Create a Virtuoso RDF Graph Replication slave Subscriber node from a master Publisher node.

What

Why

If the master Publisher node is known to contain the current state of the database, Subscriber nodes can be created from a copy of it such that they start from this known good state and keep in sync from that point forward. This could be in the cases of a subscriber failing and needing to be rebuilt or if a new Subscriber node needs to be added the Farm (note this could also be done from a copy of a subscriber node if know to be in sync with the master Publisher).

How

Stop/disable all updates to the publisher so its state remains constant when setting up the new subscriber(s)
Run a checkpoint to commit any pending updates in the trx file to the database (db) file
On each subscriber node if database files already exist then remove the following to ensure no remnants of the older subscriber node database exists:
```
   rm virtuoso.db virtuoso.trx virtuoso.log virtuoso.pxa 
```
Copy the publisher database file "virtuoso.db" to the subscriber node(s) using rsync or prefer copy method

Start the copy of the publisher database and do the following to remove the information store in the database of it being a publisher:


   delete from sys_repl_accounts; 
   delete from sys_repl_subscribers; 
   registry_remove ('DB.DBA.RDF_REPL'); 
   shutdown();

Restart the database with virtuoso-start.sh and run the following command to set it up as a subscriber node:


   repl_server ('MASTER', 'MASTER_DSN');  
   repl_subscribe ('MASTER', '__rdf_repl', 'dav', 'dav', 'dba', 'dba');  
   shutdown();

Restart the database again with virtuoso-start.sh and run the following command to continue setting it up as a subscriber node
```
   repl_sync_all ();  
   DB.DBA.SUB_SCHEDULE ('MASTER', '__rdf_repl', 1); 
```
Run the following status commands to check it is in sync with the MASTER publisher:
```
   status(); 
   repl_stat(); 
```

Trouble Shooting

There maybe occasions when Subscriber nodes are failing to sync with the master Publisher with errors of the following form being reported:

The Subscriber remains constantly in the replication state of "SYNCING" with the master Publisher as reported by the output of running the rep_stat() command on it:


SQL> repl_stat(); 
server   account  level       stat 
VARCHAR  VARCHAR  INTEGER     INTEGER 
_______________________________________________________________________________ 

MASTER   __rdf_repl  939         SYNCING 
SLAVE-1  SLAVE-1      0           OFF  

SQL>

The master Publisher report a __rdf_repl' is not valid ... error in the database log file:
```
14:19:20 Subscriber 'SLAVE-1' for '__rdf_repl' is not valid (level 2, requested level 662)
```
As can be seen from the errors above the master Publisher is at replication level 2, but the Subscriber is requesting a sync from replication level 662, which is clearly invalid, as the Subscriber should always be one replication level less than the master Publisher i.e. "1" in this case.

Thus the replication level on the master Publisher needs to be verified and the Subscriber set to be one less than it using the database sequence_set() function as follows:

On the publisher determine the replication server name with the following command:


SQL> select repl_this_server(); 
repl_this_server 
VARCHAR 
_______________________________________________________________________________ 

MASTER 

1 Rows. -- 1 msec. 
SQL>

On the publisher determine the "internal" replication server name to pass to the set_sequence function with the command:


SQL> select concat ('repl_', repl_this_server(), '_', '__rdf_repl'); 
concat 
VARCHAR 
_______________________________________________________________________________ 

repl_MASTER___rdf_repl 

1 Rows. -- 1 msec. 
SQL>

Check the publisher replication level with the command:


SQL> select sequence_set ('repl_MASTER___rdf_repl', 0, 2); 
sequence_set 
VARCHAR 
_______________________________________________________________________________ 

1685 

1 Rows. -- 0 msec. 
SQL>

Switch to the subscriber and check the publisher replication level store:


SQL> select sequence_set ('repl_MASTER___rdf_repl', 0, 2); 
sequence_set 
VARCHAR 
_______________________________________________________________________________ 

939 

1 Rows. -- 0 msec. 
SQL>

Set the publisher replication level on the subscriber to be one less than its level on the publisher itself with command:
```
SQL> sequence_set ('repl_MASTER___rdf_repl', 1684, 0); 

Done. -- 0 msec. 

SQL> 
```

Check the publisher replication level on the subscriber is one less than on the publisher:


SQL> select sequence_set ('repl_MASTER___rdf_repl', 0, 2); 
sequence_set 
VARCHAR 
_______________________________________________________________________________ 

1684 

1 Rows. -- 0 msec. 
SQL>

Back on the master Publisher, check Subscriber replication status is valid by ensuring its RS_VALID column of the SYS_REPL_SUBSCRIBERS table is set "1" and the RS_LEVEL column is set to one less than the replication level of the subscriber:


SQL>   select * from SYS_REPL_SUBSCRIBERS; 
RS_SERVER          RS_ACCOUNT         RS_SUBSCRIBER     RS_LEVEL    RS_VALID 
VARCHAR NOT NULL   VARCHAR NOT NULL   VARCHAR NOT NULL  INTEGER     INTEGER 
_______________________________________________________________________________ 

MASTER             __rdf_repl         SLAVE-1           939         0 

1 Rows. -- 49 msec. 

SQL>

If as above the RS_VALID is "0" then set it to "1" and set the RS_LEVEL column to one less than the replication level of the master Publisher as follows to activate replication from the slave subscriber node:


SQL> update SYS_REPL_SUBSCRIBERS set RS_LEVEL = 1684 , RS_VALID = 1 ;

0 Rows. -- 2 msec.
SQL>
SQL> select * from SYS_REPL_SUBSCRIBERS; 
RS_SERVER          RS_ACCOUNT         RS_SUBSCRIBER     RS_LEVEL    RS_VALID 
VARCHAR NOT NULL   VARCHAR NOT NULL   VARCHAR NOT NULL  INTEGER     INTEGER 
_______________________________________________________________________________ 

MASTER             __rdf_repl         SLAVE-1           1684        1 

1 Rows. -- 49 msec. 

SQL>

Then restart master Publisher and the subscriber for the settings to take effect, and run the following on each subscriber to force them to sync:
```
    repl_sync_all (); 
```
Finally check the Subscriber node(s) is now IN SYNC and that there are no occurrences of the __rdf_repl' is not valid ... errors in the master Publisher database log.