1. Oracle Globally Distributed Database
  2. Troubleshooting
  3. Troubleshooting Tips

Troubleshooting Tips

Use these tips to discover information about the Globally Distributed Database that you need to help you troubleshoot issues.

Pre-Deployment Network Validation

Several GDSCTL commands have a -validate_network option to detect network configuration issues as early as possible during the specification and deployment of distributed databases.

The -validate_network can be used in following GDSCTL commands for distributed databases:

  • add {invitednode | invitedsubnet}

  • add shard

  • deploy

  • start gsm

  • validate (also includes -show_errors)

Checking the Data Distribution Method

Run gdsctl config sdb to check which data distribution (sharding) method is used in the distributed database.

The data distribution method can be system-managed, composite, user-defined, directiry-based, or federated.

The distribution method is shown under "Shard type" in the output of gdsctl config sdb as shown here. 

gdsctl> config sdb
 
 
GDS Pool administrators
------------------------
 
Replication Type
------------------------
Data Guard
 
Shard type
------------------------
System-managed 
 
Shard spaces
------------------------
shd1
 
Services
------------------------
srv1

Checking the Replication Type

Run gdsctl config sdb to check which method is used for shard replication in the distributed database.

The replication type is shown under "Replication Type" in the output of gdsctl config sdb as shown here. 

gdsctl> config sdb
 
 
GDS Pool administrators
------------------------
 
Replication Type
------------------------
Data Guard
 
Shard type
------------------------
System-managed 
 
Shard spaces
------------------------
shd1
 
Services
------------------------
srv1

Table 19-1 Replication types in config sdb output

Replication Type Value Shown in Output
Oracle Data Guard Data Guard
Raft

Checking the Oracle Data Guard Protection Mode

You can run gdsctl config shardspace on a given shardspace to check the Oracle Data Guard protection mode in your GDSCTL session, rather than switching to DGMGRL.

Data Guard can be configured in three different protection modes: MaxProtection, MaxAvailability, and MaxPerformance.

The Data Guard protection mode is shown under PROTECTION MODE in the gdsctl config shardspace command output, as shown here. 

GDSCTL> config shardspace -shardspace shd1
Shard Group                   Region                        Role
-----------                   ------                        ----
dbs1                          east                          Primary
 
PROTECTION_MODE               Chunks
---------------               ------
MaxProtection                 6

Checking Which Shards Are Mapped to a Key

You can run gdsctl config chunks -key to check which shards are mapped to a sharding key.

Example 1: Single Table Family

In the following example, there is only one table family in the distributed database configuration, and the table is partitioned (sharded) on data type number.

In this example, the user is checking which chunk sharding key value "2" is mapped to. In the output it shows sharding key 2 is mapped to chunk "3" and is present in the database "aime1b".

GDSCTL> config chunks -key 2
Range Definition
------------------------
Chunks    Range Definition
------    ----------------
3         1431655764-2147483646
 
Databases
------------------------
aime1b

Similarly, this can be done for any data type sharding is done on. Also, a multiple column sharding key can be checked with comma separated values.

The range definition is the range of hash values and can be ignored.

Example 2: Multiple Table Families

In a multiple table family configuration, add the option -table_family to specify the table family to which the specified sharding key belongs.

The config chunks command lists shards from all shardgroups in the topology. This example also lists a Data Guard standby shardgroup, as shown by the addition of "aime1e" to the Databases (shards) list. 

GDSCTL> config chunks -key 1 -table_family testuserfam3.customersfam1
 
Range Definition
------------------------
Chunks    Range Definition
------    ----------------
1         0-357913941
 
Databases
------------------------
aime1b    
aime1e

Example 3: Specifying a Multiple Column Sharding Key

When a table is sharded by multiple columns, specify the sharding key value as a comma-separated list as shown here.

GDSCTL> config chunks -key 10,mary,2010-04-04
 
Range Definition
------------------------
Chunks    Range Definition
------    ----------------
4         1288490187-1717986916
 
Databases
------------------------
aime1b    
aime1e

Checking Shard Operation Mode (Read-Only or Read-Write)

You can check whether shards are running in read-only or read-write mode by running gdsctl config chunks -cross_shard.

The gdsctl config chunks -cross_shard command output shows which shards, listed under "Database", are running in read-only and read-write modes, as shown below. The command also lists the chunk ranges on those shards. 

gdsctl config chunks -cross_shard

Read-Only cross shard targets
------------------------
Database                      From To
--------                      ---- --
tst3b_cdb2_pdb1               1    3
tst3c_cdb3_pdb1               9    10
tst3d_cdb2_pdb1               4    5
tst3e_cdb3_pdb1               6    8
 
Chunks not offered for cross-shard
------------------------
Shard space    From To
-----------    ---- --
 
Read-Write cross-shard targets
------------------------
Database                      From To
--------                      ---- --
tst3b_cdb2_pdb1               1    5
tst3c_cdb3_pdb1               6    10
 
Chunks not offered for Read-Write cross-shard activity
------------------------
Data N/A

Checking DDL Text

Run gdsctl show ddl -ddl ddl_id to get the text for the specified DDL.

The DDL numeric identifier is specified with -ddl ddl_id to get the text and other details of a particular DDL, as shown here. 

gdsctl show ddl -ddl 5

DDL Text: CREATE SHARDED TABLE Customers ( CustNo NUMBER NOT NULL, Name VARCHAR2(50), Address VARCHAR2(250), Location VARCHAR2(20), Class VARCHAR2(3), CONSTRAINT RootPK PRIMARY KEY(CustNo)) PARTITION BY CONSISTENT HASH (CustNo) PARTITIONS AUTO TABLESPACE SET ts1
Owner: TESTUSER1
Object name: CUSTOMERS
DDL type: C
Obsolete: 0
Failed shards:

Note:

The show ddl command output might be truncated. You can run SELECT ddl_text FROM gsmadmin_internal.ddl_requests on the shard catalog to see the full text of the statements.

Checking Chunk Migration Status

Run gdsctl config chunks -show_reshard to check the status of chunk migration.

A chunk move is a long running operation, whether user-initiated or internal (during incremental deploy), so if you need to check the status, the gdsctl config chunks -show_reshard provides the following status indicators as the move progresses.

  • empty - indicates no chunk migration in progress

  • scheduled - chunk is pending movement, which could be because it is waiting on another chunk move to complete, or the move didn't initiate due to some error

  • running - current in progress

  • failed - chunk move failed. Check GSM traces and source and target database traces for details.

In the following example, chunk move status is shown in the "Ongoing chunk movement" table in the command output.

gdsctl config chunks -show_reshard

Chunks
------------------------
Database                      From      To
--------                      ----      --
tst3b_cdb2_pdb1               1         6
tst3c_cdb3_pdb1               7         10
tst3d_cdb2_pdb1               1         6
tst3e_cdb3_pdb1               7         10
 
Ongoing chunk movement
------------------------
Chunk     Source                        Target                        status
-----     ------                        ------                        ------
7         tst3c_cdb3_pdb1               tst3b_cdb2_pdb1               Running
8         tst3c_cdb3_pdb1               tst3b_cdb2_pdb1               scheduled
9         tst3c_cdb3_pdb1               tst3b_cdb2_pdb1               scheduled
10        tst3c_cdb3_pdb1               tst3b_cdb2_pdb1               scheduled

Checking Table Type (Sharded or Duplicated)

You can check whether tables are sharded or duplicated in dba/all/user_tables using SELECT TABLE_NAME,SHARDED,DUPLICATED FROM user_tables;.

In the following example, column "S" indicates whether a table is sharded, and column "D" indicates whether a table is duplicated.

SQL> select TABLE_NAME,SHARDED,DUPLICATED from user_tables;
 
TABLE_NAME      S D
--------------- - -
CUSTOMERS       Y N
DUP1            N Y
LINEITEMS       Y N
MLOG$_DUP1      N N
ORDERS          Y N

Checking User Type (Local or ALL_SHARD)

You can find out which users are created as local users and which are distributed database users by selecting the username and ALL_SHARD column in dba/all/user_users. 

SQL>  select USERNAME,ALL_SHARD from users_users where username='TESTUSER1';
 
USERNAME        ALL_SHARD
--------------- ---------
TESTUSER1       YES

Identifying Tables Created as Sharded Tablespaces

You can find out whether tablespaces are used for a sharded table by selecting the TABLESPACE_NAME and CHUNK_TABLESPACE columns in dba/all/user_tablespaces.

The value in the CHUNK_TABLESPACE column is Y in dba/all/user_tablespaces if it is a tablespace for a sharded table.

SQL> select TABLESPACE_NAME,CHUNK_TABLESPACE from user_tablespaces;
 
TABLESPACE_NAME                C
------------------------------ -
SYSTEM                         N
SYSAUX                         N
TEMP                           N
SYSEXT                         N
TS1                            Y

Checking if Shard DDL is Enabled or Disabled

You can check if Shard DDL is enabled or disabled in the current SQL session.

These examples show the result of checking Shard DDL status after enabling and disabling Shard DDL.

SQL> alter session enable shard ddl;
 
Session altered.
 
SQL> select shard_ddl_status from v$session where AUDSID = userenv('SESSIONID');
 
SHARD_DD
--------
ENABLED
SQL> alter session disable shard ddl;
 
Session altered.
 
SQL> select shard_ddl_status from v$session where AUDSID = userenv('SESSIONID');
 
SHARD_DD
--------
DISABLED

Filtering Data by Sharding Key

You can set the SHARD_QUERIES_RESTRICTED_BY_KEY parameter to enable or disable data filtering by a specified sharding key.

The parameter SHARD_QUERIES_RESTRICTED_BY_KEY can be set with ALTER at the system or session level. If enabled, DMLs will only display select data for specified SHARDING_KEY set in the client connection.

In the following example, the client connection is established with a shard with SHARDING_KEY specified as "1". However, when the client runs a SELECT on the customers table, all of the rows in that table in the shard are displayed. 

connection established for client with sharding_key=1

SQL> select * from customers order by custno;
 
    CUSTNO NAME       ADDRESS    LOCATION   CLA
---------- ---------- ---------- ---------- ---
         1 John       Oracle KM  Bangalore  A
        50 Larry      Oracle HQ  SFO        B
 
2 rows selected.
 
SQL>

Now, as shown below, we enable session level filtering, and the result of the same SELECT statement is restricted to only the single row that matches the SHARD_KEY specified in the client connection. 

SQL> alter session set shard_queries_restricted_by_key = true;
 
Session altered.
 
SQL> select current_shard_key from dual;
 
CURRENT_SHARD_KEY
-----------------
                1
 
1 row selected.
 
SQL> select * from customers;
 
    CUSTNO NAME       ADDRESS    LOCATION   CLA
---------- ---------- ---------- ---------- ---
         1 John       Oracle KM  Bangalore  A