On Thursday, August 23, 2012 8:34:50 PM UTC-7, Jeremy Evans wrote:
>
> On Thursday, August 23, 2012 8:05:02 PM UTC-7, Ravi wrote:
>>
>> That is good thing to know, I will check the configuration and try to
>> increase number of connection if possible.
>>
>
> Hopefully that will fix the issue.
>
>
>> You can't change the number of allowed connections from Sequel, you need
>>> to change your MySQL database configuration to increase the number of
>>> connections. The only work around in Sequel is lowering the number of
>>> connections that Sequel will create (like you saw, the default is 4). You
>>> mentioned raising the number from 4 to 30, which could be causing your
>>> problem. Do you get the same error message when running with the default
>>> of 4 connections?
>>>
>>
>> I got same error before (2-3 times) with default :max_connection settings
>> so I have increased the :max_connections variable to 30 and that worsen the
>> problem.
>> Since there are more than 50 instances/thread that update the database at
>> same time and if I lower :max_connections; wont the Sequal will throw
>> Sequel::PoolTimeout error.. if it could not connect to all 50 instances.
>>
>
> It's true that if more threads request a connection than there are
> connections in the pool, some of the threads will have to wait for a
> connection, and if they wait too long, you'll get a PoolTimeout. But you
> should not need a 50-connection pool to serve 50 threads unless each thread
> is using the database 100% of the time.
>
>
>> The other workaround I was thinking as following
>>
>> attempt = 1
>> begin
>> DB = Sequel.connect(....)
>> rescue Sequel::DatabaseConnectionError
>> if attempt<6
>> DB.disconnect #it will serve all open connections first
>> sleep(attempt*2)
>> attempt +=1
>> retry
>> else
>> raise_error("reached max attempts")
>> end
>> rescue => e
>> raise_error(...)
>> end
>>
>
> This is a bad idea, more likely to cause additional problems than fix
> existing problems.
>
>
>> Also can the other variables ( :pool_sleep_time=>0.01, :pool_timeout=>30)
>> do this work for me?
>>
>>>
>>> Note that you can have a connection pool with 4 connections serve 50
>>> threads, as long as each thread on average accesses the database 8% of the
>>> time.
>>>
>> What error will Sequel raise if it crosses the above criteria?
>>
>
> With that configuration, you'll get an error if a thread waits more than
> 30 seconds and still cannot acquire a connection.
>
The default setting is 5sec. So increasing to 30 may reduce the timeout
issue.
You might want to increase the sleep time, as otherwise you can end up with
> a lot of threads waking up, seeing there are no available connections, and
> going back to sleep.
>
This might increase the overall throughput time.
> Also the way Sequel's connection pools currently work, threads are not
> serviced in order of request. So you can end up with a situation like this:
>
> 1) thread A requests a connection from the pool, none in pool, so sleeps
> for sleep time
> 2) thread B returns a connection to the pool
> 3) thread B requests a connection from the pool, gets the one it just
> returned
> 4) go to 1)
>
> This is obviously a non-optimal situation, but it's not a major issue as
> in general practice you should make sure your connection pool is large
> enough that threads are rarely if ever waiting for connections.
>
> I'd be open to fixing this so that thread A gets the connection
> immediately when thread B checks it in, but it requires a Queue class where
> Queue#pop accepts a timeout. Currently, ruby's Queue class only takes a
> single non-block flag, so it isn't suitable.
>
There is an option called :connection_handling in
Sequel::ThreadedPoolConnection class. Changing that to :queue can also the
issue you are talking about, default is :stack.
I have not used that though, still need to figure out if I can set at the
time of connection, Sequl.connect(......., :connection_handling=>:queue).
> Jeremy
>
Ravi
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