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// Copyright (C) MongoDB, Inc. 2017-present.
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may
// not use this file except in compliance with the License. You may obtain
// a copy of the License at http://www.apache.org/licenses/LICENSE-2.0
// Package topology contains types that handles the discovery, monitoring, and selection
// of servers. This package is designed to expose enough inner workings of service discovery
// and monitoring to allow low level applications to have fine grained control, while hiding
// most of the detailed implementation of the algorithms.
package topology // import "go.mongodb.org/mongo-driver/x/mongo/driver/topology"
import (
"context"
"errors"
"math/rand"
"strings"
"sync"
"sync/atomic"
"time"
"fmt"
"go.mongodb.org/mongo-driver/bson/primitive"
"go.mongodb.org/mongo-driver/event"
"go.mongodb.org/mongo-driver/internal/randutil"
"go.mongodb.org/mongo-driver/mongo/address"
"go.mongodb.org/mongo-driver/mongo/description"
"go.mongodb.org/mongo-driver/x/mongo/driver"
"go.mongodb.org/mongo-driver/x/mongo/driver/dns"
)
// ErrSubscribeAfterClosed is returned when a user attempts to subscribe to a
// closed Server or Topology.
var ErrSubscribeAfterClosed = errors.New("cannot subscribe after closeConnection")
// ErrTopologyClosed is returned when a user attempts to call a method on a
// closed Topology.
var ErrTopologyClosed = errors.New("topology is closed")
// ErrTopologyConnected is returned whena user attempts to Connect to an
// already connected Topology.
var ErrTopologyConnected = errors.New("topology is connected or connecting")
// ErrServerSelectionTimeout is returned from server selection when the server
// selection process took longer than allowed by the timeout.
var ErrServerSelectionTimeout = errors.New("server selection timeout")
// MonitorMode represents the way in which a server is monitored.
type MonitorMode uint8
// random is a package-global pseudo-random number generator.
var random = randutil.NewLockedRand(rand.NewSource(time.Now().UnixNano()))
// These constants are the available monitoring modes.
const (
AutomaticMode MonitorMode = iota
SingleMode
)
// Topology represents a MongoDB deployment.
type Topology struct {
connectionstate int64
cfg *config
desc atomic.Value // holds a description.Topology
dnsResolver *dns.Resolver
done chan struct{}
pollingRequired bool
pollingDone chan struct{}
pollingwg sync.WaitGroup
rescanSRVInterval time.Duration
pollHeartbeatTime atomic.Value // holds a bool
updateCallback updateTopologyCallback
fsm *fsm
// This should really be encapsulated into it's own type. This will likely
// require a redesign so we can share a minimum of data between the
// subscribers and the topology.
subscribers map[uint64]chan description.Topology
currentSubscriberID uint64
subscriptionsClosed bool
subLock sync.Mutex
// We should redesign how we Connect and handle individal servers. This is
// too difficult to maintain and it's rather easy to accidentally access
// the servers without acquiring the lock or checking if the servers are
// closed. This lock should also be an RWMutex.
serversLock sync.Mutex
serversClosed bool
servers map[address.Address]*Server
id primitive.ObjectID
}
var _ driver.Deployment = &Topology{}
var _ driver.Subscriber = &Topology{}
type serverSelectionState struct {
selector description.ServerSelector
timeoutChan <-chan time.Time
}
func newServerSelectionState(selector description.ServerSelector, timeoutChan <-chan time.Time) serverSelectionState {
return serverSelectionState{
selector: selector,
timeoutChan: timeoutChan,
}
}
// New creates a new topology.
func New(opts ...Option) (*Topology, error) {
cfg, err := newConfig(opts...)
if err != nil {
return nil, err
}
t := &Topology{
cfg: cfg,
done: make(chan struct{}),
pollingDone: make(chan struct{}),
rescanSRVInterval: 60 * time.Second,
fsm: newFSM(),
subscribers: make(map[uint64]chan description.Topology),
servers: make(map[address.Address]*Server),
dnsResolver: dns.DefaultResolver,
id: primitive.NewObjectID(),
}
t.desc.Store(description.Topology{})
t.updateCallback = func(desc description.Server) description.Server {
return t.apply(context.TODO(), desc)
}
if t.cfg.uri != "" {
t.pollingRequired = strings.HasPrefix(t.cfg.uri, "mongodb+srv://") && !t.cfg.loadBalanced
}
t.publishTopologyOpeningEvent()
return t, nil
}
// Connect initializes a Topology and starts the monitoring process. This function
// must be called to properly monitor the topology.
func (t *Topology) Connect() error {
if !atomic.CompareAndSwapInt64(&t.connectionstate, disconnected, connecting) {
return ErrTopologyConnected
}
t.desc.Store(description.Topology{})
var err error
t.serversLock.Lock()
// A replica set name sets the initial topology type to ReplicaSetNoPrimary unless a direct connection is also
// specified, in which case the initial type is Single.
if t.cfg.replicaSetName != "" {
t.fsm.SetName = t.cfg.replicaSetName
t.fsm.Kind = description.ReplicaSetNoPrimary
}
// A direct connection unconditionally sets the topology type to Single.
if t.cfg.mode == SingleMode {
t.fsm.Kind = description.Single
}
for _, a := range t.cfg.seedList {
addr := address.Address(a).Canonicalize()
t.fsm.Servers = append(t.fsm.Servers, description.NewDefaultServer(addr))
}
switch {
case t.cfg.loadBalanced:
// In LoadBalanced mode, we mock a series of events: TopologyDescriptionChanged from Unknown to LoadBalanced,
// ServerDescriptionChanged from Unknown to LoadBalancer, and then TopologyDescriptionChanged to reflect the
// previous ServerDescriptionChanged event. We publish all of these events here because we don't start server
// monitoring routines in this mode, so we have to mock state changes.
// Transition from Unknown with no servers to LoadBalanced with a single Unknown server.
t.fsm.Kind = description.LoadBalanced
t.publishTopologyDescriptionChangedEvent(description.Topology{}, t.fsm.Topology)
addr := address.Address(t.cfg.seedList[0]).Canonicalize()
if err := t.addServer(addr); err != nil {
t.serversLock.Unlock()
return err
}
// Transition the server from Unknown to LoadBalancer.
newServerDesc := t.servers[addr].Description()
t.publishServerDescriptionChangedEvent(t.fsm.Servers[0], newServerDesc)
// Transition from LoadBalanced with an Unknown server to LoadBalanced with a LoadBalancer.
oldDesc := t.fsm.Topology
t.fsm.Servers = []description.Server{newServerDesc}
t.desc.Store(t.fsm.Topology)
t.publishTopologyDescriptionChangedEvent(oldDesc, t.fsm.Topology)
default:
// In non-LB mode, we only publish an initial TopologyDescriptionChanged event from Unknown with no servers to
// the current state (e.g. Unknown with one or more servers if we're discovering or Single with one server if
// we're connecting directly). Other events are published when state changes occur due to responses in the
// server monitoring goroutines.
newDesc := description.Topology{
Kind: t.fsm.Kind,
Servers: t.fsm.Servers,
SessionTimeoutMinutes: t.fsm.SessionTimeoutMinutes,
}
t.desc.Store(newDesc)
t.publishTopologyDescriptionChangedEvent(description.Topology{}, t.fsm.Topology)
for _, a := range t.cfg.seedList {
addr := address.Address(a).Canonicalize()
err = t.addServer(addr)
if err != nil {
t.serversLock.Unlock()
return err
}
}
}
t.serversLock.Unlock()
if t.pollingRequired {
go t.pollSRVRecords()
t.pollingwg.Add(1)
}
t.subscriptionsClosed = false // explicitly set in case topology was disconnected and then reconnected
atomic.StoreInt64(&t.connectionstate, connected)
return nil
}
// Disconnect closes the topology. It stops the monitoring thread and
// closes all open subscriptions.
func (t *Topology) Disconnect(ctx context.Context) error {
if !atomic.CompareAndSwapInt64(&t.connectionstate, connected, disconnecting) {
return ErrTopologyClosed
}
servers := make(map[address.Address]*Server)
t.serversLock.Lock()
t.serversClosed = true
for addr, server := range t.servers {
servers[addr] = server
}
t.serversLock.Unlock()
for _, server := range servers {
_ = server.Disconnect(ctx)
t.publishServerClosedEvent(server.address)
}
t.subLock.Lock()
for id, ch := range t.subscribers {
close(ch)
delete(t.subscribers, id)
}
t.subscriptionsClosed = true
t.subLock.Unlock()
if t.pollingRequired {
t.pollingDone <- struct{}{}
t.pollingwg.Wait()
}
t.desc.Store(description.Topology{})
atomic.StoreInt64(&t.connectionstate, disconnected)
t.publishTopologyClosedEvent()
return nil
}
// Description returns a description of the topology.
func (t *Topology) Description() description.Topology {
td, ok := t.desc.Load().(description.Topology)
if !ok {
td = description.Topology{}
}
return td
}
// Kind returns the topology kind of this Topology.
func (t *Topology) Kind() description.TopologyKind { return t.Description().Kind }
// Subscribe returns a Subscription on which all updated description.Topologys
// will be sent. The channel of the subscription will have a buffer size of one,
// and will be pre-populated with the current description.Topology.
// Subscribe implements the driver.Subscriber interface.
func (t *Topology) Subscribe() (*driver.Subscription, error) {
if atomic.LoadInt64(&t.connectionstate) != connected {
return nil, errors.New("cannot subscribe to Topology that is not connected")
}
ch := make(chan description.Topology, 1)
td, ok := t.desc.Load().(description.Topology)
if !ok {
td = description.Topology{}
}
ch <- td
t.subLock.Lock()
defer t.subLock.Unlock()
if t.subscriptionsClosed {
return nil, ErrSubscribeAfterClosed
}
id := t.currentSubscriberID
t.subscribers[id] = ch
t.currentSubscriberID++
return &driver.Subscription{
Updates: ch,
ID: id,
}, nil
}
// Unsubscribe unsubscribes the given subscription from the topology and closes the subscription channel.
// Unsubscribe implements the driver.Subscriber interface.
func (t *Topology) Unsubscribe(sub *driver.Subscription) error {
t.subLock.Lock()
defer t.subLock.Unlock()
if t.subscriptionsClosed {
return nil
}
ch, ok := t.subscribers[sub.ID]
if !ok {
return nil
}
close(ch)
delete(t.subscribers, sub.ID)
return nil
}
// RequestImmediateCheck will send heartbeats to all the servers in the
// topology right away, instead of waiting for the heartbeat timeout.
func (t *Topology) RequestImmediateCheck() {
if atomic.LoadInt64(&t.connectionstate) != connected {
return
}
t.serversLock.Lock()
for _, server := range t.servers {
server.RequestImmediateCheck()
}
t.serversLock.Unlock()
}
// SelectServer selects a server with given a selector. SelectServer complies with the
// server selection spec, and will time out after severSelectionTimeout or when the
// parent context is done.
func (t *Topology) SelectServer(ctx context.Context, ss description.ServerSelector) (driver.Server, error) {
if atomic.LoadInt64(&t.connectionstate) != connected {
return nil, ErrTopologyClosed
}
var ssTimeoutCh <-chan time.Time
if t.cfg.serverSelectionTimeout > 0 {
ssTimeout := time.NewTimer(t.cfg.serverSelectionTimeout)
ssTimeoutCh = ssTimeout.C
defer ssTimeout.Stop()
}
var doneOnce bool
var sub *driver.Subscription
selectionState := newServerSelectionState(ss, ssTimeoutCh)
for {
var suitable []description.Server
var selectErr error
if !doneOnce {
// for the first pass, select a server from the current description.
// this improves selection speed for up-to-date topology descriptions.
suitable, selectErr = t.selectServerFromDescription(t.Description(), selectionState)
doneOnce = true
} else {
// if the first pass didn't select a server, the previous description did not contain a suitable server, so
// we subscribe to the topology and attempt to obtain a server from that subscription
if sub == nil {
var err error
sub, err = t.Subscribe()
if err != nil {
return nil, err
}
defer t.Unsubscribe(sub)
}
suitable, selectErr = t.selectServerFromSubscription(ctx, sub.Updates, selectionState)
}
if selectErr != nil {
return nil, selectErr
}
if len(suitable) == 0 {
// try again if there are no servers available
continue
}
selected := suitable[random.Intn(len(suitable))]
selectedS, err := t.FindServer(selected)
switch {
case err != nil:
return nil, err
case selectedS != nil:
return selectedS, nil
default:
// We don't have an actual server for the provided description.
// This could happen for a number of reasons, including that the
// server has since stopped being a part of this topology, or that
// the server selector returned no suitable servers.
}
}
}
// FindServer will attempt to find a server that fits the given server description.
// This method will return nil, nil if a matching server could not be found.
func (t *Topology) FindServer(selected description.Server) (*SelectedServer, error) {
if atomic.LoadInt64(&t.connectionstate) != connected {
return nil, ErrTopologyClosed
}
t.serversLock.Lock()
defer t.serversLock.Unlock()
server, ok := t.servers[selected.Addr]
if !ok {
return nil, nil
}
desc := t.Description()
return &SelectedServer{
Server: server,
Kind: desc.Kind,
}, nil
}
// selectServerFromSubscription loops until a topology description is available for server selection. It returns
// when the given context expires, server selection timeout is reached, or a description containing a selectable
// server is available.
func (t *Topology) selectServerFromSubscription(ctx context.Context, subscriptionCh <-chan description.Topology,
selectionState serverSelectionState) ([]description.Server, error) {
current := t.Description()
for {
select {
case <-ctx.Done():
return nil, ServerSelectionError{Wrapped: ctx.Err(), Desc: current}
case <-selectionState.timeoutChan:
return nil, ServerSelectionError{Wrapped: ErrServerSelectionTimeout, Desc: current}
case current = <-subscriptionCh:
}
suitable, err := t.selectServerFromDescription(current, selectionState)
if err != nil {
return nil, err
}
if len(suitable) > 0 {
return suitable, nil
}
t.RequestImmediateCheck()
}
}
// selectServerFromDescription process the given topology description and returns a slice of suitable servers.
func (t *Topology) selectServerFromDescription(desc description.Topology,
selectionState serverSelectionState) ([]description.Server, error) {
// Unlike selectServerFromSubscription, this code path does not check ctx.Done or selectionState.timeoutChan because
// selecting a server from a description is not a blocking operation.
if desc.CompatibilityErr != nil {
return nil, desc.CompatibilityErr
}
// If the topology kind is LoadBalanced, the LB is the only server and it is always considered selectable. The
// selectors exported by the driver should already return the LB as a candidate, so this but this check ensures that
// the LB is always selectable even if a user of the low-level driver provides a custom selector.
if desc.Kind == description.LoadBalanced {
return desc.Servers, nil
}
var allowed []description.Server
for _, s := range desc.Servers {
if s.Kind != description.Unknown {
allowed = append(allowed, s)
}
}
suitable, err := selectionState.selector.SelectServer(desc, allowed)
if err != nil {
return nil, ServerSelectionError{Wrapped: err, Desc: desc}
}
return suitable, nil
}
func (t *Topology) pollSRVRecords() {
defer t.pollingwg.Done()
serverConfig, _ := newServerConfig(t.cfg.serverOpts...)
heartbeatInterval := serverConfig.heartbeatInterval
pollTicker := time.NewTicker(t.rescanSRVInterval)
defer pollTicker.Stop()
t.pollHeartbeatTime.Store(false)
var doneOnce bool
defer func() {
// ¯\_(ツ)_/¯
if r := recover(); r != nil && !doneOnce {
<-t.pollingDone
}
}()
// remove the scheme
uri := t.cfg.uri[14:]
hosts := uri
if idx := strings.IndexAny(uri, "/?@"); idx != -1 {
hosts = uri[:idx]
}
for {
select {
case <-pollTicker.C:
case <-t.pollingDone:
doneOnce = true
return
}
topoKind := t.Description().Kind
if !(topoKind == description.Unknown || topoKind == description.Sharded) {
break
}
parsedHosts, err := t.dnsResolver.ParseHosts(hosts, t.cfg.srvServiceName, false)
// DNS problem or no verified hosts returned
if err != nil || len(parsedHosts) == 0 {
if !t.pollHeartbeatTime.Load().(bool) {
pollTicker.Stop()
pollTicker = time.NewTicker(heartbeatInterval)
t.pollHeartbeatTime.Store(true)
}
continue
}
if t.pollHeartbeatTime.Load().(bool) {
pollTicker.Stop()
pollTicker = time.NewTicker(t.rescanSRVInterval)
t.pollHeartbeatTime.Store(false)
}
cont := t.processSRVResults(parsedHosts)
if !cont {
break
}
}
<-t.pollingDone
doneOnce = true
}
func (t *Topology) processSRVResults(parsedHosts []string) bool {
t.serversLock.Lock()
defer t.serversLock.Unlock()
if t.serversClosed {
return false
}
prev := t.fsm.Topology
diff := diffHostList(t.fsm.Topology, parsedHosts)
if len(diff.Added) == 0 && len(diff.Removed) == 0 {
return true
}
for _, r := range diff.Removed {
addr := address.Address(r).Canonicalize()
s, ok := t.servers[addr]
if !ok {
continue
}
go func() {
cancelCtx, cancel := context.WithCancel(context.Background())
cancel()
_ = s.Disconnect(cancelCtx)
}()
delete(t.servers, addr)
t.fsm.removeServerByAddr(addr)
t.publishServerClosedEvent(s.address)
}
// Now that we've removed all the hosts that disappeared from the SRV record, we need to add any
// new hosts added to the SRV record. If adding all of the new hosts would increase the number
// of servers past srvMaxHosts, shuffle the list of added hosts.
if t.cfg.srvMaxHosts > 0 && len(t.servers)+len(diff.Added) > t.cfg.srvMaxHosts {
random.Shuffle(len(diff.Added), func(i, j int) {
diff.Added[i], diff.Added[j] = diff.Added[j], diff.Added[i]
})
}
// Add all added hosts until the number of servers reaches srvMaxHosts.
for _, a := range diff.Added {
if t.cfg.srvMaxHosts > 0 && len(t.servers) >= t.cfg.srvMaxHosts {
break
}
addr := address.Address(a).Canonicalize()
_ = t.addServer(addr)
t.fsm.addServer(addr)
}
//store new description
newDesc := description.Topology{
Kind: t.fsm.Kind,
Servers: t.fsm.Servers,
SessionTimeoutMinutes: t.fsm.SessionTimeoutMinutes,
}
t.desc.Store(newDesc)
if !prev.Equal(newDesc) {
t.publishTopologyDescriptionChangedEvent(prev, newDesc)
}
t.subLock.Lock()
for _, ch := range t.subscribers {
// We drain the description if there's one in the channel
select {
case <-ch:
default:
}
ch <- newDesc
}
t.subLock.Unlock()
return true
}
// apply updates the Topology and its underlying FSM based on the provided server description and returns the server
// description that should be stored.
func (t *Topology) apply(ctx context.Context, desc description.Server) description.Server {
t.serversLock.Lock()
defer t.serversLock.Unlock()
ind, ok := t.fsm.findServer(desc.Addr)
if t.serversClosed || !ok {
return desc
}
prev := t.fsm.Topology
oldDesc := t.fsm.Servers[ind]
if oldDesc.TopologyVersion.CompareToIncoming(desc.TopologyVersion) > 0 {
return oldDesc
}
var current description.Topology
var err error
current, desc, err = t.fsm.apply(desc)
if err != nil {
return desc
}
if !oldDesc.Equal(desc) {
t.publishServerDescriptionChangedEvent(oldDesc, desc)
}
diff := diffTopology(prev, current)
for _, removed := range diff.Removed {
if s, ok := t.servers[removed.Addr]; ok {
go func() {
cancelCtx, cancel := context.WithCancel(ctx)
cancel()
_ = s.Disconnect(cancelCtx)
}()
delete(t.servers, removed.Addr)
t.publishServerClosedEvent(s.address)
}
}
for _, added := range diff.Added {
_ = t.addServer(added.Addr)
}
t.desc.Store(current)
if !prev.Equal(current) {
t.publishTopologyDescriptionChangedEvent(prev, current)
}
t.subLock.Lock()
for _, ch := range t.subscribers {
// We drain the description if there's one in the channel
select {
case <-ch:
default:
}
ch <- current
}
t.subLock.Unlock()
return desc
}
func (t *Topology) addServer(addr address.Address) error {
if _, ok := t.servers[addr]; ok {
return nil
}
svr, err := ConnectServer(addr, t.updateCallback, t.id, t.cfg.serverOpts...)
if err != nil {
return err
}
t.servers[addr] = svr
return nil
}
// String implements the Stringer interface
func (t *Topology) String() string {
desc := t.Description()
serversStr := ""
t.serversLock.Lock()
defer t.serversLock.Unlock()
for _, s := range t.servers {
serversStr += "{ " + s.String() + " }, "
}
return fmt.Sprintf("Type: %s, Servers: [%s]", desc.Kind, serversStr)
}
// publishes a ServerDescriptionChangedEvent to indicate the server description has changed
func (t *Topology) publishServerDescriptionChangedEvent(prev description.Server, current description.Server) {
serverDescriptionChanged := &event.ServerDescriptionChangedEvent{
Address: current.Addr,
TopologyID: t.id,
PreviousDescription: prev,
NewDescription: current,
}
if t.cfg.serverMonitor != nil && t.cfg.serverMonitor.ServerDescriptionChanged != nil {
t.cfg.serverMonitor.ServerDescriptionChanged(serverDescriptionChanged)
}
}
// publishes a ServerClosedEvent to indicate the server has closed
func (t *Topology) publishServerClosedEvent(addr address.Address) {
serverClosed := &event.ServerClosedEvent{
Address: addr,
TopologyID: t.id,
}
if t.cfg.serverMonitor != nil && t.cfg.serverMonitor.ServerClosed != nil {
t.cfg.serverMonitor.ServerClosed(serverClosed)
}
}
// publishes a TopologyDescriptionChangedEvent to indicate the topology description has changed
func (t *Topology) publishTopologyDescriptionChangedEvent(prev description.Topology, current description.Topology) {
topologyDescriptionChanged := &event.TopologyDescriptionChangedEvent{
TopologyID: t.id,
PreviousDescription: prev,
NewDescription: current,
}
if t.cfg.serverMonitor != nil && t.cfg.serverMonitor.TopologyDescriptionChanged != nil {
t.cfg.serverMonitor.TopologyDescriptionChanged(topologyDescriptionChanged)
}
}
// publishes a TopologyOpeningEvent to indicate the topology is being initialized
func (t *Topology) publishTopologyOpeningEvent() {
topologyOpening := &event.TopologyOpeningEvent{
TopologyID: t.id,
}
if t.cfg.serverMonitor != nil && t.cfg.serverMonitor.TopologyOpening != nil {
t.cfg.serverMonitor.TopologyOpening(topologyOpening)
}
}
// publishes a TopologyClosedEvent to indicate the topology has been closed
func (t *Topology) publishTopologyClosedEvent() {
topologyClosed := &event.TopologyClosedEvent{
TopologyID: t.id,
}
if t.cfg.serverMonitor != nil && t.cfg.serverMonitor.TopologyClosed != nil {
t.cfg.serverMonitor.TopologyClosed(topologyClosed)
}
}
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