Terrorizing REDIS with beautiful LUA

How our Redis server almost died and took the entire application down!

Swayam Raina
Dec 17, 2018 · 5 min read
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The cloud of Imagined code!

We all have worked in teams and most probably seen a large codebase. And being part of an ever-changing product we rarely get a chance to see most of the code. Which leads to something called “Code Imagination”.

Imagining code is a necessary step when you take over a large project — you will never read all the code. But if you do not distinguish code you imagined from the code you’ve seen with your own eyes, then you become prone to making decisions without evidence. When I wrote code and re-used one particular function, I only saw the method name. I imagined that this particular function would simply delete the key from REDIS, and in my complacency, I never verified that what I imagined was truly so.

Maybe it’s because we love technology so much that when many of us imagine a machine we assume each part of it has been fully and beautifully developed.

The method signature looked something like this.

public boolean deleteKeys (String key);

What actually happened?

To know what actually happened let me first tell you about LUA and then we’ll move towards the bad and the good…

What is LUA?

Lua is a powerful, efficient, lightweight, embeddable scripting language. Most of the cache server implementations like REDIS, AEROSPIKE etc provide support to run LUA scripts on servers for macro-commands.
LUA has a very low foot-print with really high speed maybe is the reason it has gained so much of attraction.
Fun Fact: Adobe Lightroom uses LUA heavily.

The Issue

We all know REDIS is a single threaded application but a very very fast one (because of the optimised code). It has something called “UDF” or User Defined Functions which is similar to PL/SQL procedures in case you ever worked on Oracle as Database or Stored Functions in case of MongoDB.
So, the method we talked about during the introduction. Yes, that’s the evil method I fell prey off.

Instead of deleting a single key as requested by the method. The implementation actually fired-off an LUA UDF which deleted the specified key. The UDF looked something like this,

local keys = redis.call('keys', '%s') 
for i,k in ipairs(keys) do
local res = redis.call('del', k)

The UDF had caused issue earlier also (about 2 years ago) and was switched off back then by sending a patch. This happened when I was not part of the organisation.

Coming back to the issue, This UDF actually iterated all of the keys in REDIS and then deleted the requested key. This is the moment when we imagine the underlying implementation of a method at the time of re-use.
Since this iterated the complete set of keys, It made all of the queries to REDIS super slow and our REDIS slow-logs was full of queries.

The Redemption

The issue was highlighted by the DevOps team as soon as the alerts went up and the application tended to become slow. As a reaction, digging started right from when exactly the issue started to happen. This clearly indicated that a new feature release which went live a few days ago was responsible.
The new feature introduced Aerospike parallel to (rewrite of) Mongo and Redis for Personalised Offer Banner display to users.

On further digging, We found out the underlying implementation used a bad UDF written somewhat 2 years ago.
The patch was sent ASAP and the servers (both REDIS and application) performed normally and everything was back to normal.

REDIS source code scanning

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As we know, we always find a silver lining to everything. With this particular issue, I started looking at the source code of REDIS and fell in ❤ with it.
After going through the source code for about 3 to 4 days I came to know the data structures REDIS uses for storing normal hashes and hash sets. Even how the commands are processed and how the output is sent out to the terminal.
I even raised improvements to existing data structures to Antirez by raising an issue on GitHub.

For people who don’t know REDIS is open source and you can read the amazing source code by visiting this page.

Chit-chat with Antirez

Stage II of this story was all about fixing the issue. And now after reading the source code, I had so many doubts. One particularly being why UDFs. So I directly reached out to Antirez (Salvatore Sanfilippo) for clarifications.

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So yes, The purpose of LUA is to give developers a mechanism by which they can perform certain aggregation queries or run a command which in itself is a combination of multiple commands in a single outbound call to the REDIS server.

Another peculiar question that daunted me was the use of kind of DLLs (Doubly linked lists) over RB Tree for HashSet storage. I pointed this out to him and He very kindly notified that RB Tree implementation was something that is coming up with the stream version of REDIS.
After brainstorming a bit, I realised why exactly DLL type of solution was better. Remember we are sitting down inside RAM which has a memory restriction. And using so many pointers is simply a waste of resource.

Learning the hard way

What I learned with this!
Well, first of all not rely on method signatures and to verify the implementations before actually using someone’s else’s code.

This particular issue also made me go through the source code and which made me more confident in the area of reading a foreign source code and grasping how the code is structured and it’s working.

Even though this issue was not something I would want to remember (because of how silly it is and how big its impact was) but yeah, certainly it taught me something which made me a better developer than the day before.

Oh Mongo!

And to just make it clear, this can happen to anyone and something similar happened with MongoDB, when the engineers outlooked a small change assuming the code in lower layers would be handling the situation properly. You should read about Assumptions Software developers made at MongoDB and how it turned out.

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