Document SDK design agreements

contracts/sdks-performance.md

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+# SDK lifecycle design
+
+This documents specifies a series of design decisions that affect the lifecycle of SDKs across languages.
+
+## Dropping events is acceptable under load
+
+Our SDKs let users subscribe to events emitted by the SDK via a callback.
+This callback can be too slow, or fail for whatever other reason.
+We've reached the decision that it's not acceptable to degrade 
+the performance of the `is_enabled/get_variant` methods for event handling.
+
+The bus of events will be lossy, because of the impact on performance that guaranteeing delivery requires.
+
+## Ready is sent at least once
+
+All SDKs emit a sort of "READY" event. We've decided that we will guarantee at-least-once
+delivery, and that the event will signify that the SDK has hydrated from at least one of its API, bootstrap or backup sources.
+
+## SDK makes no guarantees about functionality before initialize is called
+
+When an SDK provides an initialize method that is expected to guarantee 
+the SDK is in a hydrated and functional state, that SDK will not guarantee
+ proper functioning before that method is called.
+
+We cannot guarantee that any SDK will work before `initialize()` or after `stop()`.