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jortel@redhat.com, 08/29/2017 04:00 PM

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# Downloading
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In pulp3, there are two competing technologies and designs being considered. For the purposes of the discussion we'll name them **Jupiter** and **Saturn**. The *Jupiter* solution is based on *concurrent.futures* and the Saturn solution is based on *asyncio*. In addition to the underlying technology difference, the solutions meet the requirements in different ways. The *Jupiter* solution includes more classes, provides more abstraction and supports extension through object composition. The *Saturn* solution meets the requirements with the fewest classes possible and minimum abstraction.
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The two actors for our use cases is the *Importer* and the *Streamer*. The *ChangeSet* shares a subset of the Streamer requirements but not included in this discussion.
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## Use Cases
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### Importer
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As an importer, I need to download single files.  
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As an importer, I need to download files concurrently.  
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As an importer, I want to validate downloaded files.  
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As an importer, I am not required to keep all content (units) and artifacts in memory to support concurrent downloading.  
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As an importer, I need a way to link a downloaded file to an artifact without keeping all content units and artifacts in memory.  
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As an importer, I can perform concurrent downloading using a synchronous pattern.  
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As an importer, concurrent downloads must share resources such as sessions,connection pools and auth tokens across individual downloads.  
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As an importer I can customize how downloading is performed. For example, to support mirror lists  
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As an importer, concurrent downloading must limit the number of simultaneous connections. Downloading 5k artifacts cannot open 5k connections.  
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As an importer, I can terminate concurrent downlading at any point and not leak resources.  
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As an importer, I can download using any protocol. Starting with HTTP/HTTPS and FTP.
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### Streamer
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As the streamer, I need to download files related to published artifacts and metadata but delegate *the implementation* (protocol, settings, credentials) to the importer. The implementation must be a black-box.  
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As the streamer, I can download using any protocol supported by the importer.  
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As the streamer, I want to validate downloaded files.  
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As the streamer, concurrent downloads must share resources such as sessions,connection pools and auth tokens across individual downloads without having knowledge of such things.  
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As the streamer, I need to support complex downloading such as mirror lists. This complexity must be delegated to the importer.  
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As the streamer, I need to bridge the downloaded bit stream to the Twisted response. The file is not written to disk.  
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As the streamer, I need to forward HTTP headers from the download response to the twisted response.  
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As the streamer, I can download using (the same) custom logic as the importer such as supporting mirror lists