This changes the behavior of KV's range and tx range to return
current revision rather than range revision. This makes populating
range response header easier.
We want the KV to support recovering from backend to avoid
additional pointer swap. Or we have to do coordination between
etcdserver and API layer, since API layer might have access to
kv pointer and use a closed kv.
newStore() uses constants for some important parameters
e.g. batchInerval. It is not suitable for other storage users
(e.g. tools/benchmark). This commit decouples the default parameters
from storage creation. etcd should use a newly added function
newDefaultStore() for creating a store with default parameters.
Mutex is a variable, which means there needs to be only one mutex
value per scope. We don't need a separate mutex inside store struct,
**if we assume that `TxnPut` and `TxnRange` are called ONLY ONCE
per transaction (between `TxnBegin` and `TxnEnd`)**, as documented.
The point is to decouple the key-value storage layer and the
event notification layer clearly. It gives the watchableKV the
flexibility to define whatever event structure it wants without
breaking the ondisk format at key-value storage layer.
Changes:
1. change the format of key and value stored in backend
Store KeyValue struct instead of Event struct in backend value for
better abstraction as xiang suggests. And record the corresponded
action in the backend key.
2. Remove word 'event' from functions
One txn is treated as atomic, and might contain multiple Put/Delete/Range
operations. For now, between these operations, we might call forecCommit
to sync the change to disk, or backend may commit it in background.
Thus the snapshot state might contains an unfinished multiple objects
transaction, which is dangerous if database is restored from the snapshot.
This PR makes KV txn hold batchTx lock during the process and avoids
commit to happen.
When using Snapshot function, it is expected:
1. know the size of snapshot before writing data
2. split snapshot-ready phase and write-data phase. so we could cut
snapshot first and write data later.
Update its interface to fit the requirement of etcdserver.
Change to the function:
1. specify the meaning of startRev and endRev parameters
2. specify the meaning of returned nextRev
Moreover, it adds unit tests for the function.
