In this chapter we looked at how concurrency issues occur and how they can cause big problems if the application logic doesn’t account for it. You learned how to mitigate concurrency by denying other processes access to a resource until one process is finished by using locks. You also learned how to work with atomic operations, where you implement changes to the resource as part of a single operation. You should now know how to handle concurrent operations in MongoDB and MySQL.
Next, we look at ways to limit access to different resources. In the next chapter, we get cracking on building the backbone of our authentication scheme.