67 lines
2.4 KiB
Go
67 lines
2.4 KiB
Go
/*
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Copyright 2014 CoreOS, Inc.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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*/
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/*
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Package wal provides an implementation of a write ahead log that is used by
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etcd.
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A WAL is created at a particular directory and is made up of a number of
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discrete WAL files. Inside of each file the raft state and entries are appended
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to it with the Save method:
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w, err := wal.Create("/var/lib/etcd")
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...
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err := w.Save(s, ents)
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When a user has finished using a WAL it must be closed:
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w.Close()
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WAL files are placed inside of the directory in the following format:
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$seq-$index.wal
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The first WAL file to be created will be 0000000000000000-0000000000000000.wal
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indicating an initial sequence of 0 and an initial raft index of 0. The first
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entry written to WAL MUST have raft index 0.
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Periodically a user will want to "cut" the WAL and place new entries into a new
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file. This will increment an internal sequence number and cause a new file to
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be created. If the last raft index saved was 0x20 and this is the first time
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Cut has been called on this WAL then the sequence will increment from 0x0 to
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0x1. The new file will be: 0000000000000001-0000000000000021.wal. If a second
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Cut issues 0x10 entries with incremental index later then the file will be called:
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0000000000000002-0000000000000031.wal.
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At a later time a WAL can be opened at a particular raft index:
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w, err := wal.OpenAtIndex("/var/lib/etcd", 0)
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...
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The raft index must have been written to the WAL. When opening without a
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snapshot the raft index should always be 0. When opening with a snapshot
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the raft index should be the index of the last entry covered by the snapshot.
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Additional items cannot be Saved to this WAL until all of the items from 0 to
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the end of the WAL are read first:
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id, state, ents, err := w.ReadAll()
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This will give you the raft node id, the last raft.State and the slice of
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raft.Entry items in the log.
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*/
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package wal
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