The issue was a proposal to add LoadAndDelete, and therefore serves as a really convincing example for its necessity. Real world example use-cases for LoadAndDelete and LoadOrStore:Ī comment on the original issue on github proposing to add LoadAndDelete provides a really good example which is discussed below: Or, if possible, the storage can be immutable, in which case no reference from outside will ever be able to modify its value. ![]() Therefore, in cases where the stored data is mutable, we may have to use locks to synchronize the operations performed by the structure. But it cannot control the objects stored within it, the same way it can control the access to them. Therefore, when we return the loaded value after the call to Delete we cannot be certain that there weren’t any updates prior to its call so the returned value could be outdated.Īs mentioned before, async.Map guarantees that the records can be read from and written to atomically. If we have to use Load and Delete to accomplish the above, there is always a possibility that a different goroutine might read a value at some time, and delete it at a later time, in-between which any number of goroutines could have updated the value. The following method guarantees that the returned value (after deleting the record) was the most recent one, before it was deleted. If a method attempts to delete a record and retrieve its most recent value, it would be theoretically impossible without LoadAndDelete unless we use locks to synchronize Load followed by Delete. In the above example, we had used atomic.AddInt64, along with atomic.LoadInt64 to ensure the access to the stored pointer is atomic.Ī similar line of reasoning used for LoadOrStore can be applied to understand why LoadAndDelete will be useful. Sync.Map Guarantees the reads and writes to be thread-safe, but it is the user’s responsibility to make the underlying stored structure thread safe. If locks aren't used to synchronize access to the store, it must be made immutable. Therefore, whenever we may have to use a state transition operation that has its next state dependent on its previous state, the record stored in sync.Map should use an internal structure that uses locks to synchronize updates. Within this time-gap, multiple goroutines could have updated the storage multiple times all of which will be lost when the update completes (following the initial load operation). If we were to just use Load, there would always be a time-gap between calling Load and computing the next state before storing it. If the previous value was value, then the next value held by the key will be value + 1. If there was no record associated with the given key, a pointer to the new record is stored. If Add wasn’t an update operation that depends on the previous state, we might be better off with just using Store. LoadOrStore ensures that a record associated with a key is created for the first time precisely once, regardless of how many goroutines simultaneously tries to create a record. This can only be achieved with LoadAndDelete and LoadOrStore, or we may have to explicitly synchronize calls to Load and Store with locks:Įnter fullscreen mode Exit fullscreen mode Even when these values are updated concurrently, the final value stored by any key will always be accurate. These values are update-able, and lets you add arbitrary values to them. In the following example, the structure Counter is supposed to hold integer values associated with a string. Using sync.Map to store integers that can be incremented or decremented concurrently: The same way, without LoadAndDelete it is impossible for a goroutine to delete and retrieve the most recently stored (or modified) record without using additional locks to synchronize Load and Delete. ![]() Theoretically, without LoadOrStore, it is impossible to have multiple goroutines simultaneously create a record associated with a key precisely once without the use of additional locks. To anyone new to concurrency, it might not seem obvious that just the methods Load, Store and Delete cannot be used to perform the basic read, write and delete operations commonly done using the regular non-thread-safe map from within a single goroutine. Read this to understand how sync.Map works internally:
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