memcached (text)¶
The ASCII line-based protocol that telnet and most older memcached clients speak.
Framing¶
- Every command starts on its own line and ends with
\r\n. - Storage commands are followed by a separate data block, also
\r\n-terminated. - Maximum line length: 4 KiB.
- Maximum payload length: 16 MiB.
exptime semantics¶
The exptime field on set, add, replace, cas, touch, gat,
gats, and the meta T flag follows the standard memcached rules:
| Value | Meaning |
|---|---|
0 |
Never expires |
1 … 2592000 |
Relative seconds from now (up to 30 days) |
> 2592000 |
Absolute UNIX timestamp |
The 30-day threshold is the memcached convention.
Flags¶
The flags field is a 32-bit opaque integer that the server stores
verbatim and returns on get.
Two flags values are not opaque, and a decoder reads the value under them
The Redis value types that are not plain strings are tagged by their flags word,
and the two front ends share one CacheEngine — so a value written by a memcached
client is a value a Redis verb will decode:
flags |
decimal | Tagged as | Verbs that parse the value |
|---|---|---|---|
0x5E700001 |
1584398337 | Redis set | SADD, SREM, SMEMBERS, SCARD, SISMEMBER, … |
0x5E700002 |
1584398338 | Redis stream | XADD, XLEN, XRANGE, XREAD, the group verbs, … |
Setting one of those words on a set means the next such verb on the key parses
your bytes as that type's blob rather than echoing them.
That is not a privilege boundary and never was one: it is the same key space,
reached by two front ends. It is called out because the value under such a key is
parsed rather than echoed, so it is subject to the decoder's validation. A blob
declaring more elements than its bytes can supply is refused, and the verb answers
-ERR storage failure rather than the daemon attempting the allocation — for a
set's member count
(#271) and for each of
a stream's five counts, which previously clamped the reservation instead of
refusing it (#269).
Every other flags value is stored and returned untouched.
CAS¶
Every entry has a CAS token (64-bit, monotonically increasing per
storage instance). cas and meta C(token) use it for
compare-and-swap; gets, gats, and mg c return it to the client.
noreply¶
set, add, replace, append, prepend, cas, delete, incr,
decr, touch, flush_all, cache_memlimit, and verbosity accept
a trailing noreply token. When present, the response (including any
error) is suppressed. Pipelines that use noreply lose visibility
into failures.
Error tokens¶
| Token | Meaning |
|---|---|
ERROR |
Unknown command |
CLIENT_ERROR <msg> |
The client sent a malformed command |
SERVER_ERROR <msg> |
The server hit an internal failure |
Commands¶
See the Commands index for the full list.