Cache Storage Quota Limits Under Concurrent Write Requests

Quota Enforcement and Concurrent Writes

Chrome does not have a fixed maximum number of simultaneous cache.put operations that triggers silent data drops. Instead, the Cache API relies on the browser's underlying storage manager to enforce a dynamic quota based on the origin's total allocated disk space (shared with IndexedDB and FileSystem API). When the storage limit is reached, Chrome throws a QuotaExceededError; it does not silently drop writes.

Likely Behavior vs. Confirmed Facts

While the API does not drop writes due to concurrency counts, the asynchronous nature of these operations can create a perceived loss of data. Because cache.put returns a promise, multiple concurrent writes to the same key create a race condition. The final state of the cache depends on which promise resolves last, not the order in which they were initiated.

Confirmed Facts:

• Dynamic Quota: Limits are calculated as a percentage of available disk space, not a static megabyte cap.
• Error Handling: Exceeding the quota triggers a QuotaExceededError.
• Shared Pool: The quota is shared across all storage mechanisms for that origin.

Managing Concurrent Writes

To prevent race conditions and ensure quota stability, follow these implementation steps:

1. Sequentialize Critical Writes: Use a promise chain or an async queue to ensure that writes to the same resource key occur in a deterministic order.
2. Implement Try-Catch Blocks: Always wrap cache.put in a try-catch block to handle QuotaExceededError gracefully, allowing the application to trigger a manual cleanup or notify the user.
3. Monitor Usage: Use the navigator.storage.estimate() method to check remaining quota before initiating large batch writes.

Eviction Policy

Chrome employs a Least Recently Used (LRU) eviction strategy. When the global disk space becomes critically low, the browser may evict entire origins' storage to reclaim space. Concurrent writes do not accelerate this process, but they do increase the total storage footprint, potentially moving the origin closer to the threshold where the browser considers it a candidate for eviction.

Verification and Diagnostics

To verify your current environment's limits, you can run the following in the DevTools console:

const estimate = await navigator.storage.estimate();
console.log(`Usage: ${estimate.usage / 1024 / 1024} MB`);
console.log(`Quota: ${estimate.quota / 1024 / 1024} MB`);

Missing Diagnostic Detail: Are you observing specific data loss (missing keys) or receiving QuotaExceededError exceptions during your concurrent operations?