I think that Akka Persistence lacks in some areas that are essential for
using CQRS
with DDD aggregates effectively (without a lot of pain):
1) Ability to read persistent actor messages by type of Actor. This can be
as simple
as a key prefix that represents a "category/tag/topic",
e.g. <topic>-<persistenceId>. Essentially an embedded secondary index.
2) As there is no global sequence number for events across all messages in
the same
way there is for messages of a single persistent actor; there should be
a way to track the logical positions of messages. GetEventStore uses a
similar
approach.
1: Gives us ability to read across all events from a persistent actor
family, e.g. customers.
2: Gives us ability, by way of 1, to do catch-up-subscription of all
events from persistent
actors of same type, say customers, and keep track of position. This is
very useful for
maintaining a read model and integration with other systems.
I have tried many approaches and to mention a few:
a) One persistent actor for a "topic".
b) Join streams using AtLeastOnceDelivery in individual persistent actors
to
a "topic" aggregator.
c) Using a persistent actor as a journal for a "topic" and replaying that
into views, filtering events produced by "topic" siblings.
All of the above introduce unnecessary complications and work-around
gymnastics such as
dealing with state size and archival (a), worrying about out-of-order (b)
and using
background workers that move views by way of snapshotting forward (c). All
of this
because AP does not provide a way to achieve 1) and 2).
>From my point of view it is evident that AP lacks some fundamental and
essential
capabilities in order to get a decent CQRS/DDD/ES setup.
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