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Don Burleson Blog 







Using SQL*Net in a Distributed Database Environment

Oracle Tips by Burleson Consulting

6 April, 1996, Updated June 2. 2015

From Oracle Technical Journal (a.k.a. OReview Magazine)

In its most basic form, SQL*Net is a software tool that enables a network of Oracle clients and servers to communicate transparently on top of any underlying network topology or protocol via SQL. Although SQL*Net is a very robust and sophisticated tool, you must appreciate the inherent complexity that accompanies its flexibility. This article provides a no-nonsense overview of the SQL*Net architecture. All of the examples are based on Unix.

SQL*Net is not easy to install. You need the following files on the server side in order to operate SQL*Net 2.0 on a Unix system:

  • /etc/TNSNAMES.ORA -- Used for outgoing database requests, this file contains all of the database names (or service identifiers, SIDs) running on the processor, as well as domain name, protocol, host, and port information. When a new database is added to a box, you must update this file (changes to TNSNAMES.ORA become effective instantly). Note: SQL*Net version 1.0 equivalent is /etc/ORATAB.
  • /etc/LISTENER.ORA -- This file contains a list of destinations for outgoing database connections. When you add a new destination database to a Unix host, you must also add it to this file.
  • /etc/HOSTS -- This file lists all of your network addresses.
  • /etc/SERVICES -- This file lists all of the services available on a server, including SQL*Net services. It is no longer necessary with version 2.0.
  • The only file required on the client side is /etc/TNSNAMES.ORA. (On a PC client, this file is determined by the path in the c:\windows\oracle.ini file.)
  • (Although the Oracle Network Manager now sets up these file automatically, sometimes you must access or set them up manually.)

In version 2.0 and later (2.2 was due at press time, and 2.3 is due in March), Oracle added several important enhancements to SQL*Net, in addition to the badly needed bug fixes. SQL*Net now allows multiple "community" access. A community is a group of computers that share a common protocol (such as TCP/IP or LU6.2). Furthermore, the Oracle7 database engine now defines a multithreaded server (MTS) for servicing incoming data requests. In the MTS, all communication to the database is handled through dispatchers (single or multiple). In SQL*Net version 1.0, a separate process is "spawned" for each connection (you can view these connections via the Unix "ps" command).

When upgrading to SQL*Net 2.0 or later, you should be aware of subtle differences in how it handles communications as opposed to version 1.0. (See Figure 1, page 37.) Version 1.0 uses an orasrv component on the destination database to listen for incoming requests, while version 2.0 uses a process called tnslsnr (TNS listener). Also, SQL*Net 1.0 cannot use an MTS.

When you make a connection to SQL*Net, it passes the request to its underlying layer, the TNS, where the request is transmitted to the appropriate server. At the server, SQL*Net receives the request from TNS and passes the SQL to the database. TNS stands for "transparent network substrate" -- a fancy term meaning a single, common interface to all protocols that lets you connect to databases in physically separate networks. At the lowest level, TNS communicates to other databases with message-level send/receive commands.

On the client side, the User Programmatic Interface (UPI) converts SQL to associated parse, execute, and fetch statements. The UPI parses the SQL, opens the SQL cursor, binds the client application, describes the contents of returned data fields, executes the SQL, fetches the rows, and closes the cursor. Oracle attempts to minimize messages to the server by combining UPI calls whenever possible. On the server side, the Oracle Programmatic Interface (OPI) responds to all possible messages from the UPI and returns requests.

For server-to-server communication, there is no UPI; instead, a Network Programmatic Interface (NPI) resides at the initiating server. The responding server uses its OPI.

SQL*Net supports network transparency in such a way that the network structure may be changed without affecting the SQL*Net application. Location transparency is achieved via views, procedures, objects, synonyms, and so on.

Let's trace a sample data request through SQL*Net. Essentially, SQL*Net will look for the link name in the database link table (DBA_dDB_LINKS) and extract the service name. The service name is then located in the TNSNAMES.ORA file and the host name is extracted. It is a three-stage process, beginning with the link name, which references the service name, which references the host name.

In Unix environments, you can find the host name in a host file (/etc/HOSTS), and the Internal Protocol (IP) address is gathered. In the following example, london_unix translates into an IP address of The following four steps illustrate how SQL*Net takes a remote request and translates it into the IP address of the destination database:

1. Issue a remote request -- check the database link called london:


2. Database link -- obtain service name (london_unix_d) using the link_name (london):


3. TNSNAMES.ORA -- obtain the SID (london_sid) using service name (london_unix_d):

london_unix_d = (description=(address=
  (protocol=tcp) (host=marvin)
(port=1521) (connect_data=(sid=london_sid) 

4. /etc/hosts -- get IP address ( using the SID (london_sid):  london_sid

As you can see, this translation is a multistage process. The tnsnames.ora file specifies the name of the host containing the destination database. For Unix environments, this host name is then looked up in the /etc/hosts file to get the IP address of the destination box.

Listing 1 shows a sample tnsnames.ora file. The service name is looked up in tnsnames.ora, and if the service exists, the IP address is found in the /etc/hosts file and a communication request is sent to the destination IP address. Note that both of the entries in this file connect to london, but london_unix_d directs SQL*Net to spawn a dedicated process, while london_unix uses the MTS component because a shared server is specified.


london_unix_d =
(description =
        (ADDRESS =
          (PROTOCOL = TCP)
          (HOST =  tps)
          (PORT = 1521)
london_unix =
(description =
        (ADDRESS =
          (PROTOCOL = TCP)
          (HOST =  tps)
          (PORT = 1521)


After you create the tnsnames.ora and /etc/hosts files, you can include any tables from the London sites by qualifying the remote site name in the SQL query:


This query joins two tables at different locations, and the database link called london determines how the Oracle connection will be established on the destination system. Regardless of what user ID the local machine is using to make the connection to the destination, the user ID used to make the connection must have select privileges against the remote Customer table, or this query will fail.

SQL*Net for Client/Server

SQL*Net typically establishes database communications via two ways: a client-to-server connection or a server-to-server request.

Client-to-Server Connection. This is the easiest way to make a database connection. The sending database makes a request by specifying a table name suffixed with "@". SQL*Net takes it from there, seamlessly accessing the remote database and returning the data to the initiating system.

Communication is established simply by making a remote request to a remote database at one site. Within Oracle, @ specifies the remote database name, but the functionality of the @ operator depends upon where it is used. The following is an example:

sqlplus scott/tiger@london


In this request, Scott is using SQL*Plus to connect to the London database, and "@london" is the service name, as defined in the tnsnames.ora file. SQL*Net recognizes this as a client-to-server remote connection, and determines the appropriate linkage to establish communications with London. Internally, Oracle will check the tnsnames.ora file to ensure that London is a valid destination.

Server-to-Server Request. The following is another way of connecting to London from the same database using a database link:

sqlplus scott/tiger



Unlike a client-to-server connection, where the connection is made directly from SQL*Plus, a server-to-server request has Scott connecting to the local copy of SQL*Plus to specify the remote table (in this case, employee@london). In order for a server-to-server request to work, however, a database link must define "London." A database link is a connection pathway to a remote database that specifies the service name of that remote database. Without this database link, the same request will fail:

sqlplus scott/tiger

This request will give you an "ORA-02019: connection description for remote database not found" error message. The reason is the way that Oracle defines the @ operator. When entering an Oracle service such as SQL*Plus, the @ operator will go directly to the tnsnames.ora file to manage the request, while the @ operator within an Oracle program specifies a database link.

To make the code functional, you must define a database link that specifies the service name used to establish the connection. The database link name and the service name are the same in the following example, but the database link and the connect descriptor are not related in any way:



Let's take a closer look at the database link. In the previous example, there is no mention of the user ID that is used to establish the connection on the remote database. Because Scott is connecting to SQL*Plus from a remote server, scott will be the user ID when the direct connection is established with the London database. Therefore, Scott must have select privileges against the Employee table in London in order for the query to work properly. Scott's privileges on the initiating server have no bearing on the success of the query. When select security is not an issue, you can enhance the database link syntax to include a remote connect description:


This way, all users who specify the London database link will connect as scott1 and will have the same privileges that scott1 has on the London system. In other words, there is no accountability, because everyone goes through the same account. This is the issue of central accounts vs. individual accounts -- individual accounts are more secure because every account must be granted privileges explicitly, but they are also harder to maintain.

Once you have established a communications pathway to the remote database, you should implement location transparency. In relational databases such as Oracle, one way to achieve location transparency is by creating database links to the remote database and then assigning a global synonym to the remote tables. The database link specifies a link name and a SQL*Net service name. You can create database links with a location suffix that is associated with a host name (in this example, london_unix).

You can use database links to allow applications to "point" to other databases without altering the application code. For data warehouse applications, you can replicate a table on another machine and establish links to enable the application to point transparently to the new box containing the replicated table.

To see the links for a database, query the Oracle dictionary:


Keep in mind that SQL*Net bypasses all operating system connections when it connects to a database. All user accounts that are identified externally (that is, without an Oracle password) will not be allowed in SQL*Net transactions unless the init.ora parameter is changed. The "identified externally" clause (ops$) in Oracle version 6 allowed the operating system to manage passwords, but because SQL*Net bypasses the operating system, impostor accounts could be created from other platforms, thereby bypassing security. Consequently, Oracle recommends that "identified externally" accounts be forbidden for distributed connections, and version 2.0 takes care of the problem through services such as DCE.

It is interesting to note that Oracle will let you create accounts with an ops$ prefix. Therefore, the operating system can manage its passwords, and you can also have passwords within Oracle. For example, consider the following user definition:


Assuming that Scott has logged onto the operating system, Scott could enter SQL*Plus with or without a password:

sqlplus /
sqlplus scott/tiger

This ability to connect directly to Oracle presents a confounding issue with password management. Because two sets of passwords exist, one in the operating system and the other in Oracle, you may need a third-party tool to keep the passwords synchronized. (See Figure 2.)

Domains and Location Transparency

Many IS organizations recognize the need to track the locations of remote databases while providing location transparency to their users and programmers. Domains are especially important in situations of horizontal partitioning, where tables with identical names are kept at numerous locations. Domains establish a logical hierarchy of physical locations for the enterprise. (See Figure 3.)

The host name for each node in the network is appended to all of the table names within the distributed network. For duplicate table structures that exist at many locations, you can create abbreviated domains. Using the hierarchy in Figure 3 as an example, assume that both Japan and Ohio have Customer tables that are identical in structure but contain different rows. You can assign Oracle synonyms as follows:


But where is the location transparency? To make the Customer table transparent (and to make the query appear to be local), you can assign synonyms for the Customer table in London:


You can now run the following query with location transparency:


But what happens if Japan and London both have Customer tables? With SQL*Net, you can define and use a set of synonyms with a view to select all tables, regardless of their physical locations:


You can now issue a simple SQL statement to get all customer names:


Managing SQL*Net

The following examples describe some of the utilities you can use to manage SQL*Net sessions effectively. You should be aware that some of these examples are operating system-independent and may not apply to your environment.

Showing/Killing SQL*Net Sessions. On systems running SQL*Net 2.0, you can use a session script to query the number of dedicated and shared servers on the system. Although you can access this information via SQL*DBA and Server Manager, you can run the following SQL report to show all connected users:

ttitle "dbname Database|UNIX/Oracle Sessions";
spool /tmp/session
set heading off;
select 'Sessions on database '||substr(name,1,8) 
  from v$database;
set heading on;
       substr(a.spid,1,5) pid,
       substr(b.sid,1,5) sid,
       substr(b.serial#,1,5) ser#,
       substr(b.machine,1,6) box,
       substr(b.username,1,10) username,
       substr(b.osuser,1,8) os_user,
       substr(b.program,1,30) program
from v$session b, v$process a
b.paddr = a.addr
and type='USER'
order by spid;
ttitle off;
set heading off;
select 'To kill, enter SQLPLUS>  ALTER SYSTEM KILL 
''''||'SID, SER#'||''''||';' from dual;

The output from this script is shown in Listing 2.

[cad1]sh: sqlx session

                                 cert Database
                              Unix/Oracle Sessions


17723 	15	7462	cad2	OPS$JREINH  	jreinhar     unmenu50@t:cad:cad2 
18422	14	4607	cad2	OPS$LJOYCE	ljoyce       runmenu50@cad2
18524	12	5390	cad2	OPS$JSTEPH	 jstephan    runmenu50@cad2     
19047 	6	2308	cad2	ECHO_LNK	ljoyce       runmenu50@cad2     
19127 	10	2603	cad2	 OPS$BWYSZO	bwyszomi 	sqlplus@cad2	(TNS interface)       
19155 	7	4081	cad1	OPS$ORACLE	oracle       sqlplus@cad1	(Pipe Two-Task)       
10058 	7	2084	HRIS	ggallipe	C:\WINDOWS\system\KRNL386.EXE      
10091 	17	4733	HRIS	mbuddend	C:\WINDOWS\system\KRNL386.EXE
6 rows selected.


Unlike dedicated SQL*Net sessions, you cannot kill multithreaded SQL*Net sessions directly from the operating system. For example, you can identify a runaway session on a dedicated server by using the Unix "ps-ef|grep ora" command, and subsequently kill it using the "kill -9 nnn" command. With the MTS, operating system processes no longer exist for each separate task, and you must use the Oracle SQL alter system kill session command to kill the task at the Oracle subsystem level (using the "alter system kill session 'sid, ser#';" command).

To kill a user in an MTS session, enter SQL*Plus and type:


8	28	OPS$xxx
10	211	POS$yyy
13	8	dburleso

If "dburleso" is the session you want to kill, enter the alter system kill session '13, 8' command. You can also accomplish this with SQL*DBA and Server Manager.

This cumbersome method of clobbering runaway SQL*Net connections can be very annoying in development environments in which dozens of programmers are testing programs, and they must call the DBA every time they want to kill a runaway task. The only alternative, however, is to grant the programmers alter system authority on test platforms.

The type of SQL*Net connection you have is determined by the way you configure SQL*Net to connect with the database. Listing 3 shows how different two_task settings can affect Oracle sessions.

[oracle]iss: TWO_TASK=t:iss:cert  /* old SQL*Net version 1.0 */
[oracle]iss: sqlx session

SERVER Oracle username O/S User Machine Program
----------- ----------------- ---- ----- --------------------------------------
DEDICATED OPS$ORACLE oracle iss sqlplus@t:iss:iss (TCP Two-Task)

[oracle]iss: TWO_TASK=d_cert /*  dedicated service name on version two */
[oracle]iss: sqlx session

SERVER    Oracle username O/S User Machine Program                             
----------- ----------------- ---- ----- --------------------------------------
DEDICATED OPS$ORACLE      oracle   iss    sqlplus@iss (TNS interface)         

[oracle]iss: TWO_TASK=cert  /* version 2.0 service name */
[oracle]iss: sqlx session

SERVER    Oracle username O/S     User Machine Program                             
----------- ----------------- ---- ----- --------------------------------------
SHARED    OPS$ORACLE oracle   iss sqlplus@iss (TNS interface)

PC task initiated from ODBC/PowerBuilder

[oracle]iss: sqlx session

SERVER    Oracle username O/S      User     Machine Program                             
----------- ----------------- ---- ----- --------------------------------------
DEDICATED  OPS$ORACLE oracle   iss      c:\windows\krnl386.exe


Managing the MTS. One of the problems with version 1.0 was that each incoming transaction was spawned by the listener as a separate operating system task. With version 2.0, Oracle now has a method that allows the listener connection to dispatch numerous subprocesses. With the MTS, all communications to a database are handled through dispatchers, rather than with separate Unix process IDs (PIDs) on each database. This translates into faster performance for most online tasks. Even local transactions will be directed through the MTS, and you will no longer see a PID for your local task when you issue "ps-ef|grep oracle". A sample init.ora for starting the MTS is shown in Listing 4.

# ???????????-
# Multi-threaded Server
# ???????????-

   (HOST=marvin) (PORT=1521))

# ?????????????-
# Distributed systems options
# ?????????????-


However, be aware that the MTS is not a panacea. There are times when you want to invoke a dedicated process for your program. For batch processing applications, you may get better performance using a dedicated process.

In general, the MTS offers benefits such as reduced memory use, fewer processes per user, and automatic load balancing. But, it is often very confusing to know if the MTS is turned on, much less if it is working properly. Remember the following rules of thumb about the MTS:

  • The MTS is governed by the init.ora parameters. If no MTS parameters are present in init.ora, the MTS is disabled.
  • The MTS is used when the MTS parameters are in init.ora and requests are made by service name (such as @myplace). In other words, you must eliminate all version 1.0 connect strings (such as t:unix1:myplace).
  • Each user of the MTS requires 1K of storage. Plan to increase your shared_pool_size. You can also implement a resource limitation via an MTS tuning parameter.
  • The v$queue and v$dispatcher system tables indicate if the number of MTS dispatchers is too low. Even though the number of dispatchers is specified in the init.ora file, you can change it online in SQL*DBA with the alter system command:
  • If you encounter problems with the MTS, you can quickly regress to dedicated servers by issuing an alter system command. The following command turns off the MTS by setting the number of MTS servers to zero:
  • In order to use ops$, you must set two init.ora values to true (they default to false):
    remote_os_authent = true remote_os_roles = true
  • When both SQL*Net 1.0 and 2.0 are installed, users can connect to the server either via a dedicated server or via the MTS. However, you need a dedicated server for administration options such as startup and shutdown.
  • In some cases, you must shutdown and restart the server if the listener is stopped, or it will restart in dedicated mode. The listener reads the MTS parameters only if it is running before startup of the instance.

Managing the Listener. As a service request is intercepted by an Oracle server, the listener may direct the request via a dedicated server, an MTS, or an existing process (prespawned shadow). The key is whether the connection contacts the listener via a service name or bypasses the listener with the two_task connect string. If the listener is contacted as part of the connection, and the MTS parameters are defined to init.ora, the client will use the MTS.

There are five basic listener commands: reload, start, stop, status, and services. The output of the status and services commands is shown in Listing 5. The listener decides (based on the request) whether to dispatch a connection to a dedicated-server process (which it spawns) or to use the MTS. The programmer has several options when deciding how Oracle will manage the process. You can implement dedicated requests by specifying a version 1.0 connect string, or by using a service name that specifies "server=dedicated" in the tnsnames.ora file.

lsnrctl status

[oracle]cad2: lsnrctl stat

LSNRCTL for HPUX: Version - Production on 16-SEP-94 15:38:00

Copyright (c) Oracle Corporation 1993.  All rights reserved.

Connecting to (ADDRESS=(PROTOCOL=TCP)(HOST=cad2)(PORT=1521))
Alias                     LISTENER
Version                   TNSLSNR for HPUX: Version - Production
Start Date                29-AUG-94 13:50:16
Uptime                    18 days 1 hr. 47 min. 45 sec
Trace Level               off
Security                  OFF
Listener Parameter File   /etc/listener.ora
Listener Log File         /usr/oracle/network/log/listener.log
Services Summary...
  dev7db                has 1 service handlers
  cad2db                has 1 service handlers
The command completed successfully

lsnrctl services

[oracle]marvin: lsnrctl services

LSNRCTL for HPUX: Version - Production on 16-SEP-94 15:36:47

Copyright (c) Oracle Corporation 1993.  All rights reserved.

Connecting to (ADDRESS=(PROTOCOL=TCP)(HOST=marvin)(PORT=1521))
Services Summary...
  tdb000                has 4 service handlers
    DISPATCHER established:1 refused:0 current:2 max:55 state:ready
      D001 (machine: marvin, pid: 4146)

    DISPATCHER established:1 refused:0 current:2 max:55 state:ready
      D000 (machine: marvin, pid: 4145)

    DISPATCHER established:0 refused:0 current:1 max:55 state:ready
      D002 (machine: marvin, pid: 4147)

    DEDICATED SERVER established:0 refused:0
The command completed successfully

Note: Local connections will use the listener if the MTS is defined. Even internal invocations to Oracle (for example, sqlplus/) will add a connection to the MTS.

Miscellaneous Management Tips. Keep the following rules in mind:

  • Just as the /etc/oratab file for SQL*Net version 1.0 is interpreted, the tnsnames.ora file is also interpreted. This means that you can change it at any time, so you can change or add destinations during production hours. However, changes to listener.ora require that the listener be reloaded with lsnrctl reload.
  • Don't forget to change your oracle_home/bin/oraenv file to unset oracle_sid and set two_task=sid.
  • There are three logs in SQL*Net:
    listener log = /usr/oracle/network/log/listener.log
      sqlnet log = /usr/oracle/network/log/sqlnet.log trace log =destination
      set with the TRACE_DIRECTORY_LISTENER parameter of the /etc/listener.ora
  • There are three levels of tracing in SQL*Net:
    lsnrctl trace admin
      lsnrctl trace user lsnrctl trace off

    It is possible to run two listeners, one for version 1.0 and another for version 2.0. If a version 1.0 connect string is sent, a version 1.0 listener (tcpctl) will be used. Conversely, if a TNS connect description is sent, the version 2.0 listener (lsnrctl) will be used. A connect description is the name of a database (such as @mydata), which maps to the tnsnames.ora on the sending side, and listener.ora on the receiving side.

  • You should be aware that the functions of the oracle_sid and two_task variables has changed. To use the MTS while you are "local" to the database, you should unset the oracle_sid variable and set the two_task to the sid name (export two_task=mydb). If the oracle_sid is active, you will still be able to connect, but you will not be able to take advantage of the MTS. You must change all login scripts and oracle_home/bin/oraenv files to reflect this new functionality.

With MTS, there are now three ways to establish distributed database communications: by shared service-name (sqlplus /@cad2db); by dedicated service name (sqlplus /@d_cad2db -- prefixing the sid with "d_" will direct the listener to spawn a dedicated process for your program); and by a (two_task) server connect string (sqlplus /@t:host:sid). This latter approach will bypass the MTS and use a dedicated process.

Managing Two-Phase Commits (2PCs). When a distributed update (or delete) has finished processing, SQL*Net will coordinate commit processing so that if any portion of the transaction fails, the entire transaction will roll back. The first phase of this process is a "prepare phase" to each node, followed by the commit, and then terminated by a "forget phase."

If a distributed update is in the process of issuing the 2PC and a network connection breaks, Oracle will place an entry in the dba_2pc_pending table, and the recovery background process, reco, will roll back or commit the good node to match the state of the disconnected node, thereby ensuring consistency. You can activate reco via the alter system enable distributed recovery command. The reco process is applicable only when the distributed option is installed in Oracle, and is only used to manage 2PCs. If your system does not perform cross-database synchronization, you may want to disable reco.

The dba_2pc_pending table contains an "advise" column that directs the database to either commit or rollback the pending item. You can use the alter session advise syntax to direct the 2PC mechanism. For example, to force the completion of an insert, you can enter the following:


When a 2PC transaction fails, you can query the dba_2pc_pending table to check the "state" column. You can enter SQL*DBA and use the "recover in-doubt transaction" dialog box to force either a rollback or a commit of the pending transaction. If you do this, the row will disappear from dba_2pc_pending after the transaction has been resolved. If you force the transaction the wrong way (for example, to roll back when other nodes have already committed), reco will detect the problem, set the mixed column to "yes," and the row will remain in dba_2pc_pending table.

SQL*Net on the Job

Although I can only gloss over the high points, sophisticated tools such as SQL*Net require a great deal of knowledge and skill to use effectively. As systems continue to evolve into complex distributed networks, interdatabase communications will become even more complex, and require more sophisticated tools. While object orientation promises to make interdatabase communications simple, the DBA in the trenches will continue to struggle with implementing everyday distributed database communications.

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