The following text is taken from IBM Red book
http://www.redbooks.ibm.com/redpieces/abstracts/sg247488.html?Open
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7.1 Redirected Write capability
Redirected Writes provide an automatic transfer from the Secondary to the
Primary node when an update operation is to be performed. This is done
without
taking a distributed lock on the row at Secondary and the Primary.
Instead, the
Primary server uses a form of optimistic concurrency. This means that the
Primary server will consider the write operation only if the before image
is
identical between the Primary and the Secondary server. If it is not, then
the
server will consider the operation as having encountered a deadlock type
of
error
and abort the operation.
7.1.1 A Redirected Write environment
In this section we describe the behavior of Primary and Secondary servers
in a
Redirected Write environment. When a client application connects to a
Secondary server, the server begins an SQL session in which the
application can
start an SQL transaction. When any DML operations, such as INSERT, DELETE,
and UPDATE, are executed in that transaction, the Secondary server
trans****ts
those operations to the Primary server over the network. On receiving
those
operations, the Primary server applies them and sends the logical logs to
the
Secondary server. On receiving the logical logs, the Secondary server
replays
the logs and thus makes the changes performed by the client persistent at
the
Secondary server.
During the period in which the write operations are sent and then received
back
from Primary, through the logical logs, the Secondary server makes the
changes
available to the SQL transaction by caching them in memory.
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Two questions:
1. How does the secondary node determine that the before image of primary
and
secondary server are same.
2. What is meant by "During the period in which the write operations are
sent
and then received back from Primary, through the logical logs, the
Secondary
server makes the changes available to the SQL transaction by caching them
in
memory.
Can anyone from IBM explain this in bit more detail.
Thanks.
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