Merge "Add extra documentation to DiscreteRegistry." into sc-dev am: 321827b7

import java.util.List;

/**

 * This class manages information about recent accesses to ops for

 * permission usage timeline.

 * This class manages information about recent accesses to ops for permission usage timeline.

 *

 * The timeline history is kept for limited time (initial default is 24 hours) and

 * discarded after that.

 * The discrete history is kept for limited time (initial default is 24 hours, set in

 * {@link DiscreteRegistry#sDiscreteHistoryCutoff) and discarded after that.

 *

 * Discrete history is quantized to reduce resources footprint. By default quantization is set to

 * one minute in {@link DiscreteRegistry#sDiscreteHistoryQuantization}. All access times are aligned

 * to the closest quantized time. All durations (except -1, meaning no duration) are rounded up to

 * the closest quantized interval.

 *

 * When data is queried through API, events are deduplicated and for every time quant there can

 * be only one {@link AppOpsManager.AttributedOpEntry}. Each entry contains information about

 * different accesses which happened in specified time quant - across dimensions of

 * {@link AppOpsManager.UidState} and {@link AppOpsManager.OpFlags}. For each dimension

 * it is only possible to know if at least one access happened in the time quant.

 *

 * Every time state is saved (default is 30 minutes), memory state is dumped to a

 * new file and memory state is cleared. Files older than time limit are deleted

 * during the process.

 *

 * When request comes in, files are read and requested information is collected

 * and delivered.

 * and delivered. Information is cached in memory until the next state save (up to 30 minutes), to

 * avoid reading disk if more API calls come in a quick succession.

 *

 * THREADING AND LOCKING:

 * For in-memory transactions this class relies on {@link DiscreteRegistry#mInMemoryLock}. It is

 * assumed that the same lock is used for in-memory transactions in {@link AppOpsService},

 * {@link HistoricalRegistry}, and {@link DiscreteRegistry}.

 * {@link DiscreteRegistry#recordDiscreteAccess(int, String, int, String, int, int, long, long)}

 * must only be called while holding this lock.

 * {@link DiscreteRegistry#mOnDiskLock} is used when disk transactions are performed.

 * It is very important to release {@link DiscreteRegistry#mInMemoryLock} as soon as possible, as

 * no AppOps related transactions across the system can be performed while it is held.

 *

 * INITIALIZATION: We can initialize persistence only after the system is ready

 * as we need to check the optional configuration override from the settings

 * database which is not initialized at the time the app ops service is created. This class

 * relies on {@link HistoricalRegistry} for controlling that no calls are allowed until then. All

 * outside calls are going through {@link HistoricalRegistry}, where

 * {@link HistoricalRegistry#isPersistenceInitializedMLocked()} check is done.

 *

 */

final class DiscreteRegistry {