c1dc2fa624
The normalized form of the X-Timestamp header looks like a float with a fixed width to ensure stable string sorting - normalized timestamps look like "1402464677.04188" To support overwrites of existing data without modifying the original timestamp but still maintain consistency a second internal offset vector is append to the normalized timestamp form which compares and sorts greater than the fixed width float format but less than a newer timestamp. The internalized format of timestamps looks like "1402464677.04188_0000000000000000" - the portion after the underscore is the offset and is a formatted hexadecimal integer. The internalized form is not exposed to clients in responses from Swift. Normal client operations will not create a timestamp with an offset. The Timestamp class in common.utils supports internalized and normalized formatting of timestamps and also comparison of timestamp values. When the offset value of a Timestamp is 0 - it's considered insignificant and need not be represented in the string format; to support backwards compatibility during a Swift upgrade the internalized and normalized form of a Timestamp with an insignificant offset are identical. When a timestamp includes an offset it will always be represented in the internalized form, but is still excluded from the normalized form. Timestamps with an equivalent timestamp portion (the float part) will compare and order by their offset. Timestamps with a greater timestamp portion will always compare and order greater than a Timestamp with a lesser timestamp regardless of it's offset. String comparison and ordering is guaranteed for the internalized string format, and is backwards compatible for normalized timestamps which do not include an offset. The reconciler currently uses a offset bump to ensure that objects can move to the wrong storage policy and be moved back. This use-case is valid because the content represented by the user-facing timestamp is not modified in way. Future consumers of the offset vector of timestamps should be mindful of HTTP semantics of If-Modified and take care to avoid deviation in the response from the object server without an accompanying change to the user facing timestamp. DocImpact Implements: blueprint storage-policies Change-Id: Id85c960b126ec919a481dc62469bf172b7fb8549 |
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