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db: added note on performance, edited note on isolation

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Ryan Culpepper 2011-08-29 04:34:56 -06:00
parent 418985d4c4
commit f339060e55
1 changed files with 12 additions and 9 deletions
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21
collects/db/scribblings/introduction.scrbl
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@ -202,15 +202,18 @@ web-server
]
The main problem with using one connection for all requests is that
while all connection functions are thread-safe, two threads accessing
a connection concurrently may still interfere. For example, if two
threads both attempt to start a new transaction, the second one will
fail, because the first thread has already put the connection into an
``in transaction'' state. And if one thread is accessing the
connection within a transaction and another thread issues a query, the
second thread may see invalid data or even disrupt the work of the
first thread (see
@hyperlink["http://en.wikipedia.org/wiki/Isolation_%28database_systems%29"]{isolation}).
multiple threads accessing the same connection are not properly
@hyperlink["http://en.wikipedia.org/wiki/Isolation_%28database_systems%29"]{isolated}. For
example, if two threads both attempt to start a new transaction, the
second one will fail, because the first thread has already put the
connection into an ``in transaction'' state. And if one thread is
accessing the connection within a transaction and another thread
issues a query, the second thread may see invalid data or even disrupt
the work of the first thread.
A secondary problem is performance. A connection can only perform a
single query at a time, whereas most database systems are capable of
concurrent query processing.
The proper way to use database connections in a servlet is to create a
connection for each request and disconnect it when the request