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From Kristian Waagan <kristian.waa...@oracle.com>
Subject Re: Derby Locks - best practices
Date Fri, 01 Jun 2012 22:11:34 GMT
On 01.06.2012 21:50, Pavel Bortnovskiy wrote:
> Hello, David, thanks for your quick response.
>
> Usually it's one thread "per" in-memory table. Tables can be updated at random times
and their random rows may be updated, some rows deleted or new rows inserted. In some other
configuration, to avoid deletions, updates and inserts, the in-memory table is truncated and
then all the records (the new "state" of the source data) are inserted into it.

For clarity, are you referring to using Derby's memory subprotocol when 
talking about in-memory tables? As an example, that would be 
'jdbc:derby:memory:mydb;create=true', as opposed to the on-disk version 
that would be 'jdbc:derby:mydb;create=true'.
In terms of locking there is nothing special about in-memory database in 
Derby, except for the likely event that some operations may be faster 
in-memory than on-disk (which could affect timing, but many other things 
can do that too).

Two common pieces of advice when it comes to locking is to reduce the 
duration of the locks, and to reduce the scope/granularity of the locks.
There may also be  application specific considerations to take, like 
acceptable isolation levels, access patterns and schema design.

In general your application should be prepared to handle lock timeouts, 
whereas deadlocks indicate that the access/lock patterns of your 
application need to be improved.


-- 
Kristian

>
> The thread which runs SQL against all those tables frequently may do a scan of the whole
table.
>
> -----Original Message-----
> From: David Zanter [mailto:dzanter@gmail.com]
> Sent: Friday, June 01, 2012 3:46 PM
> To: Derby Discussion
> Subject: Re: Derby Locks - best practices
>
> Do mean the scenario of:
>      Multiple threads are updating the exact same rows
>
> or
>      Multiple threads doing updates to different rows, but due to queries/indexes/etc
are causing contention between each other.
>
> On Fri, Jun 1, 2012 at 3:16 PM, Pavel Bortnovskiy<pbortnovskiy@jefferies.com> 
wrote:
>> Hello, all:
>>
>>
>>
>> Derby is used in my application in the in-memory only mode. For a long
>> time Derby's lock logic caused no worries, but recently some use cases
>> failed with lock timeouts. Thus I'm looking for guidance on best
>> practices for handling locks in Derby. A use-case which may cause
>> timeouts to obtain a
>> lock: one thread is executing an SQL statement which accesses two (or
>> more) in-memory tables. Those two tables are being modified by  other
>> threads at random times. So, situations in which the SQL is executed
>> for a long time and the other threads are frequently updating the
>> tables may cause lock timeouts.
>>
>>
>>
>> Besides best practices to avoid timeouts and deadlocks, I would like
>> to ask the following questions:
>>
>> 1)      What's the default length of lock timeouts?
>>
>> 2)      Does my app need another layer of synchronization
>> mechanism/locks to avoid attempts to update in-memory tables or execute SQLs against
them?
>>
>> 3)      Can my application utilize Derby's locks through some API - to
>> query their state or to use them in making a decision of whether to
>> batch updates or to execute them, to wait or execute the SQLs?
>>
>>
>>
>> Your help would be greatly appreciated,
>>
>>
>>
>> Pavel.
>>
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