Challenge 5: 100 Likes/second - Deepstash
Challenge 5: 100 Likes/second

Challenge 5: 100 Likes/second

  • Scale horizontally again to handle more events -> Add multiple Dispatchers and move the subscription table into a key-value store so it's accessible to all Dispatchers.
  • Dispatchers are independent from Frontend nodes and don't have persistent connections between them.
  • Any Frontend node can subscribe to any Dispatcher.
  • Any Dispatcher can publish events to any Frontend node.

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MORE IDEAS FROM Streaming a Million Likes/Second: Real-Time Interactions on Live Video

Related posts
  • How LinkedIn displays Presence indicators in real-time: https://engineering.linkedin.com/blog/2018/01/now-you-see-me--now-you-dont--linkedins-real-time-presence-platf
  • How LinkedIn measures end-to-end latency across systems: https://engineering.linkedin.com/blog/2018/04/samza-aeon--latency-insights-for-asynchronous-one-way-flows
  • How LinkedIn scaled one server to handle hundreds of thousands of persistent connections: https://engineering.linkedin.com/blog/2016/10/instant-messaging-at-linkedin--scaling-to-hundreds-of-thousands-

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Challenge 2: Connection Management
  • Each connection is managed by an Akka actor.
  • Actors are so lightweight that there can be millions of them on a single system. Moreover, all of them can be served by a small number of threads, proportional to the number of cores. This is possible because a thread is assigned to an actor only when it has work to do.
  • Actors are managed by an Akka supervisor actor that sends them events (likes, comments etc.) which need to be forwarded to user devices.

Akka is a toolkit for building highly concurrent, distributed, and resilient message-driven apps.

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Final architecture
  1. A viewer likes a video and sends an HTTP request to the Likes backend, which stores it in a database.
  2. The backend forwards the like to any random Dispatcher node using an HTTP request.
  3. Dispatcher looks up in the key-value store to find out which Frontend nodes are subscribed to likes from that video.
  4. Dispatcher sends the like to the corresponding Frontend nodes.
  5. Frontend nodes send the like to client devices which are subscribed to that video.

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The Realtime Platform

LinkedIn has built the Realtime Platform to distribute multiple types of data in real-time such as:

  • Likes, comments and viewer count for Live Videos
  • Typing indicators and Read receipts for Instant Messaging
  • Presence i.e. the green online indicators

Their goal is to increase user engagement by enabling dynamic instant experiences between users, such as: likes, comments, polls, discussions etc.

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Performance and scale
  • Each Frontend node handles 100k persistent connections. It handles only this many connections because the server is doing a lot of work processing multiple types of data (likes, comments, instant messaging etc.).
  • Each Dispatcher can publish 5k events per second to the Frontend nodes.
  • End-to-end Publish Latency is 75ms at p90, from the moment the Like is received until the Like is sent to a viewer. There are only two lookups in the subscriptions tables and a series of network calls.
  • The system is completely horizontally scalable. You can add more Dispatchers and Frontend nodes to handle more viewers and more events.

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Challenge 4: 10K Concurrent Viewers
  • Scale horizontally to handle more concurrent viewers -> Add multiple Frontend nodes and coordinate them using a Dispatcher node.
  • In a similar fashion to the Frontend node, the Dispatcher has a subscriptions table to know which frontend nodes should receive which events.
  • This table is populated when Frontend nodes send subscription requests to tell the Dispatcher which live videos they're interested in (i.e. which live videos its connections are subscribed to).

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Interactive Live Videos

Having a lot of people interact on live videos poses many technical challenges. Mainly because viewers generate a lot of interactions that need to be delivered fast.

To get a sense of the scale, the top live streams in the world gathered millions of concurrent users:

  1. Cricket World Cup Semifinal 2019 - 25M concurrent viewers
  2. British Royal Wedding 2018 - 18M concurrent viewers

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Challenge 1: The Delivery Pipe
  • User devices have a persistent connection to the Realtime Platform servers.
  • The servers use server-sent events to stream data fast on this connection via the EventSource interface.

A persistent connection is an HTTP Long Poll i.e. a regular HTTP connection where the server doesn't disconnect it.

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Challenge 3: Multiple Live Videos
  • Clients are subscribing to events for a particular live video i.e. they are telling the server which live video they are watching.
  • The Frontend server stores all subscriptions in an in-memory table.
  • Every time a new event is published, the supervisor actor does a lookup in the in-memory table to determine which actors need to receive this event.

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Bonus Challenge: Multiple data-centers

Expanding to other regions requires two steps:

  1. Replicate the setup in each data-center.
  2. Have the Dispatcher broadcast the events to its peer Dispatchers from the other data-centers.

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RELATED IDEA

The Power Of The Moon On Humans
  • The belief of the moon's power to drive the emotions and feelings of human beings is well-documented.
  • Lunacy is the word describing madness, directly connecting it to Luna, the name of our moon.
  • In any country, hospital emergency units and police stations will vouch for the fact that there are more accidents, violent incidents and other emotion-related crimes on full moon days.

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The Error in Isolating Events
  • Many studies have been conducted regarding the psychological impact of a one particular event, like the trauma associated with the ongoing health crisis, or sudden job loss.
  • What researchers fail to gauge is the complex psychological experiences of various events and situations that are of different hues and colors, and happen simultaneously.

A normal person leading a full life can experience events related to death, family changes, job changes, health issues, and financial swings. Each experience is connected to the other experiences and is not isolated, making the impact on the person varied and complex.

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Dr. Seuss

How did it get so late so soon?

It’s night before it’s afternoon.

December is here before it’s June.

My goodness how time has flewn.

How did it get so late so soon?

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