Detailed explanation of common memcached commands in Java
Oct 15, 2017 pm 03:14 PM
This article mainly introduces the common commands of memcached. The editor thinks it is quite good. Now I will share it with you and give you a reference. Let’s follow the editor and take a look.
1. Common parameters for starting Memcache
-p <num> 設置TCP端口號(默認設置為: 11211) -U <num> UDP監(jiān)聽端口(默認: 11211, 0 時關閉) -l <ip_addr> 綁定地址(默認:所有都允許,無論內外網或者本機更換IP，有安全隱患，若設置為127.0.0.1就只能本機訪問) -c <num> max simultaneous connections (default: 1024) -d 以daemon方式運行 -u <username> 綁定使用指定用于運行進程<username> -m <num> 允許最大內存用量，單位M (默認: 64 MB) -P <file> 將PID寫入文件<file>，這樣可以使得后邊進行快速進程終止, 需要與-d 一起使用
For more, you can use memcached -h
Under linux: ./usr/local/bin/memcached -d -u root -l 192.168.1.197 -m 2048 -p 12121
Under window :d:\App_Serv\memcached\memcached.exe -d RunService -l 127.0.0.1 -p 11211 -m 500
Run after registering as a service under windows:
sc.exe create Memcached_srv binpath= “d:\App_Serv\memcached\memcached.exe -d RunService -p 11211 -m 500″start= auto net start Memcached
2. Connection and exit
##
telnet 127.0.0.1 11211 quit
3. Basic commands
- set ##add
- replace
- get
- delete
- The first three commands are used to operate key-value pairs stored in memcached Standard modification commands. They are both very simple and easy to use, and all use the syntax shown below:
command <key> <flags> <expiration time> <bytes> <value>
The parameter description is as follows:
command set/add/ replace
flags Can include integer parameters of key-value pairs, which are used by the client to store additional information about the key-value pair
expiration time Save the key-value pair in the cache Length of time (in seconds, 0 means forever)
bytes The byte points stored in the cache
value The value stored (always on the second line)
Now, let’s take a look Practical usage of these commands.
set command is used to add new key-value pairs to the cache. If the key already exists, the previous value will be replaced.
Note the following interaction, which uses the set command:
set userId 0 0 5 12345 STORED
If the key-value pair is correctly set using the set command, The server will respond with the word STORED. This example adds a key-value pair to the cache whose key is userId and whose value is 12345. and setting the expiration time to 0, this will notify memcached that you want this value stored in the cache until you delete it.
The add command will add a key-value pair to the cache only if the key does not exist in the cache. If the key already exists in the cache, the previous value will still be the same and you will get the response NOT_STORED.
The following is the standard interaction using the add command:
set userId 0 0 5 12345 STORED add userId 0 0 5 55555 NOT_STORED add companyId 0 0 3 564 STORED
3.3 replace
The replace command will replace a key in the cache only if the key already exists. If the key does not exist in the cache, you will receive a NOT_STORED response from the memcached server.
The following is the standard interaction using the replace command:
replace accountId 0 0 5 67890 NOT_STORED set accountId 0 0 5 67890 STORED replace accountId 0 0 5 55555 STORED
The last two basic commands are get and delete. These commands are fairly easy to understand and use similar syntax, as shown below:
command <key>
Let’s look at the application of these commands.
The get command is used to retrieve the value associated with a previously added key-value pair. You'll use get to perform most retrieval operations.
set userId 0 0 5 12345 STORED get userId VALUE userId 0 5 12345 END get bob END
As you can see, the get command is fairly simple. You call get with a key, and if the key exists in the cache, the corresponding value is returned. If it does not exist, nothing is returned.
The last basic command is delete. The delete command deletes any existing value in memcached. You would call delete with a key, and if the key exists in the cache, delete the value. If it does not exist, a NOT_FOUND message is returned.
The following is the client-server interaction using the delete command:
set userId 0 0 5 98765 STORED delete bob NOT_FOUND delete userId DELETED get userId END
The two high-level commands that can be used in memcached are gets and cas . The gets and cas commands need to be used together. You will use these two commands to ensure that an existing name/value pair is not set to a new value if the value has already been updated. Let's look at these commands individually.
The gets command functions similarly to the basic get command. The difference between the two commands is that gets returns slightly more information: a 64-bit integer value that is very much like a "version" identifier for a name/value pair.
Here is the client-server interaction using the gets command:
set userId 0 0 5 12345 STORED get userId VALUE userId 0 5 12345 END gets userId VALUE userId 0 5 4 12345 END
Consider the difference between the get and gets commands. The gets command returns an additional value—in this case, the integer value 4—that identifies the name/value pair. If another set command is executed on this name/value pair, the extra value returned by gets will change to indicate that the name/value pair has been updated. An example is shown:
set userId 0 0 5 33333 STORED gets userId VALUE userId 0 5 5 33333 END
您看到 gets 返回的值了嗎？它已經更新為 5。您每次修改名稱/值對時，該值都會發(fā)生更改。
3.7 cas
cas（check 和 set）是一個非常便捷的 memcached 命令，用于設置名稱/值對的值（如果該名稱/值對在您上次執(zhí)行 gets 后沒有更新過）。它使用與 set 命令相類似的語法，但包括一個額外的值：gets 返回的額外值。
注意以下使用 cas 命令的交互：
set userId 0 0 5 55555 STORED gets userId VALUE userId 0 5 6 55555 END cas userId 0 0 5 6 33333 STORED
如您所見，我使用額外的整型值 6 來調用 gets 命令，并且操作運行非常順序?，F在，我們來看看中的一系列命令：
使用舊版本指示符的 cas 命令
set userId 0 0 5 55555 STORED gets userId VALUE userId 0 5 8 55555 END cas userId 0 0 5 6 33333 EXISTS
注意，我并未使用 gets 最近返回的整型值，并且 cas 命令返回 EXISTS 值以示失敗。從本質上說，同時使用gets 和cas 命令可以防止您使用自上次讀取后經過更新的名稱/值對。
緩存管理命令
最后兩個 memcached 命令用于監(jiān)控和清理 memcached 實例。它們是 stats 和 flush_all 命令。
3.8 stats
stats 命令的功能正如其名：轉儲所連接的 memcached 實例的當前統計數據。在下例中，執(zhí)行 stats 命令顯示了關于當前 memcached 實例的信息：
STAT pid 22459 進程ID STAT uptime 1027046 服務器運行秒數 STAT time 1273043062 服務器當前unix時間戳 STAT version 1.4.4 服務器版本 STAT libevent 2.0.21-stable STAT pointer_size 64 操作系統字大小(這臺服務器是64位的) STAT rusage_user 0.040000 進程累計用戶時間 STAT rusage_system 0.260000 進程累計系統時間 STAT curr_connections 10 當前打開連接數 STAT total_connections 82 曾打開的連接總數 STAT connection_structures 13 服務器分配的連接結構數 STAT reserved_fds 20 STAT cmd_get 54 執(zhí)行get命令總數 STAT cmd_set 34 執(zhí)行set命令總數 STAT cmd_flush 3 指向flush_all命令總數 STAT get_hits 9 get命中次數 STAT get_misses 45 get未命中次數 STAT delete_misses 5 delete未命中次數 STAT delete_hits 1 delete命中次數 STAT incr_misses 0 incr未命中次數 STAT incr_hits 0 incr命中次數 STAT decr_misses 0 decr未命中次數 STAT decr_hits 0 decr命中次數 STAT cas_misses 0 cas未命中次數 STAT cas_hits 0 cas命中次數 STAT cas_badval 0 使用擦拭次數 STAT touch_hits 0 STAT touch_misses 0 STAT auth_cmds 0 STAT auth_errors 0 STAT bytes_read 15785 讀取字節(jié)總數 STAT bytes_written 15222 寫入字節(jié)總數 STAT limit_maxbytes 67108864 分配的內存數（字節(jié)） STAT accepting_conns 1 目前接受的鏈接數 STAT listen_disabled_num 0 STAT time_in_listen_disabled_us 0 STAT threads 4 線程數 STAT conn_yields 0 STAT hash_power_level 16 STAT hash_bytes 524288 STAT hash_is_expanding 0 STAT malloc_fails 0 STAT conn_yields 0 STAT bytes 0 存儲item字節(jié)數 STAT curr_items 0 item個數 STAT total_items 34 item總數 STAT expired_unfetched 0 STAT evicted_unfetched 0 STAT evictions 0 為獲取空間刪除item的總數 STAT reclaimed 0 STAT crawler_reclaimed 0 STAT crawler_items_checked 0 STAT lrutail_reflocked 0
此處的大多數輸出都非常容易理解。我們先來看看輸出，然后再使用新的鍵來運行一些 set 命令，并再次運行stats 命令，注意發(fā)生了哪些變化。
stats items
執(zhí)行stats items，可以看到STAT items行，如果memcached存儲內容很多，那么這里也會列出很多的STAT items行。
STAT items:1:number 3 STAT items:1:age 1698 STAT items:1:evicted 0 STAT items:1:evicted_nonzero 0 STAT items:1:evicted_time 0 STAT items:1:outofmemory 0 STAT items:1:tailrepairs 0 STAT items:1:reclaimed 0 STAT items:1:expired_unfetched 0 STAT items:1:evicted_unfetched 0 STAT items:1:crawler_reclaimed 0 STAT items:1:crawler_items_checked 0 STAT items:1:lrutail_reflocked 0 END
stats cachedump slabs_id limit_num
slabs_id:由stats items返回的結果（STAT items后面的數字）決定的
limit_num:返回的記錄數，0表示返回所有記錄
通過stats items、stats cachedump slab_id limit_num配合get命令可以遍歷memcached的記錄。
stats cachedump 1 0 ITEM userId [5 b; 1467903379 s] ITEM accountId [5 b; 1467903379 s] ITEM companyId [3 b; 1467903379 s] END stats cachedump 1 2 ITEM userId [5 b; 1467903379 s] ITEM accountId [5 b; 1467903379 s] END
stats slabs 顯示各個slab的信息，包括chunk的大小、數目、使用情況等
STAT 1:chunk_size 96 STAT 1:chunks_per_page 10922 STAT 1:total_pages 1 STAT 1:total_chunks 10922 STAT 1:used_chunks 3 STAT 1:free_chunks 10919 STAT 1:free_chunks_end 0 STAT 1:mem_requested 232 STAT 1:get_hits 9 STAT 1:cmd_set 14 STAT 1:delete_hits 1 STAT 1:incr_hits 0 STAT 1:decr_hits 0 STAT 1:cas_hits 0 STAT 1:cas_badval 0 STAT 1:touch_hits 0 STAT active_slabs 1 STAT total_malloced 1048512
stats sizes 輸出所有item的大小和個數
STAT 96 3
stats reset 清空統計數據
stats reset
RESET
3.9 flush_all
flush_all 是最后一個要介紹的命令。這個最簡單的命令僅用于清理緩存中的所有名稱/值對。如果您需要將緩存重置到干凈的狀態(tài)，則 flush_all 能提供很大的用處。下面是一個使用 flush_all 的例子：
set userId 0 0 5 55555 STORED get userId VALUE userId 0 5 55555 END flush_all OK get userId END
追加與清除命令
3.10 append
append 將數據追加到當前緩存數據的之后，當緩存數據存在時才存儲。
set username 0 0 8 wayne173 STORED get username VALUE username 0 8 wayne173 END append username 0 0 5 _ages STORED get username VALUE username 0 13 wayne173_ages END
3.11 prepend
prepend 將數據追加到當前緩存數據的之前，當緩存數據存在時才存儲。
set username 0 0 8 wayne173 STORED get username VALUE username 0 8 wayne173 END prepend username 0 0 5 name_ STORED get username VALUE username 0 13 name_wayne173 END
memcached還有很多命令，比如對于存儲為數字型的可以通過incr/decr命令進行增減操作等等，這里只列出開發(fā)和運維中經常使用的命令，其他的不再一一舉例說明。
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