MongoDB Concurrent Access: Patterns

Tip: Optimistic Concurrency

$collection->updateOne(
    ['_id' => $id, 'version' => $currentVersion],
    ['$set' => $data, '$inc' => ['version' => 1]]
);

Gotcha: Write Conflicts

MongoDB detects write conflicts at the storage engine level and retries automatically.

Tip: Find and Modify

$collection->findOneAndUpdate(
    ['status' => 'pending'],
    ['$set' => ['status' => 'processing']]
);

Atomic operation.

Gotcha: Read Isolation

MongoDB reads are not isolated. A read during a write may see partial results.

Tip: Read Concern

'readConcern' => ['level' => 'majority']

Ensures reads are acknowledged by majority.

Gotcha: Multi-Document Operations

Without transactions, multi-document operations aren't atomic.

Tip: Embed or Reference? The 80/20 Rule

If you always access data together, embed it. If you access it independently, reference it. The 16MB document size limit is the hard boundary — stay under 1MB for most documents.

Tip: Index Your Query Patterns, Not All Fields

Creating indexes on every field wastes RAM. Use explain() to find in-memory sorts and collection scans. Index only what your actual queries filter on.

Gotcha: No Transaction Rollback for Index Builds

Building an index on a large collection can take hours. If it fails midway, the partial index is silently discarded. Plan index builds during maintenance windows.

Senior Insight

MongoDB uses document-level locking (WiredTiger storage engine), which means concurrent writes to different documents don't block each other. This is a significant improvement over the older MMAPv1 engine. But operations on the same document still execute sequentially. For high-contention documents (like counters or balances), I use atomic update operators ($inc, $min, $max) instead of read-modify-write patterns to avoid race conditions.

Source: MongoDB Developer Center (https://www.mongodb.com/developer/), MongoDB Engineering Blog (https://www.mongodb.com/blog/channel/engineering-blog), Studio 3T Blog (https://studio3t.com/blog/)