Question: How often is cache saved to the database?

Answer

Caching is a technique used to speed up data retrieval by storing copies of frequently accessed data in memory. The key question regarding caching and databases is, how often is cache saved to the database? The answer depends on the cache strategy and system design.

Write-Through Cache

With write-through caching, data is written into the cache and the corresponding database simultaneously. This approach ensures data consistency but can lead to higher latency for write operations since both the cache and the database need to be updated at the same time.

# Pseudocode example for write-through cache
def save_data(key, value):
    cache.set(key, value)  # Update the cache
    database.save(key, value)  # Update the database simultaneously

Write-Around Cache

Write-around caching involves writing data directly to the database, bypassing the cache. This method reduces cache pollution with data that might not be read again soon but may result in slower read operations if the data is accessed shortly after being written, as it has to be fetched from the slower backend storage.

# Pseudocode example for write-around cache
def save_data(key, value):
    database.save(key, value)  # Data is written directly to the database

Write-Back (Write-Behind) Cache

In a write-back or write-behind scenario, writes are directed to the cache first, and the cache asynchronously saves the data to the database after a certain condition is met (e.g., after a specific time interval, when the cache reaches a certain size, or before eviction of the cached data). This approach improves write performance but risks data loss if the cache fails before it persists data to the database.

# Pseudocode example for write-back cache
def save_data(key, value):
    cache.set(key, value)  # Update the cache
    # Data will be saved to the database asynchronously based on a condition

# Possible asynchronous operation
def persist_data():
    if condition_is_met:
        for key, value in cache.items():
            database.save(key, value)
            cache.remove(key)  # Optional: remove the item after saving to the database

Conclusion

The frequency at which cached data is saved to the database highly depends on the caching strategy employed by the application. Each strategy has its trade-offs in terms of performance, complexity, and data safety. System architects and developers must choose the most appropriate caching strategy based on their specific application requirements, data consistency needs, and tolerance for data loss.