I have many different keys. some start with "user USERNAME" and many others.
is there a way to get a random key from the "user *" ?
if so, how can i get like 5 different users?
If you put all the usernames into a Redis set, then you could use the SRANDMEMBER command to randomly gets usernames from the set. Example from the documentation:
redis> SADD myset one two three
(integer) 3
redis> SRANDMEMBER myset
"three"
redis> SRANDMEMBER myset 2
1) "three"
2) "two"
The set seems to be the only Redis data type that can return random elements from the collection. You might have to process your data a little differently to do this.
You can definitely use a Set and store all your "user*" key names in it, as suggested by #Sunil D.
Alternatively, you could use a dedicated Redis database for your "user*" keys and then use the RANDOMKEY command to fetch keys from it. The nice thing about this is that you don't have to manage the Set of key names (i.e. add and remove to it whenever a key is created or deleted, respectively).
Note, however, that with both approaches, you'll have to check that the result you got wasn't returned earlier after each call to SRANDMEMBER or RANDOMKEY since both do not ensure that you'll get a different answer each time you invoke it.
Related
I am new to Redis. For example, if I have the following schema:
INCR id:product
SET product:<id:product> value
SADD color:red <id:product>
(Aside: I am not sure how to express a variable in Redis. I will just use <id:product> as the primary key value. In production, I will use golang client for this job)
To query products which have red color, I can do:
SMEMBERS color:red
But the problem is I just want to display 10 of them in the first page, and then next 10 in the second page and so on. How to let Redis return only part of them by specifying offset and limit arguments?
What do redis experts normally do for this case? Return all IDs even if I just want 10 of them? Is that efficient? What if it has millions of values in the set, but I only want 10?
Edited 1
Incidentally, I use sets instead of lists and sorted sets because I will need to do SINTER jobs for other queries.
For example:
SADD type:foo <id:product>
SINTER color:red type:foo
And then I will have pagination problem again. Because I actually just want to find 10 of the intersection at a time. (eg: if the intersection returns millions of keys, but actually I just want 10 of them at a time for pagination).
Edited 2
Should I use a sorted set instead? I am not sure if this is the expert choice or not. Something like:
ZADD color:red <id:product> <id:product>
ZADD type:foo <id:product> <id:product>
ZRANGE color:red 0 9 // for the first page of red color products
ZINTERSTORE out 2 color:red type:foo AGGREGATE MIN
ZRANGE out 0 9 // for the first page of red color and type foo products
I have no ideas if the above way is suggested or not.
What will happen if multiple clients are creating the same out sorted set?
Is that meaningful to use the same value for both score and member?
Using sorted sets is the standard way to do pagination in Redis.
The documentation of ZINTERSTORE says that: "If destination already exists, it is overwritten."
Therefore, you shouldn't use "out" as the destination key name. You should instead use a unique or sufficiently random key name and then delete it when you're done.
I'm not sure what you mean by "meaningful". It's a fine choice if that's the order you want them to be in.
On my current project I'm implementing autocompletion service on top of Redis, for it I use such approach (this article describes it more widely):
1) for storing dump of the data I have hash in which I put searchable objects as a values, for instance
HSET data 1 "{\"name\":\"Kill Bill\",\"year\":2003}"
HSET data 2 "{\"name\":\"King Kong\",\"year\":2005}"
2) for storing all possible sequences of input characters (that I generate in advance) which could be used in search I use sorted sets, like
ZADD search:index:k 0 1
ZADD search:index:ki 0 1
ZADD search:index:kil 0 1
ZADD search:index:kill 0 1
Where value stored in sorted set (in my example '1') is key for data from hash. So, for searching some data (for example where name started with 'ki') we need to make two steps:
data_keys = REDIS.zrevrange('search:index:ki', 0, -1)
matching_data = REDIS.hmget(data, *data_keys)
The issue I tried to solve - how automatically remove all data from sorted sets related to hash values when I removed it? In relational databases I can use cascade deletion for such cases, but how can I handle it in Redis?
Your design appears awkward to me, I'm unsure what you're actually trying to do with Redis and perhaps that could be the topic of another question.
That said, to address your question, Redis does offer a "cascading delete"-like behavior. Instead, if you're deleting hash "1", iterate the prefix and ZREM it from the relevant sorted sets.
Note: do not use a Lua script for this task, as it will generate key names (i.e. sorted sets by prefix) and that is against the recommendations (will not work on a cluster)
Given I have a list stored in a key "A", how can I duplicate that list in a different key "B"?
I know that for non list values, I can "get", and then "set". But for lists when I try to get it, I see the WRONGTYPE operation error.
Redis enables 5 different data structures, such as:
Key values
Strings
Hashs
Lists
Sets
Each of the data structure has its own commands.
In order to get the current list you should use the LRANGE command.
The prefix L referes to the List data structure.
(Redis Set data structure has a related command to range by using SETRANGE)
If you read the Redis LRANGE documentation you will understand how to use it.
Here is the brief code you may use:
LRANGE mylist 0 -1
Where mylist is the list you get the values from.
The offsets start and stop are zero-based indexes, with 0 being the
first element of the list (the head of the list), 1 being the next
element and so on.
-1 is used to describe the last element in the list.
Now you should use the LPUSH or RPUSH depends on the list side you wish to insert the old elements into the new list.
You should use LRANGE to get all elements of the first list, then use LPUSH or RPUSH to put these elements into the second list.
Another way to duplicate a key's value, regardless the data structure, is to DUMP it and then RESTORE it into the new key. In most cases this approach is also the quickest.
I am trying to insert time based records with multiple fields on the values (with TTL enabled).
For the multiple fields the best way to do it via Redis is using HSET:
HSET user:32 name "johns" timecreated "3333311232" address "somewhere"
I also try to read those values via time range:
for example return all history records (for example user 32) which was inserted in the last day:
so the best for that would be storing via ZADD using scores(this time I am losing the hash-map structure for easy retrieval):
ZADD user:32 3333311232 "name=johns,timecreated=3333311232,address=somewhere"
On the top of the things I want to add TTL for each record
Any idea how I could optimize my design?
I could split into two but that will requires two queries when reading:
ZADD user:32 3333311232 "user:32:3333311232"
HMSET user:32:3333311232 name “johns” timecreated “3333311232” address="somewhere"
than to retrieve ill need:
//some range
ZRANGEBYSCORE user:32 3333311232 333331123
result: 1389772850
now to get all information: HGETALL user:32:1389772850
What do you think?
Thank you,
ray.
The two methods you describe are the two common approaches. If you store the entire object in the ZSET, you would typically store it as a JSON string. If you don't need "random" access to the object, that's a valid approach.
I usually go for the other approach; a ZSET combined with hashes. the two queries are not a big deal. You could even abstract it away with a Lua script; see EVAL.
Regarding the TTL, while you cannot expire individual ZSET values, you could expire the hash, and use keyspace notifications to listen for the expired event, and remove the corresponding value from the ZSET.
Let me know if you need some more specifics.
In redis I store objects in a sorted set.
In my solution, it's important to be able to run a ranged query by dates, so I store the items with the score being the timestamp of each items, for example:
# Score Value
0 1443476076 {"Id":"92","Ref":"7ADT","DTime":1443476076,"ATime":1443901554,"ExTime":0,"SPName":"7ADT33CFSAU6","StPName":"7ADT33CFSAU6"}
1 1443482969 {"Id":"11","Ref":"DAJT","DTime":1443482969,"ATime":1443901326,"ExTime":0,"SPName":"DAJTJTT4T02O","StPName":"DAJTJTT4T02O"}
However, in other situations I need to find a single item in the set based on it's ID.
I know I can't just query this data structure as if it were a nosql db, but I tried using ZSCAN, which didn't work.
ZSCAN MySet 0 MATCH Id:92 count 1
It returns; "empty list or set"
Maybe I need to serialize different?
I have serialized using Json.Net.
How, if possible, can I achieve this; using dates as score and still be able to lookup an item by it's ID?
Many thanks,
Lars
Edit:
Assume it's not possible, but any thoughts or inputs are welcome:
Ref: http://openmymind.net/2011/11/8/Redis-Zero-To-Master-In-30-Minutes-Part-1/
In Redis, data can only be queried by its key. Even if we use a hash,
we can't say get me the keys wherever the field race is equal to
sayan.
Edit 2:
I tried to do:
ZSCAN MySet 0 MATCH *87*
127.0.0.1:6379> ZSCAN MySet 0 MATCH *87*
1) "192"
2) 1) "{\"Id\":\"64\",\"Ref\":\"XQH4\",\"DTime\":1443837798,\"ATime\":1444187707,\"ExTime\":0,\"SPName\":\"XQH4BPGW47FM\",\"StPName\":\"XQH4BPGW47FM\"}"
2) "1443837798"
3) "{\"Id\":\"87\",\"Ref\":\"5CY6\",\"DTime\":1443519199,\"ATime\":1444172326,\"ExTime\":0,\"SPName\":\"5CY6DHP23RXB\",\"StPName\":\"5CY6DHP23RXB\"}"
4) "1443519199"
And it finds the desired item, but it also finds another one with an occurance of 87 in the property ATime. Having more unique, longer IDs might work this way and I would have to filter the results in code to find the one with the exact value in its property.
Still open for suggestions.
I think it's very simple.
Solution 1(Inferior, not recommended)
Your way of ZSCAN MySet 0 MATCH Id:92 count 1 didn't work out because the stored string is "{\"Id\":\"92\"... not "{\"Id:92\".... The string has been changed into another format. So try to use MATCH Id\":\"64 or something like that to match the json serialized data in redis. I'm not familiar with json.net, so the actual string leaves for you to discover.
By the way, I have to ask you did ZSCAN MySet 0 MATCH Id:92 count 1 return a cursor? I suspect you used ZSCAN in a wrong way.
Solution 2(Better, strongly recommended)
ZSCAN is good when your sorted set is not large and you know how to save network roundtrip time by Redis' Lua transaction. This still make "look up by ID" operation O(n). Therefore, a better solution is to change you data model in the following way:
change sorted set
from
# Score Value
0 1443476076 {"Id":"92","Ref":"7ADT","DTime":1443476076,"ATime":1443901554,"ExTime":0,"SPName":"7ADT33CFSAU6","StPName":"7ADT33CFSAU6"}
1 1443482969 {"Id":"11","Ref":"DAJT","DTime":1443482969,"ATime":1443901326,"ExTime":0,"SPName":"DAJTJTT4T02O","StPName":"DAJTJTT4T02O"}
to
# Score Value
0 1443476076 Id:92
1 1443482969 Id:11
Move the rest detailed data in another set of hashes type keys:
# Key field-value field-value ...
0 Id:92 Ref-7ADT DTime-1443476076 ...
1 Id:11 Ref-7ADT DTime-1443476076 ...
Then, you locate by id by doing hgetall id:92. As to ranged query by date, you need do ZRANGEBYSCORE sortedset mindate maxdate then hgetall every id one by one. You'd better use lua to wrap these commands in one and it will still be super fast!
Data in NoSql database need to be organized in a redundant way like above. This may make some usual operation involve more than one commands and roundtrip, but it can be tackled by redis's lua feature. I strongly recommend the lua feature of redis, cause it wrap commands into one network roundtrip, which are all executed on the redis-server side and is atomic and super fast!
Reply if there's anything you don't know