// Copyright 2020 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
@available(added=7)
library fuchsia.memorypressure;

/// Indicates the memory pressure level.
type Level = strict enum {
    /// The memory pressure level is healthy.
    ///
    /// Registered clients are free to hold on to caches and allocate memory
    /// unrestricted.
    ///
    /// However, clients should take care to not proactively re-create caches on a
    /// transition back to the NORMAL level, causing a memory spike that immediately
    /// pushes the level over to WARNING again.
    NORMAL = 1;

    /// The memory pressure level is somewhat constrained, and might cross over to
    /// the critical pressure range if left unchecked.
    ///
    /// Registered clients are expected to optimize their operation to limit memory
    /// usage, rather than for best performance, for example, by reducing cache sizes
    /// and non-essential memory allocations.
    ///
    /// Clients must take care to regulate the amount of work they undertake in
    /// order to reclaim memory, and ensure that it does not cause visible
    /// performance degradation. There exists some memory pressure, but not enough
    /// to justify trading off user responsiveness to reclaim memory.
    WARNING = 2;

    /// The memory pressure level is very constrained.
    ///
    /// Registered clients are expected to drop all non-essential memory, and refrain
    /// from allocating more memory. Failing to do so might result in the job
    /// getting terminated, or the system being rebooted in the case of global
    /// memory pressure.
    ///
    /// Clients may undertake expensive work to reclaim memory if required, since
    /// failing to do so might result in termination. The client might decide that a
    /// performance hit is a fair tradeoff in this case.
    CRITICAL = 3;
};

/// Registration protocol
@discoverable
protocol Provider {
    /// Used to register for memory pressure level changes.
    /// `watcher`: memory pressure `Watcher` channel that the `Provider` will use to send
    /// level change messages to the client.
    ///
    /// The current memory pressure level is immediately sent to the watcher
    /// when this method is called.
    ///
    /// It is recommended that the root job in a component tree register for changes,
    /// rather than having individual jobs further down the tree register individually.
    /// A low client count will help minimize system churn due to a large number of
    /// memory pressure messages in transit at the same time.
    /// Also, the more context a job has, the better equipped it will be to react to
    /// memory pressure by controlling the behavior of children jobs in its tree.
    RegisterWatcher(resource struct {
        watcher client_end:Watcher;
    });
};

/// Watcher protocol
/// To be implemented by clients who wish to be notified on memory pressure level changes.
protocol Watcher {
    /// Sent to the registered client when the memory pressure level changes.
    /// `level`: indicates the current memory pressure level.
    ///
    /// Will also be invoked on initial connection via `RegisterWatcher`, so that a newly
    /// registered client can discover the current memory pressure level.
    ///
    /// The watcher must immediately reply with a message to acknowledge that it has
    /// received the level change notification, and has initiated required actions as a
    /// result. It may then continue to reclaim memory asynchronously after sending
    /// the acknowledgement.
    ///
    /// Some helpful guidelines for clients:
    /// 1. The watcher will be notified of new pressure level changes only after a reply
    /// corresponding to the previous message has been received by the provider.
    /// If multiple level transitions occur during that time, the watcher will be
    /// notified of the latest pressure level.
    ///
    /// 2. The level changes are edge-triggered, and clients are expected to maintain
    /// local state to track the current pressure level, if required. For example,
    /// a job might be notified of a CRITICAL level and drop all its caches as a result.
    /// Some time after this, it might want to trigger an activity that causes a
    /// fair amount of memory to be allocated. At this point, the job is expected to
    /// remember that the last pressure level it saw was CRITICAL, and refrain from
    /// triggering the memory-intensive activity.
    ///
    /// 3. As a performance optimization, the provider may decide to skip sending
    /// messages for some pressure level changes. For example, when oscillating across
    /// the NORMAL / WARNING boundary, it might not be worth notifying clients of every
    /// single transition. The provider might rate-limit messages in this case.
    /// On a similar note, the provider may decide to send repeated messages at the
    /// same pressure level, particularly CRITICAL, to indicate that further action
    /// needs to be taken.
    OnLevelChanged(struct {
        level Level;
    }) -> ();
};