Chapter X Replication Internals

X.1. Introduction

X.2. Replication Tables

• A replication table occupies a larger space in table space, because the update timestamp and logical delete attributes for records of a replicate table are also kept.
• Updating a Replication Table is slower than updating a Normal Table, because there is a need to update two additional attributes for each record of a Replication Table.
• An index is created implicitly for any Replication Table.

Master Table

• Update-able

Replicate Table

• Read-Only

X.3. SQL Commands

• ALTER TABLE table_name CHANGE TABLE TYPE TO MASTER; Successful execution of this command inserts a line in the system table sys_rep_tables.
• ALTER TABLE table_name CHANGE TABLE TYPE TO NORMAL; Successful execution of this command removes the record corresponding to table_name from the system table sys_rep_tables of the database containing the master table.
• DROP TABLE table_name; Successful execution of this command removes a replication table table_name and all of the replication master entries defined for table_name. It also deletes all the records from system table sys_rep_masters related to table_name.
• CREATE [AND INSERT INTO] REPLICATE TABLE replicate_table FROM master_side_access; If this command terminates successfully, a replication entry row will be added to the system table sys_rep_masters in the database containing the replicate table.
• CREATE REPLICATION REPLICATE replicate_side_access ON replication_table; Successful execution of this command inserts an entry to the system table sys_rep_replicates having replicate_side_access as a replicate of master_table.
• DROP REPLICATION REPLICATE replicate_side_access ON replication_table; Successful execution of this command removes an entry from the system table sys_rep_replicates having table accessed by replicate_side_access as replicate table of replication_table. Execution of this command does not drop any database tables.
• CREATE REPLICATION MASTER master_side_access ON replicate_table; Successful execution of this command inserts a new entry into sys_rep_masters table.
• DROP REPLICATION MASTER master_side_access ON replicate_table; It deletes replicate_table entries from sys_rep_masters table in Replicate-Side having master_side_access as its Master-Side table.
• ALTER replicate_table ENABLE REPLICATION MASTER master_side_access ; Successful execution of these commands alters the repm_order field of sys_rep_masters system table related to master_side_access in the Replicate-Side.

Synchronization

• Defining or Undefining any Replication Table (Changing the type of tables)
• Creating or Dropping any Replication Table
• Any Replication Synchronization Request
• Any Successful Replication Synchronization
• Changing the Replication Master Order or Availability (Enabling or Disabling) of a Replication Master Table
• Adding or Dropping Replication Replicate Entries or Replication Master Entries. (These are Authorizing / Unauthorizing any candidate Replication Replicate Table or Claiming / Unclaiming any candidate Replication Master Table.)

The following system tables are added for Replication purpose:

1. sys_rep_tables
   One record is kept in this table for each Replication Table in the database.

   sys_rep_masters

   One record is kept in this table for any Replication Master Entry.

   sys_rep_replicates

   One record is kept in this table for any Replication Replicate Entry.

   * DISPLAY TABLE table_name ALL;

Note:

In the explanation (Meaning) of the attributes of system table sys_rep_tables, we refer to the specific Replication Table for which the record of this system table is kept, as "T".

System Table sys_rep_tables

Attribute	Data Type	Meaning
rep_tabnum	integer Not Null	Table number the replication table T
rep_tabtype	integer Not Null	Type of the replication table T (replicate or master)
rep_tab_condition	nlstext(32,0,64,1)	A string used for Subset Replication. This string is set only in records related to Replicate Tables that participate in Subset Replication.
rep_tab_timestamp	microtimestamp(0)	Standard time that a Replicate Table has the snapshot of the Master Table of the Replication World. This attribute is not meaningful for a Master Table. If a Replicate Table R1 synchronizes with a Master Table at timestamp TS1, rep_tab_timestamp value of the record kept related to that Replicate Table is TS1. Imagine that R1, has a Replication Replicate Table called R2, such that R2 synchronizes its information with R1 at timestamp TS2, where TS2>TS1. In this case, value of rep_tab_timestamp for the record related to R2, is TS1, not TS2. Because rep_tab_timestamp contains the snapshot time of the "Master Table" of the Replication World.
rep_recov_timestamp	microtimestamp(0)	Standard Recovery Time of the replication table. This timestamp is used to recover the more recent data of a Replicate Table to the data of its current Master Table. This value is used internally when during Master Table Switch. In this case the data of all the other Replicate Tables in that Replication World come to the state of the data of their current Master Table when doing synchronization.
rep_purge_timestamp	microtimestamp(0)	Standard Purge Time of the replication table. This timestamp is used for physically deleting (purging) the logically deleted records of Replication tables. This value is used internally, and equals to the minimum of Table Timestamp (rep_tab_timestamp) of the directly related Replication Replicate tables of T. Whenever the Record Timestamp (EMPRESS_TIMESTAMP) of a logically deleted record is less than or equal to Purge Timestamp of T, then the logically "deleted" record can be purged.
rep_orig_mashost	character(32,1) Not Null	Host Identifier of the original master of T. This is the IP address of the host.
rep_orig_masdb	nlstext(32,32,32,1) Not Null	Physical name and path to the database of the original master of T. If the table itself is the original master table, this value shows its own database name.
rep_orig_mastab	nlscharacter(32,1,0) Not Null	Table Name of the original master of T. If the table itself is the original master table, this value shows its own table name.
rep_orig_mas_start_timestamp	microtimestamp(0) Not Null	Standard time that the original master table of this replication table is defined as Master. This timestamp never changes, and is constant during the life of a replication world.
rep_orig_mas_hash_value	longinteger Not Null	A hash value that simplifies detection of the members of a specific replication world. If this hash value is different from an expected value for a replication world, Empress knows that T does not belong to that specific replication world. Note that values of attributes that their name start with "rep_orig_mas" show Original Master Access information and Start Timestamp. These values are constant for T, even when Master Table Switch occurs, or if the table accessed by that access information does not exist anymore.
rep_cur_mas_start_timestamp	microtimestamp(0) Not Null	Standard time that the Current Master Table T is defined. This value changes as the result of a Master Table Switch.
rep_world	bulk(20,0,1024,1)	Contains information about the elements of the replication world that this replication table belongs to.

C.4.2. Table sys_rep_masters

Notes:

• In the Explanations, when talking about a specific record of system table sys_rep_masters, we refer to the replication table which claims to have the Replication Master Table, as T, and refer to the Replication Master Table accessed by the information in the record as RMT.
• For each record of sys_rep_masters, a combination of values in repm_massv, repm_masdb and repm_mastab forms Master-Side Access Information, which is the required information to access RMT.

System table sys_rep_masters

Attribute	Data Type	Meaning
repm_tabnum	integer Not Null	Table number of the replication Table T
repm_massv	nlscharacter(32,1) Not Null	Name of the Replication Master Server serving RMT (This attribute is 1st part of the three-part Master-Side Access Information)
repm_masdb	nlstext(32,32,32,1) Not Null	Logical or physical name of database containing RMT (This attribute is the 2nd part of the three-part Master-Side Access Information)
repm_mastab	nlscharacter(32,1) Not Null	Table name of RMT. (This attribute is the 3rd part of the three-part Master-Side Access Information).
repm_order	double precision Not Null	Order number. This is the Replication Master Order for RMT. Smaller order number means RMT will be considered first for becoming the "chosen" Replication Master Table.
repm_enabled	character(1,1) Not Null	Shows whether RMT is enabled or not.
repm_pulled_success_timestamp	microtimestamp(0)	Standard termination time of the latest successful (synchronization) request from RMT. This value is updated upon requests that are successfully terminated.
repm_pulled_timestamp	microtimestamp(0)	Standard starting time of a (synchronization) request made to RMT. The value of this attribute is updated even if the request made to RMT is not successfully terminated.
repm_pulled_status	nlstext(32,64,64,1)	The status of the latest (synchronization) request made to RMT. If the value of this attribute shows successful termination of a request, then repm_pulled_success_timestamp shows the termination timestamp of this successful request.
repm_pushed_success_timestamp	microtimestamp(0)	reserved value
repm_pushed_timestamp	microtimestamp(0)	reserved value
repm_pushed_status	nlstext (32,64,64,1)	reserved value

C.4.3. Table sys_rep_replicates

Notes:

• In the Explanations, when talking about a specific record of system table sys_rep_replicates, we refer to the replication table which authorizes a Replication Replicate Table, as T, and refer to the Replication Replicate Table accessed by the information in the record as RRT.
• For each record of sys_rep_replicates, a combination of values in repr_host, repr_repdb and repr_reptab forms Replicate-Side Access Information, which is the required information to access RRT.

System Table sys_rep_replicates

Attribute	Data type	Meaning
repr_tabnum	integer Not Null	Table number of the replication table T.
repr_rephost	character(32,1) Not Null	Name of the Host containing RRT. This is the IP address of the host. (This attribute is the 1st part of the three-part Replicate-Side Access Information)
repr_repdb	nlstext(32,32,32,1) Not Null	Name of the database containing RRT. (This attribute is the 2nd part of the three-part Replicate-Side Access Information)
repr_reptab	nlscharacter(32,1) Not Null	Table name of RRT. (This attribute is the 3rd part of the three-part Replicate-Side Access Information)
repr_condition	nlstext(32,0,64,1)	Used for Subset Replication. Contains the Condition (Restricted Subset WHERE Clause) set by T.
repr_cond_comp	bulk(20,512,512,1)	Used for Subset Replication.
repr_repptab	nlstext(32,32,32,1)	Physical RRT access information. This is the physical, resolved information to access RRT, which might be addressed by its logical path in repr_rephost, repr_repdb and repr_reptab.
repr_rephval	longinteger Not Null	Hash value of repr_repptab.
repr_pulled_success_timestamp	microtimestamp(0)	Standard termination time of the latest successful request made from RRT. The value of this attribute is updated upon requests that are successfully terminated.
repr_pulled_timestamp	microtimestamp(0)	Standard starting time of the request made from RRT. The value of this attribute is updated even if the request made from RRT is not successfully terminated.
repr_pulled_status	nlstext(32,64,64,1)	The status of the latest request from RRT. If the value of this attribute shows successful termination of a request, then repr_pulled_success_timestamp shows the termination timestamp of this successful request.
repr_repsv	nlscharacter(32,1)	reserved value
repr_repsv_enabled	character(1,1)	reserved value
repr_pushed_success_timestamp	microtimestamp(0)	reserved value
repr_pushed_timestamp	microtimestamp(0)	reserved value
repr_pushed_status	nlstext(32,64,64,1)	reserved value

Recovery