Caching POST requests

Challenge and the Ultimate Solution

Introduction

In the realm of web communication, it is important to recognize the contrasting nature of caching between GET and POST requests. By default, GET requests are eligible for caching, enabling browsers to store responses locally, thus promoting faster access to frequently requested resources. Conversely, POST requests, being more sensitive and often involving data submission, are not eligible for caching. The reason behind this distinction lies in the inherent differences between the two request methods. GET requests are designed to retrieve data from the server without causing any side effects, making them suitable for caching as they have no impact on the server state. On the other hand, POST requests involve the submission of data, potentially altering server data or triggering actions, rendering them inappropriate for caching to ensure data integrity and security.

Although POST requests are generally not cached because of the possible negative consequences they may have on the server, there may be unique circumstances when caching POST requests is required. In these situations, developers may use a number of techniques to allow caching securely without compromising the security or integrity of the data. Implementing cache-control headers, which list the suitable caching directives for POST replies, is one strategy. This gives developers additional control over caching behaviour by enabling them to specify caching criteria or expiry durations. However, care must be taken, and the nature of the application and any possible repercussions of caching POST requests should be carefully taken into account. To respect the non-idempotence principle, which prevents the same request from leading to different results and preserves the system’s dependability and consistency, caching POST requests is generally discouraged.

Why POST Requests Are Not Cached by Default?

1. Idempotency: The primary reason why POST requests are not cached by default is their idempotency. Unlike GET requests, which are designed to be safe and read-only, POST requests are intended for actions that can have side effects on the server. Caching a non-idempotent POST request can lead to unexpected and undesired consequences, as the same request might produce different outcomes if executed multiple times.
2. Data Safety: POST requests often carry sensitive information, such as user credentials or transactional data. Caching such data poses security risks, as cached responses might be accessible to other users or even malicious attackers.
3. Resourcefulness: POST requests usually include a payload with data that can be substantial, like form submissions or file uploads. Storing large cached responses could consume significant memory resources, potentially degrading the performance benefits of caching.

The Solution: Caching POST Requests with In-Memory Cache

Despite the default behaviour, there are cases where caching POST requests can greatly benefit performance and user experience. By following the steps discussed in a previous chat, we can implement an in-memory cache solution that overcomes the challenges of caching POST requests.

1. Client-Side Caching: On the client-side, we can leverage in-memory caching to store responses from POST requests. A custom cache class can be implemented to generate unique cache keys based on request parameters. This ensures that different requests are cached separately, preventing unintended side effects.
2. Server-Side Caching: The server-side implementation involves using Spring Boot’s built-in caching support. By annotating specific methods with @CachePut and @Cacheable, we can store and fetch cached POST responses based on unique cache keys. The cache manager can be configured to set expiration times or use eviction policies to manage the cached data efficiently.

Let’s focus on what can be done on client side (Mobile app)

Remember:- Always Cache Idempotent POST Requests: meaning they produce the same result for the same set of input parameters.

1. In-Memory Caching: In iOS, you can use in-memory caching to store responses from POST requests. One way to do this is by using a dictionary or a cache library to store the responses with unique keys based on the request parameters.
2. Custom Caching Logic: Implement custom caching logic in your app to determine when and for how long the cached POST responses should be considered valid. You can set a time limit or use conditional caching techniques.
3. Cache Busting: If you need to handle different entities or resources using the same POST endpoint, consider using cache busting techniques like appending a timestamp or a unique identifier to the request URL to differentiate the requests.

Sample code to generate a cache key based on the request parameters. For the sake of simplicity, let’s assume you have a dictionary of request parameters and you want to serialise it into a unique cache key:

func generateCacheKeyForRequest(_ requestParameters: [String: Any]) -> String {
    // Sort the request parameters by keys to ensure consistent cache keys
    let sortedParameters = requestParameters.sorted { $0.key < $1.key }
    
    // Create an array of key-value pairs in the format "key=value"
    let keyValuePairs = sortedParameters.map { "\($0.key)=\($0.value)" }
    
    // Concatenate all key-value pairs with a delimiter (e.g., "&")
    let cacheKey = keyValuePairs.joined(separator: "&")
    
    // You can also consider hashing the cache key for added security or to limit the length
    // of the cache key in case it becomes too long.
    // Example using SHA256 hashing:
    // return SHA256.hash(data: Data(cacheKey.utf8)).compactMap { String(format: "%02x", $0) }.joined()
    
    return cacheKey
}

Steps to implement the solution

1. Create a Cache Class: Start by creating a custom cache class that will handle caching and retrieving responses. This class can be a singleton to ensure a single instance is used throughout the app.

import Foundation

class ResponseCache {
    static let shared = ResponseCache()
    private var cache: [String: Data] = [:]
    
    private init() {}
    
    func cacheResponse(forKey key: String, data: Data) {
        cache[key] = data
    }
    
    func getCachedResponse(forKey key: String) -> Data? {
        return cache[key]
    }
}

2. Retrieve Cached Responses: Before making a new POST request, check if the response is already cached. If a cached response exists, use it instead of making the request.

func checkCachedResponse() {
    let cacheKey = generateCacheKeyForRequest(requestParameters) // Implement this function to generate a unique cache key based on request parameters.
    if let cachedData = ResponseCache.shared.getCachedResponse(forKey: cacheKey) {
        // Use the cached response instead of making the POST request
        processCachedResponse(cachedData)
    } else {
        // Make the POST request as usual
        makePostRequest()
    }
}

3. Cache Responses: After making a POST request and receiving a response, cache the response using the cache class. Use a unique key based on the request parameters to identify the response in the cache.

func makePostRequest() {
    // Your POST request code here
    // ...

    if let responseData = response.data {
        let cacheKey = generateCacheKeyForRequest(requestParameters) // Implement this function to generate a unique cache key based on request parameters.
        ResponseCache.shared.cacheResponse(forKey: cacheKey, data: responseData)
    }
}

Remember that in-memory caching is temporary and will be cleared when the app is terminated or the device restarts. If you need persistent caching, consider using other mechanisms like CoreData or FileManager to save the cached responses to disk.

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