Web applications, a lot of that expense and effort may be related to scaling, which is the capability of a system to handle an increasing amount of work gracefully or to be readily enlarged

1. why background?
App1 App2

2. why background?
App1 App2

3. why background?
App1 App2

4. why background?
Frontend Daemo
n

5. why background?
Frontend Daemon
Frontend Daemon

6. why background?
Frontend Daemon
Frontend CRASH! Daemon

7. why background?
Frontend Daemon
Frontend Daemon

8. why background?
Frontend Daemon
Frontend CRASH! Daemon

9. why background?
Frontend Daemon
Broker
Daemon
Frontend

10. why background?
Fronten
Fronten Daemon
Fronten
d
Fronten
d
Fronten
d
Frontend
d
d
Broker

11. why background?
Daemo
Fronten Daemo
Fronten Daemo
n
Fronten
d Daemo
n
Fronten
d Daemo
n
Fronten
d Daemo
n
Frontend
d Daemon
n
d n
Broker

12. why background?
Daemo
Fronten Daemo
Fronten Daemo
n
Fronten
d Daemo
n
Fronten
d Daemo
n
Fronten
d Daemo
n
Frontend
d Daemon
n
d n
Broker
CRASH!

13. why background?
Frontend Daemon
Frontend Broker Daemon
Staff Staff

14. why background?
Frontend Daemon
Frontend Broker Daemon
CRASH!
Staff Staff

15. why background?
Broker Daemon
Frontend
Workflow Daemon
Frontend
Staff Staff

16. what we have ...
1. various frontends
2. backend daemons
3. JSON messages

17. what we need
1. a way to describe message
flow
2. a easy flexible way to write and
update it

18. what is workflow
1. really only a path definition
through backends
2. some sort of condition
management
3. a backend definition

19. {
_id":"a7y9908898d5...k08b9351063",
"_rev": "6-688067711",
"name": "billing_info",
"type": "workflow",
"user": "adam",
"path": [ "logger", "billing",
"billing_jrcc_jw"]
}

20. where are we it do?
1. staff can call backends as before
2. staff can call workflows via the
workflow queue
3. staff can manage workflows in
Redis

21. problem asynchronous callbacks
1. staff starts a workflow
2. staff calls some external API
3. the external API somehow notifies
us (file attachment, email, URL
callback, ...) about progress
4. staff needs to continue in the
workflow

22. how do staff correlate messages
1. the backend daemon for the
callback polls a view in
2. staff can search for any value in
the request they sent
3. staff can search for the very value
this external API uses as correlation
_id