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From Paul Rogers <par0...@yahoo.com.INVALID>
Subject Re: RDBMS Storage Plugin Configurations
Date Wed, 15 Jan 2020 22:51:15 GMT
Hi Jiang,

Welcome to the Drill mailing list.

I think you may be making some assumptions about how Drill works, perhaps based on how other
DB-driven applications work.

Drill is not primarily a front-end for an RDBS. Instead, it is primarily designed to scan
distributed data as fast as possible to extract records of interest. Drill does support JDBC
data sources, but this is not the main use case.

In Drill, each query is stand-alone: Drill opens connections as needed to whatever data source
you use; reads data, and releases all resources. Since Drill is distributed, this happens
on each node. Since Drill is multi-threaded, this work also happens for each "minor fragment"
(thread of execution) on each node. Drill is also multi-user; each user might have their own
DB security restrictions.

This makes sense: if we want to read at maximum speed across 10 minor fragments (say) then
all 10 need their own DB connections and all will try to keep those connections 100% busy.

As a result, Drill has no DB connection pool: not within a query and not across queries. So,
there is no idle timeout. The maximum number of connections is set by the maximum "slice width"
(number of fragments per node) and the total number of nodes. Slice width is, by default,
70% of your CPU count. So, if you have 10 nodes with 8 cores each, you will have roughly 60
open DB connections for the duration of the query (assuming that the DB storage plugin knows
how to shard queries across all those minor fragments. I'm not sure that the JDBC storage
plugin knows how to do this. Can anyone clarify this point?)

It sounds like you have a particular use-case in mind that might benefit from connection caching.
Can you share that use case to help us understand? And, of course, Drill is open source; if
you find you need this ability, it can certainly be added.

Drillers: please offer corrections if I've overlooked something; I'm not super familiar with
the details of the JDBC data source.

- Paul


    On Wednesday, January 15, 2020, 01:49:21 PM PST, Jiang Wu <jiang.wu@mulesoft.com.invalid>
 Question on the RDBMS Storage Plugin: is it possible to set various options
for the database connection pool used for this storage plugin?  For
example, max number of connections, idle timeout, etc?


-- Jiang
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