insertion/updation is complete, all the records in temp object Temp are deleted. The SalesRecords object does not have any trigger on it and is a child of another object Sales. The Sales object has some rollup fields which are summing up fields from SalesRecords object.
Lately, we are getting the below error for some of the records which are updated.
UNABLE_TO_LOCK_ROW, unable to obtain exclusive access to this record
Please provide some pointers to resolve the issue
this could either be caused by conflicting DML operations in the various trigger execution or some recursive trigger execution. i would assume that the async executions cause multiple subsequent updates on the same records, probably on the SalesRecords object. I would recommend to try to simplify the process to avoid too many related trigger executions.
I'm a little surprised you were able to get this to work in the first place. After triggers should be used with caution and only when before triggers can't be. One reason for this is that you don't need to perform additional DML to make changes to records, since in before triggers you simply change the values and the insert/update commit happens automatically. But recursive trigger firings is the main problem with after triggers.
One quick way to avoid trigger re-entry is to use a public static Boolean in a class that states whether you're already in this trigger from the same thread of execution.
Something like:
public static Boolean isExecuting = false;
Once set to true, any trigger code that is a re-fire can be avoided with:
if(Class.isExecuting == false)
{
Class.isExecuting = true;
// Perform trigger logic
// ...
}
Additionally, since the order of trigger execution cannot be determined up front, you might be seeing an issue with deletions or other data changes that depend on other parts of your flow to finish first.
Also, without knowing the details of your custom unique 3-part key, I'd wonder if there's a problem there too such as whether it's truly unique or not. Case insensitivity is a common mistake and it's the reason there are 15 AND 18 character Ids in Salesforce. For example, when people export to Excel (a case-insensitive environment) and do VLOOKUPs, they would occasionally find the wrong record. The 3-digit calculated suffix was added to disambiguate for case-insensitive environments.
Googling for this same error lead me to this post:
http://boards.developerforce.com/t5/General-Development/Unable-to-obtain-exclusive-access-to-this-record/td-p/345319
Which points out some common causes for this to happen:
Sharing Rules are being calculated.
A picklist value has been replaced and replacement is in progress.
A custom index creation/removal is in progress.
Most unlikely one - someone else is already editing the same record that you are trying to access at the same time.
Posting here in case somebody else needs it.
I got this error multiple times today. Turned out one of our vendors was updating their installed package during that time in the same org. All kinds of things were going wrong also - some object validation exceptions were being thrown on DMLs, without any error message content.
Resolution
The error is shown when a field update such as a roll-up summary field is being attempted on a parent object that already had a field update to cause the roll-up summary field to calculate. This could also occur if a trigger or another apex job running on the master object and it also attempting to do an update.
You can either reduce the batch size and try again or create separate smaller files to be imported if this issue occurs.