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.)


The client may send If-Unmodified-Since or If-Modified-Since requests headers, but in the current implementation, they will be ignored.


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.

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:

If-Match: “timestamp”
Changed meanwhile HTTP 412 HTTP 412 HTTP 412 HTTP 412
Not changed Create Overwrite Modify Delete
If-None-Match: *
Id exists HTTP 412 HTTP 412 No effect No effect
Id unknown 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.


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>.