Skip to main content

REDIS

When somebody asks us to create a web application, the software requirements will be Apache, Mysql, PhP(LAMP where L stands for Linux). Imagine an application which stores data in main memory instead of disk, this will increase your application performance tremendously. One such application is Redis. Redis stores data on memory instead of disk and will periodically sync with the disks(if necessary).
Why Redis?:
  • Redis will be faster as it keeps data on memory. I read somewhere Memory is like disk and disk is like tape for redis.
  • Redis allows lot of data structures. Basically its a NOSQL database(http://en.wikipedia.org/wiki/NoSQL) . They dont support table or database schema we use traditionally. For them everything is key value pair as in hashmap. Redis allows a key to have values of types- string, set, sorted set, list, hashmap. To understand each data structure and the command they support, have a look at http://simonwillison.net/static/2010/redis-tutorial/
  • Lot of  web apps will be easier to code, as complex operations like sorting, union, aggregation can be done by redis itself
Do Redis follow ACID?
  • As of now, if redis server is crashed and data is not synced with the disk then consistency of data will be an issue
  • Redis server is a single threaded program, so no concurrency issues.
  • If you want a client to make a transaction, multi and exec commands in redis will take care of that. Unlike other databases there is no locking mechanisms involved during transaction. When a client performs exec, server wont get commands from other clients till all command queued for this transaction are done.
  • Single thread can reduce scalability of Redis. Redis supports slaves where a client can read from master and slaves but writes should first affect the master and then synced across the clients. So time consuming operations like sort can be moved to the slaves.
Some Cool Commands to Try Out:
  • BLPOP and BRPOP are some good operations that one always do on queues. Say I am waiting on a queue for a representative to pick up my call. So once a representative comes available, BLPOP will return the representative to the guy who waits for a longer time. No application logic is actually needed
  • Multi and Exec, they support transactions
Where To start?:
  • Install redis (apt-get or yum) or get a free redis instance at http://redis4you.com
  • Almost all languages support redis client coding(for Java download Jedis jar, for Ruby try redis-rb gem). For complete list check at http://redis.io
Start developing web apps without database if your data will fit in memory

Comments

Popular posts from this blog

How we have systematically improved the roads our packets travel to help data imports and exports flourish

This blog post is an account of how we have toiled over the years to improve the throughput of our interDC tunnels. I joined this company around 2012. We were scaling aggressively then. We quickly expanded to 4 DCs with a mixture of AWS and colocation. Our primary DC is connected to all these new DCs via IPSEC tunnels established from SRX. The SRX model we had, had an IPSEC throughput of 350Mbps. Around December 2015 we saturated the SRX. Buying SRX was an option on the table. Buying one with 2Gbps throughput would have cut the story short. The tech team didn't see it happening. I don't have an answer to the question, "Is it worth spending time in solving a problem if a solution is already available out of box?" This project helped us in improving our critical thinking and in experiencing the theoretical network fundamentals on live traffic, but also caused us quite a bit of fatigue due to management overhead. Cutting short the philosophy, lets jump to the story.

The server, me and the conversation

We were moving a project from AWS to our co-located DC. We have setup KVMs scheduled by Cloudstack for each of the component in the architecture. The KVMs used local storage. The VMs are provisioned with more than required resources because we have the opinion that in our DC scaling during peak load and then downscaling doesn't offer much benefits financially as we are anyways paying for the hardware in advance and its also powered on. Its going to be idle if not used. Now we found something interesting our latency in co-located DC was 2 times more than in AWS. The time for first byte at our load balancer in aws was 60ms average and at our DC was 112ms. We started our debugging mission, Mission Conquer-AWS. All the servers are newer Dell hardwares. So the initially intuition was virtualisation is causing the issue. Conversation with the Hypervisor We started with CPU optimisation, we started using the host-passthrough mode of CPU in libvirt so VMs dont see QEMU emulated CPUs,

Ptrace

Ptrace is a nice setup ( some people call dirty setup) on linux to debug running processes. This ptrace in sys/ptrace.h is used by strace and gdb. To trace a child process, the child process should call PTRACE_TRACEME. The kernel during each system call(or execution of each instruction) checks if the process is traced. If it is traced, it issues a SIGTRAP, the parent process if in wait() state, will get a signal. The parent issues a SIGSTOP to hold current state of child and can access the registers and memory of child using PEEKDATA and alter the values in register and memory using POKEDATA. Once the required job is done, parent will allow the child to run with a SIGCONT signal. Since one can access registers, the next instruction to be executed can be easily found using instruction pointer, this comes in handy when we need to set breakpoints while debugging. The entire code base can also be changed using ptrace. PTRACE_ATTACH attaches a running process. It does some hack to become