10 API caching strategies Interview Questions and Answers for api engineers

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1. How do you determine the best caching strategy for an API?

When determining the best caching strategy for an API, I consider a few key factors:

  1. Frequency of updates: If the API's data is updated frequently, a shorter cache time is better to ensure users have access to the most up-to-date information. On the other hand, if the data changes infrequently, a longer cache time can reduce API response times and server load.
  2. Data size and complexity: If the data being served by the API is large and/or complex, a cache can significantly improve response times, but finding a balance between cache time and resource usage is critical.
  3. User behavior: If the API is heavily used by a specific set of users, it may be more efficient to use a personalized cache rather than a shared one. This can reduce server load and offer quicker response times for individual users.

In a recent project, we had an API that had frequently changing data with a complex structure. We used a short-lived cache of 30 seconds to ensure accurate data but also leveraged a content delivery network (CDN) for static resources. This approach reduced server response times by 40% and improved the overall user experience.

2. What types of data do you cache and how do you handle expiration?

A common type of data that is cached using API caching strategies is response data. Response data can include data retrieved from external systems through an API call, such as weather data, financial data or news data. This data can be cached to reduce subsequent API calls, improving performance and reducing server load.

Another type of data that is often cached is metadata. Metadata includes information about the data in the cache, such as when it was last updated and how long it is valid for. This information is important for handling expiration, which is the process of removing stale data from the cache.

Expiration can be handled in a number of ways, including using time-based expiration, where data is automatically removed after a certain period of time, or using event-based expiration, where data is removed when a specific event occurs. For example, data about a stock price might be removed from the cache when a new price is available.

One approach to handling expiration is to use a combination of time-based and event-based expiration. Data that is less likely to change frequently, such as weather data, can be cached for longer periods of time, while data that changes frequently, such as stock prices, can be cached for shorter periods of time and with event-based expiration triggered by new data becoming available.

  1. Response data and metadata are common types of data that are cached.
  2. Expiration can be handled through time-based or event-based expiration.
  3. A combination of time-based and event-based expiration can be used to handle expiration for different types of data.

3. What methods do you use to monitor and maintain your API cache?

As a seasoned API developer, I use a range of strategies to monitor and maintain my API cache to ensure that it remains fast, efficient, and up-to-date. Here are some of the methods I use:

  1. Set up alerts and notifications: I use tools like DataDog and PagerDuty to receive alerts and notifications in real-time whenever there are any issues with the API cache. This helps me to quickly identify and resolve any problems before they cause any serious damage.
  2. Monitor cache hit rates: I regularly monitor the cache hit rate to ensure that the majority of requests are being served from the cache. If the hit rate drops below a certain threshold, this could indicate that the cache needs to be optimized or that there is a problem with the underlying data source.
  3. Monitor cache size: I keep a close eye on the size of the cache to make sure that it doesn't grow too large and start to impact the performance of the API. If the cache size starts to approach its limit, I will consider implementing a cache eviction strategy to remove less frequently used or outdated data.
  4. Monitor cache latency: I monitor the latency of cache requests to ensure that they are being served quickly and efficiently. If the latency starts to spike, this could indicate that there is a problem with the caching infrastructure, and I will investigate the issue further.
  5. Regularly benchmark the API: I use benchmarking tools like Apache JMeter to regularly test the performance of the API, including the cache. This helps me to identify any potential bottlenecks or performance issues before they become a problem.

Using these methods, I have been able to maintain highly performant API caches that greatly improve the overall speed and responsiveness of the API. For example, on a recent project, I was able to increase the cache hit rate from 60% to 95%, resulting in a 50% improvement in API response times, and a corresponding increase in user satisfaction.

4. What are some common API caching pitfalls and how do you avoid them?

Common API caching pitfalls include:

  1. Cache Building: Building a cache with irrelevant data can result in poor performance. It is important to identify data that should be cached and exclude data that do not need to be cached, to avoid this. This can be achieved by conducting tests on data size, relevance and freshness before building the cache.
  2. Cache Expiration: Cached data can expire and become stale if they are not updated regularly. This can lead to incorrect results or unusable data. To avoid this, periodic updating of cached data can be set up. The expiration period should be set according to the relevance of the information.
  3. Cache Invalidation: The cached data may not reflect the updated data present in the database unless the cache is invalidated or cleared. This can lead to inconsistency, errors and poor performance. To overcome this, the cache should be invalidated when new data is added or modified in the database.
  4. Cache Coordination: Large-scale systems often use multiple replicated caches to handle the load. These caches must be coordinated to avoid inconsistencies in data. Coordination can be done using a shared cache, a distributed cache or a clustered cache.
  5. Resource Utilization: Cache data takes up memory and CPU utilization. It is important to strike a balance between the size of the cache and the resources it uses. If the cache consumes too much memory, it can lead to poor system performance. If the cache size is too small, it may not be effective in speeding up performance.

Avoiding these common pitfalls can greatly increase the efficiency and reliability of API caching. It can lead to a faster and more reliable application, improved user experience and scalable functionality. For instance, a recent study found that implementing a well-optimized cache mechanism resulted in a 40% reduction in application latency, which translated into a significant improvement in customer satisfaction and retention rates.

5. How do you optimize cache hit rates for frequently accessed data?

One way to optimize cache hit rates for frequently accessed data is to implement a caching strategy that takes into account the data's expiration time and usage pattern. Here are a few techniques that I employ:

  1. Set the cache expiration time based on the data's volatility. For instance, if the data changes frequently, it's best to set a short expiration time to ensure fresh data is available to users.
  2. Use a sliding expiration mechanism to ensure the cache is kept up-to-date. With this mechanism, the expiration time is extended by a certain amount each time the data is accessed. If the data is accessed frequently, it will stay in the cache longer and reduce load on the backend.
  3. Implement a cache invalidation mechanism to remove stale data from the cache. This can be done either by periodically checking the data for changes or by relying on external events (e.g., user actions) to trigger cache invalidation.
  4. Use a content delivery network (CDN) to cache data closer to the user, reducing latency and improving performance. CDNs have servers located around the world, allowing users to access cached content from a server that is geographically closer to them.
  5. Implement a "cache aside" pattern to reduce the load on the backend. This technique involves retrieving data from the cache first, and only querying the backend if the data is not found in the cache. If the data is found in the cache, the backend is spared from unnecessary requests, reducing load and improving response time.

These strategies have proven to be effective in improving cache hit rates and reducing load on the backend. For instance, by implementing a sliding expiration mechanism for frequently-accessed data, we were able to increase our cache hit rate from 60% to 85%, resulting in a 30% reduction in overall response time.

6. Can you give an example of a time when caching negatively impacted an API’s performance? How did you address it?

A few years back, I worked on an API that was heavily reliant on caching to improve performance. There was a feature that allowed users to search for products based on various criteria such as color, size, and price. In order to speed up these searches, we cached the results of each search query for a certain period of time.

However, we soon noticed that some customers were experiencing incorrect results when searching for products. After some investigation, we discovered that the cached search results were being served up even after the product data had been updated. This meant that some customers were seeing outdated information.

To address this issue, we implemented a more sophisticated caching strategy that took into account when the product data had last been updated. If any changes had been made, the cached results would be invalidated and a fresh search would be performed to ensure the most up-to-date information was being displayed to customers.

After implementing this new caching strategy, we saw a significant improvement in performance and customer satisfaction. The number of complaints about outdated information decreased by 75%, and overall search times improved by 50% since we were no longer serving up stale data.

7. What metrics do you use to evaluate the quality of a caching strategy?

When evaluating the quality of a caching strategy, there are several metrics to consider:

  1. Cache hit rate: This measures the percentage of requests that are served directly from the cache. A higher cache hit rate implies that more requests are being served faster, which is a good indicator of a successful caching strategy. In our latest project, we were able to increase the cache hit rate from 70% to 90%, resulting in up to 50% faster page load times.
  2. Cache expiration rate: This measures the percentage of cached items that expire before being reused. A high expiration rate could indicate that the cache is not being updated frequently enough or that the expiration time is set too low. In a recent performance audit, we noticed that our caching expiration rate was around 40%, so we extended the time-to-live (TTL) for our cache to reduce its expiration rate and improve efficiency.
  3. Cache consistency: This measures whether the cache is returning correct and up-to-date data. A successful caching strategy should ensure that the cached items are consistent with the latest data, avoiding stale content. In our last project, we noticed that a caching strategy was returning outdated results, which led to several failed requests. By updating the cache coherency mechanism, we were able to maintain accurate data and avoid inconsistencies.
  4. Cache eviction: This measures the percentage of cached items that are removed due to limited cache size. A high eviction rate may indicate that the cache is not properly configured to handle high traffic or that the cache size needs to be increased. In our recent project, we optimized our cache eviction policy and increased our cache size by 50%, resulting in less frequent evictions and improved performance.

By carefully monitoring these metrics, we can evaluate the effectiveness of a caching strategy and adjust it as necessary to improve performance and meet our goals.

8. What is your experience with distributed caching and how do you overcome the challenges it poses?

As a software developer, I have extensive experience with distributed caching, having implemented it in several projects for various clients. One particular project had a distributed cache that was used to store frequently accessed data from a database, which resulted in significant performance improvements.

  1. To overcome the challenges of distributed caching, the first step is to ensure that the cache is consistent across all nodes. This can be achieved by using a consensus algorithm such as Raft or Paxos.
  2. Another challenge is ensuring that the cache remains available even if a node fails. To address this, we implemented automatic failover, where if a node goes down, another node takes over its responsibilities.
  3. We also implemented load balancing to distribute the load evenly across all nodes. This prevented any single node from becoming a bottleneck and improved the overall performance of the cache.
  4. Data consistency was also a concern, so we implemented a data synchronization mechanism that ensured that all nodes had the same data.
  5. To measure the effectiveness of our distributed caching solution, we conducted load testing and compared the response times of the application with and without the distributed cache. We found that the distributed cache improved the response times by 50% under high load conditions.

In conclusion, my experience with distributed caching has taught me the importance of addressing the challenges that come with it. I have employed various strategies such as load balancing, automatic failover, and data synchronization to ensure that the cache remains consistent and available, resulting in significant improvements in application performance.

9. How do you handle cache invalidation for APIs with frequently changing data?

There are several caching strategies that can be used for APIs with frequently changing data. One approach is to use a time-based expiration strategy. By setting a cache timeout, the cache is invalidated after a specific time period, regardless of whether the data has changed or not. This ensures that the cached data remains fresh and up-to-date.

Another approach is to use a version-based cache invalidation strategy. This involves including a version number in the API endpoint and comparing it with the cached version. If the versions match, the cached data is assumed to be up-to-date. However, if the versions don't match, the cache is invalidated, and the latest data is fetched from the API.

A third approach is to use a cache tagging strategy. This involves tagging the cached data with a specific tag and invalidating the cache when data with the same tag is updated. This is useful when working with complex datasets that have multiple dependencies.

At my previous job, we implemented a combination of these caching strategies for our APIs with frequently changing data. We set a time-based expiration of 5 minutes and also used versioning to ensure that the cache was invalidated when a new version of the API was released. Additionally, we implemented a cache tagging strategy for our more complex datasets.

As a result of these caching strategies, we were able to improve the performance of our API by up to 50%. The cache allowed us to reduce the number of requests to our servers, which reduced the load on our infrastructure and allowed us to serve more requests with the same resources.

10. What considerations should be taken into account when designing an API cache for high availability?

Designing an API cache for high availability requires careful consideration to ensure that users receive accurate and up-to-date data at all times. Here are some key considerations:

  1. Data Consistency: One of the biggest challenges with caching is ensuring that the cached data is consistent with the data stored in the database. This can be accomplished by using techniques like write-through or write-behind caching, which ensure that any changes to the database are immediately reflected in the cache. It's also important to set an appropriate cache expiration time to avoid serving stale data.
  2. Scalability: As traffic to your API grows, so does the demand on your cache. It's important to ensure that your cache is scalable to handle increasing traffic. This can be accomplished by using techniques like distributed caching or sharding.
  3. Failover: In the event that one of your cache servers goes down, it's critical to have a failover mechanism in place to ensure high availability. This can be accomplished by using techniques like clustering or replication.
  4. Cost: Caching can be expensive, particularly if you're using a cloud-based solution. It's important to carefully consider the cost of your caching solution compared to the benefits it provides in terms of increased reliability and performance.
  5. Security: Caching can introduce security risks, particularly if sensitive data is being cached. It's important to ensure that appropriate security measures are in place to protect the cached data.
  6. Monitoring: To ensure that your caching solution is functioning properly, it's important to monitor key metrics like cache hit rate, cache miss rate, and cache size. This can help you identify potential issues before they become major problems.

In our experience implementing API caching for a high availability website, we were able to significantly reduce response times and server load while maintaining data consistency. Our cache hit rate increased to over 90%, resulting in a 50% reduction in database queries and a 30% increase in overall system performance. By using a combination of write-through caching, sharding, and clustering, we were able to ensure high availability even during periods of high traffic.


Mastering API caching strategies is a crucial skill for any API engineer, and we hope this guide has been helpful to prepare you for your next interview. But the preparation doesn't stop here! To make a great impression on potential employers, don't forget to write a captivating cover letter that showcases your expertise. Check out our guide on writing a cover letter specifically for API engineers:

Crafting an outstanding cover letter

Another important aspect of your job search is to have an impressive resume that highlights your skills and experience. Our guide on how to write a resume for API engineers can help you create an eye-catching document that will catch recruiters' attention.

Preparing a winning API engineer resume

If you're ready to start searching for remote API engineer jobs, be sure to check out our job board, where you'll find plenty of exciting opportunities to put your knowledge of API caching strategies to the test:

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Good luck on your job search, and keep up the good work!
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