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src/sa/model/decisions/0008-heartbeat-as-monitor.madr
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src/sa/model/decisions/0008-heartbeat-as-monitor.madr
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## ADR #0008: Heartbeat monitors are used to monitor availability
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1. **What did you decide?**
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An Heartbeat monitor shall be used to monitor availability of the engine components of SmartHut.
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2. **What was the context for your decision?**
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Given the existent of three different engine components, the availability monitor strategy shall
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minimize network traffic so at not to overload the network between the three components. The chosen
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strategy shall also be able to effectively give a reasonable indication of availability of each engine
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component of SmartHut, so that the health of the SaaS deployment (and even of the user-controlled
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deployments) may be checked by a dashboard page.
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3. **What is the problem you are trying to solve?**
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Which is the most effective and network-concious technique to monitor availability of engine components of SmartHut?
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4. **Which alternative options did you consider?**
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- Watchdog monitor
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- Heartbeat monitor
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5. **Which one did you choose?**
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- Heartbeat monitor
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6. **What is the main reason for that?**
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An heartbeat monitor implementation requires half the amount of messages between the monitoring component and
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the component to check. With an appropriate clock, a heartbeat monitor can be as effective as a watchdog monitor
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to check availability of engine components.
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Pros:
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- Less network traffic
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- Easier to implement
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Cons:
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- Implementation of services is "active" (i.e. aware of the need to signal availability)
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- Power users with a personal deployment who do not wish to deploy an availability monitor are subjected to additional
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network traffic
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@ -1772,6 +1772,152 @@ Exceed: 1, 2, 3, 4, 5, 6, 7, 8
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}
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1. The architecture laid down for homework 11 is already compatible with a SaaS deployment. Considering the deployment view, all nodes other than the
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device the user uses to browse the UI and smart devices, all other components will be served in the cloud. Each engine component and each database could be served in a docker container, and as pointed out already in the deployment view the static assets for the frontend could be served by an existing CDN like Cloudflare.
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2. A three-tier pricing system for the SaaS deployment could be implemented in this way:
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- Up to 10 devices and 5 scenes: free
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- Up to 100 devices and 50 scenes: 5.99 CHF/mo
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- Unlimited devices and scenes: 9.99 CHF/mo
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3. A general 99.9% (three nines) availability of all engine components and the CDN shall be implemented to achieve the components. The time of application of a state change from the time it has been commanded by the UI shall not exceed 2 seconds. The time between a device included in a trigger
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and the corresponding scene being applied on the user's devices shall not exceed 5 seconds.
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4. An updated bottom up component diagram including the monitoring solution follows. The `Heartbeat monitor` component shall be
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deployed on a separate node and expose an HTTPS component which the engine components shall call periodically to signal that they are alive.
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```puml
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@startuml
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skinparam componentStyle rectangle
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!include <tupadr3/font-awesome/database>
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title Smarthut.sm Logical View
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interface " " as DIF
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interface " " as TF
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interface " " as SCF
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component "Heartbeat monitor" as MON {
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interface " " as CF
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interface " " as MF
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[Clock] as C
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[Monitor] as M
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[Dashboard] as DS
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C-CF
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M-MF
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CF)-M
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MF)-DS
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}
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component "IoT Devices" as IOT {
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interface " " as HDF
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interface " " as HSF
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interface " " as ZDF
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interface " " as ZSF
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[HomeKit smart device] as HD <<user provided>>
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[HomeKit smart sensor] as HS <<user provided>>
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[MQTT smart device] as ZD <<user provided>>
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[MQTT smart sensor] as ZS <<user provided>>
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HD--HDF
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HS--HSF
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ZD--ZDF
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ZS--ZSF
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}
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component "User Interface" as UIC {
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interface " " as WSCF
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[User Interface] as UI
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[WebSockets API (JavaScript)] <<external>> as WSC
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WSCF)--UI
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WSC--WSCF
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}
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component "Users and Devices" as DEV {
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interface " " as DIDBF
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interface " " as HAPF
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interface " " as ZIGF
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interface " " as WSSF
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[HAP-java] as HAP <<external>>
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[paho.mqtt.java] as ZIG <<external>>
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[Device Engine] as DI
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note left of DI: Already implemented using Spring Boot
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[javax.websocket API] as WSS <<external>>
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[User and Device DB <$database{scale=0.33}> (PostgreSQL)] <<external>> as DIDB
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HDF )-- HAP
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HSF )-- HAP
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ZDF )-- ZIG
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ZSF )-- ZIG
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DI--(WSSF
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WSSF--WSS
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HAP--HAPF
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ZIG--ZIGF
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HAPF )-- DI
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ZIGF )-- DI
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DIDBF-DIDB
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DI -( DIDBF
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}
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component "Triggers and Automations" as TRIG {
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interface " " as TDBF
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[Trigger and Automation Engine] as T
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[Trigger and Automation DB <$database{scale=0.33}> (PostgreSQL)] <<external>> as TDB
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TDBF-TDB
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T -( TDBF
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}
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component "Scenes" as SCENE {
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interface " " as SCDBF
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[Scene Engine] as SC
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[Scene DB <$database{scale=0.33}> (PostgreSQL)] <<external>> as SCDB
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SCDBF-SCDB
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SC -( SCDBF
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}
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DI--DIF
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TF--T
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SCF--SC
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WSS <..> WSC: WebSocket protocol (RFC 6455)
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DI --( TF
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T --( SCF
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DIF )-- SC
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DIF )-- UI
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UI --( SCF
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UI --( TF
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MF )- DI
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MF )- SC
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MF )- T
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skinparam monochrome true
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skinparam shadowing false
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skinparam defaultFontName Courier
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@enduml
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```
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8. As external dependencies are the smart devices owned by the users, the system cannot function de facto without them being available to the
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Device engine component. However, I plan to implement a "shadow" state for unavailable devices which is regularly maintained as the actual state
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of working devices, which will be applied once the device comes back online. The user interface will also make the device failure visible and notify
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that a "shadow state" is maintained while the device is not reachable.
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# Ex - Flexibility
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{.instructions
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