Release 2 (8.1.6)
Part Number A76960-01
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| Oracle8i Distributed Database Systems Release 2 (8.1.6) Part Number A76960-01 |
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This chapter describes how to manage and troubleshoot distributed transactions. Topics include:
You can set initialization parameters that control the behavior of distributed transaction processing. The following tables describes initialization parameters relevant for distributed transaction processing:
This section includes the following topics:
The initialization parameter DISTRIBUTED_TRANSACTIONS limits the number of distributed transactions in which a given instance can concurrently participate, both as a client and a server. The default value for this parameter is operating system-dependent.
If Oracle reaches this limit and a subsequent user issues a SQL statement referencing a remote database, then the system rolls back the statement and returns the following error message:
ORA-2042: too many global transactions
For example, assume that you set the parameter as follows for a given instance:
DISTRIBUTED_TRANSACTIONS = 10
In this case, a maximum of 10 sessions can concurrently process a distributed transaction. If an additional session attempts to issue a DML statement requiring distributed access, then Oracle returns an error message to the session and rolls back the statement.
Consider increasing the value of the DISTRIBUTED_TRANSACTIONS when an instance regularly participates in numerous distributed transactions and the ORA-2042 is frequently returned. Increasing the limit allows more users to concurrently issue distributed transactions.
If your site is experiencing an abnormally high number of network failures, you can temporarily decrease the value of DISTRIBUTED_TRANSACTIONS. This operation limits the number of in-doubt transactions in which your site takes part, and thereby limits the amount of locked data at your site, and the number of in-doubt transactions you might have to resolve
If DISTRIBUTED_TRANSACTIONS is set to zero, no distributed SQL statements can be issued in any session. Also, the RECO background process is not started at startup of the local instance. In-doubt distributed transactions that may be present cannot be automatically resolved by Oracle8i. Therefore, only set this initialization parameter to zero to prevent distributed transactions when a new instance is started and when it is certain that no in-doubt distributed transactions remained after the last instance shut down.
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See Also:
Oracle8i Reference for more information about the DISTRIBUTED_TRANSACTIONS initialization parameter. |
When you issue a SQL statement, Oracle8i attempts to lock the resources needed to successfully execute the statement. If the requested data is currently held by statements of other uncommitted transactions, however, and remains locked for a long time, a timeout occurs.
Consider the following scenarios involving data access failure:
A DML statement that requires locks on a remote database can be blocked if another transaction own locks on the requested data. If these locks continue to block the requesting SQL statement, then the following sequence of events occurs:
ORA-02049: time-out: distributed transaction waiting for lock
Because the transaction did not modify data, no actions are necessary as a result of the timeout. Applications should proceed as if a deadlock has been encountered. The user who executed the statement can try to re-execute the statement later. If the lock persists, then the user should contact an administrator to report the problem.
Use the initialization parameter DISTRIBUTED_LOCK_TIMEOUT to control the timeout interval, which is set in seconds (see "Specifying the Lock Timeout Interval"). For example, to set the timeout interval for an instance to 30 seconds, include the following line in the associated parameter file:
DISTRIBUTED_LOCK_TIMEOUT = 30
The default value for this parameter is 60 seconds. Normally, Oracle waits indefinitely for a lock to be released. With the above timeout interval, the timeout errors discussed in the previous section occur if a transaction cannot proceed after 30 seconds of waiting for unavailable resources.
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See Also:
Oracle8i Reference for more information about the DISTRIBUTED_LOCK_TIMEOUT initialization parameter. |
A query or DML statement that requires locks on a local database can be blocked indefinitely due to the locked resources of an in-doubt distributed transaction. In this case, Oracle issues the following error message:
ORA-01591: lock held by in-doubt distributed transaction identifier
In this case, Oracle rolls back the SQL statement immediately. The user who executed the statement can try to re-execute the statement later. If the lock persists, the user should contact an administrator to report the problem, including the ID of the in-doubt distributed transaction.
The chances of the above situations occurring are rare considering the low probability of failures during the critical portions of the two-phase commit. Even if such a failure occurs, and assuming quick recovery from a network or system failure, problems are automatically resolved without manual intervention. Thus, problems usually resolve before they can be detected by users or database administrators.
If a distributed transaction fails, then the connection from the local site to the remote site may not close immediately. Instead, it remains open in case communication can be restored quickly, without having to re-establish the connection. Use the following initialization parameter to specify the length of time to hold open a remote connection after a distributed transaction fails:
DISTRIBUTED_RECOVERY_CONNECTION_HOLD_TIME
The default value for this parameter is 200 seconds. If you set larger values, then you minimize reconnection time but also consume local resources for a longer time period. The range of values is between 0 and 1800. You can set this parameter to a value greater than 1800, however, which simply means that the connection never closes.
The database with the highest commit point strength determines which node commits first in a distributed transaction. When specifying a commit point strength for each node, ensure that the most critical server will be non-blocking if a failure occurs during a prepare or commit phase. The following initialization parameter determines a node's commit point strength:
COMMIT_POINT_STRENGTH
The default value is operating system-dependent. The range of values is any integer from 0 to 255. For example, to set the commit point strength of a database to 200, include the following line in that database's initialization parameter file:
COMMIT_POINT_STRENGTH = 200
The commit point strength only determines the commit point site in a distributed transaction.
When setting the commit point strength for a database, note the following considerations:
The data dictionary of each database stores information about all open distributed transactions. You can use data dictionary tables and views to gain information about the transactions. This section contains the following topics:
The following view show the database links that have been defined at the local database and stored in the data dictionary:
| View | Purpose |
|---|---|
|
DBA_2PC_PENDING |
Lists all in-doubt distributed transactions. The view is empty until populated by an in-doubt transaction. After the transaction is resolved, the view is purged. |
Use this view to determine the global commit number for a particular transaction ID. You can use this global commit number when manually resolving an in-doubt transaction.
The following table shows the most relevant columns (for a description of all the columns in the view, see Oracle8i Reference):
Execute the following script to query pertinent information in DBA_2PC_PENDING (sample output included):
COL local_tran_id FORMAT a13 COL global_tran_id FORMAT a30 COL state FORMAT a8 COL mixed FORMAT a3 COL host FORMAT a10 COL commit# FORMAT a10 SELECT local_tran_id, global_tran_id, state, mixed, host, commit# FROM dba_2pc_pending / SQL> @pending_txn_script LOCAL_TRAN_ID GLOBAL_TRAN_ID STATE MIX HOST COMMIT# ------------- ------------------------------ -------- --- ---------- ---------- 1.15.870 HQ.ACME.COM.ef192da4.1.15.870 commit no dlsun183 115499
This output indicates that local transaction 1.15.870 has been committed on this node, but it may be pending on one or more other nodes. Because LOCAL_TRAN_ID and the local part of GLOBAL_TRAN_ID are the same, the node is the global coordinator of the transaction.
The following view shows which in-doubt transactions are incoming from a remote client and which are outgoing to a remote server:
When a transaction is in-doubt, you may need to determine which nodes performed which roles in the session tree. Use to this view to determine:
The following table shows the most relevant columns (for an account of all the columns in the view, see Oracle8i Reference):
Execute the following script to query pertinent information in DBA_2PC_PENDING (sample output included):
COL local_tran_id FORMAT a13 COL in_out FORMAT a6 COL database FORMAT a25 COL dbuser_owner FORMAT a15 COL interface FORMAT a3 SELECT local_tran_id, in_out, database, dbuser_owner, interface FROM dba_2pc_neighbors / SQL> CONNECT sys/sys_pwd@hq.acme.com SQL> @neighbors_script LOCAL_TRAN_ID IN_OUT DATABASE DBUSER_OWNER INT ------------- ------ ------------------------- --------------- --- 1.15.870 out SALES.ACME.COM SYS C
This output indicates that the local node sent an outgoing request to remote server SALES to commit transaction 1.15.870. If SALES committed the transaction but no other node did, then you know that SALES is the commit point site--because the commit point site always commits first.
A transaction is in-doubt when there is a failure during any aspect of the two-phase commit. Distributed transactions become in-doubt in the following ways:
You can manually force the commit or rollback of a local, in-doubt distributed transaction. Because this operation can generate consistency problems, perform it only when specific conditions exist.
This section contains the following topics:
The user application that commits a distributed transaction is informed of a problem by one of the following error messages:
ORA-02050: transaction ID rolled back, some remote dbs may be in-doubt ORA-02051: transaction ID committed, some remote dbs may be in-doubt ORA-02054: transaction ID in-doubt
A robust application should save information about a transaction if it receives any of the above errors. This information can be used later if manual distributed transaction recovery is desired.
No action is required by the administrator of any node that has one or more in-doubt distributed transactions due to a network or system failure. The automatic recovery features of Oracle8i transparently complete any in-doubt transaction so that the same outcome occurs on all nodes of a session tree (that is, all commit or all roll back) after the network or system failure is resolved.
In extended outages, however, you can force the commit or rollback of a transaction to release any locked data. Applications must account for such possibilities.
Override a specific in-doubt transaction manually only when one of the following situations exists:
ORA-01591 error message interferes with user transactions.
Normally, you should make a decision to locally force an in-doubt distributed transaction in consultation with administrators at other locations. A wrong decision can lead to database inconsistencies that can be difficult to trace and that you must manually correct.
If the conditions above do not apply, always allow the automatic recovery features of Oracle8i to complete the transaction. If any of the above criteria are met, however, consider a local override of the in-doubt transaction.
If you decide to force the transaction to complete, analyze available information with the following goals in mind.
Use the DBA_2PC_PENDING view to find a node that has either committed or rolled back the transaction. If you can find a node that has already resolved the transaction, then you can follow the action taken at that node.
See if any information is given in the TRAN_COMMENT column of DBA_2PC_PENDING for the distributed transaction. Comments are included in the COMMENT parameter of the COMMIT command.
For example, an in-doubt distributed transaction's comment can indicate the origin of the transaction and what type of transaction it is:
COMMIT COMMENT 'Finance/Accts_pay/Trans_type 10B';
See if any information is given in the ADVICE column of DBA_2PC_PENDING for the distributed transaction. An application can prescribe advice about whether to force the commit or force the rollback of separate parts of a distributed transaction with the ADVISE parameter of the SQL command ALTER SESSION.
The advice sent during the prepare phase to each node is the advice in effect at the time the most recent DML statement executed at that database in the current transaction.
For example, consider a distributed transaction that moves an employee record from the EMP table at one node to the EMP table at another node. The transaction can protect the record--even when administrators independently force the in-doubt transaction at each node--by including the following sequence of SQL statements:
ALTER SESSION ADVISE COMMIT; INSERT INTO emp@hq ... ; /*advice to commit at HQ */ ALTER SESSION ADVISE ROLLBACK; DELETE FROM emp@sales ... ; /*advice to roll back at SALES*/ ALTER SESSION ADVISE NOTHING;
If you manually force the in-doubt transaction, the worst that can happen is that each node has a copy of the employee record; the record cannot disappear.
Use the COMMIT or ROLLBACK statement with the FORCE option and a text string that indicates either the local or global transaction ID of the in-doubt transaction to commit.
This section contains the following topics:
Before attempting to commit the transaction, ensure that you have the proper privileges. Note the following requirements:
| If the transaction was committed by... | Then you must have this privilege... |
|---|---|
|
You |
FORCE TRANSACTION |
|
Another user |
FORCE ANY TRANSACTION |
The following SQL statement is the command to commit an in-doubt transaction:
COMMIT FORCE 'transaction_id';
The variable transaction_id is the identifier of the transaction as specified in either the LOCAL_TRAN_ID or GLOBAL_TRAN_ID columns of the DBA_2PC_PENDING data dictionary view.
For example, assume that you query DBA_2PC_PENDING and determine the local transaction ID for a distributed transaction:
LOCAL_TRAN_ID 1.45.13
You then issue the following SQL statement to force the commit of this in-doubt transaction:
COMMIT FORCE '1.45.13';
Optionally, you can specify the SCN for the transaction when forcing a transaction to commit. This feature allows you to commit an in-doubt transaction with the SCN assigned when it was committed at other nodes.
Consequently, you maintain the synchronized commit time of the distributed transaction even if there is a failure. Specify an SCN only when you can determine the SCN of the same transaction already committed at another node.
For example, assume you want to manually commit a transaction with the following global transaction ID:
SALES.ACME.COM.55d1c563.1.93.29
First, query the DBA_2PC_PENDING view of a remote database also involved with the transaction in question. Note the SCN used for the commit of the transaction at that node. Specify the SCN when committing the transaction at the local node. For example, if the SCN is 829381993, issue:
COMMIT FORCE 'SALES.ACME.COM.55d1c563.1.93.29', 829381993;
Before attempting to roll back the in-doubt distributed transaction, ensure that you have the proper privileges. Note the following requirements:
| If the transaction was committed by... | Then you must have this privilege... |
|---|---|
|
You |
FORCE TRANSACTION |
|
Another user |
FORCE ANY TRANSACTION |
The following SQL statement is the command to roll back an in-doubt transaction:
ROLLBACK FORCE 'transaction_id';
The variable transaction_id is the identifier of the transaction as specified in either the LOCAL_TRAN_ID or GLOBAL_TRAN_ID columns of the DBA_2PC_PENDING data dictionary view.
For example, to roll back the in-doubt transaction with the local transaction ID of 2.9.4, use the following statement:
ROLLBACK FORCE '2.9.4';
Before RECO recovers an in-doubt transaction, the transaction appears in DBA_2PC_PENDING.STATE as COLLECTING, COMMITTED, or PREPARED. If you force an in-doubt transaction using COMMIT FORCE or ROLLBACK FORCE, then the states FORCED COMMIT or FORCED ROLLBACK may appear.
Automatic recovery normally deletes entries in these states. The only exception is when recovery discovers a forced transaction that is in a state inconsistent with other sites in the transaction; in this case, the entry can be left in the table and the MIXED column in DBA_2PC_PENDING has a value of YES.
If automatic recovery is not possible because a remote database has been permanently lost, then recovery cannot identify the re-created database because it receives a new database ID when it is re-created. In this case, you must use the PURGE_LOST_DB_ENTRY procedure in the DBMS_TRANSACTION package to clean up the entries. The entries do not hold up database resources, so there is no urgency in cleaning them up.
To manually remove an entry from the data dictionary, use the following syntax (where trans_id is the identifier for the transaction):
DBMS_TRANSACTION.PURGE_LOST_DB_ENTRY('trans_id');
For example, to purge pending distributed transaction 1.44.99, enter the following command in SQL*Plus:
EXECUTE DBMS_TRANSACTION.PURGE_LOST_DB_ENTRY('1.44.99');
Execute this procedure only if significant reconfiguration has occurred so that automatic recovery cannot resolve the transaction. Examples include:
Oracle8i Supplied PL/SQL Packages Reference for more information about the DBMS_TRANSACTION package.
See Also:
The following tables indicates what the various states indicate about the distributed transaction what the administrator's action should be:
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See Also:
Oracle8i Supplied PL/SQL Packages Reference for more information about the DBMS_TRANSACTION package. |
The following example, illustrated in Figure 5-1, shows a failure during the commit of a distributed transaction. It explains how to go about gaining information before manually forcing the commit or rollback of the local portion of an in-doubt distributed transaction.
In this failure case, the prepare phase completes. During the commit phase, however, the commit point site's commit confirmation never reaches the global coordinator, even though the commit point site committed the transaction.
You are the WAREHOUSE database administrator. The inventory data locked because of the in-doubt transaction is critical to other transactions. The data cannot be accessed, however, because the locks must be held until the in-doubt transaction either commits or rolls back. Furthermore, you understand that the communication link between sales and headquarters cannot be resolved immediately.
Therefore, you decide to manually force the local portion of the in-doubt transaction using the following steps:
The following sections explain each step in detail for this example:
The users of the local database system that conflict with the locks of the in-doubt transaction receive the following error message:
ORA-01591: lock held by in-doubt distributed transaction 1.21.17
In this case, 1.21.17 is the local transaction ID of the in-doubt distributed transaction. You should request and record this ID number from users that report problems to identify which in-doubt transactions should be forced.
After connecting with SQL*Plus to WAREHOUSE, query the local DBA_2PC_PENDING data dictionary view to gain information about the in-doubt transaction:
CONNECT sys/sys_pwd@warehouse.acme.com SELECT * FROM sys.dba_2pc_pending WHERE local_tran_id = '1.21.17';
Oracle returns the following information:
Column Name Value ---------------------- -------------------------------------- LOCAL_TRAN_ID 1.21.17 GLOBAL_TRAN_ID SALES.ACME.COM.55d1c563.1.93.29 STATE prepared MIXED no ADVICE TRAN_COMMENT Sales/New Order/Trans_type 10B FAIL_TIME 31-MAY-91 FORCE_TIME RETRY_TIME 31-MAY-91 OS_USER SWILLIAMS OS_TERMINAL TWA139: HOST system1 DB_USER SWILLIAMS COMMIT#
The global transaction ID is the common transaction ID that is the same on every node for a distributed transaction. It is of the form:
global_database_name.hhhhhhhh.local_transaction_id
where:
Note that the last portion of the global transaction ID and the local transaction ID match at the global coordinator. In the example, you can tell that WAREHOUSE is not the global coordinator because these numbers do not match:
LOCAL_TRAN_ID 1.21.17 GLOBAL_TRAN_ID ... 1.93.29
The transaction on this node is in a prepared state:
STATE prepared
Therefore, WAREHOUSE waits for its coordinator to send either a commit or a rollback request.
The transaction's comment or advice can include information about this transaction. If so, use this comment to your advantage. In this example, the origin and transaction type is in the transaction's comment:
TRAN_COMMENT Sales/New Order/Trans_type 10B
This information can reveal something that helps you decide whether to commit or rollback the local portion of the transaction. If useful comments do not accompany an in-doubt transaction, you must complete some extra administrative work to trace the session tree and find a node that has resolved the transaction.
The purpose of this step is to climb the session tree so that you find coordinators, eventually reaching the global coordinator. Along the way, you may find a coordinator that has resolved the transaction. If not, you can eventually work your way to the commit point site, which will always have resolved the in-doubt transaction. To trace the session tree, query the DBA_2PC_NEIGHBORS view on each node.
In this case, you query this view on the WAREHOUSE database:
CONNECT sys/sys_pwd@warehouse.acme.com SELECT * FROM dba_2pc_neighbors WHERE local_tran_id = '1.21.17' ORDER BY sess#, in_out; Column Name Value ---------------------- -------------------------------------- LOCAL_TRAN_ID 1.21.17 IN_OUT in DATABASE SALES.ACME.COM DBUSER_OWNER SWILLIAMS INTERFACE N DBID 000003F4 SESS# 1 BRANCH 0100
The DBA_2PC_NEIGHBORS view provides information about connections associated with an in-doubt transaction. Information for each connection is different, based on whether the connection is inbound (IN_OUT = in) or outbound (IN_OUT = out):
In this example, the IN_OUT column reveals that the WAREHOUSE database is a server for the SALES client, as specified in the DATABASE column:
IN_OUT in DATABASE SALES.ACME.COM
The connection to WAREHOUSE was established through a database link from the SWILLIAMS account, as shown by the DBUSER_OWNER column:
DBUSER_OWNER SWILLIAMS
Additionally, the INTERFACE column tells whether the local node or a subordinate node is the commit point site:
INTERFACE N
Neither WAREHOUSE nor any of its descendants is the commit point site, as shown by the INTERFACE column.
At this point, you can contact the administrator at the located nodes and ask each person to repeat Steps 2 and 3 using the global transaction ID.
For example, the following results are returned when Steps 2 and 3 are performed at SALES and HQ.
At this stage, the SALES administrator queries the DBA_2PC_PENDING data dictionary view:
SQL> CONNECT sys/sys_pwd@sales.acme.com SQL> SELECT * FROM sys.dba_2pc_pending > WHERE global_tran_id = 'SALES.ACME.COM.55d1c563.1.93.29'; Column Name Value ---------------------- -------------------------------------- LOCAL_TRAN_ID 1.93.29 GLOBAL_TRAN_ID SALES.ACME.COM.55d1c563.1.93.29 STATE prepared MIXED no ADVICE TRAN_COMMENT Sales/New Order/Trans_type 10B FAIL_TIME 31-MAY-91 FORCE_TIME RETRY_TIME 31-MAY-91 OS_USER SWILLIAMS OS_TERMINAL TWA139: HOST system1 DB_USER SWILLIAMS COMMIT#
Next, the SALES administrator queries DBA_2PC_NEIGHBORS to determine the global and local coordinators as well as the commit point site:
SELECT * FROM dba_2pc_neighbors WHERE global_tran_id = 'SALES.ACME.COM.55d1c563.1.93.29' ORDER BY sess#, in_out;
This query returns three rows:
Reformatted information corresponding to the rows for the WAREHOUSE connection appears below:
Column Name Value ---------------------- -------------------------------------- LOCAL_TRAN_ID 1.93.29 IN_OUT OUT DATABASE WAREHOUSE.ACME.COM DBUSER_OWNER SWILLIAMS INTERFACE N DBID 55d1c563 SESS# 1 BRANCH 1
Reformatted information corresponding to the rows for the HQ connection appears below:
Column Name Value ---------------------- -------------------------------------- LOCAL_TRAN_ID 1.93.29 IN_OUT OUT DATABASE HQ.ACME.COM DBUSER_OWNER ALLEN INTERFACE C DBID 00000390 SESS# 1 BRANCH 1
The information from the previous query reveals the following:
At this stage, the HQ administrator queries the DBA_2PC_PENDING data dictionary view:
SELECT * FROM dba_2pc_pending WHERE global_tran_id = 'SALES.ACME.COM.55d1c563.1.93.29'; Column Name Value ---------------------- -------------------------------------- LOCAL_TRAN_ID 1.45.13 GLOBAL_TRAN_ID SALES.ACME.COM.55d1c563.1.93.29 STATE COMMIT MIXED NO ACTION TRAN_COMMENT Sales/New Order/Trans_type 10B FAIL_TIME 31-MAY-91 FORCE_TIME RETRY_TIME 31-MAY-91 OS_USER SWILLIAMS OS_TERMINAL TWA139: HOST SYSTEM1 DB_USER SWILLIAMS COMMIT# 129314
At this point, you have found a node that resolved the transaction. As the view reveals, it has been committed and assigned a commit ID number:
STATE COMMIT COMMIT# 129314
Therefore, you can force the in-doubt transaction to commit at your local database. It is a good idea to contact any other administrators you know that could also benefit from your investigation.
You contact the administrator of the SALES database, who manually commits the in-doubt transaction using the global ID:
SQL> CONNECT sys/sys_pwd@sales.acme.com SQL> COMMIT FORCE 'SALES.ACME.COM.55d1c563.1.93.29';
As administrator of the WAREHOUSE database, you manually commit the in-doubt transaction using the global ID:
SQL> CONNECT sys/sys_pwd@warehouse.acme.com SQL> COMMIT FORCE 'SALES.ACME.COM.55d1c563.1.93.29';
After you manually force a transaction to commit or roll back, the corresponding row in the pending transaction table remains. The state of the transaction is changed depending on how you forced the transaction.
Every Oracle8i database has a pending transaction table. This is a special table that stores information about distributed transactions as they proceed through the two-phase commit phases. You can query a database's pending transaction table through the DBA_2PC_PENDING data dictionary view (see Table 5-1, "DBA_2PC_PENDING").
Also of particular interest in the pending transaction table is the mixed outcome flag as indicated in DBA_2PC_PENDING.MIXED. You can make the wrong choice if a pending transaction is forced to commit or roll back. For example, the local administrator rolls back the transaction, but the other nodes commit it. Incorrect decisions are detected automatically, and the damage flag for the corresponding pending transaction's record is set (MIXED=yes).
The RECO (Recoverer) background process uses the information in the pending transaction table to finalize the status of in-doubt transactions. You can also use the information in the pending transaction table to manually override the automatic recovery procedures for pending distributed transactions.
All transactions automatically resolved by RECO are automatically removed from the pending transaction table. Additionally, all information about in-doubt transactions correctly resolved by an administrator (as checked when RECO reestablishes communication) are automatically removed from the pending transaction table. However, all rows resolved by an administrator that result in a mixed outcome across nodes remain in the pending transaction table of all involved nodes until they are manually deleted.
You can force the failure of a distributed transaction for the following reasons:
The following sections describes the features available and the steps necessary to perform such operations.
You can include comments in the COMMENT parameter of the COMMIT statement. To intentionally induce a failure during the two-phase commit phases of a distributed transaction, include the following comment in the COMMENT parameter:
COMMIT COMMENT 'ORA-2PC-CRASH-TEST-n';
where n is one of the following integers:
For example, the following statement returns the following messages if the local commit point strength is greater than the remote commit point strength and both nodes are updated:
COMMIT COMMENT 'ORA-2PC-CRASH-TEST-7'; ORA-02054: transaction 1.93.29 in-doubt ORA-02059: ORA-CRASH-TEST-7 in commit comment
At this point, the in-doubt distributed transaction appears in the DBA_2PC_PENDING view. If enabled, RECO automatically resolves the transaction.
The RECO background process of an Oracle8i instance automatically resolves failures involving distributed transactions. At exponentially growing time intervals, the RECO background process of a node attempts to recover the local portion of an in-doubt distributed transaction.
RECO can use an existing connection or establish a new connection to other nodes involved in the failed transaction. When a connection is established, RECO automatically resolves all in-doubt transactions. Rows corresponding to any resolved in-doubt transactions are automatically removed from each database's pending transaction table.
You can enable and disable RECO using the ALTER SYSTEM statement with the ENABLE/DISABLE DISTRIBUTED RECOVERY options. For example, you can temporarily disable RECO to force the failure of a two-phase commit and manually resolve the in-doubt transaction.
The following statement disables RECO:
ALTER SYSTEM DISABLE DISTRIBUTED RECOVERY;
Alternatively, the following statement enables RECO so that in-doubt transactions are automatically resolved:
ALTER SYSTEM ENABLE DISTRIBUTED RECOVERY;
An important restriction exists in Oracle's implementation of distributed read consistency. The problem arises because each system has its own SCN, which you can view as the database's internal timestamp. The Oracle database server uses the SCN to decide which version of data is returned from a query.
The SCNs in a distributed transaction are synchronized at the end of each remote SQL statement and at the start and end of each transaction. Between two nodes that have heavy traffic and especially distributed updates, the synchronization is frequent. Nevertheless, no practical way exists to keep SCNs in a distributed system absolutely synchronized: a window always exists in which one node may have an SCN that is somewhat in the past with respect to the SCN of another node.
Because of the SCN gap, you can execute a query that uses a slightly old snapshot, so that the most recent changes to the remote database are not seen. In accordance with read consistency, a query can therefore retrieve consistent, but out-of-date data. Note that all data retrieved by the query will be from the old SCN, so that if a locally executed update transaction updates two tables at a remote node, then data selected from both tables in the next remote access contain data prior to the update.
One consequence of the SCN gap is that two consecutive SELECT statements can retrieve different data even though no DML has been executed between the two statements. For example, you can issue an update statement and then commit the update on the remote database. When you issue a SELECT statement on a view based on this remote table, the view does not show the update to the row. The next time that you issue the SELECT statement, the update is present.
You can use the following techniques to ensure that the SCNs of the two machines are synchronized just before a query:
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