Database  on  other  servers  can  be updated and those
transactions can be grouped together with others in a
logical unit.
Database uses a two phase commit.

We  must be able to create a after row trigger on table
(i.e., it should be not be already available )

After giving table privileges.

We  cannot  specify  snapshot  log name because oracle uses
the name of the master  table in the name of the database
objects that support its snapshot log.

The master table name should be less than or equal to 23
characters.

(The  table  name created will be MLOGS_tablename, and
trigger name will be TLOGS name).

It  is a table that maintains a record of modifications to
the master table in  a  snapshot.  It is stored  in the same
database as master table and is only  available for simple
snapshots.  It should be created before creating snapshots.

COMPLETE   -   Tables   are  completly  regenerated  using 
the  snapshot's query and the master tables every time the
snapshot referenced.

FAST  - If simple snapshot used then a snapshot log can be
used to send the changes to  the snapshot tables.

FORCE  -  Default value. If possible it performs a  FAST
refresh; Otherwise it will perform a complete refresh.

Time the references to occur when master tables are not in use.
 Peform the reference the manually immdiately locking the
master tables. We  can join  tables in snopshots by creating
a complex snapshots that will based on the master tables.

Updating or Inserting records in remote database through
database triggers. It may fail if remote database is having
any problem

-  A  simple snapshot is based on a query that does not
contains GROUP BY clauses, CONNECT BY clauses, JOINs,
sub-query or snashot of operations.
     - A complex snapshots contain atleast any one of the above.

Simple and Complex.

Snapshots  are  read-only copies of a master table located
on a remote node which  is  periodically  refreshed  to 
reflect  changes made to the master table.

Replication of data in distributed enviroonment.
Using snapshots to replicate data.
Using remote procedure calls.

To perform one of these function on an index,table, or cluster:

     -  to  collect statisties about object used by the
optimizer and store them in the data           dictionary.
     -  to  delete statistics about the object used by
object from the data dictionary.
     - to validate the structure of the object.
     - to identify migrated and chained rows of the table or
cluster.

By setting the following values in init.ora file.

LOG_ARCHIVE_FORMAT = arch %S/s/T/tarc (%S - Log sequence
number and is zero left  paded,  %s  -  Log  sequence 
number  not  padded. %T - Thread number lef-zero-paded  and
 %t -  Thread number not padded). The file name created is
arch 0001 are if %S is used.
LOG_ARCHIVE_DEST = path.

Shut the database
     Backup the database
     Modify/Include LOG_ARCHIVE_START_TRUE in init.ora file.
     Start up the databse.

1. shutdown the database with normal/emmidiate/transactional
2. modify/include the parameter log_archive_start=true,
log_archive_dest= (location to which the redo log file 
group is archived), log_archive_format=(in which format you 
want to apper the archive log file)
3. start up the database

To check database is in which mode

1. Conn sys/sys as sysdba
2. select log_mode from v$database

To set database in Archivelog mode

1. shutdown immediate;
2. startup mount;
3. alter database archivelog;
4. alter database open;

To set database in NoArchivelog Mode
1. shutdown immediate;
2. startup mount;
3. alter database noarchivelog;
4. alter database open;

If a user wants to give some privileges to 
another user or a group of users then they will create role 
and they give this role to the users.let the privileges 
be 'insert' and 'update' on 'emp' table then instead of 
specifying them by their names they will create role which 
inturn includes these methods. this role is further given 
to the users.
            If in futher if he wants to remove some 
privilege given to the users, he can just alter the role 
and remove that particular privilege such as he can remove 
update from this created role..
           Later the users cant do updations on that emp 
table.but still can insert the values into that table.
          Hence Roles are the easiest way to grant and 
manage common privileges needed by different groups of 
database users.Creating roles and assign each role to group 
of users will simplify the job of assigning privileges to 
individual users.

roles are a set of privilages that are been set to each 
user profiles. v can create a default role and use the same 
to a number of users.