Design a key-value cache to save the results of the most recent web server queries

Note: This document links directly to relevant areas found in the system design topics to avoid duplication. Refer to the linked content for general talking points, tradeoffs, and alternatives.

Step 1: Outline use cases and constraints

Gather requirements and scope the problem. Ask questions to clarify use cases and constraints. Discuss assumptions.

Without an interviewer to address clarifying questions, we'll define some use cases and constraints.

Use cases

We'll scope the problem to handle only the following use cases

User sends a search request resulting in a cache hit
User sends a search request resulting in a cache miss
Service has high availability

Constraints and assumptions

State assumptions

Traffic is not evenly distributed
- Popular queries should almost always be in the cache
- Need to determine how to expire/refresh
Serving from cache requires fast lookups
Low latency between machines
Limited memory in cache
- Need to determine what to keep/remove
- Need to cache millions of queries
10 million users
10 billion queries per month

Calculate usage

Clarify with your interviewer if you should run back-of-the-envelope usage calculations.

Cache stores ordered list of key: query, value: results
- query - 50 bytes
- title - 20 bytes
- snippet - 200 bytes
- Total: 270 bytes
2.7 TB of cache data per month if all 10 billion queries are unique and all are stored
- 270 bytes per search * 10 billion searches per month
- Assumptions state limited memory, need to determine how to expire contents
4,000 requests per second

Handy conversion guide:

2.5 million seconds per month
1 request per second = 2.5 million requests per month
40 requests per second = 100 million requests per month
400 requests per second = 1 billion requests per month

Step 2: Create a high level design

Outline a high level design with all important components.

Step 3: Design core components

Dive into details for each core component.

Use case: User sends a request resulting in a cache hit

Popular queries can be served from a Memory Cache such as Redis or Memcached to reduce read latency and to avoid overloading the Reverse Index Service and Document Service. Reading 1 MB sequentially from memory takes about 250 microseconds, while reading from SSD takes 4x and from disk takes 80x longer.¹

Since the cache has limited capacity, we'll use a least recently used (LRU) approach to expire older entries.

The Client sends a request to the Web Server, running as a reverse proxy
The Web Server forwards the request to the Query API server
The Query API server does the following:
- Parses the query
  - Removes markup
  - Breaks up the text into terms
  - Fixes typos
  - Normalizes capitalization
  - Converts the query to use boolean operations
- Checks the Memory Cache for the content matching the query
  - If there's a hit in the Memory Cache, the Memory Cache does the following:
    - Updates the cached entry's position to the front of the LRU list
    - Returns the cached contents
  - Else, the Query API does the following:
    - Uses the Reverse Index Service to find documents matching the query
      - The Reverse Index Service ranks the matching results and returns the top ones
    - Uses the Document Service to return titles and snippets
    - Updates the Memory Cache with the contents, placing the entry at the front of the LRU list

Cache implementation

The cache can use a doubly-linked list: new items will be added to the head while items to expire will be removed from the tail. We'll use a hash table for fast lookups to each linked list node.

Clarify with your interviewer how much code you are expected to write.

Query API Server implementation:

class QueryApi(object):

    def __init__(self, memory_cache, reverse_index_service):
        self.memory_cache = memory_cache
        self.reverse_index_service = reverse_index_service

    def parse_query(self, query):
        """Remove markup, break text into terms, deal with typos,
        normalize capitalization, convert to use boolean operations.
        """
        ...

    def process_query(self, query):
        query = self.parse_query(query)
        results = self.memory_cache.get(query)
        if results is None:
            results = self.reverse_index_service.process_search(query)
            self.memory_cache.set(query, results)
        return results

Node implementation:

class Node(object):

    def __init__(self, query, results):
        self.query = query
        self.results = results

LinkedList implementation:

class LinkedList(object):

    def __init__(self):
        self.head = None
        self.tail = None

    def move_to_front(self, node):
        ...

    def append_to_front(self, node):
        ...

    def remove_from_tail(self):
        ...

Cache implementation:

class Cache(object):

    def __init__(self, MAX_SIZE):
        self.MAX_SIZE = MAX_SIZE
        self.size = 0
        self.lookup = {}  # key: query, value: node
        self.linked_list = LinkedList()

    def get(self, query)
        """Get the stored query result from the cache.

        Accessing a node updates its position to the front of the LRU list.
        """
        node = self.lookup[query]
        if node is None:
            return None
        self.linked_list.move_to_front(node)
        return node.results

    def set(self, results, query):
        """Set the result for the given query key in the cache.

        When updating an entry, updates its position to the front of the LRU list.
        If the entry is new and the cache is at capacity, removes the oldest entry
        before the new entry is added.
        """
        node = self.lookup[query]
        if node is not None:
            # Key exists in cache, update the value
            node.results = results
            self.linked_list.move_to_front(node)
        else:
            # Key does not exist in cache
            if self.size == self.MAX_SIZE:
                # Remove the oldest entry from the linked list and lookup
                self.lookup.pop(self.linked_list.tail.query, None)
                self.linked_list.remove_from_tail()
            else:
                self.size += 1
            # Add the new key and value
            new_node = Node(query, results)
            self.linked_list.append_to_front(new_node)
            self.lookup[query] = new_node

When to update the cache

The cache should be updated when:

The page contents change
The page is removed or a new page is added
The page rank changes

The most straightforward way to handle these cases is to simply set a max time that a cached entry can stay in the cache before it is updated, usually referred to as time to live (TTL).

Refer to When to update the cache for tradeoffs and alternatives. The approach above describes cache-aside.

Step 4: Scale the design

Identify and address bottlenecks, given the constraints.

Important: Do not simply jump right into the final design from the initial design!

State you would 1) Benchmark/Load Test, 2) Profile for bottlenecks 3) address bottlenecks while evaluating alternatives and trade-offs, and 4) repeat. See Design a system that scales to millions of users on AWS as a sample on how to iteratively scale the initial design.

It's important to discuss what bottlenecks you might encounter with the initial design and how you might address each of them. For example, what issues are addressed by adding a Load Balancer with multiple Web Servers? CDN? Master-Slave Replicas? What are the alternatives and Trade-Offs for each?

We'll introduce some components to complete the design and to address scalability issues. Internal load balancers are not shown to reduce clutter.

To avoid repeating discussions, refer to the following system design topics for main talking points, tradeoffs, and alternatives:

Expanding the Memory Cache to many machines

To handle the heavy request load and the large amount of memory needed, we'll scale horizontally. We have three main options on how to store the data on our Memory Cache cluster:

Each machine in the cache cluster has its own cache - Simple, although it will likely result in a low cache hit rate.
Each machine in the cache cluster has a copy of the cache - Simple, although it is an inefficient use of memory.
The cache is sharded across all machines in the cache cluster - More complex, although it is likely the best option. We could use hashing to determine which machine could have the cached results of a query using machine = hash(query). We'll likely want to use consistent hashing.

Additional talking points

Additional topics to dive into, depending on the problem scope and time remaining.

SQL scaling patterns

NoSQL

Caching

Where to cache
What to cache
- Caching at the database query level
- Caching at the object level
When to update the cache

Asynchronism and microservices

Communications

Discuss tradeoffs:
- External communication with clients - HTTP APIs following REST
- Internal communications - RPC
Service discovery

Security

Refer to the security section.

Latency numbers

See Latency numbers every programmer should know.

Ongoing

Continue benchmarking and monitoring your system to address bottlenecks as they come up
Scaling is an iterative process

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