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Recently, we decided to investigate why our application ARANGOSYNC for synchronizing two ArangoDB clusters across data centers used up a lot of memory – around 2GB in certain cases. The environment contained ~1500 shards with 5000 GOroutines. Thanks to tools like pprof (to profile CPU and memory usage) it was very easy to identify the issue. The GO profiler showed us that memory was allocated in the function time.After() and it accumulated up to nearly 1GB. The memory was not released so it was clear that we had a memory leak. We will explain how memory leaks can occur using the time.After() function through three examples.

Valid usage of the time.After() function

Nothing is wrong with the above code because there is only one possibility when the select statement is finished. When it is done the timer which was created internally in the time.After() function was stopped and resources were freed.

Invalid usage of the time.After() function

It is very tempting to write the following code:

In the above select statement, if the time.After() function is finished everything works like in the first example. But if the ctx.Done() is finished earlier, then the timer which was created in the time.After function is not stopped and resources are not released – causing a memory leak (see the documentation here).

Improved usage of the time.After() function

In production code, one should use time.After() in the following way instead:

Here, one creates a new timer and when it is finished all resources created by the time.NewTimer() are released. In the other case when ctx.Done() occurs before, then resources are released using the delay.Stop() function. It may occur that the ctx.Done() finishes, and immediately afterwards the timer expires. So that is why there is an additional condition \ checking whether the timer has expired or stopped.

I hope that this finding is useful for others, it at least solved our problem immediately. Feel free to leave comments below or ping me on the ArangoDB Community Slack (@tomasz.arangodb)

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