Search and Transactions in Sharded Redis

Question

the question refers to the sharded configuration of redis. I have implemented a small test application in Java which creates 100.000 user hashes over Jedis in the form of user:userID. Each hash has the elements: name, phone, department, userID. I also created simple key-value pairs with the key phone:phone number which contains the userID whose phone number is the ID and sets for each department with the userIDs who work for that particular department. The two latter types I use only for seaching. These structures and the search are similar to Searching in values of a redis db.

In short, the data structures:

user:userID->{name, department, phone, userID}
department:department->([userID1, userID2,....])
phone:phone->userID

Use cases for the search:

1. access to user-hashes based on key i.e. userID
2. search for users with a phone number
3. search for all users of a department

Everything works all right in the single instance and sharded configuration but I would have the following questions:

• In the single instance configuration it is possible to look for phone number with a wide card e.g. with the KEY method but this is not available in the sharded configuration. How would it be possible to look for keys whose first part is known?
• The user ID is generated from a zset whose score is increased for userID. This can be doen in a transaction for the single instance configuration but transactions seem not to be supported for sharding configurations over jedis even if the participating keys are on the same instance. How would it be possible to solve this problem if multiple client threads can also do the user creation?

Thank you for your responses also in advance.

Answer 1

For the 1st part of your question:

There is no magic here, if you want to search across all your shards, you have to iterate over all shards. Jedis don't have this method, but you could extend ShardedJedis to add it (untested):

    public Set<String> keys(String pattern) {
    HashSet<String> found = new HashSet<String>();

    for (Jedis jedis : getAllShards()) {
        found.addAll(jedis.keys(pattern));
    }

    return found;
    }

For the 2nd part of your question:

AFAIK, Jedis doesn't support transactions when using Shards, even if you do force the related keys to be on the same shard (see Jedis Advanced Usage).

This same link suggest a workaround that may apply for a few scenarios:

Mixed approach

If you want easy load distribution of ShardedJedis, but still need transactions/pipelining/pubsub etc, you can also mix the normal and the sharded approach: define a master as normal Jedis, the others as sharded Jedis. Then make all the shards slaveof master. In your application, direct your write requests to the master, the read requests to ShardedJedis. Your writes don't scale anymore, but you gain good read distribution, and you have transactions/pipelining/pubsub simply using the master. Dataset should fit in RAM of master. Remember that you can improve performance of the master a lot, if you let the slaves do the persistance for the master!