The federation connector is configured to use the following projection rule
<mode:projectionRules>/source1 => /a</mode:projectionRules>

The source repository content is
[testng] Retrieving standalone repository ...
[testng] /a

{jcr:primaryType=nt:unstructured, }

[testng] /a/b

{jcr:primaryType=nt:unstructured, }

[testng] /jcr:system

{jcr:primaryType=mode:system, }

If I try to access federated repository I get
Unable to find "/{}a"; lowest existing path is "/"
org.modeshape.graph.connector.federation.JoinRequestProcessor.setPathNotFound(JoinRequestProcessor.java:400)
at org.modeshape.graph.connector.federation.JoinRequestProcessor.process(JoinRequestProcessor.java:333)
at org.modeshape.graph.request.processor.RequestProcessor.process(RequestProcessor.java:284)
at org.modeshape.graph.connector.federation.JoinRequestProcessor.process(JoinRequestProcessor.java:196)
at org.modeshape.graph.connector.federation.JoinRequestProcessor.process(JoinRequestProcessor.java:144)
at org.modeshape.graph.connector.federation.FederatedRepositoryConnection.execute(FederatedRepositoryConnection.java:225)
at org.modeshape.graph.connector.RepositoryConnectionPool$ConnectionWrapper.execute(RepositoryConnectionPool.java:1129)
at org.modeshape.graph.request.CompositeRequestChannel$2.call(CompositeRequestChannel.java:193)
at org.modeshape.graph.request.CompositeRequestChannel$2.call(CompositeRequestChannel.java:183)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:619)
1 lines not shown

If projection rule is changed to
<mode:projectionRules>/source1 => /</mode:projectionRules>

Then the federated repository correctly displays content as
[testng] Retrieving standalone repository ...
[testng] /source1

{jcr:primaryType=nt:unstructured, }

[testng] /source1/a

{jcr:primaryType=nt:unstructured, }

[testng] /source1/a/b

{jcr:primaryType=nt:unstructured, }

[testng] /jcr:system

{jcr:primaryType=mode:system, }