open-consul/agent/consul/state/memdb.go

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package state
import (
"fmt"
"github.com/hashicorp/consul/agent/consul/stream"
"github.com/hashicorp/go-memdb"
)
// ReadTxn is implemented by memdb.Txn to perform read operations.
type ReadTxn interface {
Get(table, index string, args ...interface{}) (memdb.ResultIterator, error)
First(table, index string, args ...interface{}) (interface{}, error)
FirstWatch(table, index string, args ...interface{}) (<-chan struct{}, interface{}, error)
Abort()
}
// Changes wraps a memdb.Changes to include the index at which these changes
// were made.
type Changes struct {
// Index is the latest index at the time these changes were committed.
Index uint64
Changes memdb.Changes
}
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// changeTrackerDB is a thin wrapper around memdb.DB which enables TrackChanges on
// all write transactions. When the transaction is committed the changes are
// sent to the eventPublisher which will create and emit change events.
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type changeTrackerDB struct {
db *memdb.MemDB
publisher eventPublisher
processChanges func(ReadTxn, Changes) ([]stream.Event, error)
}
type eventPublisher interface {
Publish(events []stream.Event)
}
// Txn exists to maintain backwards compatibility with memdb.DB.Txn. Preexisting
// code may use it to create a read-only transaction, but it will panic if called
// with write=true.
//
// Deprecated: use either ReadTxn, or WriteTxn.
func (c *changeTrackerDB) Txn(write bool) *txn {
if write {
panic("don't use db.Txn(true), use db.WriteTxn(idx uin64)")
}
return c.ReadTxn()
}
// ReadTxn returns a read-only transaction which behaves exactly the same as
// memdb.Txn
//
// TODO: this could return a regular memdb.Txn if all the state functions accepted
// the ReadTxn interface
func (c *changeTrackerDB) ReadTxn() *txn {
return &txn{Txn: c.db.Txn(false)}
}
// WriteTxn returns a wrapped memdb.Txn suitable for writes to the state store.
// It will track changes and publish events for the changes when Commit
// is called.
//
// The idx argument must be the index of the current Raft operation. Almost
// all mutations to state should happen as part of a raft apply so the index of
// the log being applied can be passed to WriteTxn.
// The exceptional cases are transactions that are executed on an empty
// memdb.DB as part of Restore, and those executed by tests where we insert
// data directly into the DB. These cases may use WriteTxnRestore.
func (c *changeTrackerDB) WriteTxn(idx uint64) *txn {
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t := &txn{
Txn: c.db.Txn(true),
Index: idx,
publish: c.publish,
}
t.Txn.TrackChanges()
return t
}
func (c *changeTrackerDB) publish(changes Changes) error {
readOnlyTx := c.db.Txn(false)
defer readOnlyTx.Abort()
events, err := c.processChanges(readOnlyTx, changes)
if err != nil {
return fmt.Errorf("failed generating events from changes: %v", err)
}
c.publisher.Publish(events)
return nil
}
// WriteTxnRestore returns a wrapped RW transaction that does NOT have change
// tracking enabled. This should only be used in Restore where we need to
// replace the entire contents of the Store without a need to track the changes.
// WriteTxnRestore uses a zero index since the whole restore doesn't really occur
// at one index - the effect is to write many values that were previously
// written across many indexes.
func (c *changeTrackerDB) WriteTxnRestore() *txn {
return &txn{
Txn: c.db.Txn(true),
Index: 0,
}
}
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// txn wraps a memdb.Txn to capture changes and send them to the EventPublisher.
//
// This can not be done with txn.Defer because the callback passed to Defer is
// invoked after commit completes, and because the callback can not return an
// error. Any errors from the callback would be lost, which would result in a
// missing change event, even though the state store had changed.
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type txn struct {
*memdb.Txn
// Index in raft where the write is occurring. The value is zero for a
// read-only, or WriteTxnRestore transaction.
// Index is stored so that it may be passed along to any subscribers as part
// of a change event.
Index uint64
publish func(changes Changes) error
}
// Commit first pushes changes to EventPublisher, then calls Commit on the
// underlying transaction.
//
// Note that this function, unlike memdb.Txn, returns an error which must be checked
// by the caller. A non-nil error indicates that a commit failed and was not
// applied.
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func (tx *txn) Commit() error {
// publish may be nil if this is a read-only or WriteTxnRestore transaction.
// In those cases changes should also be empty, and there will be nothing
// to publish.
if tx.publish != nil {
changes := Changes{
Index: tx.Index,
Changes: tx.Txn.Changes(),
}
if err := tx.publish(changes); err != nil {
return err
}
}
tx.Txn.Commit()
return nil
}
// TODO: may be replaced by a gRPC type.
type topic string
func (t topic) String() string {
return string(t)
}
func processDBChanges(tx ReadTxn, changes Changes) ([]stream.Event, error) {
// TODO: add other table handlers here.
return aclChangeUnsubscribeEvent(tx, changes)
}
func newSnapshotHandlers() stream.SnapshotHandlers {
return stream.SnapshotHandlers{}
}