If two distinct keys have the same value (and say the value is large) does redis store the value twice or will it use a pointer / reference. The way git does ?
Redis stores them as two independent key-value pairs.
If you want to remove the duplication, you have to build an index from multiple keys to a shared value by yourself.
What's the practical difference between keeping data in multiple hashes (HSET foo oof 1, HSET bar rab 2) and using plain keys in a hierarchy (SET foo:oof 1, SET bar:rab 2)?
According to the manual, you'd use hashes to represent a single object.
Also, it's not that efficient to iterate over Redis keys, so if you need to get all the data from a single object, HGETALL is your friend, not a KEYS thing:10:*/multiget fiasco.
However, you can't e.g. set expiry for only one key of a hash, so if you need that functionality, you'll want to use regular keys.
In cluster mode of Redis, is a piece of data with a specific key has to be stored in a specific node, no matter what data structure the it has (e.g. List/Hash)?
For example, I have a hash map:
HMSET website google www.google.com yahoo www.yahoo.com
The key of the hash map is "website", and the hash map has data {google:www.google.com, yahoo:www.yahoo.com}. In my understanding, the hash map is stored in only one node of the cluster. It will be not efficient when the hash map is large (e.g. 400M key-value pairs in one hash map).
My question is: is there a way to automatically distribute the contents of the hash map of the same key among the cluster? For example, store pair {google:www.google.com} in node 0 and store pair {yahoo www.yahoo.com} in node 1, when the key of the hash map is still "website"?
In cluster mode of Redis, is a piece of data with a specific key has to be stored in a specific node, no matter what data structure the it has (e.g. List/Hash)?
Yes - every key is mapped to a hash slot, that a single cluster instance manages.
My question is: is there a way to automatically distribute the contents of the hash map of the same key among the cluster?
No - data is distributed between nodes at key level. A given key's data structure cannot be distributed between multiple shards. To distribute the data, you'll have to model it using more keys.
Correctly modeling your needs requires knowing what type of operations you'll be performing against your distributed "hash map" and their respective frequencies. Feel free to add this information to the question, or open a new one that is more focused on your requirements.
It seems that Redis has no any entity corresponding to "table" in relational database.
For instance, I have to store:
(token, user_id)
(cart_id, token, [{product_id, count}])
If it doesn't separate store those two, the get method would search from both, which would cause chaos.
By the way, (cart_id, token, [{product_id, count}]) is a shopping cart, how to design such data structure in redis?
It seems that Redis has no any entity corresponding to "table" in relational database.
Right, because it is not a relational database. It is a data structure server which is very different and requires a different approach to be used well.
Ultimately to use Redis in the way it is intended you need to not think in relational terms, but think of the data structures you use in the code. More specifically, how do you need the data when you want to consume it? That will be the most likely way to store it in Redis.
In this case there are a few options, but the hash method works incredibly well for this one so I'll detail it here.
First, create a hash, call it users:to:tokens. Store as the key in the hash the user id, and the value the token. Next create the inverse, a hash called 'tokens:to:users'. You will probably be wanting both of these - the ability to look one up from the other - and this foundation will provide that.
Next, for your carts. This, too, will be a hash: carts:cart_id. In this hash you have the product_id and the count.
Finally up is your third hash token:to:cart which builds an index of tokens to cart id. I'd go a step further and do user:to:cart to be able to pull carts by user as well.
Now as to whether to store the keynote in the map or not, I tend to go with "no". By just storing the ID you can easily build the Redis cart key and not store the key's full path in the data store as well the saving memory usage.
Indeed, if you can do so use integers for all of your IDs. By using integers you can take advantage of Redis' integer storage optimizations to keep memory usage down. Hashes storing integers are quite efficient and very fast.
If needed you can use Redis to build your IDs. You can use the INCR command to build a counter for each data type such as userid:counter, cartid:counter, and tokenid:counter. As INCR returns the new value you make a single call to increment and get the new ID and get cartid:counter will always give you the largest ID if you wanted to quickly see how many carts have been created. Kinda neat , IMO.
Now, where it gets tricky is if you want to use expiration to automatically expire carts as opposed to leaving them to "lie around" until you want to clean things up. By setting an expiration on the cart hash (which has the product,count mapping) your carts will automatically expire. However, their references will still be hanging out in the token:to:cart hash. Removing that is a simple periodic task which treats over the members of token:to:cart and does an exists check on the cart's key. If it doesn't exist delete it from the hash.
Redis is a key-value storage. From redis.io:
Redis is an open source (BSD licensed), in-memory data structure
store, used as database, cache and message broker. It supports data
structures such as strings, hashes, lists, sets, sorted sets with
range queries, bitmaps, hyperloglogs and geospatial indexes with
radius queries.
So if you want to store two diffetent types (tokens and carts) you will need to store two keys for different datatypes. For example:
127.0.0.1:6379> hset tokens.token_id#123 user user123
(integer) 1
127.0.0.1:6379> hget tokens.token_id#123 user
"user123"
Where tokens is a namespace for tokens only. It is stored as Redis-Hash:
Redis Hashes are maps between string fields and string values, so they
are the perfect data type to represent objects
To store lists I would do the following:
127.0.0.1:6379> hmset carts.cart_1 token token_id#123 cart_contents cart_contents_key1
OK
127.0.0.1:6379> hmget carts.cart_1 token cart_contents
1) "token_id#123"
2) "cart_contents_key1" # cart_contents is a list of receipts.
cart_contents are represented as a Redis-List:
127.0.0.1:6379> rpush cart_contents.cart_contents_key1 receipt_key1
(integer) 1
127.0.0.1:6379> lrange cart_contents.cart_contents_key1 0 -1
1) "receipt_key1"
Receipt is Redis-Hash for a tuple (product_id, count):
127.0.0.1:6379> hmset receipts.receipt_key1 product_id 43 count 2
OK
127.0.0.1:6379> hmget receipts.receipt_key1 product_id count
1) "43" # Your final product id.
2) "2"
But do you really need Redis in this case?
I have a Set named 'Projects' with many key-value pairs and I want to retrieve one of the values by providing its key. I checked the redis doc but I only found how to retrieve the entire Set. Is it possible to just retrieve one value by providing its key?
Your concept of Set does not match Redis'.
All members of a set in Redis are stored in a single key. Therefore you can't access members individually by a key.
You should use hashes: http://www.redis.io/commands#hash
HSET key field value does what you are looking for.