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Merge pull request #25916 from dwijnand/tweak-persistence-doc-intro

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Tweak the introduction to akka persistence
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Patrik Nordwall 2018-11-30 16:43:54 +01:00 • committed by GitHub
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18
akka-docs/src/main/paradox/persistence.md
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@ -30,13 +30,13 @@ to see what this looks like in practice.
## Introduction ## Introduction
Akka persistence enables stateful actors to persist their internal state so that it can be recovered when an actor Akka persistence enables stateful actors to persist their state so that it can be recovered when an actor
is started, restarted after a JVM crash or by a supervisor, or migrated in a cluster. The key concept behind Akka is either restarted, such as after a JVM crash, by a supervisor or a manual stop-start, or migrated within a cluster. The key concept behind Akka
persistence is that only changes to an actor's internal state are persisted but never its current state directly persistence is that only the _events_ received by the actor are persisted, not the actual state of the actor
(except for optional snapshots). These changes are only ever appended to storage, nothing is ever mutated, which (though actor state snapshot support is also available). The events are persisted by appending to storage (nothing is ever mutated) which
allows for very high transaction rates and efficient replication. Stateful actors are recovered by replaying stored allows for very high transaction rates and efficient replication. A stateful actor is recovered by replaying the stored
changes to these actors from which they can rebuild internal state. This can be either the full history of changes events to the actor, allowing it to rebuild its state. This can be either the full history of changes
or starting from a snapshot which can dramatically reduce recovery times. Akka persistence also provides point-to-point or starting from a checkpoint in a snapshot which can dramatically reduce recovery times. Akka persistence also provides point-to-point
communication with at-least-once message delivery semantics. communication with at-least-once message delivery semantics.
@@@ note @@@ note
@ -59,14 +59,14 @@ concepts and architecture of [eventsourced](https://github.com/eligosource/event
* @scala[`PersistentActor`]@java[`AbstractPersistentActor`]: Is a persistent, stateful actor. It is able to persist events to a journal and can react to * @scala[`PersistentActor`]@java[`AbstractPersistentActor`]: Is a persistent, stateful actor. It is able to persist events to a journal and can react to
them in a thread-safe manner. It can be used to implement both *command* as well as *event sourced* actors. them in a thread-safe manner. It can be used to implement both *command* as well as *event sourced* actors.
When a persistent actor is started or restarted, journaled messages are replayed to that actor so that it can When a persistent actor is started or restarted, journaled messages are replayed to that actor so that it can
recover internal state from these messages. recover its state from these messages.
* @scala[`AtLeastOnceDelivery`]@java[`AbstractPersistentActorAtLeastOnceDelivery`]: To send messages with at-least-once delivery semantics to destinations, also in * @scala[`AtLeastOnceDelivery`]@java[`AbstractPersistentActorAtLeastOnceDelivery`]: To send messages with at-least-once delivery semantics to destinations, also in
case of sender and receiver JVM crashes. case of sender and receiver JVM crashes.
* `AsyncWriteJournal`: A journal stores the sequence of messages sent to a persistent actor. An application can control which messages * `AsyncWriteJournal`: A journal stores the sequence of messages sent to a persistent actor. An application can control which messages
are journaled and which are received by the persistent actor without being journaled. Journal maintains `highestSequenceNr` that is increased on each message. are journaled and which are received by the persistent actor without being journaled. Journal maintains `highestSequenceNr` that is increased on each message.
The storage backend of a journal is pluggable. The persistence extension comes with a "leveldb" journal plugin, which writes to the local filesystem. The storage backend of a journal is pluggable. The persistence extension comes with a "leveldb" journal plugin, which writes to the local filesystem.
Replicated journals are available as [Community plugins](http://akka.io/community/). Replicated journals are available as [Community plugins](http://akka.io/community/).
* *Snapshot store*: A snapshot store persists snapshots of a persistent actor's internal state. Snapshots are * *Snapshot store*: A snapshot store persists snapshots of a persistent actor's state. Snapshots are
used for optimizing recovery times. The storage backend of a snapshot store is pluggable. used for optimizing recovery times. The storage backend of a snapshot store is pluggable.
The persistence extension comes with a "local" snapshot storage plugin, which writes to the local filesystem. Replicated snapshot stores are available as [Community plugins](http://akka.io/community/) The persistence extension comes with a "local" snapshot storage plugin, which writes to the local filesystem. Replicated snapshot stores are available as [Community plugins](http://akka.io/community/)
* *Event sourcing*. Based on the building blocks described above, Akka persistence provides abstractions for the * *Event sourcing*. Based on the building blocks described above, Akka persistence provides abstractions for the