`metaSnapShot` enables `RootCoord` to query historical meta based on timestamp, it provides `Key-Vaule` interface. Take an example to illustrate what `metaSnapShot` is. The following figure shows a series of operations happened on the timeline.
Now assuming the Wall-Clock is `Timestamp=700`, so `B` should have been deleted from the system. But I want to know the value of `B` at `Timesamp=450`, how to do it? `metaSnapShot` is invented to solve this problem.
In `etcd`, each write operation would add 1 to `Revision`. So if we specify the `Revision` value at query, we can get the historical value under that `Revision`.
`metaSnapShot` is based on this feature of `etcd`. We will write an extra `Timestamp` on each write operation. `etcd`'s `Txn` makes sure that the `Timestamp` would have the same `Revision` with user data.
In order to speed up getting `Revision` by `Timestamp`, `metaSnapShot` would maintain an array mapping the `Timestamp` to `Revision`. The default length of this array is `1024`, which is a type of circular array.
-`minPos` points to the position where `Timestamp` and `Revision` are minimum.
- For each update operation, we first add `1` to `maxPos`. So the new `maxPos` would cover the old `minPos` position, and then add `1` to the old `minPos`
- From `0` to `maxPos` and from `minPos` to `1023`, which are two incremental arrays. We can use binary search to quickly get the `Revision` by the input `Timestamp`
- If the input `Timestamp` is greater than the `Timestamp` where the `maxPos` is located, then the `Revision` at the position of the `maxPos` will be returned
- If the input `Timestamp` is less than the `Timestamp` where `minPos` is located, `metaSnapshot` will load the historical `Timestamp` and `Revision` from `etcd` to find an appropriate `Revision` value.
For the `Read` operations (`Load` and `LoadWithPrefix`), the input parameter `typeutil.Timestamp` is used to tell `metaSnapShot` to load the value based on that `Timestamp`.
For the `Write` operations (`Save`, `MiltiSave`, `MultiSaveAndRemoveWithPrefix`), return values include `typeutil.Timestamp`, which is used to tell the caller when these write operations happened.
`additions` is an array of `func(ts typeutil.Timestamp) (string, string, error)`. So it's a function, receiving `typeutil.Timestamp` as an input, and returns two `sting` which is `key-value` pair. If `error` is `nil` in the return value, `metaSnapShot` would write this `key-value` pair into `etcd`.
Refer to the document of `CreateCollection`, a timestamp is created for `Collection`, which is the timestamp when the `Collection`'s meta have been written into `etcd`, not the timestamp when `RootCoord` receives the request. So before writing the `Collection`'s meta into `etcd`, `metaSnapshot` would allocate a timestamp, and call all the `additions`. This would make sure the timestamp created for the `Collection` is correct.