Observe.Event.Backend

This is the primary module needed to write new EventBackends.

Core interface

newtype EventBackend m r s

A backend for creating Events.

Different EventBackends will be used to emit instrumentation to different systems. Multiple backends can be combined with pairEventBackend.

A simple EventBackend for logging to a Handle can be created with jsonHandleBackend.

Typically the entrypoint for some eventuo11y-instrumented code will take an EventBackend, polymorphic in r and possibly m. Calling code can use subEventBackend to place the resulting events in its hierarchy.

From an EventBackend, new events can be created via selectors (of type s f for some field type f), typically with the resource-safe allocation functions. Selectors are values which designate the general category of event being created, as well as the type of fields that can be added to it. For example, a web service's selector type may have a ServicingRequest constructor, whose field type includes a ResponseCode constructor which records the HTTP status code.

Selectors are intended to be of a domain specific type per unit of functionality within an instrumented codebase, implemented as a GADT (but see DynamicEventSelector for a generic option).

Implementations must ensure that EventBackends and their underlying Events are safe to use across threads.

m

The monad we're instrumenting in.

r

The type of event references used in this EventBackend. See reference.

s

The type of event selectors. See newEvent.

Constructors

EventBackend

Fields

• newEvent :: forall f. s f -> m (Event m r f)

  Create a new Event, selected by the given selector.

  The selector specifies the category of new event we're creating, as well as the type of fields that can be added to it (with addField).

  Selectors are intended to be of a domain specific type per unit of functionality within an instrumented codebase, implemented as a GADT (but see DynamicEventSelector for a generic option).

  Consider the resource-safe event allocation functions instead of calling this directly.

data Event m r f

An instrumentation event.

Events are the core of the instrumenting user's interface to eventuo11y. Typical usage would be to create an Event using withEvent and add fields to the Event at appropriate points in your code with addField.

m

The monad we're instrumenting in.

r

The type of event references. See reference.

f

The type of fields on this event. See addField.

Constructors

Event

Fields

• reference :: !r

  Obtain a reference to an Event.

  References are used to link Events together, via addReference.

  References can live past when an event has been finalized.

  Code being instrumented should always have r as an unconstrained type parameter, both because it is an implementation concern for EventBackends and because references are backend-specific and it would be an error to reference an event in one backend from an event in a different backend.

• addField :: !(f -> m ())

  Add a field to an Event.

  Fields make up the basic data captured in an event. They should be added to an Event as the code progresses through various phases of work, and can be both milestone markers ("we got this far in the process") or more detailed instrumentation ("we've processed N records").

  They are intended to be of a domain specific type per unit of functionality within an instrumented codebase (but see DynamicField for a generic option).

• addReference :: !(Reference r -> m ())

  Relate another Event to this Event in the specified way

• finalize :: !(Maybe SomeException -> m ())

  Mark an Event as finished, perhaps due to an Exception.

  In normal usage, this should be automatically called via the use of the resource-safe event allocation functions.

  This is a no-op if the Event has already been finalized. As a result, it is likely pointless to call addField or addReference (or addParent / addProximate) after this call, though it still may be reasonable to call reference.

data Reference r

A reference to another Event

Constructors

Reference !ReferenceType !r

data ReferenceType

Ways in which Events can Reference each other.

Constructors

Parent

The Referenced Event is a parent of this Event.

Proximate

The Referenced Event is a proximate cause of this Event.

Instances

Eq ReferenceType

Defined in Observe.Event.Backend

Methods

(==) :: ReferenceType -> ReferenceType -> Bool

(/=) :: ReferenceType -> ReferenceType -> Bool

Backend composition

unitEventBackend :: Applicative m => EventBackend m () s

A no-op EventBackend.

This can be used if calling instrumented code from an un-instrumented context, or to purposefully ignore instrumentation from some call.

unitEventBackend is the algebraic unit of pairEventBackend.

pairEventBackend :: Applicative m => EventBackend m a s -> EventBackend m b s -> EventBackend m (a, b) s

An EventBackend which sequentially generates Events in the two given EventBackends.

This can be used to emit instrumentation in multiple ways (e.g. logs to grafana and metrics on a prometheus HTML page).

noopEventBackend :: Applicative m => r -> EventBackend m r s

A no-op EventBackend that can be integrated with other backends.

This can be used to purposefully ignore instrumentation from some call.

All events will have the given reference, so can be connected to appropriate events in non-no-op backends, but not in a way that can distinguish between different events from the same no-op backend.

Backend transformation

hoistEventBackend :: Functor m => (forall x. m x -> n x) -> EventBackend m r s -> EventBackend n r s

Hoist an EventBackend along a given natural transformation into a new monad.

hoistEvent :: (forall x. m x -> n x) -> Event m r f -> Event n r f

Hoist an Event along a given natural transformation into a new monad.

type InjectSelector s t = forall f. s f -> forall a. (forall g. t g -> (f -> g) -> a) -> a

Inject a narrower selector and its fields into a wider selector.

See injectSelector for a simple way to construct one of these.

injectSelector :: (forall f. s f -> t f) -> InjectSelector s t

Construct an InjectSelector with a straightforward injection from s to t

idInjectSelector :: InjectSelector s s

The identity InjectSelector

narrowEventBackend :: Functor m => InjectSelector s t -> EventBackend m r t -> EventBackend m r s

Narrow an EventBackend to a new selector type via a given injection function.

A typical usage, where component A calls component B, would be to have A's selector type have a constructor to take any value of B's selector type (and preserve the field) and then call narrowEventBackend with that constructor when invoking functions in B.

setDefaultReferenceEventBackend :: PrimMonad m => Reference r -> EventBackend m r s -> EventBackend m r s

Transform an EventBackend so all of its Events have a given Reference, if they haven't been given a Reference of the same ReferenceType by the time they are finalized.

See setReferenceEventBackend if the Reference should be applied unconditionally.

setAncestorEventBackend :: PrimMonad m => r -> EventBackend m r s -> EventBackend m r s

Transform an EventBackend so all of its Events have a given parent, if they are not given another parent by the time they are finalized.

See setParentEventBackend if the parent should be set unconditionally.

setInitialCauseEventBackend :: PrimMonad m => r -> EventBackend m r s -> EventBackend m r s

Transform an EventBackend so all of its Events have a given proximate cause, if they are not given another proximate cause by the time they are finalized.

See setProximateEventBackend if the proximate cause should be set unconditionally.

setReferenceEventBackend :: Monad m => Reference r -> EventBackend m r s -> EventBackend m r s

Transform an EventBackend so all of its Events have a given Reference.

You likely want setDefaultReferenceEventBackend, if your monad supports it.

setParentEventBackend :: Monad m => r -> EventBackend m r s -> EventBackend m r s

Transform an EventBackend so all of its Events have a given parent.

You likely want setAncestorEventBackend, if your monad supports it.

setProximateEventBackend :: Monad m => r -> EventBackend m r s -> EventBackend m r s

Transform an EventBackend so all of its Events have a given proximate cause.

You likely want setInitialCauseEventBackend, if your monad supports it.