Friday, 22 April 2016

Supporting transactions in REST

From the next version 5.5, Pyrrho's HTTP service and REST interface will support the RFC7232 ETag fetaure (Fielding and Reschke, 2014). The strong validator used will be compatible with Pyrrho's VERSIONED mechanism for row data, and will enable the development of layered databases using Pyrrho's REST interfaces and extended VIEWs.

I do hope other REST implementations will soon follow RFC7232 as this will make REST even more attractive for implementing linked data. There is clearly some interest in the Spring community.
ETags can be used for validating a step in a transaction.

On the other hand we need to do more to implement transaction coordination, when a Pyrrho server makes HTTP calls to another REST service. A number of systems have different mechanisms for maintaining an open transaction in such circumstances, e.g. Neo4j. But in view of Pyrrho’s rather pedantic approach to transaction isolation and durability it seems best for Pyrrho to use its own mechanism. A Pyrrho transaction coordinator will identify itself as a unique UserAgent in these circumstances, including the coordinator host, port, with a timestamp and the transaction ID. This information will enable the transaction partners to coordinate their work and it can then be used in a simple two-phase commit sequence. For example, we could make the User-Agent string contain a link with the information, such as

Pyrrho 5.5

so that a Pyrrho server participating in the transaction can keep an HTTP server thread open for operations on its local databases for this transaction, until (a) it receives a GET/POST/PUT/DELETE for / which it will interpret as Check, Prepare, Commit and Rollback respectively , or (b) the timestamp is older than 20 minutes. If anything goes wrong, it will report 409 Conflict or 410 Gone.


Fielding, R; Reschke, J, eds (2014): Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests, RFC7232,