Streams – Part II

Two weeks after the last streams post, and I need to give some updates and corrections.

1) I’ve written that:

“And you should also check which archived logs can be deleted:
SELECT r.CONSUMER_NAME,
r.NAME,
r.FIRST_SCN,
r.NEXT_SCN,
r.PURGEABLE
FROM DBA_REGISTERED_ARCHIVED_LOG r, DBA_CAPTURE c
WHERE r.CONSUMER_NAME = c.CAPTURE_NAME;"

Actually, that is not a good way to check for which archive logs can be deleted. According to Metalink note 373066.1 RMAN use dba_capture.required_checkpoint_scn to determine which logs can be purged. So I suggest to use this when doing managed backups as well:
SELECT r.CONSUMER_NAME,
r.NAME,
r.FIRST_SCN,
r.NEXT_SCN,
case when (r.next_scn > c.required_checkpoint_scn) then 'NO' else 'YES' end purgable
FROM DBA_REGISTERED_ARCHIVED_LOG r, DBA_CAPTURE c
WHERE r.CONSUMER_NAME = c.CAPTURE_NAME;

2) I’ve made some severe mistakes when trying to extend the streams implementation by replicating few more tables. I decided to start from scratch by running: dbms_streams_admin.remove_streams_configuration.

This does not work. The solution was found in Morgan’s Library:
-- the following provided by Michael M. Brennan from RSA Security who
-- reports that REMOVE_STREAMS_CONFIGURATION does not work unless the
-- propagation has been dropped first.
SELECT propagation_name FROM dba_propagation;
exec dbms_propagation_adm.drop_propagation('PROPAGATION$_143');
exec dbms_streams_adm.remove_streams_configuration;

3) Unfortunately, when I tried to start a new capture process, I find myself with the following unexpected problem:
SQL> select required_checkpoint_scn from dba_capture;

REQUIRED_CHECKPOINT_SCN
-----------------------
0

Not good. This means that my capture process is looking at the archive logs for scn 0. I have no control over required_checkpoint_scn and have no clue how this happened. Oracle support is still researching that.

But this means that I can refer you to two metalink articles that can help with debug (or at least with opening SRs on streams related issues):

Note 313279.1 “Capture Process Looping In Initializing/Dictionary Initialization State” shows how to start a trace against the capture process and how to interpret the result trace file.
Note 273674.1 “Streams Configuration Report and Health Check Script” contains cute scripts that summarize all the the streams configuration into a nice HTML page. I didn’t find the script very useful for my debugging, but it is absolutely mandatory when opening a streams related SR. They will not talk to you before you sent them the result of the script, and they’ll ask you to run it again whenever they run out of ideas on what to say next.

Oracle Streams Replication Example

I promised to show off our streams implementation, because I found so few examples out there. Keep in mind that your streams implementation will probably look nothing like mine at all, and even if it will look like mine, you can get the same results in very different way. This example is not an excuse to avoid reading the docs.

Our goal: Set up a reporting server, with replication of schemas arriving from many source DBs. We also want to filter out and not replicate some of the less critical tables.

Decisions, decisions: When implementing streams there are several things you want to ask yourself: Do you want bi-directional replications? Do you want to capture the changes on the source DB, destination DB or a third server? Do you do the instantiation (first copy of data) with DataPump or RMan?

We went with one-directional replication, capturing will be done on the destination server, and we’ll use DataPump to initially move the schemas to the destination.

Create users: You want to create a user both on source and destination server that will own all the streams related objects. Also, you want to give him his own tablespace (or at least keep his objects away from SYSTEM tablespace) because in case of errors lots of data can accumulate in stream queues. The docs recommend making the tablespace autoextend, but I’m very much against it. After creating the strmadmin user, the rest of my instructions should be run from this user.

CREATE TABLESPACE streams_tbs DATAFILE '/data/oradata/DB05/streams_tbs.dbf' SIZE 25M;

CREATE USER strmadmin
IDENTIFIED BY strmadminpw
DEFAULT TABLESPACE streams_tbs
QUOTA UNLIMITED ON streams_tbs;

GRANT DBA TO strmadmin;

BEGIN
DBMS_STREAMS_AUTH.GRANT_ADMIN_PRIVILEGE(
grantee => 'strmadmin',
grant_privileges => true);
END;

DBLinks: You will need a DBLink from source to destination DB. For convenience, I created a DBLink from destination to source which I’ll use during the configuration of Streams and will drop afterwards. Of course, the servers should be able to tnsping each other before this will work.


--on source
CONNECT strmadmin/strmadminpw;
CREATE DATABASE LINK REP01 CONNECT TO strmadmin IDENTIFIED BY strmadminpw
USING 'REP01';
select count(*) from dba_tables@REP01;

-- on target
CONNECT strmadmin/strmadminpw;
CREATE DATABASE LINK DB05 CONNECT TO strmadmin IDENTIFIED BY strmadminpw
USING 'DB05';
select count(*) from dba_tables@DB05;

Directory Objects: I decided to use Datapump for instantiation and to use DBMS_STREAMS to create a configuration scripts. This means I needed two directory objects on destination (for the script and Datapump) and one of the source (just Datapump). I used existing directories, so I had no need to create them, but in general you create a directory like so:

create directory EXP_DP_VOL as '/exp_dp';

Copying Archive redo logs: You need to get the archived redo logs from the source server to the server doing the capturing (destination server in my case, but can be a 3rd server). There are two decisions to make here: do you use ARCH or LGWR to write the log files to the remote location? And will the new archiving destination be MANDATORY or OPTIONAL (i.e. does the writing have to succeed before the corresponding redo file at the source can be overwritten)?
Also, when specifying the destination directory, you want it to be a separate directory from where REP01 redo logs are archived, and make extra sure that the directory really exists as you wrote it.
I specified a non-existent directory, this resulted in errors on the alert log about failure to archive (scary!), and after few retries the archiver stopped attempting to write to the new destination. So, after creating the directory I had to reset the log_archive_dest_2 parameter (i.e. set it to ” and then change it back again).

On the source server, we run:

ALTER SYSTEM set LOG_ARCHIVE_DEST_2='SERVICE=REP01 ARCH OPTIONAL NOREGISTER TEMPLATE=/u10/oradata/DB05_arch/DB05_arch_%t_%s_%r.redo';
ALTER SYSTEM set LOG_ARCHIVE_STATE_2=ENABLE;

Create instantiation and configuration script: Since we do replication of (almost) entire schemas, we could use DBMS_STREAMS_ADM.MAINTAIN_SCHEMAS to do both the instantiation and the configuration of Streams.
I used the option that creates a script with the instructions (instead of run everything). I had many problems with the script and ended up using only parts of it, and other parts doing in different ways. I’m not sure if it means that using a script is a bad idea (since I had so many problems) or a good idea (since the same problems would be more annoying if I did not have the script). To create the script, I ran the following command on the destination database. If you are using a 3rd server for capture, run it on the 3rd server:

BEGIN
DBMS_STREAMS_ADM.MAINTAIN_SCHEMAS(
schema_names => 'DEV110',
source_directory_object => 'EXPORTS_DIR',
destination_directory_object => 'EXP_DP_VOL',
source_database => 'DB05',
destination_database => 'REP01',
perform_actions => false,
script_name => 'src_pre_inst_streams.sql',
script_directory_object => 'EXP_DP_VOL',
dump_file_name => 'dev110.dmp',
capture_queue_table => 'rep_capture_queue_table',
capture_queue_name => 'rep_capture_queue',
capture_queue_user => NULL,
apply_queue_table => 'rep_dest_queue_table',
apply_queue_name => 'rep_dest_queue',
apply_queue_user => NULL,
capture_name => 'capture_110dev',
propagation_name => 'prop_110dev',
apply_name => 'apply_110dev',
log_file => 'export_110dev.clg',
include_ddl => true,
instantiation => DBMS_STREAMS_ADM.INSTANTIATION_SCHEMA);
END;

Running the result script: Try running the script you just created on the same DB where you ran the procedure to create it. Note that it asks you for connection details of three sites. The first should be source, the second the destination, and the third can be garbage as it is not used in the script at all.

If the stars are aligned right, maybe you run this script to the finish and now have a working schema replication. In my case, it ran fine until the Datapump part, and created all the capture processes and queues. However, the Datapump export failed with very unusual errors. I spent two days trying to get it to work, and then given up and simply used Datapump manually to export the schema from source and import into destination. After that I continued running the rest of the script, and things were fine.

Fun and games: Now that you have your own schema replication. It is time to enjoy it. Create new tables, create as select, update data, insert new data, delete data, drop tables, create procedures and types. Everything replicates!

Just remember that if you don’t commit, it won’t replicate. Reasons should be obvious. Also, if you are testing on low-volume system, you probably want to alter system switch logfile every few minutes so you can see your changes faster.

Watch the replication process: While you are replicating changes, you probably want to watch how they move from source to destination:

You can see when the last message was read from redo archive:
SELECT CAPTURE_NAME,
LOGMINER_ID,
AVAILABLE_MESSAGE_NUMBER,
TO_CHAR(AVAILABLE_MESSAGE_CREATE_TIME, 'HH24:MI:SS MM/DD/YY')
AVAILABLE_MESSAGE_CREATE_TIME
FROM V$STREAMS_CAPTURE;

You can see when the changes were entered into the capture queue:
SELECT CAPTURE_NAME,
(ENQUEUE_TIME-ENQUEUE_MESSAGE_CREATE_TIME)*86400 LATENCY_SECONDS,
TO_CHAR(ENQUEUE_MESSAGE_CREATE_TIME, 'HH24:MI:SS MM/DD/YY') CREATE_TIME,
TO_CHAR(ENQUEUE_TIME, 'HH24:MI:SS MM/DD/YY') ENQUEUE_TIME,
ENQUEUE_MESSAGE_NUMBER
FROM V$STREAMS_CAPTURE;

And you can see when the apply process started working on them:
SELECT APPLY_NAME,
(DEQUEUE_TIME-DEQUEUED_MESSAGE_CREATE_TIME)*86400 LATENCY,
TO_CHAR(DEQUEUED_MESSAGE_CREATE_TIME,'HH24:MI:SS MM/DD/YY') CREATION,
TO_CHAR(DEQUEUE_TIME,'HH24:MI:SS MM/DD/YY') LAST_DEQUEUE,
DEQUEUED_MESSAGE_NUMBER
FROM V$STREAMS_APPLY_READER;

Another nice thing to try is to generate errors. For example, you can delete a table from the destination, make a change to it in the source, and see how it doesn’t replicate. The error will be found here:
SELECT APPLY_NAME,
SOURCE_DATABASE,
LOCAL_TRANSACTION_ID,
ERROR_NUMBER,
ERROR_MESSAGE,
MESSAGE_COUNT
FROM DBA_APPLY_ERROR;

And you should also check which archived logs can be deleted:
SELECT r.CONSUMER_NAME,
r.NAME,
r.FIRST_SCN,
r.NEXT_SCN,
r.PURGEABLE
FROM DBA_REGISTERED_ARCHIVED_LOG r, DBA_CAPTURE c
WHERE r.CONSUMER_NAME = c.CAPTURE_NAME;

Excluding Tables:
The last thing I did was filter out the tables I did not need to replicate. Those are tables the application uses as queues or temps – tons of inserts and deletes, but the data is useless for reports.
I used the DBMS_STREAMS_ADM.ADD_TABLE_RULES procedure. I selected to do the filtering during the capture (you can also do it during apply).
The important bits: table_name MUST contain the schema name in it. Otherwise the procedure will assume that the table belongs to strmadmin, and nothing will get filtered.
inclusion_rule=>FALSE is the part that indicates that we don’t want this table.

begin
DBMS_STREAMS_ADM.ADD_TABLE_RULES(
table_name=>'DEV110.APP_QUEUE',
streams_type=>'capture',
streams_name=>'CAPTURE_110DEV',
queue_name=>'REP_CAPTURE_QUEUE',
include_dml=>TRUE,
include_ddl=>TRUE,
include_tagged_lcr=> FALSE,
source_database=>'DB05',
inclusion_rule=>FALSE,
and_condition=>NULL);
end;

You can then see the rule you created by running select * from DBA_RULES

What now: The process I just described got me safely past a POC. There is obviously lot more to be done before this solution goes production. The performance of the capturing, and what is the impact of lags is of concern. Also, the entire system needs to be recreated from scratch whenever we do “alter database open reset logs” on the source DB (hopefully not often). But the immediate next step for me is to prepare a nice presentation to management showing what a great solution we prepared and how useful it will be for the business and how much this will be worth the investment.