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Member functions and free functions on Ledger and LedgerEntrySet are rewritten in terms of new abstract interfaces `BasicView` and `View`, representing the set of non-decomposable primitives necessary to read and write state map items in a ledger, and to overlay a discardable view onto a Ledger that can calculate metadata during transaction processing. const-correctness is enforced through the parameter and return types. The MetaView now supports multi-level stacking: A MetaView can be stacked on top of either a Ledger or another MetaView, up to any number of levels. The getSLEi member function is removed. The CachedView wrapper replaces it, wrapping a View such that any function called with a CachedView will go through the SLECache. * Add BasicView, View, CachedView * Rename LedgerEntrySet to MetaView * Factor out free functions * Consolidate free functions in ViewAPI * Remove unused class members and free functions
How to use RPC coroutines.
Introduction.
By default, an RPC handler runs as an uninterrupted task on the JobQueue. This is fine for commands that are fast to compute but might not be acceptable for tasks that require multiple parts or are large, like a full ledger.
For this purpose, the rippled RPC handler allows suspension with continuation
- a request to suspend execution of the RPC response and to continue it after some function or job has been executed. A default continuation is supplied which simply reschedules the job on the JobQueue, or the programmer can supply their own.
The classes.
Suspension with continuation uses four std::functions in the ripple::RPC
namespace:
using Callback = std::function <void ()>;
using Continuation = std::function <void (Callback const&)>;
using Suspend = std::function <void (Continuation const&)>;
using SuspendCallback = std::function <void (Suspend const&)>;
A Callback is a generic 0-argument function. A given Callback might or might
not block. Unless otherwise advised, do not hold locks or any resource that
would prevent any other task from making forward progress when you call a
Callback.
A Continuation is a function that is given a Callback and promises to call
it later. A Continuation guarantees to call the Callback exactly once at
some point in the future, but it does not have to be immediately or even in the
current thread.
A Suspend is a function belonging to a Coroutine. A Suspend runs a
Continuation, passing it a Callback that continues execution of the
Coroutine.
And finally, a SuspendCallback is a std::function which is given a
Suspend. This is what the RPC handler gives to the coroutine manager,
expecting to get called back with a Suspend and to be able to start execution.
The flow of control.
Given these functions, the flow of RPC control when using coroutines is straight-forward.
-
The instance of
ServerHandlerreceives an RPC request. -
It creates a
SuspendCallbackand gives it to the coroutine manager. -
The coroutine manager creates a
Coroutine, starts it up, and then calls theSuspendCallbackwith aSuspend. -
Now the RPC response starts to be calculated.
-
When the RPC handler wants to suspend, it calls the
Suspendfunction with aContinuation. -
Coroutine execution is suspended.
-
The
Continuationis called with aCallbackthat the coroutine manager creates. -
The
Continuationmay choose to execute immediately, defer execution on the job queue, or wait for some resource to be free. -
When the
Continuationis finished, it calls theCallbackthat the coroutine manager gave it, perhaps a long time ago. -
This
Callbackcontinues execution on the suspendedCoroutinefrom where it left off.