DIFFERENCES BETWEEN PROCESSES, SESSIONS AND CONNECTIONS

From ASKTOM site

A process is a physical process or thread. On unix, you can see a process with "ps" for example. It is there.

There are many types of processes in Oracle -- background processes like SMON, PMON, RECO, ARCH, CKPT, EMNn, DBWR, etc.....

And user processes like dedicated servers or shared server (multi-threaded server -- aka MTS -- configuration)

A connection is a "physical circuit", a pathway to a database. You can be connected to a database yet have 0 or 1 or MORE sessions going on that connection.

We can see that with sqlplus, consider (single user system here, its all about me)

[tkyte@tkyte-pc-isdn tkyte]$ ps -auxww | grep oracleora920
[tkyte@tkyte-pc-isdn tkyte]$ sqlplus /nolog

SQL*Plus: Release 9.2.0.1.0 - Production on Sat Sep 28 10:36:03 2002

idle> !ps -auxww | grep oracleora920
tkyte 19971 0.0 0.1 2196 916 pts/1 S 10:36 0:00 /bin/bash -c ps -auxww |
grep oracleora920
tkyte 19973 0.0 0.1 1736 564 pts/1 S 10:36 0:00 grep oracleora920

no process, no nothing

idle> connect /
Connected.
idle> !ps -auxww | grep oracleora920
ora920   19974 1.5 2.2 230976 11752 ?      S    10:36   0:00 oracleora920
(DESCRIPTION=(LOCAL=YES)(ADDRESS=(PROTOCOL=beq)))
tkyte    19975 0.0 0.1 2196 916 pts/1    S    10:36   0:00 /bin/bash -c ps -auxww |
grep oracleora920
tkyte    19977 0.0 0.1 1736 564 pts/1    S    10:36   0:00 grep oracleora920

got my process now...

idle> disconnect
Disconnected from Oracle9i Enterprise Edition Release 9.2.0.1.0 - Production
With the Partitioning, OLAP and Oracle Data Mining options
JServer Release 9.2.0.1.0 - Production
idle> !ps -auxww | grep oracleora920
ora920   19974 0.6 2.3 230976 11876 ?      S    10:36   0:00 oracleora920
(DESCRIPTION=(LOCAL=YES)(ADDRESS=(PROTOCOL=beq)))
tkyte    19978 0.0 0.1 2196 916 pts/1    S    10:36   0:00 /bin/bash -c ps -auxww |
grep oracleora920
tkyte    19980 0.0 0.1 1736 564 pts/1    S    10:36   0:00 grep oracleora920

idle> select * from dual;

SP2-0640: Not connected

still have my process, but no session, the message is a little "misleading".
Technically -- I have a connection, I don't have a session

further, autotrace in sqlplus can be used to show that you can have
a) a connection
b) that uses a single process
c) to service two sessions:

ops$tkyte@ORA920.US.ORACLE.COM> select username from v$session where username is not
null;

USERNAME
------------------------------
OPS$TKYTE

one session, ME

ops$tkyte@ORA920.US.ORACLE.COM> select username, program from v$process;

USERNAME        PROGRAM
--------------- ------------------------------------------------
                PSEUDO
ora920          oracle@tkyte-pc-isdn.us.oracle.com (PMON)
ora920          oracle@tkyte-pc-isdn.us.oracle.com (DBW0)
ora920          oracle@tkyte-pc-isdn.us.oracle.com (LGWR)
ora920          oracle@tkyte-pc-isdn.us.oracle.com (CKPT)
ora920          oracle@tkyte-pc-isdn.us.oracle.com (SMON)
ora920          oracle@tkyte-pc-isdn.us.oracle.com (RECO)
ora920          oracle@tkyte-pc-isdn.us.oracle.com (CJQ0)
ora920          oracle@tkyte-pc-isdn.us.oracle.com (QMN0)
ora920          oracle@tkyte-pc-isdn.us.oracle.com (S000)
ora920          oracle@tkyte-pc-isdn.us.oracle.com (D000)
ora920          oracle@tkyte-pc-isdn.us.oracle.com (ARC0)
ora920          oracle@tkyte-pc-isdn.us.oracle.com (ARC1)
tkyte           oracle@tkyte-pc-isdn.us.oracle.com (TNS V1-V3)

14 rows selected.

you can see all of the backgrounds and my dedicated server...

ops$tkyte@ORA920.US.ORACLE.COM> set autotrace on statistics;

Autotrace for statistics uses ANOTHER session so it can query up the stats for your
CURRENT session without impacting the STATS for that session!

ops$tkyte@ORA920.US.ORACLE.COM> select username from v$session where username is not
null;

USERNAME
------------------------------
OPS$TKYTE
OPS$TKYTE

see, two sessions but....

Statistics
----------------------------------------------------------
          0 recursive calls
          0 db block gets
          0 consistent gets
          0 physical reads
          0 redo size
        418 bytes sent via SQL*Net to client
        499 bytes received via SQL*Net from client
          2 SQL*Net roundtrips to/from client
          0 sorts (memory)
          0 sorts (disk)
          2 rows processed

ops$tkyte@ORA920.US.ORACLE.COM> select username, program from v$process;

14 rows selected.

same 14 processes...

Statistics
----------------------------------------------------------
          0 recursive calls
          0 db block gets
          0 consistent gets
          0 physical reads
          0 redo size
       1095 bytes sent via SQL*Net to client
        499 bytes received via SQL*Net from client
          2 SQL*Net roundtrips to/from client
          0 sorts (memory)
          0 sorts (disk)
         14 rows processed

ops$tkyte@ORA920.US.ORACLE.COM>

I'll try to put it into a single, simple paragraph:

A connection is a physical circuit between you and the database. A connection might be
one of many types -- most popular begin DEDICATED server and SHARED server. Zero, one or
more sessions may be established over a given connection to the database as show above
with sqlplus. A process will be used by a session to execute statements. Sometimes
there is a one to one relationship between CONNECTION->SESSION->PROCESS (eg: a normal
dedicated server connection). Sometimes there is a one to many from connection to
sessions (eg: like autotrace, one connection, two sessions, one process). A process does
not have to be dedicated to a specific connection or session however, for example when
using shared server (MTS), your SESSION will grab a process from a pool of processes in
order to execute a statement. When the call is over, that process is released back to
the pool of processes.

posted @ 2014-07-16 01:53 princessd8251 阅读(164) 评论(0) 编辑收藏举报

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