Server timestamps¶
In order to avoid race conditions, each change is guaranteed to increment the timestamp of the related collection. If two changes happen at the same millisecond, they will still have two different timestamps.
The ETag
header with the current timestamp of the collection for
the current user will be given on collection endpoints.
ETag: "1432208041618"
On record enpoints, the ETag
header value will contain the timestamp of the
record.
In order to bypass costly and error-prone HTTP date parsing, ETag
headers
are not HTTP date values.
A human readable version of the timestamp (rounded to second) is provided though
in the Last-Modified
response headers:
Last-Modified: Wed May 20 17:22:38 2015 +0200
Changed in version 2.0: In previous versions, cache and concurrency control was handled using
If-Modified-Since
and If-Unmodified-Since
. But since the HTTP date
does not include milliseconds, they contained the milliseconds timestamp as
integer. The current version using ETag
is HTTP compliant (see
original discussion.)
Note
The client may send If-Unmodified-Since
or If-Modified-Since
requests
headers, but in the current implementation, they will be ignored.
Cache control¶
In order to check that the client version has not changed, a If-None-Match
request header can be used. If the response is 304 Not Modified
then
the cached version if still good.
Concurrency control¶
In order to prevent race conditions, like overwriting changes occured in the interim for example,
a If-Match
request header can be used. If the response is 412 Precondition failed
then the resource has changed meanwhile.
The client can then choose to:
- overwrite by repeating the request without
If-Match
; - reconcile the resource by fetching, merging and repeating the request.