Document API guarantees for 1.0.0 (#6626)

* Document API guarantees for 1.0.0

* [CR] Better explain "objects will continue to work" promise

* Add note about S3 API support

docs/posts/towards-1.0-sdk.md

@@ -0,0 +1,151 @@
+# lakeFS SDK in 1.0
+
+<img src="file:./images/1.0-is-coming.jpg" height="500px"
+	alt="1.0 is coming: a double rainbow over a traffic jam in Tel Aviv"/>
+
+## Goals
+
+We are putting the finishing touches on the :sparkles: lakeFS 1.0 release
+:sparkles:.  Part of the reason 1.0 is so important is that it promises
+stability to lakeFS users and to lakeFS developers.
+
+lakeFS version numbers generally follow [Semantic
+Versioning](https://semver.org/).  Version **1.0.0** will be the first major
+version, so it will be the first version to promise _major version
+stability_.  A brief summary of semantic versioning: Starting with this
+version,
+
+* **Patch** version upgrades (for instance, from 1.0.0 to 1.0.1, or from
+  1.3.11 to 1.3.12) can fix bugs but cannot add new API capabilities.  All
+  programs that worked with the previous version continue to work with the
+  new version.  All programs that work with the new version and are
+  unaffected by the fixed bugs would work with the old version.
+* **Minor** version upgrades (for instance, from 1.0.0 to 1.1.0, or from
+  1.3.11 to 1.4.0) can add API capabilities but cannot change existing APIs.
+  All programs that worked with the previous version continue to work with
+  the new version.
+* **Major** version upgrades (for instance, from 1.4.0 to 2.0.0, or from
+  1.17.12 to 2.0.0) can add, remove, or change existing APIs entirely.  A
+  program might stop working.
+
+So it's a pretty important change!  Up until now, even a bump from 0.104.0
+to 0.105.0 could break old programs.  Indeed that happened.  Starting with
+1.0.0, a minor version that breaks old programs is a bug.
+
+## 1.0.0 is a **major** version upgrade!
+
+All guarantees here are for minor version upgrades.  The upgrade to 1.0.0 is
+of course a **major version upgrade**, and it will necessarily break some
+client programs.
+
+## Interface guarantees
+
+To detail what 1.0 really promises, we need to list what are the lakeFS
+interfaces.  Obviously a program that relies on an internal implementation
+detail of lakeFS can fail at any version.
+
+Starting with the 1.0 release, we will maintain stability across the
+following interfaces:
+
+* Any call to the [REST OpenAPI API
+  endpoint](https://docs.lakefs.io/reference/api.html) is stable, except for
+  APIs tagged "experimental", "internal", or "deprecated".
+
+  If your program only calls such SDKs, it will continue to work across
+  minor version upgrades.[^1]
+* Any call to such an interface through the published, generated SDKs.
+  These include:
+
+  * [Python `lakefs-sdk`](https://pypi.org/project/lakefs-sdk/)
+  * [Java Maven `io.lakefs:lakefs-sdk`](https://mvnrepository.com/artifact/io.lakefs/lakefs-sdk)
+
+  If your program only uses such calls on the SDK, it will continue to
+  compile and function across a minor version upgrade of the SDK.
+* Any objects that were uploaded to some branch on a lakeFS server and then
+  committed will continue to work after a minor version upgrade to the
+  lakeFS server.  Running an upgrade script may be required.  However
+  running this upgrade script on a cluster of more than a single server
+  should not require downtime.[^2]
+* The lakectl CLI flags will continue to work across minor version upgrades.
+  However we do not currently guarantee the format of lakectl output.
+* Obviously we do _not_ control the S3 API or its many clients.  If your
+  program uses an S3 API that lakeFS supports, it should continue to work
+  across a minor version upgrade of lakeFS.  A minor version upgrade will
+  not remove support for a documented S3 API that is supported in the
+  previous version.
+
+## Why this is important
+
+### For users
+
+Programs that worked with one version >=1.0.0 of lakeFS should continue to
+work with all higher versions <2.0.0.  A minor version upgrade only _adds_
+to your capabilities, but you do not need to do anything if you don't use
+the added capabilities.
+
+### For developers
+
+Don't use APIs tagged "experimental", "internal", or "deprecated".  If you
+do use them, the rest of this section does not apply.  Take into account
+that you may need to modify your program to match any lakeFS server upgrade.
+
+Your program should continue to work with all lakeFS server versions that
+are minor version upgrades from the version you used to develop them.
+
+Additionally, if you depend on one of the published, generated SDKs, then
+that dependency itself also honours semantic versioning.  Switching your
+program to a minor version upgrade of the SDK dependency should be seamless.
+
+#### Java
+
+For Java and for other languages running on the JVM, the current generated
+SDK is *not* stable with respect to a minor version upgrade.  The upgrade
+from `io.lakefs:lakefs-client` to `io.lakefs:lakefs-sdk` will require you to
+rewrite API calls in a new style.
+
+Here is [one example of such a change][lakefsfs-new-sdk-sample].
+Old calls looked like this:
+
+```java
+ObjectStats objectStat = objectsApi.statObject(
+    objectLoc.getRepository(), objectLoc.getRef(), objectLoc.getPath(),
+    false, false);
+```
+
+A single call was a single function call, which had to include optional
+parameters.  If an optional parameter were added, this call would no longer
+compile.  The long argument lists were also a maintenance burden.
+
+Conceptually, API calls in the SDK now follow a fluent style:
+
+```java
+ObjectStats objectStat = objectsApi
+    .statObject(
+        objectLoc.getRepository(), objectLoc.getRef(), objectLoc.getPath()
+	)
+	.userMetadata(true)
+	.execute();
+```
+
+The `statObject` call of the objects API starts by passing all required
+parameters.  Then any optional parameters may be changed.  Here
+`userMetadata` is modified, but `presign` is kept at its default value.
+Finally, the call is _executed_.  When an optional parameter is added, this
+call will simply use its default value.  This keeps the code compatible with
+minor version upgrades of the server.
+
+### For administrators
+
+Your users and your developers no longer need to keep their program versions
+in lockstep with your lakeFS server versions, as long as you **apply only
+minor version upgrades**.
+
+And if your cluster of lakeFS servers has more than one member, you should
+be able to perform most minor version upgrades with no downtime.
+
+
+[^1]: Unless it depends on a bug that was fixed, of course.
+[^2]: Of course this may not be possible with some severe bugfixes.
+    Requiring server downtime is a measure of the severity of the bugfix.
+
+[lakefsfs-new-sdk-sample]:  https://github.com/treeverse/lakeFS/pull/6529/files#diff-4c50b9ac3bf6bfc05e3b6ff0fbe2fd3214f31afb5b449732d90efe5f97f67167R666