We can skip a error in GTID based replication by following steps:
STOP SLAVE;
set GTID_NEXT='SERVER_UUID:LAST_TRANSACTION_NUMBER+1' ;
BEGIN; COMMIT; SET GTID_NEXT="AUTOMATIC";
START SLAVE;
But if a replication is running with channel information, than how to skip the transaction for a particular channel ?
We can give "for channel" keyword in stop slave and start slave. But how to skip transaction for a particular channel, like in set GTID_NEXT command or what ?
In a replication topology GTID is a global unique identified for any transaction, therefore if transaction is required to skip, specifying channel becomes irrelevant here.
It is similar to MySQL Replication Filters [MySQL 5.7] are global , or in other words, will be applied for all running replication channels
Related
I am looking at building a database based Message Queue implementation. I will essentially have a database table which will contain a autogenerated id (bigint), a message id and message data. I will be writing a pull based consumer which will query for the oldest record (min(id)) from the table and hands it over for processing.
Now my doubt is how would I handle the querying of the oldest record when there are mulitple threads of consumer. How do I lock the first read record to the first consumer and basically not even make it visible to the next one.
One idea that I have is to add another column called locked by where I will store, lets say the thread name and select the record for update and immediately update the locked by column and then continue processing it. So that I will not select the locked columns in the next query.
Is this a workable solution?
Edit:
Essentially, this is what I want.
Connection one queries the database table for a row. Reads first row and locks it while reading for update.
Connection two queries the database table for a row. Should not be able to read the first row, should read second row if available and lock it for update.
Similar logic for connection 3, 4, etc..
Connection one updates the record with its identifier. Processes it and subsequently deletes the record.
Connection one queries the database table for a row. Reads first row and locks it while reading for update.
Connection two queries the database table for a row. Should not be able to read the first row, should read second row if available and
lock it for update.
Similar logic for connection 3, 4, etc..
Connection one updates the record with its identifier. Processes it and subsequently deletes the record.
TL;DR, see Rusanu's using tables as queues. The example DDL below is gleaned from the article.
CREATE TABLE dbo.FifoQueueTable (
Id bigint not null identity(1,1)
CONSTRAINT pk_FifoQueue PRIMARY KEY CLUSTERED
,Payload varbinary(MAX)
);
GO
CREATE PROCEDURE dbo.usp_EnqueueFifoTableMessage
#payload varbinary(MAX)
AS
SET NOCOUNT ON;
INSERT INTO dbo.FifoQueueTable (Payload) VALUES (#Payload);
GO
CREATE PROCEDURE dbo.usp_DequeueFifoTableMessage
AS
SET NOCOUNT ON;
WITH cte AS (
SELECT TOP(1) Payload
FROM dbo.FifoQueueTable WITH (ROWLOCK, READPAST)
ORDER BY Id
)
DELETE FROM cte
OUTPUT deleted.Payload;
GO
This implementation is simple but handing the unhappy path can be complex depending on the nature of the messages and the cause of the error.
When message loss is acceptable, one can simply use the default autocommit transaction and log errors.
In cases where messages must not be lost, the dequeue must be done in a client-initiated transaction and committed only after successful processing or no message read. The transaction will also ensure messages are not lost if the application or database service crashes. A robust error handling strategy depends on the type of error, nature of messages, and message processing order implications.
A poison message (i.e. an error in the payload that prevents the message from ever being successfully), one can insert the bad message into a dead letter table for subsequent manual review and commit the to transaction.
A transient error, such as a failure calling an external service, can be handled with techniques like:
Rollback the transaction so the message is first in the FIFO queue for retry next iteration.
Requeue the erred message and commit so the message is last in the FIFO queue for retry.
Enqueue the erred message in a separate retry queue along with a retry count. The message can be inserted into dead letter table once a retry limit is reached.
The app code can also include retry logic during message processing but should avoid long running database transactions and fallback to one techniques above after some retry threshold.
These same concepts can be implemented with Service Broker to facilitate a T-SQL only solution (internal activation) but adds complexity when that's not a requirement (as in your case). Note that SB queues intrinsically implement the "READPAST" requirement but, because all messages within the same conversation group are locked, the implication is that each message will need to be in a separate conversation.
I have an issue with "could not serialize access due to concurrent update". I checked logs and I can clearly see that two transactions were trying to update a row at the same time.
my sql query
UPDATE sessionstore SET valid_until = %s WHERE sid = %s;
How can I tell postgres to "try" update row without throwing any exception?
There is a caveat here which has been mentioned in comments. You must be using REPEATABLE READ transaction isolation or higher. Why? That is not typically required unless you really have a specific reason.
Your problem will go away if you use standard READ COMMITTED. But still it’s better to use SKIP LOCKED to both avoid lock waits and redundant updates and wasted WAL traffic.
As of Postgres 9.5+, there is a much better way to handle this, which would be like this:
UPDATE sessionstore
SET valid_until = %s
WHERE sid = (
SELECT sid FROM sessionstore
WHERE sid = %s
FOR UPDATE SKIP LOCKED
);
The first transaction to acquire the lock in SELECT FOR UPDATE SKIP LOCKED will cause any conflicting transaction to select nothing, leading to a no-op. As requested, it will not throw an exception.
See SKIP LOCKED notes here:
https://www.postgresql.org/docs/current/static/sql-select.html
Also the advice about a savepoint is not specific enough. What if the update fails for a reason besides a serialization error? Like an actual deadlock? You don’t want to just silently ignore all errors. But these are also in general a bad idea - an exception handler or a savepoint for every row update is a lot of extra overhead especially if you have high traffic. That is why you should use READ COMMITTED and SKIP LOCKED both to simplify the matter, and any actual error then would be truly unexpected.
The canonical way to do that would be to set a checkpoint before the UPDATE:
SAVEPOINT x;
If the update fails,
ROLLBACK TO SAVEPOINT x;
and the transaction can continue.
The default isolation level is "read committed" unless you need to change it for any specific use case.
You must be using the "repeatable read" or "serializable" isolation level. Here, the current transaction will roll-back if the already running transaction updates the value which was also supposed to be updated by current transaction.
Though this scenario can be easily handled by the "read_commit" isolation level where the current transaction accepts an updated value from other transaction and perform its instructions after the previous transaction is committed
ALTER DATABASE SET DEFAULT_TRANSACTION_ISOLATION TO 'read committed';
Ref: https://www.postgresql.org/docs/9.5/transaction-iso.html
The NServiceBus documentation lists a benefit of SQL transport as:
Queues support competing consumers (multiple instances of same
endpoint feeding off of same queue) so there is no need for
distributor in order to scale out the processing
http://docs.particular.net/nservicebus/sqlserver/design
Who does NServiceBus prevent a message from being handled by multiple consumers if multiple consumers have subscribed to the same queue?
Does NServiceBus lock the entire table until the message is handled? Or is the message marked as 'being processed' ??
The SQL Transport uses very specific lock hints in order to lock a row and cause other competing threads to ignore any row currently locked.
From NServiceBus.SqlServer 2.2.0 (current version at the time I'm writing this) the SQL used, but reformatted by me, is:
WITH message AS
(
SELECT TOP(1) *
FROM [{Schema}].[{Queue}] WITH (UPDLOCK, READPAST, ROWLOCK)
ORDER BY [RowVersion] ASC
)
DELETE FROM message
OUTPUT deleted.Id, deleted.CorrelationId, deleted.ReplyToAddress,
deleted.Recoverable, deleted.Expires, deleted.Headers, deleted.Body;
It uses a Common Table Expression to limit the source data to the one row to return, then uses the following lock hints:
UPDLOCK - Hold the data under lock with intent to update it.
READPAST - Ignore locked rows and fetch the next unlocked one.
ROWLOCK - Force row-level locks and don't escalate to page locks or table locks.
By executing the entire thing as a delete but then outputting the data about to be deleted, we can read the data, and if the transaction commits, the row is removed. Otherwise, if the transaction rolls back, then the lock is released and the row waits to be picked up by the next competing consumer.
SQL Server is SQL Azure, basically it's SQL Server 2008 for normal process.
I have a table, called TASK, constantly have new data in (new task), and removed (task complete)
For new data in, I use INSERT INTO .. SELECT ..., most of time takes very long, lets say dozen of minutes.
For old data out, I first use SELECT (WITH NOLOCK) to get task, UPDATE to let other thread know this task already starts to process, then DELETE once finished.
Dead lock sometime happens on SELECT, most time happens on UPDATE and DELETE.
this is not time critical task, so I can start process the new data once all INSERT finished. Is there any kind of LOCK to ask SELECT not to select it before the INSERT finished? Or any kind of other suggestion to avoid Conflict. I can redesign table if needed.
later the sqlserver2005,resolve lock is easy.
for conflict
1.you can use the service broker.
2.use the isolution level.
dbcc useroptions ,at last row ,you can see the deflaut isolution level is read_committed,this is the session level.
we can change the level to read_committed_snapshot for conflict,in sqlserver, not realy row lock like oracle.but we can use this method implement.
ALTER DATABASE DBName
SET READ_COMMITTED_SNAPSHOT ON;
open this feature,must in single user schame.
and you can test it.
for session A ,session B.
A:update table1 set name = 'new' with(Xlock) where id = 1
B:you still update other row and select all the data from table.
my english is not very good,but for lock ,i know.
in sqlserver,for function ,there are three locks.
1.optimistic lock ,use the timestamp(rowversion) control.
2.pessimism lock ,force lock when use the date.use Ulock,Xlock and so on.
3.virtual lock,use the proc getapplock().
if you need lock schame in system architecture,please me email : mjjjj2001#163.com
Consider using service broker if this is a processing queue.
There are a number of considerations that affect performance and locking. I surmise that the data is being updated and deleted in a separate session. Which transaction isolation level is in use for the insert session and the delete session.
Has the insert session and all transactions committed and closed when the delete session runs? Are there multiple delete sessions running concurrently? It is very important to have an index on the columns you are using to identify a task for the SELECT/UPDATE/DELETE statements, especially if you move to a higher isolation level such as REPEATABLE READ or SERIALIZED.
All of these issues could be solved by moving to Service Broker if it is appropriate.
I'm going to make up some sql here. What I want is something like the following:
select ... for update priority 2; // Session 2
So when I run in another session
select ... for update priority 1; // Session 1
It immediately returns, and throws an error in session 2 (and hence does a rollback), and locks the row in session 1.
Then, whilst session 1 holds the lock, running the following in session 2.
select ... for update priority 2; // Session 2
Will wait until session 1 releases the lock.
How could I implement such a scheme, as the priority x is just something I've made up. I only need something that can do two priority levels.
Also, I'm happy to hide all my logic in PL/SQL procedures, I don't need this to work for generic SQL statements.
I'm using Oracle 10g if that makes any difference.
I'm not aware of a way to interrupt an atomic process in Oracle like you're suggesting. I think the only thing you could do would be to programmaticaly break down your larger processes into smaller ones and poll some type of sentinel table. So instead of doing a single update for 1 million rows perhaps you could write a proc that would update 1k, check a jobs table (or something similar) to see if there's a higher priority process running, and if a higher priority process is running, to pause its own execution through a wait loop. This is the only thing I can think that would keep your session alive during this process.
If you truly want to abort the progress of your currently running, lower priority thread and losing your session is acceptable, then I would suggest a jobs table again that registered the SQL that was being run and the session ID that it is run on. If you run a higher priority statement it should again check the jobs table and then issue a kill command to the low priority session (http://www.oracle-base.com/articles/misc/KillingOracleSessions.php) along with inserting a record into the jobs table to note the fact that it was killed. When a higher-priority process finishes it could check the jobs table to see if it was responsible for killing anything and if so, reissue it.
That's what resource manager was implemented for.