Shadowing Practice: Distributed Task Scheduler: System Design Interview (Stripe & Amazon Offers) - Learn English Speaking with Video

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in this video we are going to design a distributed task

scheduler like aws step functions a distributed task scheduler is a system

that is designed to execute arbitrary code

or trigger events at a specific time in the future either once

or on a recurring basis designing a robust task scheduler requires solving complex distributed systems problems including high precision

so a tasks should execute exactly when they are supposed to, maintaining fault tolerance so no tasks are lost if a worker node crashes,

and avoiding duplicate executions in a highly concurrent environment.

And so this is a really popular system design that is asked in interviews by Meta, Amazon, and Microsoft.

And they love to ask it because it exposes your understanding of distributed system trade -offs, which is a great way of differentiating the good candidates from the great ones.

For those of you

that are struggling with system design I highly recommend you check

out Tech Prep where you can see the full write -up

and the interactive AI whiteboard

that walks you through the design exactly like how an interviewer would

and it helped me go from projections to offers from the likes of Stripe and Amazon.

There are also a lot of system design concepts and patterns

that may be new to you in this system design and

so one way to keep up is to use the AI

highlighter where you can highlight anything in the write -up

and it will give you a full explanation based on the context of this system design.

So without further ado let's jump into the requirements.

So for the functional requirements firstly we will need task submission

so users can submit one -off delayed tasks or recurring tasks e .g cron expressions.

For task execution the system must execute the provided task payload so this could be a HTTP webhook, a script or a message push at the scheduled time.

In terms of task management users can query the state of a task so it can be pending,

running, completed, failed or cancelled and users should be able to cancel an upcoming task.

We should also then have an execution history

so the system must store the output and execution logs of completed tasks for auditing purposes.

Then for the non -functional requirements firstly scalability the system must handle 10 000 plus task submissions per second

and peak execution bursts of 50 000 tasks per second in

terms of high availability we'll want 99 .99 uptime for task submission

and execution routing and then for reliability we want no data loss

so once the task is acknowledged as accepted it must be durably stored

and guaranteed to execute and then finally in terms of precision

and latency tasks should begin execution within 200 milliseconds of their scheduled time.

So now that we've looked at the requirements let's look at the data model we need to build to fulfill those requirements.

So to handle the contrasting demand of strict state management

and the massive write -heavy logging I think a polyglot persistence strategy is required for a system of this scale

and so what polyglot persistence simply means is

that we have to use different data storage types for different things and I'm going to walk through each of them now.

So the first thing will be our relational database which you can implement with Postgres.

So this will serve as the system's source of truth for task definitions, schedules and metadata and we rely heavily on its asset properties

and row level locking to manage the state machine of a task without race conditions.

And the core entities will include a task definition

so this is the core configuration object it stores the user's

intent the payload to be executed for example http webhook url

and body the retry policy and the scheduling logic it's either one off

or a cron expression next will be our task execution

so this represents a single distinct run of a task definition

and it acts as the primary state machine

so we'll go from pending to queued to running to either success

or failed and it includes an item potency key and tracks exactly

when the task was picked up by a worker and when it finished.

So this is really important and is the kind of core of our architecture.

Next we'll also need an in -memory store which we can implement with Redis and this is also super important.

So this will act as the high speed timer and scheduling buffer

and we will also utilize Redis sorted sets

so z sets where the score is in the epoch execution time

and this will allow the system to query for tasks

that need to run right now in o log n time bypassing the need to constantly scan the relational database.

Now if you don't know what red assorted sets are don't worry about

that we're going to explain those in detail later on.

We'll also need a message broker so something implemented with either Kafka

or RabbitMQ which will serve as the durable execution queue and

so it decouples the scheduling components from the worker pool allowing the system to absorb massive spikes in task executions

so back pressure without overwhelming the database

and finally we'll also need a wide column store

so something like Cassandra and this is used exclusively for execution logs

and history since a single recurring task might generate millions of

logs over its lifetime a high write throughput horizontally scalable no sql database is ideal for this

and so that's the data model so

if you understand these four data storage techniques

and why they're used you will be in a really good place to move on to our api design

so for a distributed task scheduler we can just use a classic restful API to interact with the data.

RESTful API is simple, widely used, stateless and they support caching which makes it a great candidate for our system.

So the first endpoint can be a create task endpoint.

So this will be a post to a v1 tasks.

It accepts a payload, execution time or a cron and a target and will return a task ID.

Next will be the get task status.

This will be a get request to the v1 tasks task id

and it will return the current state and metadata of that task.

A cancel task endpoint so this will be a delete at v1 tasks task id

and it marks a pending task as cancelled

and then finally for logs we can get a get logs endpoint

which will be a get to a v1 tasks task id executions

and this will return a paginated execution history of that task.

Obviously production system will have a lot more endpoints

but these are the kind of core endpoints to outline to your interviewer what's kind of core to this system

and what we'll need to cover as we go through our high -level system design.

So now that we've mentioned our data model and API design let's look at our high -level design.

So at a high level the system architecture can be separated in terms of accepting a task as well as executing it.

So the first part will be the submission path.

So this path focuses on securely receiving the task and durably persisting its schedule.

So here a client will upload it'll go to the gateway

this will route the incoming request it'll also authenticate the client

and enforce rate limits this will then get pushed to our task management service

which validates the payload and cron expressions

and it writes the task definition to the task database again Postgres with this state of pending and then

if the task is scheduled to run in the very near

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future e .g within the next 15 minutes the service could

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also push a lightweight reference to the task directly into the redis zset timer next is the execution path

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and so this path focuses on time triggering the task

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and hand and handing it off to worker nodes

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so firstly we'll have relay nodes this will be a background batch process

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that periodically queries the task database postgres for tasks scheduled to run in the near future for example 10 minutes

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and stages them in the redis zset timer this prevents the database from being continuously hammered by high frequency polling

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and then what we can have is we can have a fleet of lightweight scheduler nodes

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which will continuously run a process let's say every 100 milliseconds

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to pull the redis z set for tasks whose scheduled time is less than

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or equal to the current time once we've pulled those tasks

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off the redis z set we can then add them to a task queue

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and so the task payload is fetched

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and pushed onto this highly available message queue

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and to prevent noisy neighbors where one customer submitting 50 000 tasks saturates the queue

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and delays others tasks are rooted to tenant specific queues using a hash of the tenant id

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and this allows workers to round robin between tenant queues ensuring

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equitable compute distribution among our customers next we'll have our worker nodes

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and so this is a fleet of horizontally scaled compute nodes

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that consume messages from the queue execute the business logic eg

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make the request to the target system the http webhook

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and then report the successor failure to the task database

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and then keep a historical log in cassandra

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so pretty simple system it's not too crazy and so

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while this high level design does separate our concerns it does contain critical flaws

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so pulling our database efficiently we can dive deeper into

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that workers can crash mid -execution leaving tasks stuck in limbo

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which we'll have to deal with

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and network retries could cause a payment task to execute twice

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and so we have to deep diaper into all of these and show how we can resolve them.

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So firstly let's look at Redis Z sets and how they're implemented so we have a better understanding.

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So Redis sorted sets is an in -memory data structure that automatically orders unique elements by a specific score.

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In this case the Unix execution timestamp allowing the scheduler to instantly identify

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and extract tasks run in o log n time without constantly polling

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and overwhelming our primary database and

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so under the hood the way it's implemented is two data structures the first one is a hash map

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and this is used for o1 point lookups

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so the purpose of this is to get

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that instant access via task id and

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so the situation this would be used is like

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if a user wants to cancel an upcoming task

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or the system needs to check its exact scheduled time can check via the task

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id to get its execution timestamp

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and redis ignores time -based sorting entirely by consulting this internal hash map to find by key

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and instantly return the execution timestamp in o of one time

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so it doesn't have to traverse any list the second data structure is the skip list

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and this is for o log n time -based polling and

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so here this is the purpose of this is for fast traversal by execution timestamp.

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And so the scenario we use this is like the schedule node polls for tasks ready to execute right now.

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So we'll check for tasks that's execution time is less than or equal to right now.

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And so obviously Redis cannot use the hash map here because it doesn't know which specific task IDs are right for execution.

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Instead it goes to this skip list. And

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so it uses the upper level express lanes to quickly skip

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over millions of tasks scheduled far into the future like task C

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and it drops down to the base level only

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when it nears the target timestamp landing directly on the threshold of ready tasks to instantly extract them for the message queue.

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So if we look at a quick example let's say we're polling for ready tasks

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and the current timestamp is this one here on the screen.

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Okay well the first thing we're going to do is start at level three start at our head

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and then look right and when we look right we can see we have task c

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which as you can see is greater than our current time stamp as we can see that's in the future

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so we can ignore it so what do we do

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so we drop down to the next level

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and again we start out our head and we look to the right

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and as you can see we move across

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and we can see the time stamp is exactly equal to our current time stamp

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and so therefore we know we have found our upper bound

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and so what we now need to do is find all the elements up to

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that upper bound and the way we do

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that is drop down again a level and

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because we're at the bottom level here we know this is a perfectly sorted link list

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and so redis instantly knows

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that any node from the head to task b is right for execution

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so it sequentially traverses the level one extracting task a

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and task b and returning them to the scheduler to be pushed to the kafka queue

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so hopefully that makes sense

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and again the primary architectural benefit here is we are able

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to extract ready to run tasks in o log n time without constantly polling and overwhelming the primary database.

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So next we need a look at high precision scheduling.

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So running a select all from the tasks table in our

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Postgres database containing billions of rows every second will instantly melt the database.

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We need a way to track time efficiently to achieve that less than 200 milliseconds execution precision without database bottlenecking.

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And so we will implement a push -pull time partitioned architecture.

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So the first step, again, we have our luck ahead cron.

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So this background relay worker will wake up every five minutes

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and it'll query the Postgres database for all tasks scheduled to run in the next 10 minutes.

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Then we'll have our Redis sorted set.

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The relay will push these upcoming task IDs into the Redis Z set.

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And the score in the Z set again is that exact Unix epoch timestamp of when the task should run.

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Then we will have sub second polling.

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So a fleet of lightweight scheduler nodes will pull the Redis Z set every 100 milliseconds.

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looking for tasks where the score is less than or equal to the current timestamp.

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Then we will have an atomic handoff, so using a Lua script.

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So to prevent multiple scheduler nodes from pulling the exact same task during concurrent polling, we use a custom Lua script.

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And because Redis executes Lua scripts as a single indivisible transaction on its single threaded event loop, it guarantees an atomic check and pop.

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And so this means that the script queries for the ready tasks and immediately deletes them,

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safely handing the tasks to exactly one scheduler to then push onto the message queue.

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And then finally, to mitigate thundering herds.

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So, for example, humans will naturally schedule cron jobs at clean intervals, for example, every midnight.

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And so to prevent massive artificial latency spikes at the top of the hour, the schedule can inject optional execution jitter,

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which will add a random number of seconds between, let's say, 0 and 300 delay for tasks

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that do not require strict millisecond precision so that we can smooth out that load curve,

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ensuring that we don't have massive spikes in executions.

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And so the architectural benefit here is the heavy relational database is only queried sequentially in predictable batches,

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while the high frequency sub -second polling and lock frequency concurrency control is handled entirely in memory by Redis.

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Next, we need to look at our worker fault tolerance.

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So a major distributed system problem is the stuck task.

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So what happens if a worker node pulls a critical task from the queue, but the server suffers a hard crash before finishing?

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Well, the task must not be lost, nor can it be duplicated prematurely.

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So to guarantee at least once delivery and fault tolerance,

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we rely on visibility timeouts and explicit acknowledgements natively supported by brokers like AWS SQS or RabbitMQ.

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what we need is

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when a worker pulls a message the broker does not delete

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it instead it will update the metadata attribute on the message

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for example we could have a field invisible underscore until timestamp and so

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when other workers pull the queue the broker's internal query simply

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filters out messages where the invisible until is greater than the current time

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because that is currently being executed

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and we should not touch it we'll also have the receipt handle

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and so upon pulling the message the broker also gives the worker a unique temporary identifier

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so this receipt handle or delivery tag and we'll see why that's important in just a second.

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We'll also have heartbeats so this is the visibility extension so

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if a task naturally takes a long time a background thread on the worker periodically pings the broker to overwrite

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and extend the invisible until timestamp actively maintaining its exclusive lock and preventing other workers from touching it.

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We'll also have the ACK which is the final deletion and

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so upon successful completion the worker sends the receipt handle

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which we saw previously back to the broker in an ACK request

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and the broker uses this handle to locate the message in O of one time

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and permanently hard delete it from its memory or disk so that it is never picked up again by any other worker.

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And then in the case of handling the crash well

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if a worker crashes it stops sending the heartbeats

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and never sends the ACK and so once the invisible until timestamp expires

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and it's in the past the message naturally becomes visible again for a healthy worker node to claim

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and process and so this is a really nice way of handling

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that worker fault tolerance that most candidates won't be able to answer.

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Next we need to look at execution guarantees and

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because our fault tolerance mechanism relies on timeouts

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and retries we guarantee at least once execution however this means

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that a task might run twice and

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so strict exactly once delivery is theoretically impossible over an unreliable

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network therefore we must implement idempotency to make duplicate executions harmless

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so we need optimistic concurrency control

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so before a worker executes a payload it attempts to update the task status in postgres

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so you'll update the status to running

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and increment its version number by one right and so this means

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if the worker crashed previously

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and a duplicate worker picks up the same task the database will reject the second update

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because the status is no longer pending

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and the worker knows to safely discard the duplicate and again

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that version number acts as a unique constraint

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so once one worker increments it from one to two the second worker's update fails

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because it will still be looking for version one and

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so this implements the optimistic locking ensuring exactly one worker can claim a task even under high concurrency

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and the version number means the check and update operation truly is atomic

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and we'll also need client side item potency keys

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so for HTTP webhook tasks the scheduler will inject an item

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potency key header generated from the task id plus the execution timestamp into the outbound request

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and the downstream system receiving the webhook is responsible for using this key to ignore duplicate HTTP calls

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and so by using these mechanisms we are improving our execution guarantees.

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And then finally looking at scaling our storage layer

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because handling 10 ,000 plus continuous task commissions per second pushes the limits of single node architectures.

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This translates to 10 ,000 plus database inserts operations per second plus B3 index updates all

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while serving continuous select queries from the relay components.

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And so to prevent severe CPU and disk IO bottlenecks both the relational database

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and the in -memory Redis cluster must be horizontally partitioned

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so firstly looking at postgres you know

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while a heavily provisioned single postgres primary can technically peak at 10 000 tps running it at

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that threshold and production guarantees latency spikes and lock contention and

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so firstly we'll need horizontal sharding

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so we distribute the right load across multiple independent postgresql clusters

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for a multi -tenant sas platform we shard based on tenant id

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and this ensures data locality

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so all tasks for specific customer reside on the same database node keeping transactions highly localized

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and by provisioning 10 distinct nodes the load drops to a highly manageable 1000 tps per primary

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and then we'll also have time -based table partitioning

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so as the system scales to billions of executed tasks the

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active database shard will become bloated slowing down the look ahead cron queries

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and so we can mitigate this by partitioning the task tables by month

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or week and

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so this allows the system to quickly drop historical partitions archiving

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them to cold storage like s3 without running expensive delete queries and keeping the active b3 index incredibly small and fast

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next looking at redis and so

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because redis operates entirely in memory a single instance can handle

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100 000 simple operations per second however a single node creates a single point of failure

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and a massive single sorted set degrades performance and

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so we can see time complexity duration so because we rely on zadd

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and z range by score queries both of

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which operate o log n time as a single z set

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grows to millions of elements the single threaded redis event loop will begin to block

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and so therefore we will need time bucketed keys

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so instead of one global schedule task zset we can partition

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zsets by time bucket for example tasks minute you know 2 30

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and tasks minute 2 31 and as well as

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that we can also have a hash slot distribution

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so by deploying a redis cluster these time bucketed keys are deterministically hashed

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and distributed across redis primary nodes and this horizontally scales both the memory footprint

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and the olog n compute overhead ensuring this layer never blocks under massive concurrency

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and so look we've gone deep on you know high precision scheduling worker fault tolerance execution guarantees

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and scaling the storage layer so

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if you need know these things these are the core things interviewers are going to ask you

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so knowing them in depth will really bulletproof you

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when you go into your interview and

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so to finish up let's just walk through the complete architecture one last time

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so you've kind of got that one mental snapshot of understanding how everything works together

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so a client will submit a payload to the API gateway

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that will then get routed to the task management service which writes

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that persistent configuration to Postgres the relay service every few minutes

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in a batch process will pull tasks scheduled for the near future from Postgres

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and stage them to our Redis Z set keyed by their

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precise execution epoch time high frequency scheduler nodes then continuously pull Redis

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and when a task's time arrives a Lua script atomically pops it from redis

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and pushes it onto a partitioned kafka

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or rabbit mqq utilizing tenant -based routing to avoid noisy neighbors

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a worker node claims a task triggering a visibility timeout the

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worker then updates the database state to running using optimistic locking

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to prevent concurrent execution races the worker then executes the arbitrary code

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or http webhook injecting an item potency key to protect downstream systems from network retries

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and the worker then sends an act to the queue to permanently delete the message updates the state

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and postgres to complete it

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and flushes the execution logs asynchronously into the cassandra wide column

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store for permanent auditing to check out the full write -up

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and the ai whiteboard head over to techprep .app

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and hopefully you got some value out of this

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if you did please like and subscribe

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and share with a friend it helps the channel out a lot and i will see you in the next one

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Vocabulary and speaking notes for this lesson

This C1 speaking lesson is built on the video “Distributed Task Scheduler: System Design Interview”. The speaker keeps coming back to these words: task, execution, worker, redis, database. This video has 324 sentences and 4079 words to shadow. The speech runs for 22:56. The speaker talks at a natural 178 words per minute, close to everyday conversation. Only 73% of the words are among the 3,000 most common in English, so the vocabulary is demanding.

Key vocabulary in this video

The 15 most advanced words in the video, with pronunciation and meaning:

Word	Pronunciation	Meaning
node noun	/noʊd/	A knot, knob, protuberance or swelling.
queue noun	/kju/	A line of people, vehicles or other objects, usually one to be dealt with in sequence (i.e., the one at the front end is dealt with first, the one behind is…
execute verb	/ˈɛksɪˌkjuːt/	To kill, especially as punishment for a capital crime.
scheduler noun	/ˈʃɛdjələ/	A person or device that determines a schedule, that determines the order that tasks are to be done.
timestamp noun	/ˈtaɪmˌstæmp/	A variable containing the date and time at which an event occurred, often included in a log to track the sequence of events.
query noun	/ˈkwɪɹ.i/	A question, an inquiry (US), an enquiry (UK).
cron noun		A scheduler or timer that automatically starts a job, program, task.
payload noun	/ˈpeɪloʊd/	That part of a cargo that produces revenue.
distribute verb	/dɪˈstɹɪbjuːt/	To divide into portions and dispense.
endpoint noun		Either of the two points at the ends of a line segment.
webhook noun	/ˈwɛb.hʊk/	A method of augmenting or altering the behavior of a web application or a web page using custom callbacks, usually triggered by some external event.
firstly adverb	/ˈfɜɹstli/	In the first place; before anything else; first.
tenant noun	/ˈtɛ.nənt/	One who holds a lease (a tenancy).
broker noun	/ˈbɹoʊkɚ/	A mediator between a buyer and seller.
precision noun	/pɹɪˈsɪʒ.ən/	The state of being precise or exact; especially, both exact and accurate.

Phrasal verbs you will hear

Word	Pronunciation	Meaning
pick up verb		To lift; to grasp and raise.
write up verb	/ˈɹaɪ̯t ˌʌp/	To produce by writing.
walk through verb		To explain (something) to (someone), step by step.
check out verb		To record the departure or withdrawal of someone or something (such as guests, employees, books, etc.).
drop down verb		To sail, move, or row down a coast, or down a river to the sea.
finish up verb		To complete the last details of a task.
go through verb		To travel from one end of something to the other.
keep up verb		To maintain; to preserve; to prevent from deteriorating or falling.

Grammar in this video

The structures the speaker uses most, with the exact words from the video:

Structure	In the video
Passive voice be + past participle — the focus is on what happens, not who does it	is designed · are lost · is asked
Present perfect have/has + past participle — a past action that still matters now	we've looked · we've mentioned · we've pulled
Relative clauses who / which + clause — extra information about a person or thing	offs, which is · RabbitMQ which will · caching which makes

Pronunciation to watch

The speaker uses 40 contractions and reduced forms, such as we'll, we're, we've. Say them the short way, as you hear them.

The “sh” and “zh” sounds: scheduler /ˈʃɛdjələ/, precision /pɹɪˈsɪʒ.ən/, hash /ˈhæʃ/, partition /pɑɹˈtɪ.ʃən/, submission /səbˈmɪʃən/
Long words — get the stress right: concurrency /kəŋˈkʌɹənsi/, millisecond /ˈmɪlɪˌsɛkənd/, invisible /ɪnˈvɪzəb(ə)l/, metadata /ˈmɛt.əˌdeɪ.tə/, visibility /ˌvɪz.əˈbɪl.ɪ.ti/

How to practise with this video

Listen to the whole video once without speaking and note the words you do not know.
Start at 0.75× speed, shadow it sentence by sentence, then go back to normal speed once it feels easy.
Record yourself and compare with the original, paying attention to words like node, queue, execute.

What is the Shadowing Technique?

Shadowing is a science-backed language learning technique originally developed for professional interpreter training and popularized by polyglot Dr. Alexander Arguelles. The method is simple but powerful: you listen to native English audio and immediately repeat it out loud — like a shadow following the speaker with just a 1–2 second delay. Unlike passive listening or grammar drills, shadowing forces your brain and mouth muscles to simultaneously process and reproduce real speech patterns. Research shows it significantly improves pronunciation accuracy, intonation, rhythm, connected speech, listening comprehension, and speaking fluency — making it one of the most effective methods for IELTS Speaking preparation and real-world English communication.

Shadowing technique: read the full step-by-step guide →

Shadowing Practice: Distributed Task Scheduler: System Design Interview (Stripe & Amazon Offers) - Learn English Speaking with Video

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Vocabulary and speaking notes for this lesson

Key vocabulary in this video

Phrasal verbs you will hear

Grammar in this video

Pronunciation to watch

How to practise with this video

What is the Shadowing Technique?