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snapshot.go 5.58 KB
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// snapshot manages the interactions between Consul and Raft in order to take
// and restore snapshots for disaster recovery. The internal format of a
// snapshot is simply a tar file, as described in archive.go.
package snapshot
import (
"compress/gzip"
"fmt"
"io"
"io/ioutil"
"log"
"os"
"github.com/hashicorp/raft"
)
// Snapshot is a structure that holds state about a temporary file that is used
// to hold a snapshot. By using an intermediate file we avoid holding everything
// in memory.
type Snapshot struct {
file *os.File
index uint64
}
// New takes a state snapshot of the given Raft instance into a temporary file
// and returns an object that gives access to the file as an io.Reader. You must
// arrange to call Close() on the returned object or else you will leak a
// temporary file.
func New(logger *log.Logger, r *raft.Raft) (*Snapshot, error) {
// Take the snapshot.
future := r.Snapshot()
if err := future.Error(); err != nil {
return nil, fmt.Errorf("Raft error when taking snapshot: %v", err)
}
// Open up the snapshot.
metadata, snap, err := future.Open()
if err != nil {
return nil, fmt.Errorf("failed to open snapshot: %v:", err)
}
defer func() {
if err := snap.Close(); err != nil {
logger.Printf("[ERR] snapshot: Failed to close Raft snapshot: %v", err)
}
}()
// Make a scratch file to receive the contents so that we don't buffer
// everything in memory. This gets deleted in Close() since we keep it
// around for re-reading.
archive, err := ioutil.TempFile("", "snapshot")
if err != nil {
return nil, fmt.Errorf("failed to create snapshot file: %v", err)
}
// If anything goes wrong after this point, we will attempt to clean up
// the temp file. The happy path will disarm this.
var keep bool
defer func() {
if keep {
return
}
if err := os.Remove(archive.Name()); err != nil {
logger.Printf("[ERR] snapshot: Failed to clean up temp snapshot: %v", err)
}
}()
// Wrap the file writer in a gzip compressor.
compressor := gzip.NewWriter(archive)
// Write the archive.
if err := write(compressor, metadata, snap); err != nil {
return nil, fmt.Errorf("failed to write snapshot file: %v", err)
}
// Finish the compressed stream.
if err := compressor.Close(); err != nil {
return nil, fmt.Errorf("failed to compress snapshot file: %v", err)
}
// Sync the compressed file and rewind it so it's ready to be streamed
// out by the caller.
if err := archive.Sync(); err != nil {
return nil, fmt.Errorf("failed to sync snapshot: %v", err)
}
if _, err := archive.Seek(0, 0); err != nil {
return nil, fmt.Errorf("failed to rewind snapshot: %v", err)
}
keep = true
return &Snapshot{archive, metadata.Index}, nil
}
// Index returns the index of the snapshot. This is safe to call on a nil
// snapshot, it will just return 0.
func (s *Snapshot) Index() uint64 {
if s == nil {
return 0
}
return s.index
}
// Read passes through to the underlying snapshot file. This is safe to call on
// a nil snapshot, it will just return an EOF.
func (s *Snapshot) Read(p []byte) (n int, err error) {
if s == nil {
return 0, io.EOF
}
return s.file.Read(p)
}
// Close closes the snapshot and removes any temporary storage associated with
// it. You must arrange to call this whenever NewSnapshot() has been called
// successfully. This is safe to call on a nil snapshot.
func (s *Snapshot) Close() error {
if s == nil {
return nil
}
if err := s.file.Close(); err != nil {
return err
}
return os.Remove(s.file.Name())
}
// Verify takes the snapshot from the reader and verifies its contents.
func Verify(in io.Reader) (*raft.SnapshotMeta, error) {
// Wrap the reader in a gzip decompressor.
decomp, err := gzip.NewReader(in)
if err != nil {
return nil, fmt.Errorf("failed to decompress snapshot: %v", err)
}
defer decomp.Close()
// Read the archive, throwing away the snapshot data.
var metadata raft.SnapshotMeta
if err := read(decomp, &metadata, ioutil.Discard); err != nil {
return nil, fmt.Errorf("failed to read snapshot file: %v", err)
}
return &metadata, nil
}
// Restore takes the snapshot from the reader and attempts to apply it to the
// given Raft instance.
func Restore(logger *log.Logger, in io.Reader, r *raft.Raft) error {
// Wrap the reader in a gzip decompressor.
decomp, err := gzip.NewReader(in)
if err != nil {
return fmt.Errorf("failed to decompress snapshot: %v", err)
}
defer func() {
if err := decomp.Close(); err != nil {
logger.Printf("[ERR] snapshot: Failed to close snapshot decompressor: %v", err)
}
}()
// Make a scratch file to receive the contents of the snapshot data so
// we can avoid buffering in memory.
snap, err := ioutil.TempFile("", "snapshot")
if err != nil {
return fmt.Errorf("failed to create temp snapshot file: %v", err)
}
defer func() {
if err := snap.Close(); err != nil {
logger.Printf("[ERR] snapshot: Failed to close temp snapshot: %v", err)
}
if err := os.Remove(snap.Name()); err != nil {
logger.Printf("[ERR] snapshot: Failed to clean up temp snapshot: %v", err)
}
}()
// Read the archive.
var metadata raft.SnapshotMeta
if err := read(decomp, &metadata, snap); err != nil {
return fmt.Errorf("failed to read snapshot file: %v", err)
}
// Sync and rewind the file so it's ready to be read again.
if err := snap.Sync(); err != nil {
return fmt.Errorf("failed to sync temp snapshot: %v", err)
}
if _, err := snap.Seek(0, 0); err != nil {
return fmt.Errorf("failed to rewind temp snapshot: %v", err)
}
// Feed the snapshot into Raft.
if err := r.Restore(&metadata, snap, 0); err != nil {
return fmt.Errorf("Raft error when restoring snapshot: %v", err)
}
return nil
}
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