MEMCACHED_BEHAVIOR(3)

📦
version 1.1.4 · 12 versions
, in fedora 43

💖 NAME 💖

memcached_behavior — libmemcached Documentation 💅

okay so bestie, welcome to MEMCACHED_BEHAVIOR(3) from libmemcached-awesome. This page is iconic because it's literally the docs for memcached_behavior — the thing that lets you manipulate the behavior of a memcached_st structure. periodt. 🫶

✨ SYNOPSIS ✨

#include <libmemcached/memcached.h>

Compile and link with -lmemcached

🧠 memcached_behavior_get

uint64_t memcached_behavior_get(memcached_st *ptr, memcached_behavior_t flag)

Parameters

  • ptr — pointer to initialized memcached_st struct
  • flag — memcached_behavior_t to query

Returns — the vaue set for flag

🧠 memcached_behavior_set

memcached_return_t memcached_behavior_set(memcached_st *ptr, memcached_behavior_t flag, uint64_t data)

Parameters

  • ptr — pointer to initialized memcached_st struct
  • flag — memcached_behavior_t to set
  • data — the value to set for flag

Returns — memcached_return_t indicating success

📝 Changed in version 0.17

The data argument of memcached_behavior_set() was changed from taking a pointer to data value, to taking a uin64_t.

🏷️ typedef enum memcached_behavior_t memcached_behavior_t

typedef enum memcached_behavior_t memcached_behavior_t

🌈 enum memcached_behavior_t

MEMCACHED_BEHAVIOR_USE_UDP

Causes libmemcached to use the UDP transport when communicating with a memcached server. Not all I/O operations are tested when this behavior is enabled. periodt.

The following operations will return MEMCACHED_NOT_SUPPORTED when executed with MEMCACHED_BEHAVIOR_USE_UDP enabled:

  • memcached_version(),
  • memcached_stat(),
  • memcached_get(),
  • memcached_get_by_key(),
  • memcached_mget(),
  • memcached_mget_by_key(),
  • memcached_fetch(),
  • memcached_fetch_result(),
  • memcached_fetch_execute().

All other operations are tested but are executed in a 'fire-and-forget' mode, in which once the client has executed the operation, no attempt will be made to ensure the operation has been received and acted on by the server.

libmemcached does not allow TCP and UDP servers to be shared within the same libmemcached client 'instance'. An attempt to add a TCP server when this behavior is enabled will result in a MEMCACHED_INVALID_HOST_PROTOCOL, as will attempting to add a UDP server when this behavior has not been enabled.

MEMCACHED_BEHAVIOR_NO_BLOCK

This enables SO_LINGER only, so the close(2) call on the socket returns immediately. I/O is always handled asynchronously in recent versions of libmemcached. slay.

MEMCACHED_BEHAVIOR_SND_TIMEOUT

This sets the microsecond behavior of the socket against the SO_SNDTIMEO flag.

MEMCACHED_BEHAVIOR_RCV_TIMEOUT

This sets the microsecond behavior of the socket against the SO_RCVTIMEO flag.

MEMCACHED_BEHAVIOR_TCP_NODELAY

Disables Nagle's algorithm. See RFC 896.

MEMCACHED_BEHAVIOR_HASH

Set the hash algorithm used for keys.

Each hash has its advantages and its weaknesses. If you don't know or don't care, just go with the default. iykyk.

MEMCACHED_BEHAVIOR_DISTRIBUTION

Setting a memcached_server_distribution_t you can enable different means of distributing values to servers.

The default method is MEMCACHED_DISTRIBUTION_MODULA (hash of the key modulo number of servers).

You can enable consistent hashing by setting MEMCACHED_DISTRIBUTION_CONSISTENT. Consistent hashing delivers better distribution and allows servers to be added to the cluster with minimal cache losses.

Currently MEMCACHED_DISTRIBUTION_CONSISTENT is an alias for the value MEMCACHED_DISTRIBUTION_CONSISTENT_KETAMA.

MEMCACHED_BEHAVIOR_CACHE_LOOKUPS

Deprecated since version 0.46(?): DNS lookups are now always cached until an error occurs with the server.

Memcached can cache named lookups so that DNS lookups are made only once.

MEMCACHED_BEHAVIOR_SUPPORT_CAS

Support CAS operations (this is not enabled by default at this point in the server since it imposes a slight performance penalty).

MEMCACHED_BEHAVIOR_KETAMA

Sets the default distribution to MEMCACHED_DISTRIBUTION_CONSISTENT_KETAMA and the hash to MEMCACHED_HASH_MD5.

MEMCACHED_BEHAVIOR_KETAMA_WEIGHTED

Sets the default distribution to MEMCACHED_DISTRIBUTION_CONSISTENT_KETAMA with the weighted tests. Makes the default hashing algorithm for keys use MEMCACHED_HASH_MD5.

MEMCACHED_BEHAVIOR_KETAMA_HASH

Sets the hashing algorithm for host mapping on continuum.

MEMCACHED_BEHAVIOR_KETAMA_COMPAT

Sets the compatibility mode. The value can be set to either MEMCACHED_KETAMA_COMPAT_LIBMEMCACHED (this is the default) or MEMCACHED_KETAMA_COMPAT_SPY to be compatible with the SPY Memcached client for Java.

MEMCACHED_BEHAVIOR_POLL_TIMEOUT

Modify the timeout in milliseconds value that is used by poll. The default value is -1. An signed int must be passed to memcached_behavior_set() to change this value (this requires casting). For memcached_behavior_get() a 'signed int' value will be cast and returned as 'unsigned long long'.

MEMCACHED_BEHAVIOR_USER_DATA

Deprecated since version <: 0.30

MEMCACHED_BEHAVIOR_BUFFER_REQUESTS

Enabling buffered IO causes commands to "buffer" instead of being sent. Any action that gets data causes this buffer to be be sent to the remote connection. Quiting the connection or closing down the connection will also cause the buffered data to be pushed to the remote connection. no cap.

MEMCACHED_BEHAVIOR_VERIFY_KEY

Enabling this will cause libmemcached to test all keys to verify that they are valid keys.

MEMCACHED_BEHAVIOR_SORT_HOSTS

Enabling this will cause hosts that are added to be placed in the host list in sorted order. This will defeat consistent hashing.

MEMCACHED_BEHAVIOR_CONNECT_TIMEOUT

Set the timeout during socket connection in milliseconds. Specifying -1 means an infinite time‐out.

MEMCACHED_BEHAVIOR_BINARY_PROTOCOL

Enable the use of the binary protocol. Please note that you cannot toggle this flag on an open connection.

MEMCACHED_BEHAVIOR_IO_MSG_WATERMARK

Set this value to tune the number of messages that may be sent before libmemcached should start to automatically drain the input queue.

Setting this value to high, may cause libmemcached to deadlock (trying to send data, but the send will block because the input buffer in the kernel is full).

MEMCACHED_BEHAVIOR_IO_BYTES_WATERMARK

Set this value to tune the number of bytes that may be sent before libmemcached should start to automatically drain the input queue (need at least 10 IO requests sent without reading the input buffer).

Setting this value to high, may cause libmemcached to deadlock (trying to send data, but the send will block because the input buffer in the kernel is full).

MEMCACHED_BEHAVIOR_IO_KEY_PREFETCH

The binary protocol works a bit different than the textual protocol in that a multiget is implemented as a pipe of single get-operations which are sent to the server in a chunk.

If you are using large multigets from your application, you may improve the latency of the gets by setting this value so you send out the first chunk of requests when you hit the specified limit. It allows the servers to start processing the requests to send the data back while the rest of the requests are created and sent to the server.

MEMCACHED_BEHAVIOR_NOREPLY

Set this value to specify that you really don't care about the result from your storage commands (set, add, replace, append, prepend).

MEMCACHED_BEHAVIOR_NUMBER_OF_REPLICAS

Specify the number of replicas libmemcached should store of each item (on different servers).

This replication does not dedicate certain memcached servers to store the replicas in, but instead it will store the replicas together with all of the other objects (on the 'n' next servers specified in your server list).

Requires the binary protocol and only supports (M)GET/SET/DELETE.

NOTE: libmemcached does not guarantee nor enforce any consistency.

MEMCACHED_BEHAVIOR_RANDOMIZE_REPLICA_READ

Allows randomizing the replica reads starting point. Normally the read is done from primary server and in case of failure the read is done from primary + 1, then primary + 2 all the way to 'n' replicas.

This allows distributing read load to multiple servers with the expense of more write traffic.

NOTE: Only errors to communicate with a server are considered failures, so MEMCACHED_NOTFOUND does not account for failure.
  • MEMCACHED_BEHAVIOR_CORK — deprecated, bestie, RIP 💀
Deprecated since version ?: this one got deprecated because the behavior is now built in and gets applied appropriately on selected platforms. So like... it just happens now. You don't gotta ask.
  • MEMCACHED_BEHAVIOR_KEEPALIVE — turns on TCP_KEEPALIVE behavior. Keeping that connection alive and thriving ✨
  • MEMCACHED_BEHAVIOR_KEEPALIVE_IDLE — sets the time, in seconds, after which a connection gets marked as idle. Heads up: this is only available as an option on Linux. Linux girlies only 🐧
  • MEMCACHED_BEHAVIOR_SOCKET_SEND_SIZE — finds the current size of SO_SNDBUF. A value of 0 means either an error happened or no hosts were available. If that's what you get, it's totally safe to just assume the system default.

And if it was an error? You can check the last cached errno to find out the specific error. Detective work, but make it cute 🔍

  • MEMCACHED_BEHAVIOR_SOCKET_RECV_SIZE — finds the current size of SO_RCVBUF. Same deal: a value of 0 means either an error occurred or there were no hosts available, and it's safe to assume the system default in that case.

Same energy on the error front too — check the last cached errno to find the specific error.

  • MEMCACHED_BEHAVIOR_SERVER_FAILURE_LIMIT — deprecated since version 0.48, go look at MEMCACHED_BEHAVIOR_REMOVE_FAILED_SERVERS instead.

Okay but for the lore: set this value and it enables a server to be removed after MEMCACHED_BEHAVIOR_SERVER_FAILURE_LIMIT times of continuous connection failure. Like a three-strikes rule but make it configurable.

  • MEMCACHED_BEHAVIOR_AUTO_EJECT_HOSTS — deprecated since version 0.48, see MEMCACHED_BEHAVIOR_REMOVE_FAILED_SERVERS.

For the record though: if enabled, any hosts that have been flagged as disabled get removed from the list of servers in the memcached_st structure. And you must use this together with MEMCACHED_BEHAVIOR_SERVER_FAILURE_LIMIT. They're a package deal, no solo acts 🫶

  • MEMCACHED_BEHAVIOR_REMOVE_FAILED_SERVERS — if enabled, any hosts flagged as disabled get removed from the list of servers in the memcached_st structure. That's it, that's the whole vibe. Clean. Simple. Iconic. 💅
  • MEMCACHED_BEHAVIOR_RETRY_TIMEOUT — when this is enabled, a host that's being problematic will only get checked for usage based on the amount of time set by this behavior. The value is in seconds. It's giving "you're on timeout, go think about what you did" ⏳
  • MEMCACHED_BEHAVIOR_HASH_WITH_PREFIX_KEY — when enabled, the prefix key gets added to the key when determining the server by hash. Want more tea on this? Go read MEMCACHED_CALLBACK_NAMESPACE. 🔑

Then there's the whole other type, and it's a whole separate vibe:

typedef enum memcached_server_distribution_t
memcached_server_distribution_t

enum memcached_server_distribution_t
  • MEMCACHED_DISTRIBUTION_MODULA — distributes keys by hash modulo number of servers. Old school, does the job.
  • MEMCACHED_DISTRIBUTION_CONSISTENT — an alias for MEMCACHED_DISTRIBUTION_CONSISTENT_KETAMA. Same girl, different name 💖
  • MEMCACHED_DISTRIBUTION_CONSISTENT_KETAMA — unweighted consistent key distribution.
  • MEMCACHED_DISTRIBUTION_RANDOM — distributes keys by rand(3) modulo number of servers. Chaos mode, honestly, but valid 🎲
  • MEMCACHED_DISTRIBUTION_CONSISTENT_KETAMA_SPY — unweighted consistent key distribution, but compatible with the SPY client.
  • MEMCACHED_DISTRIBUTION_CONSISTENT_WEIGHTED — weighted consistent key distribution. Now with ✨proportion✨.
  • MEMCACHED_DISTRIBUTION_VIRTUAL_BUCKET — consistent key distribution by virtual buckets 🪣

🌸 DESCRIPTION 🌸

okay so the tea is: libmemcached behavior can be modified by using memcached_behavior_set(). Default behavior is that the library strives to be quick and accurate. But ngl, some behavior, while being faster, can also result in not entirely accurate behavior — for instance, memcached_set() will always respond with MEMCACHED_SUCCESS. Yeah. It'll just say success. Every time. Make of that what you will 😭

memcached_behavior_get() takes a behavior flag and returns whether or not that behavior is currently enabled in the client.

memcached_behavior_set() changes the value of a particular option of the client. It takes both a flag and a value. For simple on-or-off options you just need to pass in a value of 1. And heads up bestie: calls to memcached_behavior_set() will flush and reset all connections. Every single one. Not a drill 🚨

🦋 RETURN VALUE 🦋

memcached_behavior_get() returns either the current value of the key, or 0 or 1 on simple flag behaviors (1 being enabled).

memcached_behavior_set() returns failure or success. Binary. No in-between. Periodt. 💅

👑 NOTES 👑

The data argument of memcached_behavior_set() was changed in version 0.17 — it went from taking a pointer to a data value, to taking a uin64_t. Glow-up, honestly. (Yes, that's spelled uin64_t in here. Write it down exactly like that, don't be clever 🙈)

🌈 SEE ALSO 🌈

and that's the whole page, bestie, we did it 🎀 this was libmemcached version 1.1, dated Jul 24, 2025, and yeah — it's MEMCACHED_BEHAVIOR(3). go forth and behave (or don't, it's configurable) 💖✨

📦 where this page comes from