[ZODB-Dev] RelStorage: Clearing temp_store in replication-friendly way

[Stefan H. Holek]

This is not a problem of the ZODB or relstorage, but specific to how  
MySQL handles a) replication and b) temporary tables.

MySQL employs a log-based replication mechanism. This means the  
replication slave replays the commands performed on the master to  
keep its copy of the database in sync.

In MySQL, temporary tables live in RAM. So when a slave goes down,  
its copy of the table vanishes. When the slave comes back up the log  
may still contain commands using the temporary table however, causing  
execution to barf (and replication to stop). To fix this condition,  
and get replication going again, we have to perform a manual copy of  
the master's database to the affected slave.

We believe the solution is to avoid temporary tables altogether, and  
to recreate the needed semantics in a replication-safe way. The  
refactoring for temp_store could look like:

1) Create 'temp_store' as a permanent table.
2) Add a 'connid' column, storing the MySQL connection id.
3) Use CONNECTION_ID() in all inserts to populate 'connid'.
4) Qualify all updates and queries using 'temp_store' with
    WHERE connid = CONNECTION_ID() or equivalent.
5) Clear entries from 'temp_store' at transaction boundaries with
    DELETE FROM temp_store WHERE connid = CONNECTION_ID().

I plan to work on this in the near future.

Stefan


On 24. Jul 2008, at 18:33, Shane Hathaway wrote:

>> We hope to tackle the main issue (a.k.a. better-not-use-temporary-  
>> tables-with-mysql-replication-at-all) in a later installment.
>
> Ok.  Conceptually, what we need is a way for each connection to  
> write to a scratch table that no other connection can see.  Is  
> there a better way to do that than temporary tables?

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