I have some scripts that touch a handful of keys. What happens if Elasticache decides to reshard while my script is running? Will it wait for my script to complete before it moves the underlying keys? Or should I assume that it is not the case and design my application with this edge case in mind?
One example would be a script that increment 2 keys at once. I could receive a "cluster error" which means something went wrong and I have to execute my script again (and potentially end up with one key being incremented twice and the other once)
Assuming you are talking about a Lua script, for as long as you're passing the keys in the arguments (and not hardcoded in the script) you should be good. It will be all or nothing. If you are not using a Lua script - consider doing so
From EVAL command:
All Redis commands must be analyzed before execution to determine
which keys the command will operate on. In order for this to be true
for EVAL, keys must be passed explicitly. This is useful in many ways,
but especially to make sure Redis Cluster can forward your request to
the appropriate cluster node.
From AWS ElastiCache - Best Practices: Online Cluster Resizing:
During resharding, we recommend the following:
Avoid expensive commands – Avoid running any computationally and I/O
intensive operations, such as the KEYS and SMEMBERS commands. We
suggest this approach because these operations increase the load on
the cluster and have an impact on the performance of the cluster.
Instead, use the SCAN and SSCAN commands.
Follow Lua best practices – Avoid long running Lua scripts, and always
declare keys used in Lua scripts up front. We recommend this approach
to determine that the Lua script is not using cross slot commands.
Ensure that the keys used in Lua scripts belong to the same slot.
Related
In the documentation about scripting in redis the following is stated:
Redis guarantees the script's atomic execution. While executing the script, all server activities are blocked during its entire runtime.
Do I understand that correctly that really all server activities are blocked? In the call to EVAL you state the keys that are modified, so why would Redis not only block activities for those keys?
Thanks in advance for any clarification!
Because Redis runs commands one-by-one in a single thread. When it runs your script, i.e. EVAL, it cannot run other commands until it finishes the script.
So, NEVER run complicated Lua script, since it might block Redis for a long time, and hurt performance.
I have a Redis cluster that is still active and not sure if it is still been used. Logged in and tried command KEYS * and found around 30k+ keys.
How to identify if it is still been used if not can process to terminate?
First of all, you shouldn't use KEYS command in production, it can jam your Redis. Use SCAN instead.
If you mean you would like to know if some queries are still being made against Redis, you can use the Monitor command. Be careful though, it reduces your throughput by about 50%, so only use when necessary.
One way to execute commands in REDIS, is via the EVAL script.
Redis uses the same Lua interpreter to run all the commands. Also
Redis guarantees that a script is executed in an atomic way: no other
script or Redis command will be executed while a script is being
executed.
Since redis is single threaded, why do we need EVAL to offer atomicity? I would expect that this is implied by the one running thread.
Am I missing something? Apologies if my question is pretty simple, I am quite new to redis
Every (data path) command in Redis is indeed atomic. EVAL allows you to compose an "atomic" command with a script that can include many Redis commands, not to mention control structures and some other utilities that are helpful to implement server-side logic. To achieve the similar "atomicity" of multiple commands you can also use MULTI/EXEC blocks (i.e. transactions) by the way.
Without an EVAL or a MULTI/EXEC block, your commands will run one after another, but other clients' commands may interleave between them. Using a script or transaction eliminates that.
Redis uses a single thread to execute commands from many different clients. So if you want a group of commands from one client to be executed in sequence, you need a way to direct Redis to do that. That's what EVAL is for. Without it, Redis could interleave the execution of commands from other clients in with yours.
As I know redis is single threaded solution from client point of view.
But what about the general architecture?
Amuse we have some lua script that going to execute several commands on keys that has some TTL.
How does redis garbage collections works? Could it interrupt the EVAL execution & evict some value or internal tasks share the single thread with user tasks?
Lua is majik, and because that is the case time stops when Redis is doing Lua. Put differently, expiration stops once you start running the script in the sense that time does not advance. However, if a key expired before the script started, it will not be available for the script to use.
We're having a severe config/product bug on our installation. We've been experiencing concurrency related errors that we've been blaming to Jedis usage, but it seems that it might be a product / config issue.
This is a single redis installation with over 4M keys. Whenever we run a long running command from redis-cli, like a keys *, our client code (Jedis based) starts to throw errors, like trying to cast a string into a binary (typical concurrency errors in Jedis conf). The worst scenario is that sometimes it seems that it returns wrong keys. We were using a Jedis instance in each actor instance, so it shouldn't be an issue but we changed to JedisPool nevertheless. But the problem remained (we are using Jedis 2.6.2).
But the main thing was when trying from different redis-cli. We run KEYS * that stays a long time running and then a GET command which returned. It was our understanding that the KEYS * should block everyone, but the GET command keeps working. This also happens with a SLEEP command.
Is this related to a config setting or this is something that shouldn't happen, or the KEYS command isn't blocking and my problem lies elsewhere?
Redis.io documentation for KEYS clearly states that KEYS is a debug command and should not be used in production:
Warning: consider KEYS as a command that should only be used in
production environments with extreme care. It may ruin performance
when it is executed against large databases. This command is intended
for debugging and special operations, such as changing your keyspace
layout. Don't use KEYS in your regular application code. If you're
looking for a way to find keys in a subset of your keyspace, consider
using SCAN or sets.