Today I Learned

TIL, 2018-03-31, Designing Data-Intensive Applications


  • Active Job outside of Rails:
require 'sidekiq'
require 'active_job'

ActiveJob::Base.queue_adapter = :sidekiq

class Foo < ActiveJob::Base
  queue_as :default

  def perform
    sleep 10

Foo.perform_later # => undefined method 'perform_later'

Introduction to Architecting Systems for Scale


  • Load balancing
    • User to web server.
    • Web server to an internal platform layer.
    • Internal platform layer to your database.
  • Smart clients: Because devs are devs, they are used to writing software to solve their problems, and smart clients are software.
    • These take in a pool of service hosts and balanced load across them, detects downed hosts and avoids sending requests their way.
  • Hardware load balancer: expensive and non-trivial to configure.
  • Software: HAProxy.
    • Runs locally on your box, and each service you want to load balance has a locally bound port.
    • Platform machine: 9000, DB read: 9001, DB write: 9002.
  • Caching: Pre-calculating results, pre-generating expensive index (search history), storing copies of frequently accessed data in a faster back-end (Memcache instead of PostgreSQL).
    • Caching is important earlier in the dev process rather than load-balancing, and starting with a consistent caching strat saves you time early on.
    • App vs db caching: requires explicit integration in the app code itself. Check if value is in cache, if not, retrieve the value from the DB, then write that value into the cache.
    • Database caching: just improving the DB for performance.
    • CDN.
    • Cache invalidation: these require consistency between your caches and the source of truth (cache invalidation).
  • Off-line processing.
    • Message queues.
      • These allow your web applications to quickly publish messages to the queue, and have other consumer processes perform the processing outside the scope and timeline of the client request.
      • You can inform the user that the task will occur offline, or perform enough work in-line to make it appear that it’s worked.
  • Scheduling periodic tasks: cron.
  • Map-reduce.
  • Platform layer.

Musings, Designing Data-Intensive Applications

  • More data-intensive applications (vs compute-intensive). They need to:
    • Store data (databases).
    • Remember results of expensive operations (caching).
    • Allow users to search data by keyword/filter it in various ways (search indexes).
    • Send messages that are to be handled asynchronously (stream processing).
    • Crunch a large amount of accumulated data (batch processing).

Chapter 1: Reliable, Scalable, and Maintainable Applications

  • Data systems:
    • Data stores that are also used as message queues (Redis).
    • Message queues with database-like durability guarantees (Kafka).
    • Application-managed caching layer via Memcached.
    • Full-text search server using ElasticSearch or Solr.
  • Reliability: You can put a fault inside just to make sure that they would still work even with poor error handling.
  • Scalability:
    • Approaches: database join + update everyone’s timeline caches. Timeline write = a single tweet can result in x million writes to home timelines!
    • The new approach is that most users tweets are fanned out, but the celebrities are exempted.
  • Performance:
    • Service’s response time, and you also need to think about the distribution.
    • Even if you have slow response times in the 99th percentile, you still have to think about them, because they usually have the most data and thus, most purchases. (Tail latency).
    • Service level objectives and service level agreements.

Chapter 2: Data Models and Query Languages

  • Object-relational mismatch: There is an awkward translation layer required between the objects in the application code and the database model of tables, rows, and columns.
  • Possible approaches for storing resume information:
    • Put position, education, and contact information in separate tables.
    • Structured data types/XML data, storing JSON in a single row.
    • Encode jobs, education, and contact info as a JSON or XML document, store it on a text column, and let the app interpret.
    • JSON: Better locality (it’s easier to construct a profile than in the relational, where you have to perform multiple joins).
  • ID or plain-text strings?
    • For drop-down lists, we have consistent style, we avoid ambiguity, we have ease of updating.
    • When you store ID, no duplicated info.
    • If DB doesn’t support joins, you have to emulate a join in application code by making multiple queries to the DB.
  • RDBMS vs document databases:
    • If the data in your app has a document-like structure, use a document model.
    • The code that reads a document database usually assumes some sort of schema.
    • RDBMS: schema on write, document db: schema on read.
    • Schema changes have a bad reputation of being slow and requiring downtime.
  • MapReduce Querying:
    • The logic of the query is expressed with snippets of code, which are called repeatedly by the processing framework, using map and reduce (fold or inject).
    • map and reduce functions must be pure functions.

Chapter 3: Storage and Retrieval

  • The simplest database: 2 bash functions (one to set, and one to get). Key-value pair, some kind of CSV file.
  • Most databases internally use a log.

This project is maintained by daryllxd