Thursday 8 September 2011

REST and transactions

Much has been said recently about transactions and databases in the cloud. We really need transactions, at almost any price, and Pyrrho supports long-running transactions for as long as a connection to the server stays open.
But Http is supposed to be a stateless protocol, and connections are closed on each request.

In recent years there has been a trend away from using session state as it creates server affinity and so is not scalable. One option is to use stored procedures to wrap several steps into an action that can be called using REST. Another is to offer transaction scripts, and these are available in Pyrrho v4.5 as described in this posting.

Recall that a REST URL in Pyrrho has the form

If no Selector or Processing components are provided, the target is the Role itself, to which a REST transaction script can be POSTed. This is envisaged for application use (not for an interactive user). Here is an example (POST to https://mybank:8133/MyBank/Teller):

(POST /CurrentAccount (AC:567123,AMT:655.43,REASON:’From Malcolm’))
(POST /CurrentAccount (AC:423991,AMT:-655.43,REASON:’To Fred’))

In this syntax embedded spaces in the url must be encoded as %20. In XML this could look like:

<?xml version=”1.0”?>
<POST Path=”/CurrentAccount” AC=”567123” AMT=”655.43”>
<REASON>From Malcolm</REASON>
<POST Path=”/CurrentAccount” AC=”423991” AMT=”-655.43”>

As usual with transactions, from within the transaction each step is seen to complete, but no external or persistent changes are made unless the COMMIT executes without conflict. The SQL syntax for a transaction script is thus:

Script = ‘(‘ ‘(‘ Step ‘)’ { [‘,’] ‘(‘ Step ‘)’ } ‘)’ .
Step = POST url Row_Value
| PUT url ‘(‘ Row_Value {[‘,’] Row_Value } ‘)’
| GET url
| DELETE url

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