SQL ERROR "00D31034"

You have a web application connecting to DB2 for z/OS and it times out with the message:

SQL30081N A communication error has been detected. 
Communication protocol being used: "TCP/IP". Communication
 API being used: "SOCKETS". Location where the error was
detected: ip-address", Communication function detecting
the error: "recv". Protocol specific error code(s):
 "*", "10054", "*". SQLSTATE=08001

There could be many reasons for this error. One that I found recently is here.

Look in your z/OS DB2 MSTR log for a message like this:

DSNL030I  +DB2x DSNLILNR DDF PROCESSING FAILURE FOR
LUWID=xxxxxxxx.GE21.000000000000
AUTHID=-N/A-, REASON=00D31034

As per the DB2 manual, "00D31034" means

The MAX REMOTE CONNECTED limit was reached and the
conversation was abnormally deallocated. The thread
was not allowed to connect to DB2.

Now, as the message says, there may be too many remote database connections and the number of remote connections exceeded the ZPARM CONDBAT(MAX REMOTE CONNECTED) limit.

But, before jumping to the conclusion that all the connections are genuine and asking your DBA to bump this number, look at your connection threads closely.

The web application might be using an old version of the arch.jar (arch-2_2_3.jar). This version of arch.jar does not close the database connection successfully on a few occasions.

The resolution is a simple upgrade to the latest arch.jar (arch-3_0_1.jar) which has the fix to close the connection.