Allowing Form Runner to cache form definitions is important to help with performance. Form Runner interfaces with the persistence layer through a REST API. Standard HTTP caching mechanisms are introduced to allow caching at that level.
The idea is that when Form Runner requests a form definition, it is able, if it holds the form definition in its local cache, to perform a so-called HTTP conditional
GET. This is exactly the same thing that web browsers do to revalidate some resources that they cache on the client. But here it is about Form Runner caching form definitions obtained from the persistence layer.
This page is intended for programmers implementing their own persistence layer (or for anybody curious about how things work!).
There is minimal support in Form Runner for conditional
GETs with the
If-Modified-Since HTTP headers. However the built-in eXist and relational persistence layer implementations do not support these headers properly yet.
As we work towards the improvements described in issue #1239, we hope to see even better performance when the persistence layer supports these headers. We also plan to add support for
If-None-Match, which will be required for persistence layers that support versioning.
The idea is very simple: the HTTP request may contain an
If-Modified-Since HTTP header, for example:
If-Modified-Since: Thu, 16 Jan 2014 01:16:27 GMT
The server (here the persistence layer), when seeing this header, SHOULD first try to check whether the resource requested (typically the form definition) has been modified since the given date (this can happen for example if the form is republished by Form Builder).
If the resource is newer, then the persistence layer must do the usual and serve the form definition with an HTTP
200 status code.
If the resource has the same date or is older, the persistence layer should simply return a
304 ("not modified") status code, without the need to send any response body.
The benefits are that:
The persistence layer doesn't need to actually retrieve the form definition from a database.
The form definition doesn't need to be sent over the wire and parsed by Form Runner.
For this to work, your persistence layer MUST send, with every form definition, a correct
Last-Modified header, for example:
Last-Modified: Thu, 16 Jan 2014 01:16:27 GMT
The above only needs to be supported for the HTTP
GET method and, optionally, for the
HEAD method (which Form Runner doesn't use).