1 // A Volume is an interface representing a Keep back-end storage unit:
2 // for example, a single mounted disk, a RAID array, an Amazon S3 volume,
13 type Volume interface {
14 // Get a block. IFF the returned error is nil, the caller must
15 // put the returned slice back into the buffer pool when it's
18 // loc is guaranteed to consist of 32 or more lowercase hex
21 // Get should not verify the integrity of the returned data:
22 // it should just return whatever was found in its backing
25 // If an error is encountered that prevents it from
26 // retrieving the data, that error should be returned so the
27 // caller can log (and send to the client) a more useful
30 // If the error is "not found", and there's no particular
31 // reason to expect the block to be found (other than that a
32 // caller is asking for it), the returned error should satisfy
33 // os.IsNotExist(err): this is a normal condition and will not
34 // be logged as an error (except that a 404 will appear in the
35 // access log if the block is not found on any other volumes
38 // If the data in the backing store is bigger than BLOCKSIZE,
39 // Get is permitted to return an error without reading any of
41 Get(loc string) ([]byte, error)
43 // Put writes a block to an underlying storage device.
45 // loc is as described in Get.
47 // len(block) is guaranteed to be between 0 and BLOCKSIZE.
49 // If a block is already stored under the same name (loc) with
50 // different content, Put must either overwrite the existing
51 // data with the new data or return a non-nil error.
53 // Put must return a non-nil error unless it can guarantee
54 // that the entire block has been written and flushed to
55 // persistent storage. Of course, this guarantee is only as
56 // good as the underlying storage device, but it is Put's
57 // responsibility to at least get whatever guarantee is
58 // offered by the storage device.
60 // Put should not verify that loc==hash(block): this is the
61 // caller's responsibility.
62 Put(loc string, block []byte) error
64 // Touch sets the timestamp for the given locator to the
67 // loc is as described in Get.
69 // Touch must return a non-nil error unless it can guarantee
70 // that a future call to Mtime() will return a timestamp newer
71 // than {now minus one second}.
72 Touch(loc string) error
74 // Mtime returns the stored timestamp for the given locator.
76 // loc is as described in Get.
78 // Mtime must return a non-nil error if the given block is not
79 // found or the timestamp could not be retrieved.
80 Mtime(loc string) (time.Time, error)
82 // IndexTo writes a complete list of locators with the given
83 // prefix for which Get() can retrieve data.
85 // prefix consists of zero or more lowercase hexadecimal
88 // Each locator must be written to the given writer using the
91 // loc "+" size " " timestamp "\n"
95 // - size is the number of bytes of content, given as a
96 // decimal number with one or more digits
98 // - timestamp is the timestamp stored for the locator,
99 // given as a decimal number of seconds after January 1,
102 // IndexTo must not write any other data to writer: for
103 // example, it must not write any blank lines.
105 // If an error makes it impossible to provide a complete
106 // index, IndexTo must return a non-nil error. It is
107 // acceptable to return a non-nil error after writing a
108 // partial index to writer.
110 // The resulting index is not expected to be sorted in any
112 IndexTo(prefix string, writer io.Writer) error
114 // Delete deletes the block data from the underlying storage
117 // loc is as described in Get.
119 // If the timestamp for the given locator is newer than
120 // blob_signature_ttl, Delete must not delete the data.
122 // If callers in different goroutines invoke overlapping
123 // Delete() and Touch() operations on the same locator, the
124 // implementation must guarantee that Touch() returns a
125 // non-nil error, or Delete() does not delete the block, or
127 Delete(loc string) error
129 // Status() returns a *VolumeStatus representing the current
130 // in-use and available storage capacity and an
131 // implementation-specific volume identifier (e.g., "mount
132 // point" for a UnixVolume).
133 Status() *VolumeStatus
135 // String() returns an identifying label for this volume,
136 // suitable for including in log messages. It should contain
137 // enough information to uniquely identify the underlying
138 // storage device, but should not contain any credentials or
142 // Writable() returns false if all future Put(), Mtime(), and
143 // Delete() calls are expected to fail.
145 // If the volume is only temporarily unwritable -- or if Put()
146 // will fail because it is full, but Mtime() or Delete() can
147 // succeed -- then Writable() should return false.
151 // A VolumeManager tells callers which volumes can read, which volumes
152 // can write, and on which volume the next write should be attempted.
153 type VolumeManager interface {
154 // AllReadable returns all volumes.
155 AllReadable() []Volume
156 // AllWritable returns all volumes that aren't known to be in
157 // a read-only state. (There is no guarantee that a write to
158 // one will succeed, though.)
159 AllWritable() []Volume
160 // NextWritable returns the volume where the next new block
161 // should be written. A VolumeManager can select a volume in
162 // order to distribute activity across spindles, fill up disks
163 // with more free space, etc.
164 NextWritable() Volume
165 // Close shuts down the volume manager cleanly.
169 type RRVolumeManager struct {
175 func MakeRRVolumeManager(volumes []Volume) *RRVolumeManager {
176 vm := &RRVolumeManager{}
177 for _, v := range volumes {
178 vm.readables = append(vm.readables, v)
180 vm.writables = append(vm.writables, v)
186 func (vm *RRVolumeManager) AllReadable() []Volume {
190 func (vm *RRVolumeManager) AllWritable() []Volume {
194 func (vm *RRVolumeManager) NextWritable() Volume {
195 if len(vm.writables) == 0 {
198 i := atomic.AddUint32(&vm.counter, 1)
199 return vm.writables[i%uint32(len(vm.writables))]
202 func (vm *RRVolumeManager) Close() {