7179: Improve description of how races must be handled.

[arvados.git] / services / keepstore / volume.go

// A Volume is an interface representing a Keep back-end storage unit:
// for example, a single mounted disk, a RAID array, an Amazon S3 volume,
// etc.

package main

import (
        "io"
        "sync/atomic"
        "time"
)

type Volume interface {
        // Get a block. IFF the returned error is nil, the caller must
        // put the returned slice back into the buffer pool when it's
        // finished with it.
        //
        // loc is guaranteed to consist of 32 or more lowercase hex
        // digits.
        //
        // Get should not verify the integrity of the returned data:
        // it should just return whatever was found in its backing
        // store.
        //
        // If an error is encountered that prevents it from
        // retrieving the data, that error should be returned so the
        // caller can log (and send to the client) a more useful
        // message.
        //
        // If the error is "not found", and there's no particular
        // reason to expect the block to be found (other than that a
        // caller is asking for it), the returned error should satisfy
        // os.IsNotExist(err): this is a normal condition and will not
        // be logged as an error (except that a 404 will appear in the
        // access log if the block is not found on any other volumes
        // either).
        //
        // If the data in the backing store is bigger than BLOCKSIZE,
        // Get is permitted to return an error without reading any of
        // the data.
        Get(loc string) ([]byte, error)

        // Put writes a block to an underlying storage device.
        //
        // loc is as described in Get.
        //
        // len(block) is guaranteed to be between 0 and BLOCKSIZE.
        //
        // If a block is already stored under the same name (loc) with
        // different content, Put must either overwrite the existing
        // data with the new data or return a non-nil error.
        //
        // Put also sets the timestamp for the given locator to the
        // current time.
        //
        // Put must return a non-nil error unless it can guarantee
        // that the entire block has been written and flushed to
        // persistent storage, and that its timestamp is current. Of
        // course, this guarantee is only as good as the underlying
        // storage device, but it is Put's responsibility to at least
        // get whatever guarantee is offered by the storage device.
        //
        // Put should not verify that loc==hash(block): this is the
        // caller's responsibility.
        Put(loc string, block []byte) error

        // Touch sets the timestamp for the given locator to the
        // current time.
        //
        // loc is as described in Get.
        //
        // Touch must return a non-nil error unless it can guarantee
        // that a future call to Mtime() will return a timestamp newer
        // than {now minus one second}.
        Touch(loc string) error

        // Mtime returns the stored timestamp for the given locator.
        //
        // loc is as described in Get.
        //
        // Mtime must return a non-nil error if the given block is not
        // found or the timestamp could not be retrieved.
        Mtime(loc string) (time.Time, error)

        // IndexTo writes a complete list of locators with the given
        // prefix for which Get() can retrieve data.
        //
        // prefix consists of zero or more lowercase hexadecimal
        // digits.
        //
        // Each locator must be written to the given writer using the
        // following format:
        //
        //   loc "+" size " " timestamp "\n"
        //
        // where:
        //
        //   - size is the number of bytes of content, given as a
        //     decimal number with one or more digits
        //     
        //   - timestamp is the timestamp stored for the locator,
        //     given as a decimal number of seconds after January 1,
        //     1970 UTC.
        //
        // IndexTo must not write any other data to writer: for
        // example, it must not write any blank lines.
        //
        // If an error makes it impossible to provide a complete
        // index, IndexTo must return a non-nil error. It is
        // acceptable to return a non-nil error after writing a
        // partial index to writer.
        //
        // The resulting index is not expected to be sorted in any
        // particular order.
        IndexTo(prefix string, writer io.Writer) error

        // Delete deletes the block data from the underlying storage
        // device.
        //
        // loc is as described in Get.
        //
        // If the timestamp for the given locator is newer than
        // blob_signature_ttl, Delete must not delete the data.
        //
        // If a Delete operation overlaps with any Touch or Put
        // operations on the same locator, the implementation must
        // ensure one of the following outcomes:
        //
        //   - Touch and Put return a non-nil error, or
        //   - Delete does not delete the block, or
        //   - Both of the above.
        //
        // If it is possible for the storage device to be accessed by
        // a different process or host, the synchronization mechanism
        // should also guard against races with other processes and
        // hosts. If such a mechanism is not available, there must be
        // a mechanism for detecting unsafe configurations, alerting
        // the operator, and aborting or falling back to a read-only
        // state. In other words, running multiple keepstore processes
        // with the same underlying storage device must either work
        // reliably or fail outright.
        //
        // Corollary: A successful Touch or Put guarantees a block
        // will not be deleted for at least blob_signature_ttl
        // seconds.
        Delete(loc string) error

        // Status returns a *VolumeStatus representing the current
        // in-use and available storage capacity and an
        // implementation-specific volume identifier (e.g., "mount
        // point" for a UnixVolume).
        Status() *VolumeStatus

        // String returns an identifying label for this volume,
        // suitable for including in log messages. It should contain
        // enough information to uniquely identify the underlying
        // storage device, but should not contain any credentials or
        // secrets.
        String() string

        // Writable returns false if all future Put, Mtime, and Delete
        // calls are expected to fail.
        //
        // If the volume is only temporarily unwritable -- or if Put
        // will fail because it is full, but Mtime or Delete can
        // succeed -- then Writable should return false.
        Writable() bool
}

// A VolumeManager tells callers which volumes can read, which volumes
// can write, and on which volume the next write should be attempted.
type VolumeManager interface {
        // AllReadable returns all volumes.
        AllReadable() []Volume
        // AllWritable returns all volumes that aren't known to be in
        // a read-only state. (There is no guarantee that a write to
        // one will succeed, though.)
        AllWritable() []Volume
        // NextWritable returns the volume where the next new block
        // should be written. A VolumeManager can select a volume in
        // order to distribute activity across spindles, fill up disks
        // with more free space, etc.
        NextWritable() Volume
        // Close shuts down the volume manager cleanly.
        Close()
}

type RRVolumeManager struct {
        readables []Volume
        writables []Volume
        counter   uint32
}

func MakeRRVolumeManager(volumes []Volume) *RRVolumeManager {
        vm := &RRVolumeManager{}
        for _, v := range volumes {
                vm.readables = append(vm.readables, v)
                if v.Writable() {
                        vm.writables = append(vm.writables, v)
                }
        }
        return vm
}

func (vm *RRVolumeManager) AllReadable() []Volume {
        return vm.readables
}

func (vm *RRVolumeManager) AllWritable() []Volume {
        return vm.writables
}

func (vm *RRVolumeManager) NextWritable() Volume {
        if len(vm.writables) == 0 {
                return nil
        }
        i := atomic.AddUint32(&vm.counter, 1)
        return vm.writables[i%uint32(len(vm.writables))]
}

func (vm *RRVolumeManager) Close() {
}