Replicating Oracle Webinar Question Follow-up

We had really great webinar on Replicating to/from Oracle earliest this month, and you can view the recording of that Webinar here.

A good sign of how great a Webinar was is the questions that come afterwards, and we didn’t get through them all. so here are all the questions and answers for the entire webinar.

Q: What is the overhead of Replicator on source database with asynchronous CDC?

A: With asynchronous operation there is no substantial CPU overhead (as with synchronous), but the amount of generated redo logs becomes bigger requiring more disk space and better log management to ensure that the space is used effectively.

Q: Do you support migration from Solaris/Oracle to Linux/Oracle?

A: The replication is not certified for use on Solaris, however, it is possible to configure a replicator to operate remotely and extract from a remote Oracle instance. This is achieved by installing Tungsten Replicator on Linux and then extracting from the remote Oracle instance.

Q: Are there issues in supporting tables without Primary Keys on Oracle to Oracle replication?

A: Non-primary key tables will work, but it is not recommended for production as it implies significant overhead when applying to a target database.

Q: On Oracle->Oracle replication, if there are triggers on source tables, how is this handled?

A: Tungsten Replicator does not automatically disable triggers. The best solution is to remove triggers on slaves, or rewrite triggers to identify whether a trigger is being executed on the master or slave and skip it accordingly, although this requires rewriting the triggers in question.

Q: How is your offering different/better than Oracle Streams replication?

A: We like to think of ourselves as GoldenGate without the price tag. The main difference is the way we extract the information from Oracle, otherwise, the products offer similar functionality. For Tungsten Replicator in particular, one advantage is the open and flexible nature, since Tungsten Replicator is open source, released under a GPL V2 license, and available at https://code.google.com/p/tungsten-replicator/.

Q: How is the integrity of the replica maintained/verified?

A: Replicator has built-in real-time consistency checks: if an UPDATE or DELETE doesn’t update any rows, Replicator will go OFFLINE:ERROR, as this indicates an inconsistent dataset.

Q: Can configuration file based passwords be specified using some form of encrypted value for security purposes to keep them out of the clear?

A: We support an INI file format so that you do not have to use the command-line installation process. There is currently no supported option for an encrypted version of these values, but the INI file can be secured so it is only readable by the Tungsten user.

Q: Our source DB is Oracle RAC with ~10 instances. Is coherency maintained in the replication from activity in the various instances?

A: We do not monitor the information that has been replicated; but CDC replicates row-based data, not statements, so typical sequence insertion issues that might occur with statement based replication should not apply.

Q: Is there any maintenance of Oracle sequence values between Oracle and replicas?

A: Sequence values are recorded into the row data as extracted by Tungsten Replicator. Because the inserted values, not the sequence itself, is replicated, there is no need to maintain sequences between hosts.

Q: How timely is the replication? Particularly for hot source tables receiving millions of rows per day?

A: CDC is based on extracting the data at an interval, but the interval can be configured. In practice, assuming there are regular inserts and updates on the Oracle side, the data is replicated in real-time. See https://docs.continuent.com/tungsten-replicator-3.0/deployment-oracle-cdctuning.html for more information on how this figure can be tuned.

Q: Can parallel extractor instances be spread across servers rather than through threads on the same server (which would be constrained by network or HBA)?

A: Yes. We can install multiple replicators and tune the extraction of the parallel extractor accordingly. However, that selection would need to be manual, but certainly that is possible.

Q: Do you need the CSV file (to select individual tables with the setupCDC.sh configuration) on the master setup if you want all tables?

A: No.

Q: If you lose your slave down the road, do you need to re-provision from the initial SCN number or is there a way to start from a later point?

A: This is the reason for the THL Sequence Number introduced in the extractor. If you lose your slave, you can install a new slave and have it start at the transaction number where the failed slave stopped if you know it, since the information will be in the THL. If not, you can usually determine this by examining the THL directly. There should be no need to re-provision – just to restart from the transaction in the THL on the master.

Q: Regarding a failed slave, what if it failed such that we don’t have a backup or wanted to provision a second slave such that it had no initial data.

A: If you had no backups or data, yes, you would need to re-provision with the parallel extractor in order to seed the target database.

Q: Would you do that with the original SCN? If it had been a month or two, is there a way to start at a more recent SCN (e.g. you have to re-run the setupCDC process)?

A: The best case is to have two MySQL slaves and when one fails, you re-provision it from the healthy one. This avoids setupCDC stage.

However, the replication can always be started from a specific event (SCN) provided that SCN is available in the Oracle undo log space.

Q: How does Tungsten handle Oracle’s CLOB and BLOB data types

A: Providing you are using asynchronous CDC these types are supported; for synchronous CDC these types are not supported by Oracle.

Q: Can different schemas in Oracle be replicated at different times?

A: Each schema is extracted by a separate service in Replicator, so they are independent.

Q: What is the size limit for BLOB or CLOB column data types?

A: This depends on the CDC capabilities in Oracle, and is not limited within Tungsten Replicator. You may want to refer to the Oracle Docs for more information on CDC: http://docs.oracle.com/cd/B28359_01/server.111/b28313/cdc.htm

Q: With different versions of Oracle e.g. enterprise edition and standard edition one be considered heterogeneous environments?

A: Essentially yes, although the nomenclature is really only a categorization, it does not affect the operation, deployment or functionality of the replicator. All these features are part of the open source product.

Q: Can a 10g database (master) send the data to a 11g database (slave) for use in an upgrade?

A: Yes.

Q: Does the Oracle replicator require the Oracle database to be in archive mode?

A: Yes. This is a requirement for Oracle’s CDC implementation.

Q: How will be able to revisit this recorded webinar?

A: Slides and a recording from today’s webinar will be available at http://www.slideshare.net/Continuent_Tungsten

 

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