Re: BAST in RAC

In RAC the lock related operations are handled by Global Enqueue Services (GES) layer. Asynchronous System Traps (AST) are just like generic interrupt service routine. There are two types of interrupts or ASTs in the RAC. One is AAST (Acquisition AST) and another one is BAST (Blocking AST).This will be visible in the X$ structure X$LE
(Lock Element?)

When a process wants to acesss a resurce in RAC environment it needs to acquire lock and this will be given by GES. Before granting the lock to the process, GES will send the global AST to the holder of that resource and the
holder may grant access to the requesting process. Once you have got the required locks on that resource GES will send an another AST
(AAST) to tell that you have aqcuired the lock.

GES will have the queue of requests/converts and will be delievering AST on an ongoing basis. BAST is the special kind of interrupt
(Blocking AST) that will be sent to the process holding the resource
that may lead some conflicts in the request conversions.

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Best Regards,
K Gopalakrishnan 
Co-Author: Oracle Wait Interface, Oracle Press 2004
http://www.amazon.com/exec/obidos/tg/detail/-/007222729X/
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