Hi there,
We?re extending the use case of RADOSGW to store some lightweight metadata which is accessed concurrently by multiple clients. The metadata is stored as a RGW object in CEPH. The payload data is JSON array encoded as string. The following is a sample payload.
[
{
"id": "foo",
"property": "pro-foo",
},
{
"id": "bar",
"property": "pro-bar",
}
]
Each client 1) reads the payload out first, 2) makes some modification in memory, and then 3) writes the updated payload to CEPH.
The issue we are encoutering here is that when there are multiple clients adding items concurrently to the metadata, some updates may be lost because the concurrent update. Apparently, we need some mechanism to implement an optimistic concurrency control. We?d like to have a simple solution that If-Match on ETag only. The API is of course compatible with S3.
Suppose we have If-Match ETag on PUT, the client mentioned above will work like this 1) reads the payload nd ETag out first, 2) makes some modification in memory, and 3) writes the updated payload to CEPH with If-Match: <ETag>, 4) If the operation fails, it retry from the first step.
One possible issue for this solution is on consistency between RGW replications. But since the current RGW replication is a can-write-to-master-only scheme, it seems not a real issue right now.
We are happy to enhance RGW to make it unique from AWS S3 by supporting this use case. And suggestion/comments from you guys are highly welcomed :-) We have been heading to the feature and opened a feature ticket on tracker: http://tracker.ceph.com/issues/8562
Thanks,
Ray
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