86 lines
3.7 KiB
Go
86 lines
3.7 KiB
Go
package cache
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import (
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"time"
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)
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// Type implements the logic to fetch certain types of data.
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type Type interface {
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// Fetch fetches a single unique item.
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//
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// The FetchOptions contain the index and timeouts for blocking queries. The
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// MinIndex value on the Request itself should NOT be used as the blocking
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// index since a request may be reused multiple times as part of Refresh
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// behavior.
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//
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// The return value is a FetchResult which contains information about the
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// fetch. If an error is given, the FetchResult is ignored. The cache does not
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// support backends that return partial values. Optional State can be added to
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// the FetchResult which will be stored with the cache entry and provided to
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// the next Fetch call but will not be returned to clients. This allows types
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// to add additional bookkeeping data per cache entry that will still be aged
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// out along with the entry's TTL.
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//
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// On timeout, FetchResult can behave one of two ways. First, it can return
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// the last known value. This is the default behavior of blocking RPC calls in
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// Consul so this allows cache types to be implemented with no extra logic.
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// Second, FetchResult can return an unset value and index. In this case, the
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// cache will reuse the last value automatically. If an unset Value is
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// returned, the State field will still be updated which allows maintaining
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// metadata even when there is no result.
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Fetch(FetchOptions, Request) (FetchResult, error)
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// RegisterOptions are used when the type is registered to configure the
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// behaviour of cache entries for this type.
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RegisterOptions() RegisterOptions
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}
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// FetchOptions are various settable options when a Fetch is called.
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type FetchOptions struct {
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// MinIndex is the minimum index to be used for blocking queries.
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// If blocking queries aren't supported for data being returned,
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// this value can be ignored.
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MinIndex uint64
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// Timeout is the maximum time for the query. This must be implemented
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// in the Fetch itself.
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Timeout time.Duration
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// LastResult is the result from the last successful Fetch and represents the
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// value currently stored in the cache at the time Fetch is invoked. It will
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// be nil on first call where there is no current cache value. There may have
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// been other Fetch attempts that resulted in an error in the mean time. These
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// are not explicitly represented currently. We could add that if needed this
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// was just simpler for now.
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//
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// The FetchResult read-only! It is constructed per Fetch call so modifying
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// the struct directly (e.g. changing it's Index of Value field) will have no
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// effect, however the Value and State fields may be pointers to the actual
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// values stored in the cache entry. It is thread-unsafe to modify the Value
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// or State via pointers since readers may be concurrently inspecting those
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// values under the entry lock (although we guarantee only one Fetch call per
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// entry) and modifying them even if the index doesn't change or the Fetch
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// eventually errors will likely break logical invariants in the cache too!
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LastResult *FetchResult
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}
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// FetchResult is the result of a Type Fetch operation and contains the
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// data along with metadata gathered from that operation.
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type FetchResult struct {
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// Value is the result of the fetch.
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Value interface{}
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// State is opaque data stored in the cache but not returned to clients. It
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// can be used by Types to maintain any bookkeeping they need between fetches
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// (using FetchOptions.LastResult) in a way that gets automatically cleaned up
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// by TTL expiry etc.
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State interface{}
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// Index is the corresponding index value for this data.
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Index uint64
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// NotModified indicates that the Value has not changed since LastResult, and
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// the LastResult value should be used instead of Value.
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NotModified bool
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}
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