70 lines
2.5 KiB
Go
70 lines
2.5 KiB
Go
// Copyright 2015 CoreOS, Inc.
|
|
//
|
|
// Licensed under the Apache License, Version 2.0 (the "License");
|
|
// you may not use this file except in compliance with the License.
|
|
// You may obtain a copy of the License at
|
|
//
|
|
// http://www.apache.org/licenses/LICENSE-2.0
|
|
//
|
|
// Unless required by applicable law or agreed to in writing, software
|
|
// distributed under the License is distributed on an "AS IS" BASIS,
|
|
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
|
|
// See the License for the specific language governing permissions and
|
|
// limitations under the License.
|
|
|
|
/*
|
|
Package wal provides an implementation of a write ahead log that is used by
|
|
etcd.
|
|
|
|
A WAL is created at a particular directory and is made up of a number of
|
|
discrete WAL files. Inside of each file the raft state and entries are appended
|
|
to it with the Save method:
|
|
|
|
metadata := []byte{}
|
|
w, err := wal.Create("/var/lib/etcd", metadata)
|
|
...
|
|
err := w.Save(s, ents)
|
|
|
|
After saving an raft snapshot to disk, SaveSnapshot method should be called to
|
|
record it. So WAL can match with the saved snapshot when restarting.
|
|
|
|
err := w.SaveSnapshot(walpb.Snapshot{Index: 10, Term: 2})
|
|
|
|
When a user has finished using a WAL it must be closed:
|
|
|
|
w.Close()
|
|
|
|
WAL files are placed inside of the directory in the following format:
|
|
$seq-$index.wal
|
|
|
|
The first WAL file to be created will be 0000000000000000-0000000000000000.wal
|
|
indicating an initial sequence of 0 and an initial raft index of 0. The first
|
|
entry written to WAL MUST have raft index 0.
|
|
|
|
Periodically a user will want to "cut" the WAL and place new entries into a new
|
|
file. This will increment an internal sequence number and cause a new file to
|
|
be created. If the last raft index saved was 0x20 and this is the first time
|
|
Cut has been called on this WAL then the sequence will increment from 0x0 to
|
|
0x1. The new file will be: 0000000000000001-0000000000000021.wal. If a second
|
|
Cut issues 0x10 entries with incremental index later then the file will be called:
|
|
0000000000000002-0000000000000031.wal.
|
|
|
|
At a later time a WAL can be opened at a particular snapshot. If there is no
|
|
snapshot, an empty snapshot should be passed in.
|
|
|
|
w, err := wal.Open("/var/lib/etcd", walpb.Snapshot{Index: 10, Term: 2})
|
|
...
|
|
|
|
The snapshot must have been written to the WAL.
|
|
|
|
Additional items cannot be Saved to this WAL until all of the items from the given
|
|
snapshot to the end of the WAL are read first:
|
|
|
|
metadata, state, ents, err := w.ReadAll()
|
|
|
|
This will give you the metadata, the last raft.State and the slice of
|
|
raft.Entry items in the log.
|
|
|
|
*/
|
|
package wal
|