Scaling Memcache at Facebook
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Questions and Answers

What is the primary purpose of using memcache in a web server architecture?

To lighten the read load on databases.

Explain the difference between ‘memcached’ and ‘memcache’ as stated in the text.

‘Memcached’ refers to the source code or running binary, while ‘memcache’ describes the distributed system.

What happens when a web server makes a request for data that is not present in memcache?

The web server retrieves the data directly from the database.

How does the scaling of server clusters relate to memcache and database interactions?

<p>As servers scale into multiple clusters, memcache helps manage data replication between these clusters.</p> Signup and view all the answers

What part of the architecture does Figure 1 illustrate regarding the interactions with memcache?

<p>It illustrates the read path on a cache miss and the write path.</p> Signup and view all the answers

What kind of workload is primarily addressed by utilizing memcache?

<p>A read-heavy workload.</p> Signup and view all the answers

In the context of the discussed architecture, what role does the string key serve?

<p>It is used by the web server to request specific data from memcache.</p> Signup and view all the answers

What is the significance of addressing data replication across clusters in a memcache setup?

<p>It ensures consistency and availability of data across multiple server clusters.</p> Signup and view all the answers

What is the impact of using stale data in applications that rely on memcached?

<p>Using stale data allows applications to continue making progress without waiting for the latest values from the database.</p> Signup and view all the answers

How does splitting requests for keys improve performance in a memcached setup?

<p>By splitting requests for 100 keys into two parallel requests for approximately 50 keys, each server processes a reduced load, enabling better scalability.</p> Signup and view all the answers

What is the advantage of replicating keys to multiple servers in a memcached environment?

<p>Replicating keys to multiple servers allows any replica to handle client requests, which decreases the load on individual servers to 500k requests per second.</p> Signup and view all the answers

What challenges arise from using a shared infrastructure for different application workloads in memcached?

<p>Different access patterns and memory footprints can result in negative interference, leading to decreased hit rates for some applications.</p> Signup and view all the answers

How do pools in a memcached cluster help with accommodating varying application needs?

<p>Pools allow for the partitioning of servers based on applications' specific requirements, helping to improve efficiency and performance.</p> Signup and view all the answers

What happens when the memcached fails to fetch data and how can it affect backend services?

<p>Failure to fetch data from memcached can cause excessive load on backend services, potentially leading to cascading failures.</p> Signup and view all the answers

What are the two scales at which failures must be addressed in a memcached setup?

<p>Failures must be addressed at both the level of a small number of hosts and across the broader cluster.</p> Signup and view all the answers

What role does the wildcard pool play in a memcached clustering strategy?

<p>The wildcard pool serves as the default pool for memcached servers, accommodating keys whose residence might cause issues.</p> Signup and view all the answers

Why does the web server issue a delete request to memcache after write operations?

<p>The delete request invalidates stale data because deletes are idempotent, allowing for easier management of cache consistency.</p> Signup and view all the answers

What design choice was made to address excessive read traffic on MySQL databases?

<p>The decision was made to use memcache as the caching layer to reduce load on the MySQL database.</p> Signup and view all the answers

How does separating the caching layer from the persistence layer benefit system optimization?

<p>It allows independent adjustments to each layer as workload changes, enhancing overall performance and flexibility.</p> Signup and view all the answers

What is the stance on accepting transient stale data according to the design goals mentioned?

<p>The design is willing to expose slightly stale data in exchange for reducing the load on backend storage services.</p> Signup and view all the answers

What role does memcache play beyond caching in the described system?

<p>Memcache is leveraged as a generic key-value store for pre-computed results from machine learning algorithms.</p> Signup and view all the answers

What are the two major design goals prioritized during system evolution?

<p>The goals are to ensure any change impacts a user-facing or operational issue and to optimize for broader scope changes.</p> Signup and view all the answers

Why is memcache not considered the authoritative source of data?

<p>Memcache is not authoritative because it allows eviction of cached data, which means it can serve stale information.</p> Signup and view all the answers

How does the architecture manage updates to non-master regions?

<p>A master region provides a data stream to keep non-master regions up-to-date.</p> Signup and view all the answers

What is the purpose of the 'mcsqueal' daemon in the described architecture?

<p>The 'mcsqueal' daemon inspects SQL statements, extracts deletes, and broadcasts these deletes to memcache deployments.</p> Signup and view all the answers

Why is simply adding more web and memcached servers not effective for scaling?

<p>Adding more servers does not eliminate all problems, as highly requested items can become even more popular, worsening incast congestion.</p> Signup and view all the answers

What percentage of issued deletes actually result in invalidation of cached data?

<p>Only 4% of all deletes issued result in the actual invalidation of cached data.</p> Signup and view all the answers

What does the architecture's division into frontend clusters and a storage cluster address?

<p>It reduces the size of failure domains and creates a tractable network configuration.</p> Signup and view all the answers

What problem arises from having many databases and memcached servers communicating across a cluster boundary?

<p>The problem is an unacceptably high rate of packets sent from the backend cluster to frontend clusters.</p> Signup and view all the answers

How do invalidation daemons optimize the process of sending deletes to memcached servers?

<p>They batch deletes into fewer packets and send them to dedicated servers running mcrouter instances.</p> Signup and view all the answers

What is the main trade-off made by the region architecture in terms of data replication?

<p>The main trade-off is reducing data replication in favor of having more independent failure domains.</p> Signup and view all the answers

Explain the relationship between user traffic and the popularity of requested items in this architecture.

<p>As user traffic increases, highly requested items become more popular, leading to greater demand on the system.</p> Signup and view all the answers

How do leases affect memcache requests compared to TCP windows?

<p>Leases prevent stale sets for all memcache requests independently, while TCP windows apply only to a single stream.</p> Signup and view all the answers

What role do tokens play in memcached servers with leases?

<p>Tokens regulate the rate at which memcached servers return responses, typically issuing a token once every 10 seconds per key.</p> Signup and view all the answers

According to Little's Law, what is the relationship between the number of queued requests and processing time?

<p>Little's Law states that the number of queued requests (L) is proportional to the average time a request takes to process (W).</p> Signup and view all the answers

What impact do leases have on the database query rate during cache misses?

<p>With leases, the peak database query rate was reduced to 1.3K/s compared to 17K/s without leases.</p> Signup and view all the answers

What is a possible consequence of a lower window size in the application?

<p>A lower window size requires more groups of memcache requests to be dispatched serially, increasing the web request duration.</p> Signup and view all the answers

How does the lease mechanism help mitigate the issue of thundering herds?

<p>Leases notify waiting clients to retry requests, ensuring that data is more likely to be present in the cache.</p> Signup and view all the answers

What was observed about web requests waiting to be scheduled without leases?

<p>The time web requests waited to be scheduled indicated a high number of queued requests in the system.</p> Signup and view all the answers

What happens when a client with a lease retrieves a key's value just after a token is issued?

<p>When a client retries a request shortly after a token is issued, it typically finds the data already present in the cache.</p> Signup and view all the answers

What condition necessitates a slab class to increase its memory allocation?

<p>A slab class increases memory allocation if it is evicting items and the next item to be evicted was used at least 20% more recently than the average of the least recently used items in other slab classes.</p> Signup and view all the answers

How does the proposed algorithm differ from other allocators in terms of handling slab classes?

<p>The proposed algorithm focuses on balancing the age of the oldest items among classes rather than solely adjusting eviction rates.</p> Signup and view all the answers

What is the significance of the cumulative distribution figure mentioned in the context of memcached servers?

<p>The cumulative distribution figure illustrates the number of distinct memcached servers accessed, indicating workload characteristics.</p> Signup and view all the answers

What issue arises from data entries in memcache regarding their expiration times?

<p>Entries in memcache may persist in memory well after they have expired, leading to potential inefficiencies.</p> Signup and view all the answers

What common goal do the algorithms discussed seek to achieve in terms of resource management?

<p>The algorithms aim to balance memory usage and eviction rates across slab classes to optimize resource management.</p> Signup and view all the answers

In the context of memory management, why is it beneficial to focus on the age of items rather than just eviction rates?

<p>Focusing on the age of items offers a more nuanced understanding of access patterns, leading to better retention of frequently accessed data.</p> Signup and view all the answers

What does the phrase 'transient item cache' imply regarding the nature of stored data?

<p>The term 'transient item cache' suggests that the data held may only be temporarily relevant and subject to expiration.</p> Signup and view all the answers

What impact does access pattern have on the proposed algorithm's memory management strategy?

<p>Access patterns significantly influence evacuation rates, which the algorithm considers when balancing memory allocation across slab classes.</p> Signup and view all the answers

Study Notes

Scaling Memcache at Facebook

  • Memcached is a widely used in-memory caching solution
  • Facebook uses Memcached to create a distributed key-value store supporting billions of requests per second
  • This system stores trillions of items
  • Popular social networking sites face significant infrastructure challenges including real-time communication, content aggregation (from multiple sources), access/updates to popular content, and scaling to handle millions of requests/second

Introduction

  • Social networks require infrastructure to handle massive concurrent user activity
  • Memcached is a critical component for efficient data access
  • Facebook's system scales from a single cluster to geographically distributed clusters

Overview

  • User consumption of content significantly outweighs creation, favoring caching
  • Data fetching comes from diverse sources (MySQL, HDFS, backend services) requiring a flexible caching strategy
  • Memcached's operations (set, get, delete) are suitable for distributed systems
  • Facebook's implementation builds on the open-source memcached with efficiency improvements and distributed architecture

Latency and Load

  • Memcache latency (hit or miss) is crucial for user experience
  • Facebook focuses on reducing latency by employing mechanisms like parallel requests, batching, and client-side management of communication (UDP/TCP)
  • A sliding window mechanism controls the number of outstanding requests

Reducing Load

  • Leases provide a mechanism to manage stale data and thundering herds
  • Memcached servers are partitioned into pools for diversified workloads (high/low churn)

Replication Within Pools

  • Replication is used to improve latency when the application frequently fetches multiple keys from the same pool
  • Replication strategies depend on the volume of keys and request rates optimizing memory usage

Handling Failures

  • The system includes a mechanism, "Gutter", to handle host failures and divert traffic to redundant hosts
  • This minimizes impact on overall performance

Regional Replication

  • Data is replicated across geographical clusters (regions) for increased availability and reduced latency
  • A critical part of this is the handling of invalidation messages (using daemons) maintaining consistency

Single Server Improvements

  • Utilizing UDP instead of TCP improves initial performance (at the client level) by 13-20% (in the case of testing)
  • An adaptive slab allocator dynamically balances memory usage based on access patterns
  • Enhanced memory efficiency by improving resource usage within the memcached server
  • Optimizations like automatic hash table expansion and multi-threading enhance overall server performance

Memcache Workload

  • Measurements on the production workload show a significant get request volume (i.e hundreds of key requests per user page request)
  • A wide variation in response sizes (key values) is observed
  • Measurements of invalidation latency reveal bottlenecks in the system(for consistency across regions)
  • The design draws from existing distributed systems and caching techniques
  • Memcache's architecture aligns with broader distributed systems research, adapting existing concepts to Facebook's specific demands

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Scaling Memcache at Facebook PDF

Description

Explore how Facebook utilizes Memcached as a critical component of its infrastructure to efficiently handle billions of requests per second. This quiz covers the scaling challenges faced by social networks and the role of Memcached in managing massive concurrent user activity.

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