open-consul/agent/cache/cache.go

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// Package cache provides caching features for data from a Consul server.
//
// While this is similar in some ways to the "agent/ae" package, a key
// difference is that with anti-entropy, the agent is the authoritative
// source so it resolves differences the server may have. With caching (this
// package), the server is the authoritative source and we do our best to
// balance performance and correctness, depending on the type of data being
// requested.
//
// The types of data that can be cached is configurable via the Type interface.
// This allows specialized behavior for certain types of data. Each type of
// Consul data (CA roots, leaf certs, intentions, KV, catalog, etc.) will
// have to be manually implemented. This usually is not much work, see
// the "agent/cache-types" package.
package cache
import (
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"container/heap"
"fmt"
"sync"
"time"
"github.com/armon/go-metrics"
)
//go:generate mockery -all -inpkg
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// Constants related to refresh backoff. We probably don't ever need to
// make these configurable knobs since they primarily exist to lower load.
const (
CacheRefreshBackoffMin = 3 // 3 attempts before backing off
CacheRefreshMaxWait = 1 * time.Minute // maximum backoff wait time
)
// Cache is a agent-local cache of Consul data. Create a Cache using the
// New function. A zero-value Cache is not ready for usage and will result
// in a panic.
//
// The types of data to be cached must be registered via RegisterType. Then,
// calls to Get specify the type and a Request implementation. The
// implementation of Request is usually done directly on the standard RPC
// struct in agent/structs. This API makes cache usage a mostly drop-in
// replacement for non-cached RPC calls.
//
// The cache is partitioned by ACL and datacenter. This allows the cache
// to be safe for multi-DC queries and for queries where the data is modified
// due to ACLs all without the cache having to have any clever logic, at
// the slight expense of a less perfect cache.
//
// The Cache exposes various metrics via go-metrics. Please view the source
// searching for "metrics." to see the various metrics exposed. These can be
// used to explore the performance of the cache.
type Cache struct {
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// types stores the list of data types that the cache knows how to service.
// These can be dynamically registered with RegisterType.
typesLock sync.RWMutex
types map[string]typeEntry
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// entries contains the actual cache data. Access to entries and
// entriesExpiryHeap must be protected by entriesLock.
//
// entriesExpiryHeap is a heap of *cacheEntry values ordered by
// expiry, with the soonest to expire being first in the list (index 0).
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//
// NOTE(mitchellh): The entry map key is currently a string in the format
// of "<DC>/<ACL token>/<Request key>" in order to properly partition
// requests to different datacenters and ACL tokens. This format has some
// big drawbacks: we can't evict by datacenter, ACL token, etc. For an
// initial implementation this works and the tests are agnostic to the
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// internal storage format so changing this should be possible safely.
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entriesLock sync.RWMutex
entries map[string]cacheEntry
entriesExpiryHeap *expiryHeap
}
// typeEntry is a single type that is registered with a Cache.
type typeEntry struct {
Type Type
Opts *RegisterOptions
}
// ResultMeta is returned from Get calls along with the value and can be used
// to expose information about the cache status for debugging or testing.
type ResultMeta struct {
// Return whether or not the request was a cache hit
Hit bool
}
// Options are options for the Cache.
type Options struct {
// Nothing currently, reserved.
}
// New creates a new cache with the given RPC client and reasonable defaults.
// Further settings can be tweaked on the returned value.
func New(*Options) *Cache {
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// Initialize the heap. The buffer of 1 is really important because
// its possible for the expiry loop to trigger the heap to update
// itself and it'd block forever otherwise.
h := &expiryHeap{NotifyCh: make(chan struct{}, 1)}
heap.Init(h)
c := &Cache{
types: make(map[string]typeEntry),
entries: make(map[string]cacheEntry),
entriesExpiryHeap: h,
}
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// Start the expiry watcher
go c.runExpiryLoop()
return c
}
// RegisterOptions are options that can be associated with a type being
// registered for the cache. This changes the behavior of the cache for
// this type.
type RegisterOptions struct {
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// LastGetTTL is the time that the values returned by this type remain
// in the cache after the last get operation. If a value isn't accessed
// within this duration, the value is purged from the cache and
// background refreshing will cease.
LastGetTTL time.Duration
// Refresh configures whether the data is actively refreshed or if
// the data is only refreshed on an explicit Get. The default (false)
// is to only request data on explicit Get.
Refresh bool
// RefreshTimer is the time between attempting to refresh data.
// If this is zero, then data is refreshed immediately when a fetch
// is returned.
//
// RefreshTimeout determines the maximum query time for a refresh
// operation. This is specified as part of the query options and is
// expected to be implemented by the Type itself.
//
// Using these values, various "refresh" mechanisms can be implemented:
//
// * With a high timer duration and a low timeout, a timer-based
// refresh can be set that minimizes load on the Consul servers.
//
// * With a low timer and high timeout duration, a blocking-query-based
// refresh can be set so that changes in server data are recognized
// within the cache very quickly.
//
RefreshTimer time.Duration
RefreshTimeout time.Duration
}
// RegisterType registers a cacheable type.
//
// This makes the type available for Get but does not automatically perform
// any prefetching. In order to populate the cache, Get must be called.
func (c *Cache) RegisterType(n string, typ Type, opts *RegisterOptions) {
if opts == nil {
opts = &RegisterOptions{}
}
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if opts.LastGetTTL == 0 {
opts.LastGetTTL = 72 * time.Hour // reasonable default is days
}
c.typesLock.Lock()
defer c.typesLock.Unlock()
c.types[n] = typeEntry{Type: typ, Opts: opts}
}
// Get loads the data for the given type and request. If data satisfying the
// minimum index is present in the cache, it is returned immediately. Otherwise,
// this will block until the data is available or the request timeout is
// reached.
//
// Multiple Get calls for the same Request (matching CacheKey value) will
// block on a single network request.
//
// The timeout specified by the Request will be the timeout on the cache
// Get, and does not correspond to the timeout of any background data
// fetching. If the timeout is reached before data satisfying the minimum
// index is retrieved, the last known value (maybe nil) is returned. No
// error is returned on timeout. This matches the behavior of Consul blocking
// queries.
func (c *Cache) Get(t string, r Request) (interface{}, ResultMeta, error) {
info := r.CacheInfo()
if info.Key == "" {
metrics.IncrCounter([]string{"consul", "cache", "bypass"}, 1)
// If no key is specified, then we do not cache this request.
// Pass directly through to the backend.
return c.fetchDirect(t, r)
}
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// Get the actual key for our entry
key := c.entryKey(&info)
// First time through
first := true
// timeoutCh for watching our timeout
var timeoutCh <-chan time.Time
RETRY_GET:
// Get the current value
c.entriesLock.RLock()
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entry, ok := c.entries[key]
c.entriesLock.RUnlock()
// If we have a current value and the index is greater than the
// currently stored index then we return that right away. If the
// index is zero and we have something in the cache we accept whatever
// we have.
if ok && entry.Valid {
if info.MinIndex == 0 || info.MinIndex < entry.Index {
meta := ResultMeta{}
if first {
metrics.IncrCounter([]string{"consul", "cache", t, "hit"}, 1)
meta.Hit = true
}
// Touch the expiration and fix the heap.
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c.entriesLock.Lock()
entry.Expiry.Reset()
c.entriesExpiryHeap.Fix(entry.Expiry)
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c.entriesLock.Unlock()
// We purposely do not return an error here since the cache
// only works with fetching values that either have a value
// or have an error, but not both. The Error may be non-nil
// in the entry because of this to note future fetch errors.
return entry.Value, meta, nil
}
}
// If this isn't our first time through and our last value has an error,
// then we return the error. This has the behavior that we don't sit in
// a retry loop getting the same error for the entire duration of the
// timeout. Instead, we make one effort to fetch a new value, and if
// there was an error, we return.
if !first && entry.Error != nil {
return entry.Value, ResultMeta{}, entry.Error
}
if first {
// We increment two different counters for cache misses depending on
// whether we're missing because we didn't have the data at all,
// or if we're missing because we're blocking on a set index.
if info.MinIndex == 0 {
metrics.IncrCounter([]string{"consul", "cache", t, "miss_new"}, 1)
} else {
metrics.IncrCounter([]string{"consul", "cache", t, "miss_block"}, 1)
}
}
// No longer our first time through
first = false
// Set our timeout channel if we must
if info.Timeout > 0 && timeoutCh == nil {
timeoutCh = time.After(info.Timeout)
}
// At this point, we know we either don't have a value at all or the
// value we have is too old. We need to wait for new data.
waiterCh, err := c.fetch(t, key, r, true, 0)
if err != nil {
return nil, ResultMeta{}, err
}
select {
case <-waiterCh:
// Our fetch returned, retry the get from the cache
goto RETRY_GET
case <-timeoutCh:
// Timeout on the cache read, just return whatever we have.
return entry.Value, ResultMeta{}, nil
}
}
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// entryKey returns the key for the entry in the cache. See the note
// about the entry key format in the structure docs for Cache.
func (c *Cache) entryKey(r *RequestInfo) string {
return fmt.Sprintf("%s/%s/%s", r.Datacenter, r.Token, r.Key)
}
// fetch triggers a new background fetch for the given Request. If a
// background fetch is already running for a matching Request, the waiter
// channel for that request is returned. The effect of this is that there
// is only ever one blocking query for any matching requests.
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//
// If allowNew is true then the fetch should create the cache entry
// if it doesn't exist. If this is false, then fetch will do nothing
// if the entry doesn't exist. This latter case is to support refreshing.
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func (c *Cache) fetch(t, key string, r Request, allowNew bool, attempt uint) (<-chan struct{}, error) {
// Get the type that we're fetching
c.typesLock.RLock()
tEntry, ok := c.types[t]
c.typesLock.RUnlock()
if !ok {
return nil, fmt.Errorf("unknown type in cache: %s", t)
}
// We acquire a write lock because we may have to set Fetching to true.
c.entriesLock.Lock()
defer c.entriesLock.Unlock()
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entry, ok := c.entries[key]
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// If we aren't allowing new values and we don't have an existing value,
// return immediately. We return an immediately-closed channel so nothing
// blocks.
if !ok && !allowNew {
ch := make(chan struct{})
close(ch)
return ch, nil
}
// If we already have an entry and it is actively fetching, then return
// the currently active waiter.
if ok && entry.Fetching {
return entry.Waiter, nil
}
// If we don't have an entry, then create it. The entry must be marked
// as invalid so that it isn't returned as a valid value for a zero index.
if !ok {
entry = cacheEntry{Valid: false, Waiter: make(chan struct{})}
}
// Set that we're fetching to true, which makes it so that future
// identical calls to fetch will return the same waiter rather than
// perform multiple fetches.
entry.Fetching = true
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c.entries[key] = entry
metrics.SetGauge([]string{"consul", "cache", "entries_count"}, float32(len(c.entries)))
// The actual Fetch must be performed in a goroutine.
go func() {
// Start building the new entry by blocking on the fetch.
result, err := tEntry.Type.Fetch(FetchOptions{
MinIndex: entry.Index,
Timeout: tEntry.Opts.RefreshTimeout,
}, r)
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// Copy the existing entry to start.
newEntry := entry
newEntry.Fetching = false
if result.Value != nil {
// A new value was given, so we create a brand new entry.
newEntry.Value = result.Value
newEntry.Index = result.Index
if newEntry.Index < 1 {
// Less than one is invalid unless there was an error and in this case
// there wasn't since a value was returned. If a badly behaved RPC
// returns 0 when it has no data, we might get into a busy loop here. We
// set this to minimum of 1 which is safe because no valid user data can
// ever be written at raft index 1 due to the bootstrap process for
// raft. This insure that any subsequent background refresh request will
// always block, but allows the initial request to return immediately
// even if there is no data.
newEntry.Index = 1
}
// This is a valid entry with a result
newEntry.Valid = true
}
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// Error handling
if err == nil {
metrics.IncrCounter([]string{"consul", "cache", "fetch_success"}, 1)
metrics.IncrCounter([]string{"consul", "cache", t, "fetch_success"}, 1)
if result.Index > 0 {
// Reset the attempts counter so we don't have any backoff
attempt = 0
} else {
// Result having a zero index is an implicit error case. There was no
// actual error but it implies the RPC found in index (nothing written
// yet for that type) but didn't take care to return safe "1" index. We
// don't want to actually treat it like an error by setting
// newEntry.Error to something non-nil, but we should guard against 100%
// CPU burn hot loops caused by that case which will never block but
// also won't backoff either. So we treat it as a failed attempt so that
// at least the failure backoff will save our CPU while still
// periodically refreshing so normal service can resume when the servers
// actually have something to return from the RPC. If we get in this
// state it can be considered a bug in the RPC implementation (to ever
// return a zero index) however since it can happen this is a safety net
// for the future.
attempt++
}
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} else {
metrics.IncrCounter([]string{"consul", "cache", "fetch_error"}, 1)
metrics.IncrCounter([]string{"consul", "cache", t, "fetch_error"}, 1)
// Increment attempt counter
attempt++
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// Always set the error. We don't override the value here because
// if Valid is true, then we can reuse the Value in the case a
// specific index isn't requested. However, for blocking queries,
// we want Error to be set so that we can return early with the
// error.
newEntry.Error = err
}
// Create a new waiter that will be used for the next fetch.
newEntry.Waiter = make(chan struct{})
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// Set our entry
c.entriesLock.Lock()
// If this is a new entry (not in the heap yet), then setup the
// initial expiry information and insert. If we're already in
// the heap we do nothing since we're reusing the same entry.
if newEntry.Expiry == nil || newEntry.Expiry.HeapIndex == -1 {
newEntry.Expiry = &cacheEntryExpiry{
Key: key,
TTL: tEntry.Opts.LastGetTTL,
}
newEntry.Expiry.Reset()
heap.Push(c.entriesExpiryHeap, newEntry.Expiry)
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}
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c.entries[key] = newEntry
c.entriesLock.Unlock()
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// Trigger the old waiter
close(entry.Waiter)
// If refresh is enabled, run the refresh in due time. The refresh
// below might block, but saves us from spawning another goroutine.
if tEntry.Opts.Refresh {
c.refresh(tEntry.Opts, attempt, t, key, r)
}
}()
return entry.Waiter, nil
}
// fetchDirect fetches the given request with no caching. Because this
// bypasses the caching entirely, multiple matching requests will result
// in multiple actual RPC calls (unlike fetch).
func (c *Cache) fetchDirect(t string, r Request) (interface{}, ResultMeta, error) {
// Get the type that we're fetching
c.typesLock.RLock()
tEntry, ok := c.types[t]
c.typesLock.RUnlock()
if !ok {
return nil, ResultMeta{}, fmt.Errorf("unknown type in cache: %s", t)
}
// Fetch it with the min index specified directly by the request.
result, err := tEntry.Type.Fetch(FetchOptions{
MinIndex: r.CacheInfo().MinIndex,
}, r)
if err != nil {
return nil, ResultMeta{}, err
}
// Return the result and ignore the rest
return result.Value, ResultMeta{}, nil
}
// refresh triggers a fetch for a specific Request according to the
// registration options.
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func (c *Cache) refresh(opts *RegisterOptions, attempt uint, t string, key string, r Request) {
// Sanity-check, we should not schedule anything that has refresh disabled
if !opts.Refresh {
return
}
// If we're over the attempt minimum, start an exponential backoff.
if attempt > CacheRefreshBackoffMin {
waitTime := (1 << (attempt - CacheRefreshBackoffMin)) * time.Second
if waitTime > CacheRefreshMaxWait {
waitTime = CacheRefreshMaxWait
}
time.Sleep(waitTime)
}
// If we have a timer, wait for it
if opts.RefreshTimer > 0 {
time.Sleep(opts.RefreshTimer)
}
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// Trigger. The "allowNew" field is false because in the time we were
// waiting to refresh we may have expired and got evicted. If that
// happened, we don't want to create a new entry.
c.fetch(t, key, r, false, attempt)
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}
// runExpiryLoop is a blocking function that watches the expiration
// heap and invalidates entries that have expired.
func (c *Cache) runExpiryLoop() {
var expiryTimer *time.Timer
for {
// If we have a previous timer, stop it.
if expiryTimer != nil {
expiryTimer.Stop()
}
// Get the entry expiring soonest
var entry *cacheEntryExpiry
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var expiryCh <-chan time.Time
c.entriesLock.RLock()
if len(c.entriesExpiryHeap.Entries) > 0 {
entry = c.entriesExpiryHeap.Entries[0]
expiryTimer = time.NewTimer(entry.Expires.Sub(time.Now()))
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expiryCh = expiryTimer.C
}
c.entriesLock.RUnlock()
select {
case <-c.entriesExpiryHeap.NotifyCh:
// Entries changed, so the heap may have changed. Restart loop.
case <-expiryCh:
c.entriesLock.Lock()
// Entry expired! Remove it.
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delete(c.entries, entry.Key)
heap.Remove(c.entriesExpiryHeap, entry.HeapIndex)
// This is subtle but important: if we race and simultaneously
// evict and fetch a new value, then we set this to -1 to
// have it treated as a new value so that the TTL is extended.
entry.HeapIndex = -1
// Set some metrics
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metrics.IncrCounter([]string{"consul", "cache", "evict_expired"}, 1)
metrics.SetGauge([]string{"consul", "cache", "entries_count"}, float32(len(c.entries)))
c.entriesLock.Unlock()
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}
}
}