Overview

High-level architecture

The diagram below describes the high-level architecture of WebMon, including both internal resources that are considered part of WebMon and external systems that WebMon interacts with. The arrows represent relationships between these services and resources.

        flowchart LR
    FileArchive[("`SNS/HFIR
    File archive`")]
    subgraph DAS
        DASMON
        TranslationService["`Streaming
        Translation
        Client
        (STC)`"]
        SMS["`Stream
        Management
        Service
        (SMS)`"]
    end
    WorkflowManager[Workflow Manager]
    DasmonListener[Dasmon listener]
    Database[(Workflow DB)]
    PostProcessingAgent[Post-Processing Agent]-->ONCat
    LiveDataServer-->WebMon
    LiveDataServer<-->LiveDataDB[(LiveData DB)]
    LiveReduce
    WebMon["`WebMon
    frontend`"]
    SMS-->LiveReduce
    TranslationService-->WorkflowManager
    DASMON-->DasmonListener
    DASMON-->Database
    WorkflowManager-->Database
    WorkflowManager<-->PostProcessingAgent
    PostProcessingAgent-->LiveDataServer
    Database-->WebMon
    ONCat-->WebMon
    LiveReduce-->LiveDataServer
    DasmonListener-->Database
    TranslationService-->FileArchive
    FileArchive<-->PostProcessingAgent
    style DAS fill:#D3D3D3, stroke-dasharray: 5 5
    classDef externalStyle fill:#faf2e6, stroke:#f2e3cb
    class DASMON,TranslationService,SMS,FileArchive,ONCat externalStyle

    subgraph Legend
        direction LR
        Internal["Internal resource"]
        External["External resource"]
        Internal ~~~ External
    end
    LiveReduce ~~~ Internal
    style Legend fill:#FFFFFF,stroke:#000000
    class External externalStyle
    

The gray box labeled “DAS” are services managed by the Data Acquisition System team that pass information to WebMon. Post-Processing Agent interacts with the HFIR/SNS file archive to access instrument-specific reduction scripts and experiment data files. Post-Processing Agent also writes reduced data files and reduction log files back to the file archive.

Another external component is the experiment data catalog, ONCat, where Post-Processing Agent catalogs experiment metadata. The WebMon frontend retrieves and displays this metadata from ONCat.

The section communication_flows includes sequence diagrams that show how the services interact.

Inter-service communication

WebMon uses an ActiveMQ message broker as the main method of communication between services. The message broker also serves as a load balancer by distributing post-processing jobs to the available instances of Post-Processing Agent in a round-robin fashion. Communication with Live Data Server and ONCat occurs via their respective REST API:s.

Table 1 lists the type of communication between pairs services, which are loosely categorized as “client” and “service” in that interaction.

Table 1: Inter-service communication types

“Client”	“Server”	Communication type
Post-Processing Agent	Dasmon Listener	Message queue
Post-Processing Agent	Live Data Server	REST API
Post-Processing Agent	ONCat	REST API
DASMON	Dasmon Listener	Message queue
DASMON	Workflow DB	Direct database
Dasmon Listener	Workflow DB	Direct database
Live Data Server	Live Data DB	Direct database
Livereduce	Live Data Server	REST API
Livereduce	Stream Management Service	Stream socket
Process Variable Streaming Daemon (PVSD)	Dasmon Listener	Message queue
Stream Management Service (SMS)	Dasmon Listener	Message queue
Streaming Translation Client (STC)	Dasmon Listener	Message queue
Streaming Translation Client (STC)	Workflow Manager	Message queue
Workflow Manager	Post-Processing Agent	Message queue
Workflow Manager	Dasmon Listener	Message queue
Workflow Manager	Workflow DB	Direct database
WebMon frontend	Live Data Server	REST API
WebMon frontend	ONCat	REST API
WebMon frontend	Workflow DB	Direct database