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Redis (consistent hashing based) sharding made easy

emre.github.io

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Re: Redis (consistent hashing based) sharding made easy

#11

Post by janerik »

emre-yilmaz wrote: post #6
janerik wrote: post #3

The TCP server example shows a different protocol than the default redis protocol. (As I could not install it correctly I couldn't try it.) If this is the case current redis client libraries cannot be used.

What was the problem for installation? I use redis-router in production as a library . some of my non-pythoneer friends asked to use it in PHP, so I made a little wrapper with gevent. It's probably not compatible with current clients. Needs testing :)

Had problems installing "ketama".

I re-read the README now and it explicitely states that libketama should be installed from the git repository. I'll try it again later.

Re: Redis (consistent hashing based) sharding made easy

#12

Post by unwind »

The "abstract" text (copied from the Wikipedia page) lacks the variable names, making the text incomprehensible:

When a hash table is resized and consistent hashing is used, only keys need to be remapped on average, where is the number of keys [...]

it should end "only K/n keys need to be remapped on average, where K is the number of keys [...]". Just thought I'd point this out so it could get fixed, should only take a few seconds to edit.

Re: Redis (consistent hashing based) sharding made easy

#13

Post by emre-yilmaz »

unwind wrote: post #12

The "abstract" text (copied from the Wikipedia page) lacks the variable names, making the text incomprehensible: When a hash table is resized and consistent hashing is used, only keys need to be remapped on average, where is the number of keys [...] it should end "only K/n keys need to be remapped on average, where K is the number of keys [...]". Just thought I'd point this out so it could get fixed, should only take…

thanks! https://github.com/emre/redis-router/commit/0d9307436e4c3b69...

Re: Redis (consistent hashing based) sharding made easy

#14

Post by antirez »

janerik wrote: post #9
antirez wrote: post #5

Earlier quoted context omitted.

Hello, Redis accepts both a structured protocol, and a simple space-separated tokens protocol (called the inline protocol) that helps sysadmins to avoid a disaster just because they lack a proper redis-cli but are in need to run a Redis command ASAP. Perhaps this proxy is also supporting both forms. You can do that with any modern enough Redis server: Escape character is '^]'. ping +PONG set foo bar +OK get foo $3 ba…

I know of that simple protocol. But the responses in the example telnet session on that side return "True" and "13" where it should be "+OK" and ":13" with the redis protocol.

good point...

Re: Redis (consistent hashing based) sharding made easy

#16

Post by mmcnickle »

janerik wrote: post #3

The TCP server example shows a different protocol than the default redis protocol. (As I could not install it correctly I couldn't try it.) If this is the case current redis client libraries cannot be used.

The servers are returning the output of the redis-py library, which maps returned values from redis into python objects (hashes -> dicts etc).

I had a quick look but I don't see an easy way to have redis-py pass forward the raw return values, though I think it could be done with some effort. So this isn't a drop in proxy for a redis server just yet.

Edit: referring to the TCP/HTTP Servers above, the library itself can be used as a drop in replacement for redis-py in python.

Re: Redis (consistent hashing based) sharding made easy

#17

Post by kijin »

> Q: What about data invalidation if I move servers, change the config etc.

> A: It's minimum. At least better than: Node = Hash(key) MOD N

Seriously?

Redis is not just a cache. Let me repeat that. Redis is not just a cache. Lots of people use Redis as a primary data store. If Redis is your primary data store, you can't afford to have any of your keys invalidated, ever. "Minimum" might be good enough when you're talking to Memcached, but it's not enough when you're taking to Redis.

Consistent hashing has been a solved problem for a long time if you can afford to misplace a few keys from time to time, which will happen every time a node is added to or removed from the pool. There are Redis client libraries implementing consistent hashing in nearly every language, and most of them work just fine if you use Redis as a cache or if your pool size never changes. Solving this problem again isn't particularly interesting.

What really would be interesting is a server that sits between Redis nodes and clients and intelligently moves keys from one node to another in the background so that no key is ever invalidated even when the pool size changes. I believe that project is called Redis Cluster or something. That might be worth an extra TCP connection. But right now, I'm not seeing why I should prefer Redis-Router to any tried-and-true client library with built-in lossy consistent hashing.

Re: Redis (consistent hashing based) sharding made easy

#18

Post by nasalgoat »

kijin wrote: post #17

> Q: What about data invalidation if I move servers, change the config etc. > A: It's minimum. At least better than: Node = Hash(key) MOD N Seriously? Redis is not just a cache. Let me repeat that. Redis is not just a cache. Lots of people use Redis as a primary data store. If Redis is your primary data store, you can't afford to have any of your keys invalidated, ever. "Minimum" might be good enough when you're talk…

What you're describing sounds a lot like mongodb - it has the server (mongos) that sits between the client and the upstream databases (mongod) and stores metadata about where data is located, and it moves that data around based on shard keys.

Unfortunately it's the weakest part of mongodb.

Re: Redis (consistent hashing based) sharding made easy

#19

Post by nasalgoat »

peterhunt wrote: post #15

Somewhat off-topic, but just wondering: is anyone using consistent hashing for their DB masters? Every setup I've seen uses a manual sharding table.

Yes, and it works fine as long as you accept that data will need to be migrated at some point when you run out of memory and/or instances.

To put this off as long as possible, you can max out the memory in your boxes and use a large number of servers - 100s in some cases - running on those servers, hopefully with less than 1 per CPU core to maximize performance.

Then if you begin maxing out memory, you can easily split those servers onto their own hardware. My calculations showed that would allow scaling to trillions of keys without problems.

Re: Redis (consistent hashing based) sharding made easy

#20

Post by emre-yilmaz »

kijin wrote: post #17

> Q: What about data invalidation if I move servers, change the config etc. > A: It's minimum. At least better than: Node = Hash(key) MOD N Seriously? Redis is not just a cache. Let me repeat that. Redis is not just a cache. Lots of people use Redis as a primary data store. If Redis is your primary data store, you can't afford to have any of your keys invalidated, ever. "Minimum" might be good enough when you're talk…

okay, tell me a python library comes with consistent hashing?

(for the record = HASH(key) MOD N is not 'consistent hashing'.)

redis-router is just a library that wraps redis-py with consistent-hashing. nothing more.

I use it in production heavily since it solves the client-side sharding problem for me. When I wrote this, there was no trustable client library comes with consistent-hashing.

I don't know what do you want to see actually. "saving the world" is a todo though. wait for the new releases. you might like it. :)

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