// Copyright 2015 The etcd Authors
//
// 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
segmented 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(zap.NewExample(), "/var/lib/etcd", metadata)
	...
	err := w.Save(s, ents)

After saving a 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()

Each WAL file is a stream of WAL records. A WAL record is a length field and a wal record
protobuf. The record protobuf contains a CRC, a type, and a data payload. The length field is a
64-bit packed structure holding the length of the remaining logical record data in its lower
56 bits and its physical padding in the first three bits of the most significant byte. Each
record is 8-byte aligned so that the length field is never torn. The CRC contains the CRC32
value of all record protobufs preceding the current record.

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.

WAL will cut its current tail wal file if its size exceeds 64MB. 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
