Do not use global (package-level) variables for mutable state whenever possible. Instead, encapsulate state in structs or pass it as parameters. If you must use a global variable (for configuration or caching), protect it with mutexes if concurrent access is possible and document its usage.
Global mutable state can be modified from anywhere, which makes the program harder to understand, test, and maintain. It can introduce hidden dependencies between components and is prone to concurrency issues. Encapsulating state helps maintain invariants and makes code more modular.
Side-by-side examples engineers can pattern-match during review.
var cache = map[string]string{}
func SetCache(key, value string) {
cache[key] = value // modifies global map, not thread-safe
}type Cache struct {
mu sync.Mutex
data map[string]string
}
func (c *Cache) Set(key, value string) {
c.mu.Lock()
c.data[key] = value
c.mu.Unlock()
}
// or pass *Cache around instead of using a globalvar cache = map[string]string{}
func SetCache(key, value string) {
cache[key] = value // modifies global map, not thread-safe
}type Cache struct {
mu sync.Mutex
data map[string]string
}
func (c *Cache) Set(key, value string) {
c.mu.Lock()
c.data[key] = value
c.mu.Unlock()
}
// or pass *Cache around instead of using a globalFrom the same buckets as this rule.