@@ -67,7 +67,7 @@ The output of above test program should look like this:
--- PASS: TestMVCC (0.01s)
```
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`.
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.
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@@ -102,7 +102,7 @@ For the `Write` operations (`Save`, `MultiSave`, `MultiSaveAndRemoveWithPrefix`)
You might be curious about the parameter `additions` of `MultiSave` and `MultiSaveAndRemoveWithPrefix`: What does `additions` do, and why?
`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`.
`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 `string` 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.
