ORACLE/ASP.NET: ORA-2020 - Too many database links... what's causing this?

Question

Here's the scenario...

We have an internal website that is running the latest version of the ODAC (Oracle Client). It opens database connections, runs a stored procedure or packaged method, then disconnects. Connection pooling is turned on, and we are currently under version 11g in both our development and test environments, but under 10gR2 in our production environment. This happens on Production.

A few days ago, a process began firing off a ORA-2020 error. The process is called from a webpage on our internal website. The user simply sets a date, hits a button, and a job is started on another system that is separate from the website. The call itself, however, uses a database link to run a function.

We've scoured the SQL to find that it only uses that one database link. And since these links are on a per session basis and the user isn't exceeding the default limit of 4, how is it possible that we are getting a ORA-2020 error.

We have ran a number of tests to try to push over the default limit of 4. ODAC, from what I recall, runs a commit after each connection, and I can't seem to run 4 DB links, then run a piece of SQL with 1 DB link directly after with any errors. The only way I can bring up this error is if I run a query with 4 DB links, then a function or piece of dynamic SQL with a database link within it. We don't have that problem as this issue is sporadic. It isn't ALWAYS happening.

Questions

1. Is it possible that connection pooling is allowing User B to use User A's connection after the initial process was run, thus adding to the open links number if User B runs a SQL statement with more database links?
2. Is this a scenario where we should up our limit past 4? What are the disadvantages of increasing the number?
3. Do I need to explicitly close open database links before disconnecting from the database? Oracle documentation seems to suggest it should automatically happen, but "on occasion"... doesn't.

Answer 1

Firstly, the simple solution: I'd double check that in the production database the number of default links is actually 4.

select *
  from v$system_parameter
 where name = 'OPEN_LINKS'

Assuming you're not going to get off that lightly:

Is it possible that connection pooling is allowing User B to use User A's connection after the initial process was run, thus adding to the open links number if User B runs a SQL statement with more database links?

You say that you explicitly close the session, which, according to the documentation, should mean that all links associated with that session are closed. Other than that I confess complete ignorance on this point.

Is this a scenario where we should up our limit past 4? What are the disadvantages of increasing the number?

There aren't any disadvantages that I can think of. Tom Kyte suggests, albeit a long time ago, that each open database link uses 500k of PGA memory. If you don't have any then this will obviously cause a problem but it should be more than fine for most situations.

There are, however, unintended consequences: Imagine that you up this number to 100. Somebody codes something that continually opens links and draws a lot of data through all them select * from my_massive_table or similar. Instead of 4 sessions doing this you have 100, which is attempting to transfer hundreds of gigabytes simultaneously. Your network dies under the strain...

There's probably more but you get the picture.

Do I need to explicitly close open database links before disconnecting from the database? Oracle documentation seems to suggest it should automatically happen, but "on occasion"... doesn't.

As you've noted the best answer is "probably not", which isn't much help. You don't mention exactly how you're terminating the session but if you're killing it rather than closing gracefully then definitely.

Using a database link spawns a child process on the remote server. Because your server is no longer in absolute charge of this process there's a myriad of things that could cause it to become orphaned or otherwise not close on termination of the parent process. By no means does this happen the whole time but it can and does.

I would do two things.

1. In your process, if an exception is encountered, e-mail the results of the following query to yourself.

   select * 
     from v$dblink
   

   As a minimum at least you will know what database links are open in the session and give you some way of tracing them.

2. Follow the documentations advice; specifically the following:

   "You may have occasion to close the link manually. For example, close links when:

   • The network connection established by a link is used infrequently in an application.
   • The user session must be terminated."

The first seems to exactly fit your situation. Unless your process is time-sensitive, which doesn't seem to be the case, then what have you got to lose? The syntax is:

alter session close database link <linkname>