Обсуждение: Restrict number of connections to specific table

Sam Franklin, 13.05.2014 12:50:
> Hi – apologies if this question has a painfully obvious answer.  I
> put the N in novice regarding postgresql.
>
> I use postgres with the spatial extension, postgis.
>
> I have some licenced geospatial data. The licence terms state that
> only “one user can access the data at one time”.
>
> I want to add this data to the collection of spatially-enabled
> postgis tables that I serve to my enterprise, where users access and
> consume postgis data tables via GIS clients.
>
>
> Can I put a constraint on a specific table to restrict the number of
> concurrent connections to 1, which will allow me to adhere to the
> data licencing terms?
>
> If not, is there an alternative solution to serve up the data via a
> PostgreSQL table and still keep within the data licence terms?

There is no configuration option if you mean that.

One thing I can think of is to allow access to that table only through a function. Inside that function lock the table
exclusivelyso that no other session can access it.  

The question then is: _when_ do you release this lock?

If you release the lock after the select has finished, two users could query the data one after the other. But if the
firstuser doesn't clear the results within the tool that is used to display the data you would still wind up with two
users"accessing" the data at the same time. 

Thomas

"Sam Franklin" <Sam.Franklin@senergyworld.com> writes:
> I have some licenced geospatial data. The licence terms state that only
> "one user can access the data at one time".

> Can I put a constraint on a specific table to restrict the number of
> concurrent connections to 1, which will allow me to adhere to the data
> licencing terms?

Not on a specific table.  You could possibly use a dedicated Postgres
instance with max_connections set to 1.  If you need to serialize access
to just this table while allowing concurrent access to some other data
in the same DB, then there's no built-in answer.

            regards, tom lane

Thomas, Keith and Tom - many thanks for your replies.  Plenty of ideas
to try.
Best regards
Sam

-----Original Message-----
From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
Sent: 13 May 2014 15:28
To: Sam Franklin
Cc: pgsql-novice@postgresql.org
Subject: Re: [NOVICE] Restrict number of connections to specific table

"Sam Franklin" <Sam.Franklin@senergyworld.com> writes:
> I have some licenced geospatial data. The licence terms state that
> only "one user can access the data at one time".

> Can I put a constraint on a specific table to restrict the number of
> concurrent connections to 1, which will allow me to adhere to the data

> licencing terms?

Not on a specific table.  You could possibly use a dedicated Postgres
instance with max_connections set to 1.  If you need to serialize access
to just this table while allowing concurrent access to some other data
in the same DB, then there's no built-in answer.

            regards, tom lane

On Tue, May 13, 2014 at 10:27:38AM -0400, Tom Lane wrote:
> "Sam Franklin" <Sam.Franklin@senergyworld.com> writes:
> > I have some licenced geospatial data. The licence terms state that only
> > "one user can access the data at one time".
>
> > Can I put a constraint on a specific table to restrict the number of
> > concurrent connections to 1, which will allow me to adhere to the data
> > licencing terms?
>
> Not on a specific table.  You could possibly use a dedicated Postgres
> instance with max_connections set to 1.  If you need to serialize access
> to just this table while allowing concurrent access to some other data
> in the same DB, then there's no built-in answer.

Seems they could set the CONNECTION LIMIT to 1 on CREATE/ALTER DATABASE
also, but as you said, it is per database.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + Everyone has their own god. +