go内存缓存BigCache源码阅读，如何new一个bigcache对象

一、下载源码

在github上，地址github.com/allegro/bigcache，我们可以把代码源码clone到本地。
这里选择分支v3.1.0的代码。

二、源码目录

我们打开源码

.
├── LICENSE
├── README.md
├── assert_test.go
├── bigcache.go
├── bigcache_bench_test.go
├── bigcache_test.go
├── bytes.go
├── bytes_appengine.go
├── clock.go
├── config.go
├── encoding.go
├── encoding_test.go
├── entry_not_found_error.go
├── examples_test.go
├── fnv.go
├── fnv_bench_test.go
├── fnv_test.go
├── go.mod
├── go.sum
├── hash.go
├── hash_test.go
├── iterator.go
├── iterator_test.go
├── logger.go
├── queue
│   ├── bytes_queue.go
│   └── bytes_queue_test.go
├── server
│   ├── README.md
│   ├── cache_handlers.go
│   ├── middleware.go
│   ├── middleware_test.go
│   ├── server.go
│   ├── server_test.go
│   └── stats_handler.go
├── shard.go
├── stats.go
└── utils.go

2 directories, 36 files

比较重要的几个文件
queue目录
shard.go 分片
fnv.go Fnv hash算法
bigcache.go 初始化new结构体，以及set，get方法。

我们可以看上篇文档的示例：

func TestSetGet(t *testing.T) {
    // new一个bigCache对象
    cache, _ := bigcache.New(context.Background(), bigcache.DefaultConfig(10*time.Minute))

    // get获取一个无值的key
    vNil, err := cache.Get("key")
    t.Log(vNil, err) // [] Entry not found 值为空的[]字节slice

    // set 存储数据
    cache.Set("key", []byte("value"))
    // get 获取数据
    v, _ := cache.Get("key")
    t.Log(v)         // 输出 [118 97 108 117 101]
    t.Log(string(v)) // 输出 value
}

那么我们先找到这个
第一行对应的

cache, _ := bigcache.New(context.Background(), bigcache.DefaultConfig(10*time.Minute))

在源码的代码如下:

// New initialize new instance of BigCache
// New用来初始化一个新的BigCache实例。
func New(ctx context.Context, config Config) (*BigCache, error) {
    return newBigCache(ctx, config, &systemClock{})
}

我们在使用的时候，主要又涉及到一个 bigcache.DefaultConfig(10*time.Minute))
我们看一下他的方法：

// DefaultConfig initializes config with default values.
// DefaultConfig 初始化Config，给定默认值。
// When load for BigCache can be predicted in advance then it is better to use custom config.
// 当load加载 BigCache 的时候，如果在使用之前就能预估到使用量，然后预设置config，它好于使用默认config。
func DefaultConfig(eviction time.Duration) Config {
    return Config{
        Shards:             1024,
        LifeWindow:         eviction,
        CleanWindow:        1 * time.Second,
        MaxEntriesInWindow: 1000 * 10 * 60,
        MaxEntrySize:       500,
        StatsEnabled:       false,
        Verbose:            true,
        Hasher:             newDefaultHasher(),
        HardMaxCacheSize:   0,
        Logger:             DefaultLogger(),
    }
}

这里设计一个 Config 结构体

// Config for BigCache
type Config struct {
    // Number of cache shards, value must be a power of two
    Shards int
    // Time after which entry can be evicted
    // LifeWindow后,缓存对象被认为不活跃,但并不会删除对象
    LifeWindow time.Duration
    // Interval between removing expired entries (clean up).
    // If set to <= 0 then no action is performed. Setting to < 1 second is counterproductive — bigcache has a one second resolution.
    // CleanWindow后，会删除被认为不活跃的对象（超过LifeWindow时间的对象），0代表不操作；
    CleanWindow time.Duration
    // Max number of entries in life window. Used only to calculate initial size for cache shards.
    // When proper value is set then additional memory allocation does not occur.
    // 设置最大存储对象数量，仅在初始化时可以设置
    MaxEntriesInWindow int
    // Max size of entry in bytes. Used only to calculate initial size for cache shards.
    // 缓存对象的最大字节数，仅在初始化时可以设置
    MaxEntrySize int
    // StatsEnabled if true calculate the number of times a cached resource was requested.
    StatsEnabled bool
    // Verbose mode prints information about new memory allocation
    // 是否打印内存分配信息
    Verbose bool
    // Hasher used to map between string keys and unsigned 64bit integers, by default fnv64 hashing is used.
    Hasher Hasher
    // HardMaxCacheSize is a limit for BytesQueue size in MB.
    // It can protect application from consuming all available memory on machine, therefore from running OOM Killer.
    // Default value is 0 which means unlimited size. When the limit is higher than 0 and reached then
    // the oldest entries are overridden for the new ones. The max memory consumption will be bigger than
    // HardMaxCacheSize due to Shards' s additional memory. Every Shard consumes additional memory for map of keys
    // and statistics (map[uint64]uint32) the size of this map is equal to number of entries in
    // cache ~ 2×(64+32)×n bits + overhead or map itself.
    // 设置缓存最大值(单位为MB),0表示无限制
    HardMaxCacheSize int
    // OnRemove is a callback fired when the oldest entry is removed because of its expiration time or no space left
    // for the new entry, or because delete was called.
    // Default value is nil which means no callback and it prevents from unwrapping the oldest entry.
    // ignored if OnRemoveWithMetadata is specified.
    // 在缓存过期或者被删除时,可设置回调函数，参数是(key、val)，默认是nil不设置
    OnRemove func(key string, entry []byte)
    // OnRemoveWithMetadata is a callback fired when the oldest entry is removed because of its expiration time or no space left
    // for the new entry, or because delete was called. A structure representing details about that specific entry.
    // Default value is nil which means no callback and it prevents from unwrapping the oldest entry.
    OnRemoveWithMetadata func(key string, entry []byte, keyMetadata Metadata)
    // OnRemoveWithReason is a callback fired when the oldest entry is removed because of its expiration time or no space left
    // for the new entry, or because delete was called. A constant representing the reason will be passed through.
    // Default value is nil which means no callback and it prevents from unwrapping the oldest entry.
    // Ignored if OnRemove is specified.
    // 在缓存过期或者被删除时,可设置回调函数，参数是(key、val,reason)默认是nil不设置
    OnRemoveWithReason func(key string, entry []byte, reason RemoveReason)

    onRemoveFilter int

    // Logger is a logging interface and used in combination with `Verbose`
    // Defaults to `DefaultLogger()`
    Logger Logger
}

BigCache的实现是 一种索引和数据分离的多阶hash。
主索引（多阶hash数组）的value保存的是在数据段的位置，通过二次定位拿到某个key对应的真实的value。