adr: Add ADR for query option to filter flow segment get_urls

docs/README.md

@@ -30,6 +30,8 @@ For more information on how we use ADRs, see [here](./adr/README.md).
 | [0018](./adr/0018-restrict-direct-source-modification.md)          | Restrict direct Source modification                                        |
 | [0019](./adr/0019-consolidate-modified-updated-terms.md)           | Rename `modified_by` properties in Source and Flow schemas to `updated_by` |
 | [0020](./adr/0020-version-signalling.md)                           | Improving the signalling of the supported API version in implementations   |
+| [0021](./adr/0021-storage-label-format.md)                         | Guidance for use of `get_urls` labels on Flow Segments                     |
+| [0023](./adr/0023-filter-segment-get-urls.md)                      | Add query option to filter Flow Segment `get_urls` by `label`              |
 
 \* Note: ADR 0004a was the unintended result of a number clash in the early development of TAMS which wasn't caught before publication
 

docs/adr/0023-filter-segment-get-urls.md

@@ -0,0 +1,78 @@
+---
+status: "proposed"
+---
+# Query Option to Filter get_urls
+
+## Context and Problem Statement
+
+Each Flow Segment returned by the `/flows/<flow id>/segment` endpoint includes a `get_urls` propery that may include (labelled) URLs for fetching media objects.
+The calculation of pre-signed URLs for example can result in an not insignificant increased request time.
+Clients do not need the `get_urls` if they don't intend to fetch the media objects.
+This ADR proposes adding a query option to filter the URLs that need to be included by a TAMS.
+
+It was found that calculating a pre-signed URL for fetching a media object takes around 0.35 milliseconds when using the Python [boto3](https://github.com/boto/boto3) library.
+That means a request to get 5000 Flow Segments will include around 1.75 seconds overhead to calculate the pre-signed URLs.
+This was observed in a request to a TAMS which took around 4 seconds to return Flow Segments with pre-signed URLs and only around 2 seconds to return the same Flow Segments without pre-signed URLs.
+
+The [boto3](https://github.com/boto/boto3) method to generate pre-signed URLs could also be optimised to reduce the time taken.
+It was found that the older deprecated [boto](https://github.com/boto/boto) library took around 0.14 milliseconds to calculate a pre-signed URL, a 60% speed reduction when compared to [boto3](https://github.com/boto/boto3).
+There are likely even more optimisations that could be made.
+
+The pre-signed URL calculation may use a highly optimised implementation that avoids the large time overhead.
+This would still result in larger response sizes because the pre-signed URLs tend to be quite large as they include signatures, keys and tokens.
+
+It is difficult to predict what the expected number of flow segments will be in practice.
+If it is low in all cases then including or not including pre-signed URLs doesn't make much difference.
+If it is unknown then having an option to avoid the pre-signed calculation overhead may be useful in some cases.
+
+## Considered Options
+
+* Option 1: Always return all `get_urls`
+* Option 2: Add a query option to return none or all `get_urls`
+* Option 3: Add a simple query option to filter `get_urls` based on the `label`
+* Option 4: Add more complex query option(s) to filter `get_urls`
+
+## Decision Outcome
+
+Chosen option: "Option 3: Add a simple query option to filter `get_urls` based on the `label`", because this covers the requirement to disable calculation of pre-signed URLs and at the same time allows other URLs to be retained.
+
+### Implementation
+
+Implemented in [PR #88](https://github.com/bbc/tams/pull/88).
+
+## Pros and Cons of the Options
+
+### Option 1: Always return all `get_urls`
+
+This is the current state.
+
+* Bad, because TAMS is doing work to calculate pre-signed URLs even though clients may not need them
+
+### Option 2: Add a query option to return none or all `get_urls`
+
+This adds a boolean option named `include_get_urls` that if set to `false` results in no `get_urls` in the response and therefore no pre-signed URLs are calculated by TAMS.
+
+* Good, because it allows clients to indicate that `get_urls` are not required
+* Bad, because some TAMS implementations may provide different types of URLs and a client does not have the option to filter out the ones that cause increased request time whilst retaining ones that don't
+
+### Option 3: Add a simple query option to filter `get_urls` based on the `label`
+
+This extends [Option 2](#option-2-add-a-query-option-to-return-none-or-all-get_urls) to add a (comma-separated) list option named `accept_get_urls` that specifies the `labels` associated with URLS to include in the response.
+Omitting `accept_get_urls` will result in all URLs in the response.
+Setting `accept_get_urls` to an empty string will result in no URLs in the response.
+If `accept_get_urls` includes a `default` label then URLs with label `default` or no label are included in the response.
+
+* Good, because it allows clients to indicate that `get_urls` are not required
+* Good, because clients can retain URLs that don't cause increased request times
+* Neutral, because clients may want more complex filters based on what is available
+
+### Option 4: Add more complex query option(s) to filter `get_urls`
+
+This changes [Option 3](#option-3-add-a-simple-query-option-to-filter-get_urls-based-on-the-label) to allow more complex filtering based on what URLs are available.
+E.g. include a URL for direct access to the object if available and otherwise a pre-signed URL.
+
+* Good, because it allows clients to indicate that `get_urls` are not required
+* Good, because clients can retain URLs that don't cause increased request times
+* Neutral, because clients can use more complex filters based on what is available
+* Neutral, because it isn't clear whether there is a requirement for more complex filters
+* Neutral, because [0021](./0021-storage-label-format.md) thus far only proposes an App note for guidance on the use of `get_urls` labels on flow segments