Server timestamps¶
In order to avoid race conditions, each change is guaranteed to increment the timestamp of the related list. If two changes happen at the same millisecond, they will still have two different timestamps.
The ETag
header with the current timestamp of the list for
the current user will be given on list endpoint.
ETag: "1432208041618"
On object enpoints, the ETag
header value will contain the timestamp of the
object.
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.
Important
When the list is empty, its timestamp remains the same until new objects are created.
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 is still good.
GET | |
---|---|
If-None-Match: “<timestamp>”
|
|
Changed meanwhile | Return response content |
Not changed | Empty HTTP 304 |
Concurrency control¶
In order to prevent race conditions, like overwriting changes occured in the interim for example,
a If-Match: "timestamp"
request header can be used. If the response is 412 Precondition Failed
then the resource has changed meanwhile.
Concurrency control also allows to make sure a creation won’t overwrite any object using
the If-None-Match: *
request header.
The following table gives a summary of the expected behaviour of a resource:
GET | POST | PUT | PATCH | DELETE | |
---|---|---|---|---|---|
If-Match: “timestamp”
|
|||||
Not changed | Fetch | Fetch | Overwrite | Modify | Delete |
Changed meanwhile | HTTP 412 |
HTTP 412 |
HTTP 412 |
HTTP 412 |
HTTP 412 |
If-Match: *
|
|||||
Id exists | Fetch | Fetch | Overwrite | Modify | Delete |
Id unknown | HTTP 412 |
HTTP 412 |
HTTP 412 |
HTTP 412 |
HTTP 412 |
If-None-Match: *
|
|||||
Id exists | HTTP 412 |
HTTP 412 |
HTTP 412 |
HTTP 412 |
HTTP 412 |
Id unknown | No effect | Create | Create | No effect | No effect |
When the client receives a 412 Precondition Failed
, it can then choose to:
- overwrite by repeating the request without concurrency control;
- reconcile the resource by fetching, merging and repeating the request.
Replication¶
In order to replicate the timestamps when importing existing records, it is possible to force the last modified values.
When an object is created (via POST or PUT), the specified timestamp becomes the new list timestamp if it is in the future (i.e. greater than current one). If it is in the past, the record is created with the timestamp in the past but the list timestamp is bumped into the future as usual.
When an object is replaced, modified or deleted, if the specified timestamp is less or equal than the existing object, the value is simply ignored and the timestamp is bumped into the future as usual.
When an object is deleted, a last_modified
timestamp can be forced
by passing it in the query string using ?last_modified=<value>
.