I use redis to cache my web blog.My article has a field "checked",if this field changed in database,I also need to set the new value to redis,here is code
if redis_conn.exists("article"):
redis_conn.hset("article", "checked",1)
it seems like ok,but if article key expired after exists and before hset,there will be some problems.the article key will only has one field of checked,other field like title,content,etc...will gone.
how to hset only if key exists,if key is expired just do nothing.
You can use a Lua script for that, i.e. (pseudo NodeJS):
redis_conn.eval("if redis.call('EXISTS', KEYS[1])==1 then redis.call('HSET', KEYS[1], ARGV[1], ARGV[2]) end", 1, "article", "checked", 1)
Server-side Lua scripts are atomic, so you are assured that the key will not expire in between calls.
Note: Redis does have the HSETNX command, but not the HSETEX command, which is apparently what you're looking for.
Related
I have created a cache key on redis cluster with cacheKey as "citylist_[object Object]"
When i am trying to get the keys on cluster mode with pattern keys citylist* on redis. It is returning me (empty array)
but when i am trying to get the value directly using get "citylist_[object Object]" , it returns me the value.
how to search the keys using the pattern for above mentioned cache key.
Very simple test on my local Redis (v6.2.4):
127.0.0.1:6379> set "citylist_[object Object]" test_value
OK
127.0.0.1:6379> keys citylist*
1) "citylist_[object Object]"
i'm currently working about Redis Expire Event
My goal : get the Value, Field to do something in next process after Data in Redis already expire
so i had found Redis Keyspace Notifications Feature
That Allow client to Subscribe to Channel in Redis to Recieve Event affecting Data in reset like Expire
so i have some example code : https://github.com/toygame/nodejs-redis-keyspace-notifications
subscriber.subscribe("__keyevent#0__:expired")
subscriber.on('message', async(channel, message) => {
// do somethings
console.log(message);
})
Result : Key0
And this work find but the Result i got is only Key that i have set into redis and expired
I have already do some research
https://medium.com/nerd-for-tech/redis-getting-notified-when-a-key-is-expired-or-changed-ca3e1f1c7f0a
but its found only Event that maybe i can get as result but not for Value, Field that i expect
is their anyway to get Those Value and Field ?
FYI. Document https://redis.io/topics/notifications
UPDATE
according to this https://stackoverflow.com/a/42556450/11349357
Keyspace notifications do not report the value, only the key's name and/or command performed are included in the published message.The main underlaying reasoning for this is that Redis values can become quite large.
If you really really really need this kind of behavior, well that's pretty easy actually. Because keyspace notifications are using Pub/Sub messages, you can just call PUBLISH yourself after each relevant operation, and with the information that you're interested in.
look like i can't use this Redis Keyspace but i have to publish its on my own
You can use RedisGears to process keyspace notification and get both key and value.
You can write your processing code in python and register it in Redis.
e.g. Capture each keyspace event and store to a Stream
GearsBuilder() \
.foreach(lambda x: execute('XADD', "notifications-stream", '*', *sum([[k,v] for k,v in x.items()],[]))) \
.register(prefix="person:*", eventTypes=['hset', 'hmset'], mode='sync')
You can read more about this example here: https://oss.redis.com/redisgears/examples.html#reliable-keyspace-notification
I'm trying to push an entry in a list in Redis and also want to update the TTL of the list every time a new entry comes in. I can do that my simple calling the EXPIRE "my-list" ttl using Redis. Since my application is receiving heavy traffic, I want to reduce the number of calls to redis.
Can I set my TTL during the push operation in Redis, i.e RPUSH "mylist" I1 I2...IN ex "TTL", does redis support this time of command functionality. I can see that it does support this for the String data structures.
Redis does not have dedicated commands to push and expire the List, although as you've mentioned it does have something like that for the String data type.
The way you'd go about this challenge is to compose your own "command" from existing ones. Instead of having your application call these commands, however, you would use a Lua script as explained in the EVAL documentation page.
Lua scripts are cached and run atomically on the server. One such as the following would probably help in your case - it expects to get the key name, the pushed element and the expiry value:
local reply = redis.call('RPUSH', KEYS[1], ARGV[1])
redis.call('EXPIRE', KEYS[1], ARGV[2])
return reply
I am using Jedis java client for redis. My requirement is that when someone add item to list, say mylist by doing jedisClient.lpush("mylist", "this is my msg"), I need to get notification.
Is this possible ?
Yes, it is possible to achieve that in one of two ways.
The first approach is to use Redis' keyspace notifications. Configure Redis to generate list events with the following configuration directive:
CONFIG SET notify-keyspace-events El
Then, subscribe to the relevant channel/channels. If you want to subscribe only to mylist's changes, do:
SUBSCRIBE __keyevent#0__:mylist
Or, use PSUBSCRIBE and listen to events to matching key names that match a pattern.
Note, however, that keysapce notifications will not provide the actual pushed value. You can use Lua scripts as an alternate approach and implement your own notifications mechanism. For example, use the following script to push and publish a custom message to a custom channel:
local l = redis.call("LPUSH", KEYS[1], ARGS[1])
redis.call("PUBLISH", "mylistnotif:" .. KEYS[1], "Pushed value " .. ARGS[1])
return l
Make sure that "someone" uses that script to do the actual list-pushing and subscribe to the relevant channel/channels.
i am using the Booksleeve hash api for Redis. I am doing the following:
CurrentConnection.Hashes.Set(0, "item:1", "priority", task.priority.ToString());
var taskResult = CurrentConnection.Hashes.GetString(0, "item:1", "priority");
taskResult.Wait();
var priority = Int32.Parse(taskResult.Result)
However i am getting an Aggregate exception:
"ERR Operation against a key holding the wrong kind of value"
I am not sure what i am doing wrong here (except of blocking the task :)).
Note: CurrentConnection is an instance of BookSleeve.RedisConnection
Please help!
Thanks
That is not a Booksleeve issue - it is a redis error; in fact, the full error message you should be seeing is:
Redis server: ERR Operation against a key holding the wrong kind of value
(where I try to make it clear that this error has come from redis, not Booksleeve)
As for what causes this: each key in redis has a designated type; string, hash, list, etc. You cannot use hash operations on something that is not a hash.
My guess, then, is that "item:1" already exists, but as something other than a hash. I have unit tests that confirm this from Booksleeve (i.e. with/without a pre-existing non-hash value).
You can investigate this in redis using redis-cli or any other client (telnet works, at a push), with the command:
type item:1
(thanks #Sripathi)