redis源码分析之十三内存DB管理

一、数据库DB

REDIS是一款内存型数据库，那么数据的最终处理是如何处理的呢，前面提到了数据的挺久化，那么持久化的什么内容，其实就是数据的处理过程。也就是说，持久化的数据也就是Redis需要操作的数据，这些数据才是它真正有用的部分，前面提到的一切一切，甚至以后再提到的一切一切，其实都是为这些数据服务的，保证这些数据的安全、高效和稳定。
REDIS的所有数据都存储在redisDb这个数据结构体中，它在server.h文件中，即：

typedef struct redisDb {
    dict *dict;                 /* The keyspace for this DB */
    dict *expires;              /* Timeout of keys with a timeout set */
    dict *blocking_keys;        /* Keys with clients waiting for data (BLPOP)*/
    dict *ready_keys;           /* Blocked keys that received a PUSH */
    dict *watched_keys;         /* WATCHED keys for MULTI/EXEC CAS */
    int id;                     /* Database ID */
    long long avg_ttl;          /* Average TTL, just for stats */
    unsigned long expires_cursor; /* Cursor of the active expire cycle. */
    list *defrag_later;         /* List of key names to attempt to defrag one by one, gradually. */
} redisDb;

struct redisServer {
    /* General */
    pid_t pid;                  /* Main process pid. */
    char *configfile;           /* Absolute config file path, or NULL */
......
    redisDb *db;  
......
}

而整个REDIS其实就是这个数组结构体对象的一个动态数组。这个数据结构体在前面也接触过，今天开始正式对它进行分析.通过上面的数据结构体就可以整理整个REDIS的数据结构的构成方式了：
redisServer包含指定个redisDb，而它本身又是键值对dict的数组指针，而dict又是dictht的指针数组，最终指向dictEntry，它包含最终的由前面提到的SDS等,而list,sds,zset等又统一由RedisObject来管理。也即是：
一个数据库服务redisServer，含指定个redisDb数据库，每个数据库由dict字典组成，字典的KEY和VALUE都是RedisObject(根据实际情况创建SDS，LIST等)。
但整体数据的存储是在db.c这个文件，所以基本是围绕这个文件展开分析的。

二、源码分析

数据库常见的功能有哪些，“CRUD”增删改查，这是最基础的几个命令，其它还有数据库切换、各种定期删除、策略等等，下面看一下主要的源码：
1、选择数据库，查找

//指定哪个数据库，然后开始工作
int selectDb(client *c, int id) {
    if (id < 0 || id >= server.dbnum)
        return C_ERR;
    c->db = &server.db[id];
    return C_OK;
}
/* Low level key lookup API, not actually called directly from commands
 * implementations that should instead rely on lookupKeyRead(),
 * lookupKeyWrite() and lookupKeyReadWithFlags(). */
 //一个标准的底层调用查找函数，其它如lookupKeyReadWithFlags都调用其来实现
 //数据库中取指定键值的对象，不存在返回NULL
robj *lookupKey(redisDb *db, robj *key, int flags) {
    //在字典中查找
    dictEntry *de = dictFind(db->dict,key->ptr);
    if (de) {
        robj *val = dictGetVal(de);

        /* Update the access time for the ageing algorithm.
         * Don't do it if we have a saving child, as this will trigger
         * a copy on write madness. */
         //检查是否重复操作，防止IO密集引起效率下降
        if (!hasActiveChildProcess() && !(flags & LOOKUP_NOTOUCH)){
            if (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {
                updateLFU(val);
            } else {
                val->lru = LRU_CLOCK();
            }
        }
        return val;
    } else {
        return NULL;
    }
}

/* Lookup a key for read operations, or return NULL if the key is not found
 * in the specified DB.
 *
 * As a side effect of calling this function:
 * 1. A key gets expired if it reached it's TTL.
 * 2. The key last access time is updated.
 * 3. The global keys hits/misses stats are updated (reported in INFO).
 * 4. If keyspace notifications are enabled, a "keymiss" notification is fired.
 *
 * This API should not be used when we write to the key after obtaining
 * the object linked to the key, but only for read only operations.
 *
 * Flags change the behavior of this command:
 *
 *  LOOKUP_NONE (or zero): no special flags are passed.
 *  LOOKUP_NOTOUCH: don't alter the last access time of the key.
 *
 * Note: this function also returns NULL if the key is logically expired
 * but still existing, in case this is a slave, since this API is called only
 * for read operations. Even if the key expiry is master-driven, we can
 * correctly report a key is expired on slaves even if the master is lagging
 * expiring our key via DELs in the replication link. */
robj *lookupKeyReadWithFlags(redisDb *db, robj *key, int flags) {
    robj *val;

    if (expireIfNeeded(db,key) == 1) {
        /* Key expired. If we are in the context of a master, expireIfNeeded()
         * returns 0 only when the key does not exist at all, so it's safe
         * to return NULL ASAP. */
        if (server.masterhost == NULL) {
            server.stat_keyspace_misses++;
            notifyKeyspaceEvent(NOTIFY_KEY_MISS, "keymiss", key, db->id);
            return NULL;
        }

        /* However if we are in the context of a slave, expireIfNeeded() will
         * not really try to expire the key, it only returns information
         * about the "logical" status of the key: key expiring is up to the
         * master in order to have a consistent view of master's data set.
         *
         * However, if the command caller is not the master, and as additional
         * safety measure, the command invoked is a read-only command, we can
         * safely return NULL here, and provide a more consistent behavior
         * to clients accessign expired values in a read-only fashion, that
         * will say the key as non existing.
         *
         * Notably this covers GETs when slaves are used to scale reads. */
        if (server.current_client &&
            server.current_client != server.master &&
            server.current_client->cmd &&
            server.current_client->cmd->flags & CMD_READONLY)
        {
            server.stat_keyspace_misses++;
            notifyKeyspaceEvent(NOTIFY_KEY_MISS, "keymiss", key, db->id);
            return NULL;
        }
    }
    val = lookupKey(db,key,flags);
    if (val == NULL) {
        server.stat_keyspace_misses++;
        notifyKeyspaceEvent(NOTIFY_KEY_MISS, "keymiss", key, db->id);
    }
    else
        server.stat_keyspace_hits++;
    return val;
}

/* Like lookupKeyReadWithFlags(), but does not use any flag, which is the
 * common case. */
robj *lookupKeyRead(redisDb *db, robj *key) {
    return lookupKeyReadWithFlags(db,key,LOOKUP_NONE);
}

/* Lookup a key for write operations, and as a side effect, if needed, expires
 * the key if its TTL is reached.
 *
 * Returns the linked value object if the key exists or NULL if the key
 * does not exist in the specified DB. */
robj *lookupKeyWriteWithFlags(redisDb *db, robj *key, int flags) {
    expireIfNeeded(db,key);
    return lookupKey(db,key,flags);
}

robj *lookupKeyWrite(redisDb *db, robj *key) {
    return lookupKeyWriteWithFlags(db, key, LOOKUP_NONE);
}

robj *lookupKeyReadOrReply(client *c, robj *key, robj *reply) {
    robj *o = lookupKeyRead(c->db, key);
    if (!o) addReply(c,reply);
    return o;
}

robj *lookupKeyWriteOrReply(client *c, robj *key, robj *reply) {
    robj *o = lookupKeyWrite(c->db, key);
    if (!o) addReply(c,reply);
    return o;
}

其实就是第一个函数是实在的干活的，其它都是加了各种辅助和限制条件，比如有的写，有的读，有的删除TTL，有的返回对象操作的回复等等。

2、增加

增加也类似：

/* Add the key to the DB. It's up to the caller to increment the reference
 * counter of the value if needed.
 *
 * The program is aborted if the key already exists. */
void dbAdd(redisDb *db, robj *key, robj *val) {
    sds copy = sdsdup(key->ptr);
    int retval = dictAdd(db->dict, copy, val);

    serverAssertWithInfo(NULL,key,retval == DICT_OK);
    if (val->type == OBJ_LIST ||
        val->type == OBJ_ZSET)
        signalKeyAsReady(db, key);
    if (server.cluster_enabled) slotToKeyAdd(key);
}


/* Return a random key, in form of a Redis object.
 * If there are no keys, NULL is returned.
 *
 * The function makes sure to return keys not already expired. */
robj *dbRandomKey(redisDb *db) {
    dictEntry *de;
    int maxtries = 100;
    int allvolatile = dictSize(db->dict) == dictSize(db->expires);

    while(1) {
        sds key;
        robj *keyobj;

        de = dictGetFairRandomKey(db->dict);
        if (de == NULL) return NULL;

        key = dictGetKey(de);
        //这就是前面提到的，创建使用了一层隔离，增加了RedisObject
        keyobj = createStringObject(key,sdslen(key));
        if (dictFind(db->expires,key)) {
            if (allvolatile && server.masterhost && --maxtries == 0) {
                /* If the DB is composed only of keys with an expire set,
                 * it could happen that all the keys are already logically
                 * expired in the slave, so the function cannot stop because
                 * expireIfNeeded() is false, nor it can stop because
                 * dictGetRandomKey() returns NULL (there are keys to return).
                 * To prevent the infinite loop we do some tries, but if there
                 * are the conditions for an infinite loop, eventually we
                 * return a key name that may be already expired. */
                return keyobj;
            }
            if (expireIfNeeded(db,keyobj)) {
                decrRefCount(keyobj);
                continue; /* search for another key. This expired. */
            }
        }
        return keyobj;
    }
}

简单看一下dictAdd这个函数：

int dictAdd(dict *d, void *key, void *val)
{
    dictEntry *entry = dictAddRaw(d,key,NULL);

    if (!entry) return DICT_ERR;
    dictSetVal(d, entry, val);
    return DICT_OK;
}

回到了对字典的增加，其实最后就是指针操作。包括是否要进行Rehash等等的操作，这些在前面都分析过了，现在是不是可以关联起来了。

3、修改
修改和增加区别没啥，只不过一个要原生出来，一个需要找一下当前的，然后改一下值就可以了。

/* Overwrite an existing key with a new value. Incrementing the reference
 * count of the new value is up to the caller.
 * This function does not modify the expire time of the existing key.
 *
 * The program is aborted if the key was not already present. */
void dbOverwrite(redisDb *db, robj *key, robj *val) {
    dictEntry *de = dictFind(db->dict,key->ptr);

    serverAssertWithInfo(NULL,key,de != NULL);
    dictEntry auxentry = *de;
    robj *old = dictGetVal(de);
    if (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {
        val->lru = old->lru;
    }
    dictSetVal(db->dict, de, val);

    if (server.lazyfree_lazy_server_del) {
        freeObjAsync(old);
        dictSetVal(db->dict, &auxentry, NULL);
    }

    dictFreeVal(db->dict, &auxentry);
}
/* High level Set operation. This function can be used in order to set
 * a key, whatever it was existing or not, to a new object.
 *
 * 1) The ref count of the value object is incremented.
 * 2) clients WATCHing for the destination key notified.
 * 3) The expire time of the key is reset (the key is made persistent).
 *
 * All the new keys in the database should be created via this interface. */
void setKey(redisDb *db, robj *key, robj *val) {
    if (lookupKeyWrite(db,key) == NULL) {
        dbAdd(db,key,val);
    } else {
        dbOverwrite(db,key,val);
    }
    incrRefCount(val);
    removeExpire(db,key);
    signalModifiedKey(db,key);
}

int dbExists(redisDb *db, robj *key) {
    return dictFind(db->dict,key->ptr) != NULL;
}

修改对照增加就可以了。

4、删除

/* Delete a key, value, and associated expiration entry if any, from the DB */
int dbSyncDelete(redisDb *db, robj *key) {
    /* Deleting an entry from the expires dict will not free the sds of
     * the key, because it is shared with the main dictionary. */
    if (dictSize(db->expires) > 0) dictDelete(db->expires,key->ptr);
    if (dictDelete(db->dict,key->ptr) == DICT_OK) {
        if (server.cluster_enabled) slotToKeyDel(key);
        return 1;
    } else {
        return 0;
    }
}

/* This is a wrapper whose behavior depends on the Redis lazy free
 * configuration. Deletes the key synchronously or asynchronously. */
int dbDelete(redisDb *db, robj *key) {
    return server.lazyfree_lazy_server_del ? dbAsyncDelete(db,key) :
                                             dbSyncDelete(db,key);
}
/* Remove all keys from all the databases in a Redis server.
 * If callback is given the function is called from time to time to
 * signal that work is in progress.
 *
 * The dbnum can be -1 if all the DBs should be flushed, or the specified
 * DB number if we want to flush only a single Redis database number.
 *
 * Flags are be EMPTYDB_NO_FLAGS if no special flags are specified or
 * EMPTYDB_ASYNC if we want the memory to be freed in a different thread
 * and the function to return ASAP.
 *
 * On success the fuction returns the number of keys removed from the
 * database(s). Otherwise -1 is returned in the specific case the
 * DB number is out of range, and errno is set to EINVAL. */
long long emptyDbGeneric(redisDb *dbarray, int dbnum, int flags, void(callback)(void*)) {
    int async = (flags & EMPTYDB_ASYNC);
    long long removed = 0;

    if (dbnum < -1 || dbnum >= server.dbnum) {
        errno = EINVAL;
        return -1;
    }

    /* Fire the flushdb modules event. */
    RedisModuleFlushInfoV1 fi = {REDISMODULE_FLUSHINFO_VERSION,!async,dbnum};
    moduleFireServerEvent(REDISMODULE_EVENT_FLUSHDB,
                          REDISMODULE_SUBEVENT_FLUSHDB_START,
                          &fi);

    /* Make sure the WATCHed keys are affected by the FLUSH* commands.
     * Note that we need to call the function while the keys are still
     * there. */
    signalFlushedDb(dbnum);

    int startdb, enddb;
    if (dbnum == -1) {
        startdb = 0;
        enddb = server.dbnum-1;
    } else {
        startdb = enddb = dbnum;
    }

    for (int j = startdb; j <= enddb; j++) {
        removed += dictSize(dbarray[j].dict);
        if (async) {
            emptyDbAsync(&dbarray[j]);
        } else {
            dictEmpty(dbarray[j].dict,callback);
            dictEmpty(dbarray[j].expires,callback);
        }
    }
    if (server.cluster_enabled) {
        if (async) {
            slotToKeyFlushAsync();
        } else {
            slotToKeyFlush();
        }
    }
    if (dbnum == -1) flushSlaveKeysWithExpireList();

    /* Also fire the end event. Note that this event will fire almost
     * immediately after the start event if the flush is asynchronous. */
    moduleFireServerEvent(REDISMODULE_EVENT_FLUSHDB,
                          REDISMODULE_SUBEVENT_FLUSHDB_END,
                          &fi);

    return removed;
}

long long emptyDb(int dbnum, int flags, void(callback)(void*)) {
    return emptyDbGeneric(server.db, dbnum, flags, callback);
}

删除其实也比较好理解，除了精准删除还有整库整库的删除。

5、时间过期处理

过期时间处理其实主要就是三个，删除过期，设置过期，查询过期，

int removeExpire(redisDb *db, robj *key) {
    /* An expire may only be removed if there is a corresponding entry in the
     * main dict. Otherwise, the key will never be freed. */
    serverAssertWithInfo(NULL,key,dictFind(db->dict,key->ptr) != NULL);
    return dictDelete(db->expires,key->ptr) == DICT_OK;
}

/* Set an expire to the specified key. If the expire is set in the context
 * of an user calling a command 'c' is the client, otherwise 'c' is set
 * to NULL. The 'when' parameter is the absolute unix time in milliseconds
 * after which the key will no longer be considered valid. */
void setExpire(client *c, redisDb *db, robj *key, long long when) {
    dictEntry *kde, *de;

    /* Reuse the sds from the main dict in the expire dict */
    kde = dictFind(db->dict,key->ptr);
    serverAssertWithInfo(NULL,key,kde != NULL);
    de = dictAddOrFind(db->expires,dictGetKey(kde));
    dictSetSignedIntegerVal(de,when);

    int writable_slave = server.masterhost && server.repl_slave_ro == 0;
    if (c && writable_slave && !(c->flags & CLIENT_MASTER))
        rememberSlaveKeyWithExpire(db,key);
}

/* Return the expire time of the specified key, or -1 if no expire
 * is associated with this key (i.e. the key is non volatile) */
long long getExpire(redisDb *db, robj *key) {
    dictEntry *de;

    /* No expire? return ASAP */
    if (dictSize(db->expires) == 0 ||
       (de = dictFind(db->expires,key->ptr)) == NULL) return -1;

    /* The entry was found in the expire dict, this means it should also
     * be present in the main dict (safety check). */
    serverAssertWithInfo(NULL,key,dictFind(db->dict,key->ptr) != NULL);
    return dictGetSignedIntegerVal(de);
}

/* Propagate expires into slaves and the AOF file.
 * When a key expires in the master, a DEL operation for this key is sent
 * to all the slaves and the AOF file if enabled.
 *
 * This way the key expiry is centralized in one place, and since both
 * AOF and the master->slave link guarantee operation ordering, everything
 * will be consistent even if we allow write operations against expiring
 * keys. */
void propagateExpire(redisDb *db, robj *key, int lazy) {
    robj *argv[2];

    argv[0] = lazy ? shared.unlink : shared.del;
    argv[1] = key;
    incrRefCount(argv[0]);
    incrRefCount(argv[1]);

    if (server.aof_state != AOF_OFF)
        feedAppendOnlyFile(server.delCommand,db->id,argv,2);
    replicationFeedSlaves(server.slaves,db->id,argv,2);

    decrRefCount(argv[0]);
    decrRefCount(argv[1]);
}

6、过期策略处理

在REDIS中，过期有三种策略，即定时删除，惰性删除也就是前面看到的访问时判断是否TTL过期删除，另外一个就是定期删除。第一种和第三种方式雷同，只不过前者使用策略覆盖自己，后者是全覆盖。

/* Check if the key is expired. */
int keyIsExpired(redisDb *db, robj *key) {
    mstime_t when = getExpire(db,key);
    mstime_t now;

    if (when < 0) return 0; /* No expire for this key */

    /* Don't expire anything while loading. It will be done later. */
    if (server.loading) return 0;

    /* If we are in the context of a Lua script, we pretend that time is
     * blocked to when the Lua script started. This way a key can expire
     * only the first time it is accessed and not in the middle of the
     * script execution, making propagation to slaves / AOF consistent.
     * See issue #1525 on Github for more information. */
    if (server.lua_caller) {
        now = server.lua_time_start;
    }
    /* If we are in the middle of a command execution, we still want to use
     * a reference time that does not change: in that case we just use the
     * cached time, that we update before each call in the call() function.
     * This way we avoid that commands such as RPOPLPUSH or similar, that
     * may re-open the same key multiple times, can invalidate an already
     * open object in a next call, if the next call will see the key expired,
     * while the first did not. */
    else if (server.fixed_time_expire > 0) {
        now = server.mstime;
    }
    /* For the other cases, we want to use the most fresh time we have. */
    else {
        now = mstime();
    }

    /* The key expired if the current (virtual or real) time is greater
     * than the expire time of the key. */
    return now > when;
}

/* This function is called when we are going to perform some operation
 * in a given key, but such key may be already logically expired even if
 * it still exists in the database. The main way this function is called
 * is via lookupKey*() family of functions.
 *
 * The behavior of the function depends on the replication role of the
 * instance, because slave instances do not expire keys, they wait
 * for DELs from the master for consistency matters. However even
 * slaves will try to have a coherent return value for the function,
 * so that read commands executed in the slave side will be able to
 * behave like if the key is expired even if still present (because the
 * master has yet to propagate the DEL).
 *
 * In masters as a side effect of finding a key which is expired, such
 * key will be evicted from the database. Also this may trigger the
 * propagation of a DEL/UNLINK command in AOF / replication stream.
 *
 * The return value of the function is 0 if the key is still valid,
 * otherwise the function returns 1 if the key is expired. */
int expireIfNeeded(redisDb *db, robj *key) {
    if (!keyIsExpired(db,key)) return 0;

    /* If we are running in the context of a slave, instead of
     * evicting the expired key from the database, we return ASAP:
     * the slave key expiration is controlled by the master that will
     * send us synthesized DEL operations for expired keys.
     *
     * Still we try to return the right information to the caller,
     * that is, 0 if we think the key should be still valid, 1 if
     * we think the key is expired at this time. */
    if (server.masterhost != NULL) return 1;

    /* Delete the key */
    server.stat_expiredkeys++;
    propagateExpire(db,key,server.lazyfree_lazy_expire);
    notifyKeyspaceEvent(NOTIFY_EXPIRED,
        "expired",key,db->id);
    return server.lazyfree_lazy_expire ? dbAsyncDelete(db,key) :
                                         dbSyncDelete(db,key);
}

过期处理的具体函数在expire.c这个文件中：

void activeExpireCycle(int type) {
    /* Adjust the running parameters according to the configured expire
     * effort. The default effort is 1, and the maximum configurable effort
     * is 10. */
    unsigned long
    effort = server.active_expire_effort-1, /* Rescale from 0 to 9. */
    config_keys_per_loop = ACTIVE_EXPIRE_CYCLE_KEYS_PER_LOOP +
                           ACTIVE_EXPIRE_CYCLE_KEYS_PER_LOOP/4*effort,
    config_cycle_fast_duration = ACTIVE_EXPIRE_CYCLE_FAST_DURATION +
                                 ACTIVE_EXPIRE_CYCLE_FAST_DURATION/4*effort,
    config_cycle_slow_time_perc = ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC +
                                  2*effort,
    config_cycle_acceptable_stale = ACTIVE_EXPIRE_CYCLE_ACCEPTABLE_STALE-
                                    effort;

    /* This function has some global state in order to continue the work
     * incrementally across calls. */
    static unsigned int current_db = 0; /* Last DB tested. */
    static int timelimit_exit = 0;      /* Time limit hit in previous call? */
    static long long last_fast_cycle = 0; /* When last fast cycle ran. */

    int j, iteration = 0;
    int dbs_per_call = CRON_DBS_PER_CALL;
    long long start = ustime(), timelimit, elapsed;

    /* When clients are paused the dataset should be static not just from the
     * POV of clients not being able to write, but also from the POV of
     * expires and evictions of keys not being performed. */
    if (clientsArePaused()) return;

    if (type == ACTIVE_EXPIRE_CYCLE_FAST) {
        /* Don't start a fast cycle if the previous cycle did not exit
         * for time limit, unless the percentage of estimated stale keys is
         * too high. Also never repeat a fast cycle for the same period
         * as the fast cycle total duration itself. */
        if (!timelimit_exit &&
            server.stat_expired_stale_perc < config_cycle_acceptable_stale)
            return;

        if (start < last_fast_cycle + (long long)config_cycle_fast_duration*2)
            return;

        last_fast_cycle = start;
    }

    /* We usually should test CRON_DBS_PER_CALL per iteration, with
     * two exceptions:
     *
     * 1) Don't test more DBs than we have.
     * 2) If last time we hit the time limit, we want to scan all DBs
     * in this iteration, as there is work to do in some DB and we don't want
     * expired keys to use memory for too much time. */
    if (dbs_per_call > server.dbnum || timelimit_exit)
        dbs_per_call = server.dbnum;

    /* We can use at max 'config_cycle_slow_time_perc' percentage of CPU
     * time per iteration. Since this function gets called with a frequency of
     * server.hz times per second, the following is the max amount of
     * microseconds we can spend in this function. */
    timelimit = config_cycle_slow_time_perc*1000000/server.hz/100;
    timelimit_exit = 0;
    if (timelimit <= 0) timelimit = 1;

    if (type == ACTIVE_EXPIRE_CYCLE_FAST)
        timelimit = config_cycle_fast_duration; /* in microseconds. */

    /* Accumulate some global stats as we expire keys, to have some idea
     * about the number of keys that are already logically expired, but still
     * existing inside the database. */
    long total_sampled = 0;
    long total_expired = 0;

    for (j = 0; j < dbs_per_call && timelimit_exit == 0; j++) {
        /* Expired and checked in a single loop. */
        unsigned long expired, sampled;

        redisDb *db = server.db+(current_db % server.dbnum);

        /* Increment the DB now so we are sure if we run out of time
         * in the current DB we'll restart from the next. This allows to
         * distribute the time evenly across DBs. */
        current_db++;

        /* Continue to expire if at the end of the cycle more than 25%
         * of the keys were expired. */
        do {
            unsigned long num, slots;
            long long now, ttl_sum;
            int ttl_samples;
            iteration++;

            /* If there is nothing to expire try next DB ASAP. */
            if ((num = dictSize(db->expires)) == 0) {
                db->avg_ttl = 0;
                break;
            }
            slots = dictSlots(db->expires);
            now = mstime();

            /* When there are less than 1% filled slots, sampling the key
             * space is expensive, so stop here waiting for better times...
             * The dictionary will be resized asap. */
            if (num && slots > DICT_HT_INITIAL_SIZE &&
                (num*100/slots < 1)) break;

            /* The main collection cycle. Sample random keys among keys
             * with an expire set, checking for expired ones. */
            expired = 0;
            sampled = 0;
            ttl_sum = 0;
            ttl_samples = 0;

            if (num > config_keys_per_loop)
                num = config_keys_per_loop;

            /* Here we access the low level representation of the hash table
             * for speed concerns: this makes this code coupled with dict.c,
             * but it hardly changed in ten years.
             *
             * Note that certain places of the hash table may be empty,
             * so we want also a stop condition about the number of
             * buckets that we scanned. However scanning for free buckets
             * is very fast: we are in the cache line scanning a sequential
             * array of NULL pointers, so we can scan a lot more buckets
             * than keys in the same time. */
            long max_buckets = num*20;
            long checked_buckets = 0;

            while (sampled < num && checked_buckets < max_buckets) {
                for (int table = 0; table < 2; table++) {
                    if (table == 1 && !dictIsRehashing(db->expires)) break;

                    unsigned long idx = db->expires_cursor;
                    idx &= db->expires->ht[table].sizemask;
                    dictEntry *de = db->expires->ht[table].table[idx];
                    long long ttl;

                    /* Scan the current bucket of the current table. */
                    checked_buckets++;
                    while(de) {
                        /* Get the next entry now since this entry may get
                         * deleted. */
                        dictEntry *e = de;
                        de = de->next;

                        ttl = dictGetSignedIntegerVal(e)-now;
                        if (activeExpireCycleTryExpire(db,e,now)) expired++;
                        if (ttl > 0) {
                            /* We want the average TTL of keys yet
                             * not expired. */
                            ttl_sum += ttl;
                            ttl_samples++;
                        }
                        sampled++;
                    }
                }
                db->expires_cursor++;
            }
            total_expired += expired;
            total_sampled += sampled;

            /* Update the average TTL stats for this database. */
            if (ttl_samples) {
                long long avg_ttl = ttl_sum/ttl_samples;

                /* Do a simple running average with a few samples.
                 * We just use the current estimate with a weight of 2%
                 * and the previous estimate with a weight of 98%. */
                if (db->avg_ttl == 0) db->avg_ttl = avg_ttl;
                db->avg_ttl = (db->avg_ttl/50)*49 + (avg_ttl/50);
            }

            /* We can't block forever here even if there are many keys to
             * expire. So after a given amount of milliseconds return to the
             * caller waiting for the other active expire cycle. */
            if ((iteration & 0xf) == 0) { /* check once every 16 iterations. */
                elapsed = ustime()-start;
                if (elapsed > timelimit) {
                    timelimit_exit = 1;
                    server.stat_expired_time_cap_reached_count++;
                    break;
                }
            }
            /* We don't repeat the cycle for the current database if there are
             * an acceptable amount of stale keys (logically expired but yet
             * not reclained). */
        } while ((expired*100/sampled) > config_cycle_acceptable_stale);
    }

    elapsed = ustime()-start;
    server.stat_expire_cycle_time_used += elapsed;
    latencyAddSampleIfNeeded("expire-cycle",elapsed/1000);

    /* Update our estimate of keys existing but yet to be expired.
     * Running average with this sample accounting for 5%. */
    double current_perc;
    if (total_sampled) {
        current_perc = (double)total_expired/total_sampled;
    } else
        current_perc = 0;
    server.stat_expired_stale_perc = (current_perc*0.05)+
                                     (server.stat_expired_stale_perc*0.95);
}

7、命令处理函数
另外，在db.c中还有大量的命令处理函数，只举几个例子：

void renameGenericCommand(client *c, int nx) {
    robj *o;
    long long expire;
    int samekey = 0;

    /* When source and dest key is the same, no operation is performed,
     * if the key exists, however we still return an error on unexisting key. */
    if (sdscmp(c->argv[1]->ptr,c->argv[2]->ptr) == 0) samekey = 1;

    if ((o = lookupKeyWriteOrReply(c,c->argv[1],shared.nokeyerr)) == NULL)
        return;

    if (samekey) {
        addReply(c,nx ? shared.czero : shared.ok);
        return;
    }

    incrRefCount(o);
    expire = getExpire(c->db,c->argv[1]);
    if (lookupKeyWrite(c->db,c->argv[2]) != NULL) {
        if (nx) {
            decrRefCount(o);
            addReply(c,shared.czero);
            return;
        }
        /* Overwrite: delete the old key before creating the new one
         * with the same name. */
        dbDelete(c->db,c->argv[2]);
    }
    dbAdd(c->db,c->argv[2],o);
    if (expire != -1) setExpire(c,c->db,c->argv[2],expire);
    dbDelete(c->db,c->argv[1]);
    signalModifiedKey(c->db,c->argv[1]);
    signalModifiedKey(c->db,c->argv[2]);
    notifyKeyspaceEvent(NOTIFY_GENERIC,"rename_from",
        c->argv[1],c->db->id);
    notifyKeyspaceEvent(NOTIFY_GENERIC,"rename_to",
        c->argv[2],c->db->id);
    server.dirty++;
    addReply(c,nx ? shared.cone : shared.ok);
}

void renameCommand(client *c) {
    renameGenericCommand(c,0);
}

void renamenxCommand(client *c) {
    renameGenericCommand(c,1);
}

void moveCommand(client *c) {
    robj *o;
    redisDb *src, *dst;
    int srcid;
    long long dbid, expire;

    if (server.cluster_enabled) {
        addReplyError(c,"MOVE is not allowed in cluster mode");
        return;
    }

    /* Obtain source and target DB pointers */
    src = c->db;
    srcid = c->db->id;

    if (getLongLongFromObject(c->argv[2],&dbid) == C_ERR ||
        dbid < INT_MIN || dbid > INT_MAX ||
        selectDb(c,dbid) == C_ERR)
    {
        addReply(c,shared.outofrangeerr);
        return;
    }
    dst = c->db;
    selectDb(c,srcid); /* Back to the source DB */

    /* If the user is moving using as target the same
     * DB as the source DB it is probably an error. */
    if (src == dst) {
        addReply(c,shared.sameobjecterr);
        return;
    }

    /* Check if the element exists and get a reference */
    o = lookupKeyWrite(c->db,c->argv[1]);
    if (!o) {
        addReply(c,shared.czero);
        return;
    }
    expire = getExpire(c->db,c->argv[1]);

    /* Return zero if the key already exists in the target DB */
    if (lookupKeyWrite(dst,c->argv[1]) != NULL) {
        addReply(c,shared.czero);
        return;
    }
    dbAdd(dst,c->argv[1],o);
    if (expire != -1) setExpire(c,dst,c->argv[1],expire);
    incrRefCount(o);

    /* OK! key moved, free the entry in the source DB */
    dbDelete(src,c->argv[1]);
    signalModifiedKey(src,c->argv[1]);
    signalModifiedKey(dst,c->argv[1]);
    notifyKeyspaceEvent(NOTIFY_GENERIC,
                "move_from",c->argv[1],src->id);
    notifyKeyspaceEvent(NOTIFY_GENERIC,
                "move_to",c->argv[1],dst->id);

    server.dirty++;
    addReply(c,shared.cone);
}

从代码中可以看到，如果深入数据修改处理，就会调用相关的字典操作和具体的robj对象进行操作，如果对DB操作，则要进行相关的参数调整配置即可。

三、总结

其实对于新手来说，看这么多确实挺复杂，不过如果自己尝试着写一个简单的内存池，你就会明白过来，REDIS其实你就可以理解成一个大的内存池，只不过增加很多的辅助功能，保障这个池子内存应用的合理以及分布式应用的安全等等。等明白了这一点，就会发现，内存池就变成了内存型数据库。当然，这样说只是对于初学者的一个基本情况的展示，它是不准确的。
分析到这里，应该是核心的主流程就分析的基本完成了，但是仍然还有许多重要的辅助部分会在接下来逐一分析。因为疫情，困在帝都，源码分析就停了下来，趁着春节休息，赶紧把它剩下的部分搞定。