Update comments to reflect reality
This commit is contained in:
Sean Chittenden
parent
1a09a5b2cf
commit
b906e40811
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@ -59,23 +59,23 @@ type ConsulClusterInfo interface {
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// serverCfg is the thread-safe configuration structure that is used to
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// maintain the list of consul servers in Client.
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//
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// NOTE(sean@): We are explicitly relying on the fact that this is copied.
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// Please keep this structure light.
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// NOTE(sean@): We are explicitly relying on the fact that serverConfig will
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// be copied onto the stack. Please keep this structure light.
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type serverConfig struct {
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// servers tracks the locally known servers
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// servers tracks the locally known servers. List membership is
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// maintained by Serf.
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servers []*server_details.ServerDetails
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}
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type ServerManager struct {
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// serverConfig provides the necessary load/store semantics to
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// serverConfig
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// serverConfig provides the necessary load/store semantics for the
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// server list.
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serverConfigValue atomic.Value
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serverConfigLock sync.Mutex
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// shutdownCh is a copy of the channel in consul.Client
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shutdownCh chan struct{}
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// logger uses the provided LogOutput
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logger *log.Logger
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// serf is used to estimate the approximate number of nodes in a
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@ -89,8 +89,10 @@ type ServerManager struct {
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}
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// AddServer takes out an internal write lock and adds a new server. If the
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// server is not known, it adds the new server and schedules a rebalance. If
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// it is known, we merge the new server details.
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// server is not known, appends the server to the list. The new server will
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// begin seeing use after the rebalance timer fires or enough servers fail
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// organically. If the server is already known, merge the new server
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// details.
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func (sm *ServerManager) AddServer(server *server_details.ServerDetails) {
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sm.serverConfigLock.Lock()
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defer sm.serverConfigLock.Unlock()
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@ -130,8 +132,7 @@ func (sm *ServerManager) AddServer(server *server_details.ServerDetails) {
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func (sc *serverConfig) cycleServer() (servers []*server_details.ServerDetails) {
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numServers := len(sc.servers)
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if numServers < 2 {
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// No action required
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return servers
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return servers // No action required
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}
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newServers := make([]*server_details.ServerDetails, 0, numServers)
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@ -141,7 +142,11 @@ func (sc *serverConfig) cycleServer() (servers []*server_details.ServerDetails)
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}
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// FindHealthyServer takes out an internal "read lock" and searches through
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// the list of servers to find a healthy server.
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// the list of servers to find a "healthy" server. If the server is actually
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// unhealthy, we rely on Serf to detect this and remove the node from the
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// server list. If the server at the front of the list has failed or fails
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// during an RPC call, it is rotated to the end of the list. If there are no
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// servers available, return nil.
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func (sm *ServerManager) FindHealthyServer() *server_details.ServerDetails {
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serverCfg := sm.getServerConfig()
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numServers := len(serverCfg.servers)
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@ -149,7 +154,10 @@ func (sm *ServerManager) FindHealthyServer() *server_details.ServerDetails {
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sm.logger.Printf("[ERR] consul: No servers found in the server config")
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return nil
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} else {
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// Return whatever is at the front of the list
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// Return whatever is at the front of the list because it is
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// assumed to be the oldest in the server list (unless -
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// hypothetically - the server list was rotated right after a
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// server was added).
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return serverCfg.servers[0]
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}
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}
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@ -170,28 +178,29 @@ func (sm *ServerManager) getServerConfig() serverConfig {
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// NewServerManager is the only way to safely create a new ServerManager
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// struct.
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func NewServerManager(logger *log.Logger, shutdownCh chan struct{}, cci ConsulClusterInfo) (sm *ServerManager) {
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func NewServerManager(logger *log.Logger, shutdownCh chan struct{}, clusterInfo ConsulClusterInfo) (sm *ServerManager) {
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// NOTE(sean@): Can't pass *consul.Client due to an import cycle
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sm = new(ServerManager)
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sm.logger = logger
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sm.clusterInfo = cci
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sm.clusterInfo = clusterInfo
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sm.shutdownCh = shutdownCh
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sc := serverConfig{}
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sc.servers = make([]*server_details.ServerDetails, 0)
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sm.serverConfigValue.Store(sc)
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sm.saveServerConfig(sc)
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return sm
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}
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// NotifyFailedServer is an exported convenience function that allows callers
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// to pass in a server that has failed an RPC request and mark it as failed.
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// If the server being failed is not the first server on the list, this is a
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// noop. If, however, the server is failed and first on the list, acquire
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// the lock, retest, and take the penalty of moving the server to the end of
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// the list.
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// NotifyFailedServer marks the passed in server as "failed" by rotating it
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// to the end of the server list.
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func (sm *ServerManager) NotifyFailedServer(server *server_details.ServerDetails) {
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serverCfg := sm.getServerConfig()
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// If the server being failed is not the first server on the list,
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// this is a noop. If, however, the server is failed and first on
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// the list, acquire the lock, retest, and take the penalty of moving
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// the server to the end of the list.
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// Use atomic.CAS to emulate a TryLock().
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if len(serverCfg.servers) > 0 && serverCfg.servers[0] == server &&
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atomic.CompareAndSwapInt32(&sm.notifyFailedBarrier, 0, 1) {
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@ -212,10 +221,12 @@ func (sm *ServerManager) NotifyFailedServer(server *server_details.ServerDetails
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// RebalanceServers takes out an internal write lock and shuffles the list of
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// servers on this agent. This allows for a redistribution of work across
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// consul servers and provides a guarantee that the order list of
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// ServerDetails isn't actually ordered, therefore we can sequentially walk
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// the array to pick a server without all agents in the cluster dog piling on
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// a single node.
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// consul servers and provides a guarantee that the order of the server list
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// isn't related to the age at which the node was added to the cluster.
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// Elsewhere we rely on the position in the server list as a hint regarding
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// the stability of a server relative to its position in the server list.
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// Servers at or near the front of the list are more stable than servers near
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// the end of the list.
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func (sm *ServerManager) RebalanceServers() {
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sm.serverConfigLock.Lock()
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defer sm.serverConfigLock.Unlock()
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@ -237,11 +248,7 @@ func (sm *ServerManager) RebalanceServers() {
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}
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// RemoveServer takes out an internal write lock and removes a server from
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// the server list. No rebalancing happens as a result of the removed server
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// because we do not want a network partition which separated a server from
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// this agent to cause an increase in work. Instead we rely on the internal
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// already existing semantics to handle failure detection after a server has
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// been removed.
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// the server list.
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func (sm *ServerManager) RemoveServer(server *server_details.ServerDetails) {
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sm.serverConfigLock.Lock()
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defer sm.serverConfigLock.Unlock()
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@ -264,7 +271,9 @@ func (sm *ServerManager) RemoveServer(server *server_details.ServerDetails) {
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}
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}
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// refreshServerRebalanceTimer is called
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// refreshServerRebalanceTimer is only called once the rebalanceTimer
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// expires. Historically this was an expensive routine and is intended to be
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// run in isolation in a dedicated, non-concurrent task.
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func (sm *ServerManager) refreshServerRebalanceTimer(timer *time.Timer) {
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serverCfg := sm.getServerConfig()
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numConsulServers := len(serverCfg.servers)
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@ -289,8 +298,11 @@ func (sm *ServerManager) saveServerConfig(sc serverConfig) {
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}
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// Start is used to start and manage the task of automatically shuffling and
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// rebalance the list of consul servers. This maintenance happens either
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// when a new server is added or when a duration has been exceed.
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// rebalancing the list of consul servers. This maintenance only happens
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// periodically based on the expiration of the timer. Failed servers are
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// automatically cycled to the end of the list. New servers are appended to
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// the list. The order of the server list must be shuffled periodically to
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// distribute load across all known and available consul servers.
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func (sm *ServerManager) Start() {
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var rebalanceTimer *time.Timer = time.NewTimer(time.Duration(initialRebalanceTimeoutHours * time.Hour))
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var rebalanceTaskDispatched int32
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