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Redis Source Code Analysis - Server

[TOC]

Command execution flow

Key fields of redisCommand

struct redisCommand { /* Redis command */
		char *name;             /* command name */
		redisCommandProc *proc; /* command callback */
		int arity;              /* number of arguments */
		char *sflags;           /* string flags (human readable) */
		int flags;              /* parsed binary flags */
		redisGetKeysProc *getkeys_proc; /* function to extract key arguments from argv */
		int firstkey;                   /* position of first key (0 = no keys) */
		int lastkey;                    /* position of last key (may be negative: argc+lastkey) */
		int keystep;                    /* step between first and last key */
		long long microseconds, calls;  /* total microseconds spent and call count */
};

  • name: command name (lookup is case-insensitive)

  • proc: pointer to the implementation function

  • arity: used to validate argument count (negative value -N indicates >= N args)

  • sflags: human-readable flags

    flag binary flag constant meaning example commands
    w REDIS_CMD_WRITE write command (may modify dataset) SET, RPUSH, DEL
    r REDIS_CMD_READONLY read-only command GET, STRLEN, EXISTS
    m REDIS_CMD_DENYOOM may use a lot of memory; check OOM before executing SET, APPEND, RPUSH, LPUSH, SADD
    a REDIS_CMD_ADMIN administrative command SAVE, BGSAVE, SHUTDOWN
    p REDIS_CMD_PUBSUB pub/sub related command PUBLISH, SUBSCRIBE, PUBSUB
    s REDIS_CMD_NOSCRIPT not allowed inside Lua scripts BRPOP, BLPOP, SPOP
    R REDIS_CMD_RANDOM non-deterministic: same args on same dataset may return different result SPOP, SRANDMEMBER, RANDOMKEY
    S REDIS_CMD_SORT_FOR_SCRIPT sort output when used in Lua scripts to make results deterministic SINTER, SMEMBERS, KEYS
    l REDIS_CMD_LOADING allowed while server is loading data INFO, SHUTDOWN, PUBLISH
    t REDIS_CMD_STALE allowed on replicas with stale data SLAVEOF, PING, INFO
    M REDIS_CMD_SKIP_MONITOR not auto-propagated in MONITOR mode EXEC
    k REDIS_CMD_ASKING (internal use)
    F REDIS_CMD_FAST (internal use)

  • flags: binary flags derived from sflags

  • getkeys_proc: function to extract keys from argv

  • firstkey, lastkey, keystep: describe key positions

  • calls, microseconds: command statistics

serverCron

serverCron runs every 100ms by default and manages server resources and maintenance tasks.

Overview flow

graph TD
updateCachedTime(Update cached time) --> getLRUClock(Update LRU clock)
getLRUClock --> zmalloc_user_memory(Update memory usage stats)
zmalloc_user_memory --> prepareForShutdown(Check graceful shutdown)
prepareForShutdown --> clientCron(Handle clients)
clientCron --> databasesCron(Handle DB tasks)
databasesCron --> rewriteAppendOnlyFileBackground(Schedule BGREWRITEAOF)
rewriteAppendOnlyFileBackground --> checkPersistence(Check background persistence)
checkPersistence --> flushAppendOnlyFile(Flush AOF buffer to disk)
flushAppendOnlyFile --> freeClientsInAsyncFreeQueue(Free clients in async queue)
freeClientsInAsyncFreeQueue --> incrCronloops(Increment server.cronloops)
Loading

Main tasks include:

  1. Update cached server time
  2. Update LRU clock
  3. Update commands-per-second counters
  4. Update memory peak records
  5. Handle SIGTERM
  6. Manage client resources
  7. Manage database resources
  8. Run deferred BGREWRITEAOF if scheduled
  9. Check background persistence status (rdb_child_pid != -1 || aof_child_pid != -1)
  10. Flush AOF buffer to AOF file
  11. Close clients scheduled for asynchronous close
  12. Increment server.cronloops

Decision flow for persistence tasks:

no_persistent=>operation: No background persistence running
is_delay=>condition: Is BGREWRITEAOF delayed?
is_autosave=>condition: Are autosave conditions met?
exec_bgrewriteaof=>operation: Execute BGREWRITEAOF
exec_bgsave=>operation: Execute BGSAVE
is_aofrewrite=>condition: Is AOF rewrite needed?
do_nothing=>operation: Do nothing

no_persistent->is_delay
is_delay(yes)->exec_bgrewriteaof
is_delay(no)->is_autosave
is_autosave(yes)->exec_bgsave
is_autosave(no)->is_aofrewrite
is_aofrewrite(yes)->exec_bgrewriteaof
is_aofrewrite(no)->do_nothing

Server initialization

  1. Initialize server state structure
  2. Load configuration options
  3. Initialize internal data structures
  4. Restore dataset state (load RDB/AOF as configured)
  5. Start the event loop

References

[1] Huang Jianhong. Redis Design and Implementation

[2] Redis5 design and source analysis — Chapter 9: command processing lifecycle