services/syncbase/store/model.go

// Copyright 2015 The Vanadium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package store defines the API for the syncbase storage engine.
// Currently, this API and its implementations are meant to be internal.
package store

// StoreReader reads data from a CRUD-capable storage engine.
type StoreReader interface {
	// Get returns the value for the given key. The returned slice may be a
	// sub-slice of valbuf if valbuf was large enough to hold the entire value.
	// Otherwise, a newly allocated slice will be returned. It is valid to pass a
	// nil valbuf.
	// If the given key is unknown, valbuf is returned unchanged and the function
	// fails with ErrUnknownKey.
	//
	// It is safe to modify the contents of the key after Get returns.
	Get(key, valbuf []byte) ([]byte, error)

	// Scan returns all rows with keys in range [start, limit). If limit is "",
	// all rows with keys >= start are included.
	// Concurrency semantics: It is legal to perform writes concurrently with
	// Scan. The returned stream may or may not reflect subsequent writes to keys
	// not yet reached by the stream.
	//
	// It is safe to modify the contents of the arguments after Scan returns.
	Scan(start, limit []byte) Stream
}

// StoreWriter writes data to a CRUD-capable storage engine.
type StoreWriter interface {
	// Put writes the given value for the given key.
	//
	// WARNING: For performance reasons, a Put inside a transaction doesn't make
	// a defensive copy of the value. The client MUST keep the value unchanged
	// until the transaction commits or aborts.
	//
	// It is safe to modify the contents of the key after Put returns.
	Put(key, value []byte) error

	// Delete deletes the entry for the given key.
	// Succeeds (no-op) if the given key is unknown.
	//
	// It is safe to modify the contents of the key after Delete returns.
	Delete(key []byte) error
}

// Store is a CRUD-capable storage engine that supports transactions.
type Store interface {
	StoreReader
	StoreWriter

	// Close closes the store.
	Close() error

	// NewTransaction creates a transaction.
	// TODO(rogulenko): add transaction options.
	NewTransaction() Transaction

	// NewSnapshot creates a snapshot.
	// TODO(rogulenko): add snapshot options.
	NewSnapshot() Snapshot
}

// SnapshotOrTransaction represents a Snapshot or a Transaction.
type SnapshotOrTransaction interface {
	StoreReader

	// Abort closes the snapshot or transaction.
	// Any subsequent method calls will fail.
	// NOTE: this method is also used to distinguish between StoreReader and
	// SnapshotOrTransaction.
	Abort() error
}

// Snapshot is a handle to particular state in time of a Store.
//
// All read operations are executed against a consistent view of Store commit
// history. Snapshots don't acquire locks and thus don't block transactions.
type Snapshot interface {
	SnapshotOrTransaction

	// __snapshotSpec is a utility method to distinguish between Snapshot and
	// SnapshotOrTransaction. This is a no-op.
	__snapshotSpec()
}

// Transaction provides a mechanism for atomic reads and writes. Instead of
// calling this function directly, clients are encouraged to use the
// RunInTransaction() helper function, which detects "concurrent transaction"
// errors and handles retries internally.
//
// Default concurrency semantics:
// - Reads (e.g. gets, scans) inside a transaction operate over a consistent
//   snapshot taken during NewTransaction(), and will see the effects of prior
//   writes performed inside the transaction.
// - Commit() may fail with ErrConcurrentTransaction, indicating that after
//   NewTransaction() but before Commit(), some concurrent routine wrote to a
//   key that matches a key or row-range read inside this transaction.
// - Other methods will never fail with error ErrConcurrentTransaction, even if
//   it is known that Commit() will fail with this error.
//
// Once a transaction has been committed or aborted, subsequent method calls
// will fail with no effect.
type Transaction interface {
	SnapshotOrTransaction
	StoreWriter

	// Commit commits the transaction.
	// Fails if writes from outside this transaction conflict with reads from
	// within this transaction.
	Commit() error
}

// Stream is an interface for iterating through a collection of key-value pairs.
type Stream interface {
	// Advance stages an element so the client can retrieve it with Key or Value.
	// Advance returns true iff there is an element to retrieve. The client must
	// call Advance before calling Key or Value. The client must call Cancel if it
	// does not iterate through all elements (i.e. until Advance returns false).
	// Advance may block if an element is not immediately available.
	Advance() bool

	// Key returns the key of the element that was staged by Advance. The returned
	// slice may be a sub-slice of keybuf if keybuf was large enough to hold the
	// entire key. Otherwise, a newly allocated slice will be returned. It is
	// valid to pass a nil keybuf.
	// Key may panic if Advance returned false or was not called at all.
	// Key does not block.
	Key(keybuf []byte) []byte

	// Value returns the value of the element that was staged by Advance. The
	// returned slice may be a sub-slice of valbuf if valbuf was large enough to
	// hold the entire value. Otherwise, a newly allocated slice will be returned.
	// It is valid to pass a nil valbuf.
	// Value may panic if Advance returned false or was not called at all.
	// Value does not block.
	Value(valbuf []byte) []byte

	// Err returns a non-nil error iff the stream encountered any errors. Err does
	// not block.
	Err() error

	// Cancel notifies the stream provider that it can stop producing elements.
	// The client must call Cancel if it does not iterate through all elements
	// (i.e. until Advance returns false). Cancel is idempotent and can be called
	// concurrently with a goroutine that is iterating via Advance/Key/Value.
	// Cancel causes Advance to subsequently return false. Cancel does not block.
	Cancel()
}