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Guest33873 |
hi |
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Gtron |
Hi all, what will be the target level of the session? |
| 16:58 |
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millrossjez |
evening all :) |
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zakaria |
when does this starts? |
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sjmaple |
starting in 1 min! |
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AgentSmithers |
in one minute? |
| 16:59 |
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sjmaple |
yep |
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zakaria |
cool |
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shelajev |
we're starting in a couple of seconds |
| 17:00 |
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shelajev |
:) all ready and just doing the last sound checks |
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| 17:05 |
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shelajev |
oh, he put me on the spot |
| 17:05 |
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shelajev |
the video runs |
| 17:05 |
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| 17:06 |
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shelajev |
please use the cog icon on the video to pick the best quality you cna |
| 17:06 |
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| 17:06 |
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shelajev |
Sebastian will show some code |
| 17:06 |
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shelajev |
so you'd like to go as high as you can to have a crystal crisp image of that |
| 17:06 |
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zakaria |
cool setup |
| 17:06 |
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zakaria |
how do you put slides behind you |
| 17:07 |
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zakaria |
do you have a projector |
| 17:07 |
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sjmaple |
he has another layer that streams to hangouts :) I'm not sure if his current setup is software or hardware. |
| 17:08 |
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sjmaple |
Sebastien also does a bunch of nighthacking stuff so has some cool kit |
| 17:08 |
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KubaZguba |
CAP theory and CRUD scalablity - is it a joke ? |
| 17:08 |
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zakaria |
awesome:) |
| 17:09 |
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shelajev |
I think it's just xsplit doing picture in picture |
| 17:09 |
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shelajev |
I saw the xsplit logo for a second there |
| 17:10 |
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zakaria |
@shelajev good to know |
| 17:11 |
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| 17:23 |
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shelajev |
oh, here's the link to the RebelLabs survey we mentioned: https://rebellabs.typeform.com/to/IrpKbp |
| 17:24 |
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KubaZguba |
why command part (CmdSrv) has access to DB ? |
| 17:26 |
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| 17:27 |
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Eirini_gr |
because i was a bit late, we assume that the 'order', 'beans' and 'barista' are independent applications? |
| 17:27 |
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KubaZguba |
@Eirini_gr yes |
| 17:28 |
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Eirini_gr |
thank's |
| 17:29 |
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seakayone |
Are Commands shared with the Eventstore, or are they simply http calls? |
| 17:30 |
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| 17:31 |
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seakayone |
why not model as event as well? |
| 17:31 |
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ejan |
Questions: so the query service updates its database when an event arrives and later can answer questions about the data it stores. How's it different than a normal CRUD app. This query service stills creates data in its database and queries it, although it won't update and delete stuff. Just trying to understand the "big difference". |
| 17:32 |
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| 17:32 |
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KubaZguba |
command and events can look similiar but the main difference is that events are something that just happened in the past (was accepted, validated and can be carved in stone) |
| 17:32 |
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Murthy1 |
how are command and query correlated? |
| 17:32 |
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ejan |
Question 2: If everything is calculated off of events, how do we deal with performance issues? For example, if you have many entities with lots and lots of events, calculating their histories would be a real drain on performance. |
| 17:33 |
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| 17:33 |
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KubaZguba |
@ejan you can use snapshoting |
| 17:36 |
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Suman |
@KubaZguba How does snapshoting work? Can you please provide more information on this? |
| 17:37 |
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KubaZguba |
is about aggregating multiple events into a one "entity" - instead of doing it ad hoc you do it on regular basis so you can avoid applying all events that happened from initial beginning |
| 17:38 |
|
KubaZguba |
so to build a "state of entity" you take a snapshot for that entity and apply all "new events" (these which are not in current snapshot) on top of it |
| 17:39 |
|
KubaZguba |
so it is a pure performance optimisation |
| 17:41 |
|
Suman |
@KubaZguba Thanks for the explanation. So this is kind of materialized view which is incrementally updated. |
| 17:41 |
|
KubaZguba |
@Suman exactly |
| 17:42 |
|
KubaZguba |
basically the whole idea behind EventSourcing is instead of store a state of let say entity |
| 17:42 |
|
KubaZguba |
is to process commands to that entity |
| 17:42 |
|
KubaZguba |
and if business rules says ok |
| 17:43 |
|
KubaZguba |
than you know you can change the state - but instead of updating your DB, you publish an event |
| 17:43 |
|
KubaZguba |
this event can be than used by other applications (or Read part of your app) |
| 17:43 |
|
KubaZguba |
but also when new command for same entity (actually i shall call it aggregate ) |
| 17:44 |
|
KubaZguba |
you need to "rebuild" your aggregate state - and you do it by applying all events |
| 17:44 |
|
KubaZguba |
for that aggregate |
| 17:44 |
|
KubaZguba |
if you find the "rebuild" being too slow |
| 17:45 |
|
KubaZguba |
than you go with snapshottinh |
| 17:45 |
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Suman |
@KubaZguba ok. To support multiple query I guess we need to maintain multiple aggregates. |
| 17:45 |
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Suman |
one aggregate per query |
| 17:45 |
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KubaZguba |
queries are different |
| 17:46 |
|
KubaZguba |
as they do not have "aggregates" just projections |
| 17:46 |
|
KubaZguba |
but you are right |
| 17:46 |
|
KubaZguba |
for each query you have really optimised structure (maybe even in different NoSql or SQL database) |
| 17:46 |
|
KubaZguba |
so reads are very fast |
| 17:46 |
|
KubaZguba |
and read models are built from events |
| 17:47 |
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Suman |
Also I guess we need to maintain the snapshot in persistence storage |
| 17:47 |
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| 17:47 |
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Suman |
because in case of system restart |
| 17:47 |
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KubaZguba |
yes |
| 17:47 |
|
Suman |
it will take a lot of time to rebuid the snapshot |
| 17:47 |
|
KubaZguba |
but it depenends on case |
| 17:47 |
|
KubaZguba |
it depends on how many events you have |
| 17:47 |
|
KubaZguba |
in total |
| 17:48 |
|
Suman |
over time number of events will increase |
| 17:48 |
|
KubaZguba |
and for a particualar entity |
| 17:48 |
|
KubaZguba |
sorry aggregate |
| 17:48 |
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| 17:48 |
|
Suman |
so if we want to build the snapshot from scratch |
| 17:48 |
|
Suman |
or the state of entity from scratch |
| 17:48 |
|
Suman |
there will be performance penalty |
| 17:49 |
|
KubaZguba |
yes. but from my experience you do not need to go immediately |
| 17:49 |
|
KubaZguba |
with snapshot |
| 17:49 |
|
KubaZguba |
i saw systems with millions of events |
| 17:49 |
|
angJobs |
hi |
| 17:49 |
|
KubaZguba |
and no snapshotting |
| 17:50 |
|
Suman |
ok. How do we handle system crash |
| 17:50 |
|
Suman |
I mean how we recover once the system comes back from crash |
| 17:50 |
|
Suman |
do we run the logic to build state again |
| 17:50 |
|
KubaZguba |
you reapply events |
| 17:50 |
|
Suman |
ok |
| 17:51 |
|
Suman |
if there are millions of events |
| 17:51 |
|
Suman |
won't it slow down |
| 17:51 |
|
Suman |
? |
| 17:51 |
|
millrossjez |
keep in mind you're talking about the read model, it's entirely legitimate to persist your read model and just mark it with the version |
| 17:51 |
|
Suman |
ok |
| 17:51 |
|
millrossjez |
then as long as you can start your read from a particular point on the timeline, you only have to apply events since your latest version of the read model |
| 17:52 |
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Suman |
@millrossjez thanks. It is clear now |
| 17:54 |
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| 17:55 |
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sjmaple |
any final questions/unanswered questions I can send to Sebastian? |
| 17:56 |
|
demassinner |
What if business logic change a lot and all events needs to be replayed often.. how to adress that? Is CQRS a way forward in that case? |
| 18:02 |
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millrossjez |
how does using Kafka compare with e.g. Greg Young's EventStore software? |
| 18:02 |
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KubaZguba |
Kafka is just an EventStore |
| 18:03 |
|
KubaZguba |
which is like GY event store on steroids |
| 18:03 |
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millrossjez |
how easy is it to select events for just a particular aggregate, I guess, is my question |
| 18:03 |
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| 18:03 |
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KubaZguba |
not so easy in Kafka you can go with Kafka partitions |
| 18:04 |
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shelajev |
sorry about the sound issues |
| 18:04 |
|
sat |
if this example available before this session this would have been better more |
| 18:04 |
|
millrossjez |
thanks @KubaZguba |
| 18:09 |
|
sjmaple |
we've run out of time for questions - but please reach out to Sebastian on twitter for more info or to ask further questions http://twitter.com/DaschnerS |
| 18:11 |
|
jyeary |
thank you for the very nice talk. |
| 18:11 |
|
jyeary |
I like the live code personally too. |
| 18:12 |
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Eirini_gr |
thank you! |
| 18:12 |
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jyeary |
Really pro |
| 18:12 |
|
sat |
thanks |
| 18:12 |
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jyeary |
That looks slick. |
| 18:13 |
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Anonymous10 |
thank you for the session. |
| 18:17 |
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