Update components documentation (#1090)

* Update components documentation
* Add fixes based on feedback

docs/src/user-manual/components.md

@@ -1,30 +1,143 @@
 # Components
 
-## MinIO
-
-[MinIO](https://min.io/) is a flexible, high performance object storage
-platform. Enduro uses MinIO as its storage back-end for both uploading
-submission information packages (SIPs) and storing archival information packages
-(AIPs). Material intended for preservation can be uploaded to MinIO either
-through the user interface or via command line using the [MinIO
-client](https://min.io/docs/minio/linux/reference/minio-mc.html). Any time new
-content is uploaded to a designated bucket in MinIO, a transfer is started in
-Enduro.
-
-## Temporal
-
-[Temporal](https://temporal.io/) is responsible for orchestrating Enduro's
-workflows - that is, for kicking off tasks, managing them, and recording them as
-auditable events. It also manages retries and timeouts, resulting in a reliable
-platform that can process digital objects for preservation in a highly automated
-environment.
-
-## a3m
-
-[a3m](https://github.com/artefactual-labs/a3m) is a streamlined version of
-[Archivematica](https://archivematica.org) that is wholly focused on AIP
-creation. It does not have external dependencies, integration with access
-systems, search capabilities, or a graphical interface. It was designed to
-reduce the bulk of Archivematica's extraneous functions for users operating at a
-large scale who are more focused on throughput of digital objects for
-preservation.
+Enduro Ingest is composed of a number of different components used to provide
+ingest and preservation functionality.
+
+![enduro components diagram](screenshots/enduro-ingest-components.jpg)
+
+## Processing storage service
+
+The processing storage service acts as Enduro's storage back-end for any local
+package interactions that are needed as part of ingest and preservation
+workflows. Examples include:
+
+* Uploading Submission Information Packages (SIPs) for ingest
+* Depositing failed ingest packages after a workflow error
+* Passing other package types (such as a search metadata bundle) to external
+  systems
+
+If desired, it can also be configured for the long-term storage of Archival
+Information Packages (AIPs) after preservation has completed succesfully.
+
+Currently, Enduro uses [MinIO](https://min.io/) as its processing storage
+service. MinIO is a flexible, high performance object storage platform. Material
+intended for preservation can be uploaded to MinIO either through its user
+interface or via command line using the [MinIO
+client](https://min.io/docs/minio/linux/reference/minio-mc.html).
+
+## Messaging queue
+
+The messaging queue acts as a watcher for SIPs deposited for ingest into the
+[processing storage service](#processing-storage-service). Thanks to this
+watcher, any time new content is uploaded to a designated bucket in MinIO, an
+ingest workflow is started in Enduro.
+
+Enduro Ingest uses [Redis](https://redis.io/) as a messaging queue, to ensure
+reliable communication between the ingest application and the processing storage
+service.
+
+## Workflow engine
+
+The workflow engine is responsible for orchestrating Enduro's workflows - for
+kicking off preservation actions and other tasks, managing them, and recording
+them as auditable events. It also manages retries and timeouts, resulting in a
+reliable platform that can process digital objects for preservation in a highly
+automated environment.
+
+Currently, Enduro Ingest uses [Temporal](https://temporal.io/) as its workflow
+engine.
+
+## Application code
+
+The application code is where the business logic guiding Enduro's workflows is
+written. It is the glue that helps each component in the Enduro Ingest stack act
+in concert, to assist operators achieve their preservation goals.
+
+Enduro  uses the Go programming language ([GoLang](https://go.dev/)) for its
+application code.
+
+## Application Programming Interface (API)
+
+The [API](https://en.wikipedia.org/wiki/API) is how Enduro Ingest communicates
+with its [user interface](#user-interface), and how external users and
+applications can interact with Enduro. The Enduro Ingest API defines multiple
+services which map to resources in [REST](https://en.wikipedia.org/wiki/REST).
+Services define their own methods, errors, and so on.
+
+Enduro's application code uses the [Goa design language](https://goa.design/) to
+structure its API design. The API itself uses [Swagger](https://swagger.io/) to
+generate its documentation and ensure it conforms to
+[OpenAPI](https://www.openapis.org/) design specifications.
+
+## Identity & access management (IAM)
+
+The Identity & access management (IAM) component provides authentication as well
+as role-based access control
+([RBAC](https://en.wikipedia.org/wiki/Role-based_access_control)) to manage user
+permissions and roles in Enduro Ingest.
+[API](#application-programming-interface-api) requests must be authorized by the
+IAM component before their contents are delivered or displayed in the [user
+interface](#user-interface), ensuring that sensitive information is only shared
+with authorized users and/or applications.
+
+[Keycloak](https://www.keycloak.org/) is currently used for Enduro Ingest's IAM
+component. Keycloak is based on standard protocols and provides support for
+OpenID Connect, OAuth 2.0, and SAML.
+
+## User interface
+
+Enduro Ingest's web-based user interface is the graphical method by which
+operators can interact with Enduro to see and manage ingest and preservation
+workflows, view information about packages, and more. Key information from other
+components and applications (such as tasks executed by the [preservation
+engine](#preservation-engine) in a workflow) is also collected via API and shown
+in the Enduro Ingest user interface when relevant.
+
+Enduro's user interface is built using [Vue.js](https://vuejs.org/) and
+[Bootstrap](https://getbootstrap.com/).
+
+## Preservation engine
+
+The preservation engine manages the transformation of Processing Information
+Packages (PIPs) into Archival Information Packages (AIPs). The preservation
+engine is equipped with a number of tools and microservices to perform
+preservation tasks (such as characterization, metadata capture, and
+normalization, etc.), and communicates the outcome of these actions back to
+Enduro for display in the [user interface](#user-interface).
+
+Enduro Ingest currently supports two different preservation engines:
+
+* [Archivematica](https://archivematica.org) is an open-source digital
+  preservation system that is designed to maintain standards-based, long-term
+  access to collections of digital objects. It provides an integrated suite of
+  free and open-source tools that allows users to process digital objects and
+  generate Archival Information Packages (AIPs) in compliance with the Open
+  Archival Information System
+  ([OAIS](https://en.wikipedia.org/wiki/Open_Archival_Information_System))
+  functional model and other digital preservation standards and best practices,
+  define preservation policies, manage processing configurations, and more.
+  Archivematica includes a web-based dashboard for user interaction, as well as
+  a set of [API](https://en.wikipedia.org/wiki/API) endpoints that Enduro uses
+  to integrate with Archivematica and display preservation task outcomes in the
+  Enduro user interface.
+* [a3m](https://github.com/artefactual-labs/a3m) is a streamlined version of
+  Archivematica that is wholly focused on AIP creation. It does not have
+  external dependencies, integration with access systems, search capabilities,
+  or a graphical user interface. a3m was designed  to remove many Archivematica
+  features that are not required for operating at a large scale to improve AIP
+  creation throughput.
+
+## Preservation storage service
+
+The preservation storage service acts as an interface between the [preservation
+engine](#preservation-engine) and whatever storage devices have been configured
+for the long-term preservation of AIPs produced by Enduro workflows. The storage
+service manages the storage, movement, replication, and deletion of packages,
+and other related functions such as location management, encryption, fixity
+checking, and more.
+
+Currently Enduro uses the [Archivematica](https://archivematica.org) Storage
+Service ([AMSS](https://github.com/artefactual/archivematica-storage-service))
+when using Archivematica as the system's preservation engine. When using a3m,
+Enduro uses its own lightweight storage functionality to cover basic storage
+service requirements.