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Published: Jul 22, 2021 License: MIT Imports: 16 Imported by: 0



Package abstractlist provides a utility for managing peer availability with a separate implementation of peer selection from just among available peers. The peer list implements the peer.ChooserList interface and accepts an abstractlist.Implementation to provide the implementation-specific concern of, for example, a *roundrobin.List.

The example is an implementation of peer.ChooserList using a random peer selection strategy, returned by newRandomListImplementation(), implementing abstractlist.Implementation.

type List struct {

func New(transport peer.Transport) *List {
	return &List{
		List: abstractlist.New(

The abstract peer list is designed to take responsibility for concurrently communicating with three parties: the outbound (which sees it as a peer.Chooser), the peer list updater (which see it as a peer.List), and the transport (which sees it as a bank of peer.Subscriber). By taking care of concurrency, the abstract peer list frees the Implementation from the concern of thread safety.



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type Implementation

type Implementation interface {
	Add(peer.StatusPeer, peer.Identifier) Subscriber
	Remove(peer.StatusPeer, peer.Identifier, Subscriber)
	Choose(*transport.Request) peer.StatusPeer

Implementation is a collection of available peers, with its own subscribers for pending request count change notifications. The abstract list uses the implementation to track peers that can be returned by Choose, as opposed to those that were added but reported unavailable by the underlying transport. Use "go.uber.org/yarpc/peer/abstractlist".List in conjunction with an Implementation to produce a "go.uber.org/yarpc/api/peer".ChooserList.

The abstractlist.List calls Add, Remove, and Choose under a write lock so the implementation is free to perform mutations on its own data without locks.

It should be noted that since the abstractlist.List is using a write lock for Add, Remove and Choose, performances of the abstract.List might be limited for a given implementation. For instance, a tworandomchoices implementation would not need a write lock for the method Choose but only a read lock.

Choose must return nil immediately if the collection is empty. The abstractlist.List guarantees that peers will only be added if they're absent, and only removed they are present. Choose should not block.

type List

type List struct {
	// contains filtered or unexported fields

List is an abstract peer list, backed by an Implementation to determine which peer to choose among available peers. The abstract list manages available versus unavailable peers, intercepting these notifications from the transport's concrete implementation of peer.StatusPeer with the peer.Subscriber API. The peer list will not choose an unavailable peer, prefering to block until one becomes available.

The list is a suitable basis for concrete implementations like round-robin.

This abstract list does not participate in the transport’s pending request count tracking. The list tracks pending request counts for the peers that it chooses, does not inform the transport of these choices, and ignores notifications from the transport about choices other peer lists that share the same peers have made.

func New

func New(name string, transport peer.Transport, implementation Implementation, opts ...Option) *List

New creates a new peer list with an identifier chooser for available peers.

func (*List) Available

func (pl *List) Available(pid peer.Identifier) bool

Available returns whether the identifier peer is available for traffic.

func (*List) Choose

func (pl *List) Choose(ctx context.Context, req *transport.Request) (peer.Peer, func(error), error)

Choose selects the next available peer in the peer list.

func (*List) Introspect

func (pl *List) Introspect() introspection.ChooserStatus

Introspect returns a ChooserStatus with a summary of the Peers.

func (*List) IsRunning

func (pl *List) IsRunning() bool

IsRunning returns whether the peer list is running.

func (*List) Name

func (pl *List) Name() string

Name returns the name of the list.

func (*List) NotifyStatusChanged

func (pl *List) NotifyStatusChanged(pid peer.Identifier)

NotifyStatusChanged receives status change notifications for peers in the list.

This function exists only as is necessary for dispatching connection status changes from tests.

func (*List) NumAvailable

func (pl *List) NumAvailable() int

NumAvailable returns how many peers are available.

func (*List) NumUnavailable

func (pl *List) NumUnavailable() int

NumUnavailable returns how many peers are unavailable while the list is running.

func (*List) NumUninitialized

func (pl *List) NumUninitialized() int

NumUninitialized returns how many peers are unavailable because the peer list was stopped or has not yet started.

func (*List) Peers

func (pl *List) Peers() []peer.StatusPeer

Peers returns a snapshot of all retained (available and unavailable) peers.

func (*List) Start

func (pl *List) Start() error

Start notifies the List that requests will start coming

func (*List) Stop

func (pl *List) Stop() error

Stop notifies the List that requests will stop coming

func (*List) Transport

func (pl *List) Transport() peer.Transport

Transport returns the underlying transport for retaining and releasing peers.

func (*List) Uninitialized

func (pl *List) Uninitialized(pid peer.Identifier) bool

Uninitialized returns whether the identifier peer is present but uninitialized.

func (*List) Update

func (pl *List) Update(updates peer.ListUpdates) error

Update applies the additions and removals of peer Identifiers to the list it returns a multi-error result of every failure that happened without circuit breaking due to failures.

Updates must be serialized so no peer is removed if it is absent and no peer is added if it is present. Updates should not have overlapping additions and removals, but the list will tollerate this case, but may cause existing connections to close and be replaced.

Update will return errors if its invariants are violated, regardless of whether updates are sent while the list is running. Updates may be interleaved with Start and Stop in any order any number of times.

type Option

type Option interface {
	// contains filtered or unexported methods

Option customizes the behavior of a list.

func Capacity

func Capacity(capacity int) Option

Capacity specifies the default capacity of the underlying data structures for this list

Defaults to 10.

func DefaultChooseTimeout added in v1.46.0

func DefaultChooseTimeout(timeout time.Duration) Option

DefaultChooseTimeout specifies the default timeout to add to 'Choose' calls without context deadlines. This prevents long-lived streams from setting calling deadlines.

Defaults to 500ms.

func FailFast

func FailFast() Option

FailFast indicates that the peer list should not wait for peers to be added, when choosing a peer.

This option is particularly useful for proxies.

func Logger

func Logger(logger *zap.Logger) Option

Logger specifies a logger.

func NoShuffle

func NoShuffle() Option

NoShuffle disables the default behavior of shuffling peer list order.

func Seed

func Seed(seed int64) Option

Seed specifies the random seed to use for shuffling peers

Defaults to approximately the process start time in nanoseconds.

type Subscriber

type Subscriber interface {

Subscriber is a callback that implementations of peer list data structures must provide.

The peer list uses the Subscriber to send notifications when a peer’s pending request count changes. A peer list implementation may have a single subscriber or a subscriber for each peer. UpdatePendingRequestCount is thread safe to be called

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