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Re: HDR question once again

Posted By: Art Kagel
Date: Thursday, 9 February 2012, at 10:31 a.m.

In Response To: Re: HDR question once again (JACK PAPA)


You are missing a lot. A bit of recent history then. Informix from
version 7.10 through 7.31 and 9.10 through version 10.00, supported two
overall replication types:

1. HDR or High availability Data Replication - This supported one

read/write primary server and one read-only HDR secondary server. In this

scenario the entire server (or at least any logged databases in the server)

were replicated by making an initial copy of the primary using ontape or

onbar and using onmode -d to tell each server about the other. After that

the primary sent copies of each logical log buffer to the secondary before

writing it to disk locally. The replication could be synchronous

(transactions on the primary wait for the secondary to acknowledge that the

log record containing its COMMIT had been applied on the secondary before

applying the COMMIT on the primary) or asynchronous (the primary does not

wait to commit the local transaction). HDR can only support servers which

are all on the same platform and running the same exact version of the

Informix software.

2. Enterprise Replication or ER. This replicates data at the table

level (and even at the row level as filters can determine which rows to

replicate) in many different configurations of from two to hundreds of

replicated servers. Replication can be master-to-slave, peer-to-peer and

can replicate all of the data or only part of the data in one or all

tables. It is more complex to set up, but far more flexible and does not

require an archive and restore to initiate the replication (an new server

can start out empty and ER will provision it with data). All server are

read-write and data can be distributed to many nodes from a single root

source, gathered from many nodes to a central target, or replicated in a

complex network of peer servers. ER supported servers on multiple

platforms running different versions of the Informix software (within

reason - the oldest versions couldn't keep up with more recent ones, but IB

that from 7.30 & 9.21 on cross version replication was possible). ER nodes

and replicated tables are configured using a utility called cdr. (You have

it. If you run it cdr with no args will print out the commands that it


3. Combinations of ER and HDR can be configured where any ER node can

actually be an HDR pair.

With the release of Informix 11.10 IBM introduced two new kinds of
secondary servers and renamed the whole package "MACH11" an acronym which I
refuse to expand. ;-) The new server types were:

1. RSS or Remote Secondary Server - This is identical to an HDR

secondary except that one can still have only one HDR secondary but can

have zero to many RSS secondaries. While the communications with HDR

secondaries was and remains half duplex (each packet has to be acked before

another is sent) RSS secondaries operate under a full duplex protocol

permitting more efficient operations over slower and less reliable

communications. (An RSS secondary is more likely to work in a remote data

center at the other end of the world than an HDR secondary.) Logical log

buffers are shipped to the RSS secondaries just as if they were HDR

secondaries, but, the primary does not wait for any acks and will spool the

buffers locally and reship them later if the RSS secondaries fall behind.

RSS secondaries can only be promoted to an HDR secondary, not directly to a


2. SDS or Shared Disk Secondary Server - These secondaries share the

disks used by the primary. They can be instances running on the same

physical host connecting locally to the disk chunks, running on different

hosts connecting to the chunks on a SAN or NAS that is sharable, or running

in a remote data center connected to chunk files that are replicated at the

SAN level between data centers. When the primary modifies a page on disk

it send ONLY the page's address to all SDS secondaries that it knows about

which then reread the page into their own private cache if they had a stale

version of the page in cache at the time. No data moves to the SDS

secondaries. SDS secondaries have private TEMP dbspaces for their own

sorting and temp tables which can be on local disk. Any SDS secondary can

replace a down primary and be promoted to primary over the remaining SDS,

RSS, and HDR secondaries. (You can still have the HDR secondary take over

by converting an SDS secondary to primary and then switching roles with the

HDR secondary). Since there is no lock sharing, unlike Oracle RAC

clusters, SDS secondaries do not require a high speed backplane to work

efficiently and SDS scales to well over a hundred secondaries in the lab

and I have heard of dozens of secondaries to a single primary being

configured in the field.

RSS secondaries are initialized just like HDR secondaries. SDS secondaries
just require the Informix software, an appropriately configured ONCONFIG
file, and shared access to the primary's chunk files. They are enabled
using onmode -d sub-commands. ER was improved with the concept of
templates which let you more easily replicate many tables at once.

With the release of Informix 11.50 RSS and SDS secondary servers became
writable! Now, if you want, users can connect to any secondary server,
whether it is an HDR, RSS, or SDS secondary and modify the data in their
tables. The secondary sends the update to the primary which performs the
work and updates the secondaries appropriately. Once the primary
acknowledges the transaction the secondary will notify the user. Version
11.50 also introduced the ability to create a new secondary server as a
clone of an existing server in a single step with the new ifxclone utility.

Informix version 11.70 introduced the concept of Informix Flexible Grid
which improved the interoperability of ER, HDR, RSS, and SDS so that you
can manage an entire grid of heterogeneous primary, secondary, and peer
servers using new 'grid' commands added to the cdr utility. You can manage
the grid as a whole including CREATEing, DROPping, and ALTERing tables,
creating indexes, etc. across every server in the grid with a single
command. Flexible grid also allows you to perform rolling upgrades of the
servers in the grid with no down time. Finally, 11.70 introduced the
concept of Transaction Survival. Transactions started by a client
connected to a secondary server (so HDR, RSS, or SDS) continue
uninterrupted if the primary server crashes. The transactions are picked
up by whichever secondary server takes over as primary (HDR or SDS) and
continue after a brief pause as if nothing happened.


Art S. Kagel
Advanced DataTools (

Disclaimer: Please keep in mind that my own opinions are my own opinions
and do not reflect on my employer, Advanced DataTools, the IIUG, nor any
other organization with which I am associated either explicitly,
implicitly, or by inference. Neither do those opinions reflect those of
other individuals affiliated with any entity with which I am affiliated nor
those of the entities themselves.

On Thu, Feb 9, 2012 at 9:17 AM, JACK PAPA <> wrote:

> Hi Art,
> Let me first thank you for all of your answers.
> I think you have answered my questions and what i want to achieve is to
> have a
> (2) two secondary servers connected to 1 primary server. I am a little bit
> confused on what is the difference between an Enterprise Replication and
> High
> Availability Data Replication.
> So far, what i have seen is 1 primary server and (1) Secondary Server
> connected which performs log shipping. The primary server is in Read Write
> mode, while the Secondary server is in Read Only mode. I've never used
> "cdr"
> command before, instead i always used "onmode -d" command.
> I am reading Enterprise Replication Guide v. 9.4 to understand more of
> what i
> need, but still can't differentiate ER and HDR.
> Anyways, I'll do a follow up post on this of what i read.
> Regards,
> *******************************************************************************
> Forum Note: Use "Reply" to post a response in the discussion forum.


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